THE INFLUENCE OF PROFESSIONAL STATUS ON MAXIMAL AND RAPID ISOMETRIC TORQUE CHARACTERISTICS IN ELITE SOCCER REFEREES TY B. PALMER,1 MATT J. HAWKEY,1 DOUG B. SMITH,1
AND
BRENNAN J. THOMPSON2
1
Applied Musculoskeletal and Human Physiology Research Laboratory, Department of Health and Human Performance, Oklahoma State University, Stillwater, Oklahoma; and 2Human Performance Laboratory, Department of Health, Exercise, and Sports Sciences, Texas Tech University, Lubbock, Texas ABSTRACT
INTRODUCTION
Palmer, TB, Hawkey, MJ, Smith, DB, and Thompson, BJ. The influence of professional status on maximal and rapid isometric torque characteristics in elite soccer referees. J Strength Cond Res 28(5): 1310–1318, 2014—The purpose of this study was to examine the effectiveness of maximal and rapid isometric torque characteristics of the posterior muscles of the hip and thigh and lower-body power to discriminate between professional status in full-time and part-time professional soccer referees. Seven full-time (mean 6 SE: age = 36 6 2 years; mass = 82 6 4 kg; and height = 179 6 3 cm) and 9 part-time (age = 34 6 2 years; mass = 84 6 2 kg; and height = 181 6 2 cm) professional soccer referees performed 2 isometric maximal voluntary contractions (MVCs) of the posterior muscles of the hip and thigh. Peak torque (PT) and absolute and relative rate of torque development (RTD) were calculated from a torque-time curve that was recorded during each MVC. Lower-body power output was assessed through a vertical jump test. Results indicated that the rapid torque characteristics were greater in the full-time compared with the part-time referees for absolute RTD (p = 0.011) and relative RTD at 1/2 (p = 0.022) and 2/3 (p = 0.033) of the normalized torque-time curve. However, no differences were observed for PT (p = 0.660) or peak power (Pmax, p = 0.149) between groups. These findings suggest that rapid torque characteristics of the posterior muscles of the hip and thigh may be sensitive and effective measures for discriminating between full-time and part-time professional soccer referees. Strength and conditioning coaches may use these findings to help identify professional soccer referees with high explosive strengthrelated capacities and possibly overall refereeing ability.
KEY WORDS peak torque, rate of torque development, hamstrings, hip extension, vertical jump Address correspondence to Dr. Brennan J. Thompson, brennan. [email protected]. 28(5)/1310–1318 Journal of Strength and Conditioning Research Ó 2014 National Strength and Conditioning Association
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eferees play an important role in the quality and development of the sport of soccer (7). During competition, soccer referees are required to regulate the actions of players by keeping pace with the intensity of play (12,26). Over recent years, the intensity of soccer play has increased (33), which has subjected referees to greater physiological demands in keeping pace with player movements and monitoring playing actions from optimal viewing positions (39). In light of these increasing demands, previous studies (6,7,11,28,39) have suggested that the fitness level of referees may be an important component for providing high-quality officiating and, therefore, may be used to discriminate between referees of varying professional and competitive statuses. For example, Bartha et al. (7) showed that the aerobic fitness level demonstrated in a 12-minute run was significantly higher for elite-level soccer referees than lower-level referees, and thus was an effective measure at determining referee status. Furthermore, in a study of Greek referees of different competitive levels, anaerobic sprint performance was shown to be significantly related to refereeing ability (28). Previous authors (6,11,39) have indicated that refereeing ability is influenced by the capacity to perform intense intermittent exercise. Consequently, performance attributes resulting in a greater capacity to perform high-intensity sprints, including rapid acceleration and quick changes of direction may be an important aspect of refereeing performance ability and could help referees in achieving higher professional status. Taken together, the previous studies suggest that both aerobic and anaerobic dynamic fitness measures are important predictors of refereeing performance; however, considering the potential influence of explosive strength on these parameters (40), it may be important to investigate the use of maximal and explosive strength measures to differentiate between referee status in an elite professional referee population. Maximal and explosive strength are considered essential physiological performance parameters for successful performance in many sports (4,32) and are typically measured in vivo from an isometric torque-time curve that is recorded during a voluntary
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Journal of Strength and Conditioning Research isometric maximal contraction (42). Maximal torque production (i.e., peak torque [PT]) has been shown to be an important predictor of performance in power-type sports, such as weightlifting (10) and sprint cycling (32); however, because maximal torque requires .300 milliseconds to achieve (36), it may not be functionally relevant for intermittent-type sports (i.e., soccer, tennis) that involve movement durations that last ,250 milliseconds (i.e., sprinting, accelerating, and cutting) (4,34). Alternatively, rapid torque characteristics, which include rate of torque development (RTD), have functional significance in fast (i.e., #250 milliseconds) and forceful muscle contractions (2), and thus, may be better discriminators of performance in sports that include high-intensity sprints and quick changes of direction. To obtain an ideal viewing position, which involves being in a close proximity to the playing actions, elite-level soccer referees are often required to perform a large number of sprint-type movements during competition (19,33). Consequently, given the association with explosive type tasks, rapid torque characteristics may be particularly effective for determining refereeing status, in comparison with maximal strength. Although maximal and explosive strength have been examined extensively in athletes (15,24,30,32,34,38), previous authors have acknowledged an important research gap regarding strength-related characteristics in referees. For example, Castagna et al. (12) previously stated that “no research has been published on the strength performance of soccer referees. [which] would be of importance for the development of proper training strategies.” (p. 640). Thus, future studies are needed to examine the characteristics of strength-related measures in professional soccer referees and to evaluate the ability of these parameters to discriminate among referee status, which could provide insight regarding implications in the development of proper training programs for this population. In addition to isometric strength-related measures being potentially useful performance characteristics, dynamic muscle power has also been suggested as being an important variable for explosive-type activities. Previous authors have reported that lower-body muscle power was effective at discriminating among athletic performances in explosive-type sports including volleyball (29), weightlifting (10), and sprint cycling (32). Given its dynamic relevance, muscle power may be an important performance-related variable and assessment tool when examining referee performance abilities. Because the hamstrings are more commonly recognized for their actions as leg flexors rather than hip extensors (37), the contribution of the hamstrings in extending the thigh at the hip is often overlooked (25). Moreover, previous studies (3,5,35) examining the strength capacities of the hamstrings have traditionally used an isometric leg flexion test; however, leg flexion does not take into consideration the strength contributions of the other hip extensor muscles surrounding the posterior hip and thigh (i.e., gluteus maximus, adductor magnus) (37). Hip extensor strength has been reported to be a large contributor to important functional-related tasks
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including walking (9), running (14), and jumping (18). Therefore, from a functional standpoint, using an isometric hip extension assessment may provide a highly relevant, safe, and effective method for examining the maximal and explosive strength characteristics of the posterior muscles of the hip and thigh, which may offer the advantages of encompassing a more complete functional aspect of these muscles with the practical utility of a relatively simple and yet portable (potential for field-based testing) strength-related performance test. Thus, the purpose of this study was to perform a strength-based profiling assessment of an elite professional referee population and to examine the effectiveness of maximal and rapid isometric torque characteristics during an isometric hip extension test to discriminate between referee status in full-time and part-time professional soccer referees. A secondary aim of this study was to compare vertical jump power between the 2 referee groups and to examine the relationships between rapid torque development and lower-body muscle power and strength.
METHODS Experimental Approach to the Problem
A cross-sectional research design was used to investigate the ability of maximal and rapid isometric torque characteristics and vertical jump performance to discriminate between professional referee status (PRS) in full-time and part-time professional soccer referees. Previous studies (6,7,11,28,39) have demonstrated that dynamic measures of performance (i.e., speed, endurance, and agility) may discriminate between soccer referees of different professional levels. However, we are aware of no studies that have examined maximal and rapid isometric torque characteristics to differentiate between PRS in this population. Thus, full-time and parttime professional soccer referees performed isometric strength testing of the hip extensors and a vertical jump test to assess maximal and rapid isometric torque characteristics and lower-body power output, respectively. It was hypothesized that the field-based rapid torque characteristics would discriminate between the full-time and part-time referees and that rapid torque development would also be related to lower-body power output and strength. Subjects
Sixteen healthy, professional men soccer referees employed by the Professional Referee Organization volunteered for this investigation. All demographic data are presented in Table 1. Each referee had at least 7 years of officiating experience, of which 3 were spent in major league soccer (MLS). Of the 16 participants, 7 were identified as full-time professional referees and 9 as part-time professional referees. Classification of PRS (“full-time” vs. “part-time”) was made qualitatively based on a review of the biographical sketches of each referee in accordance with the number of games officiated. Specifically, the full-time referees officiated an average of 2 games per month, from March to December (MLS competitive season), whereas the part-time referees only officiated an average of 1 game per VOLUME 28 | NUMBER 5 | MAY 2014 |
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Isometric Testing and Professional Referee Status month. None of the referees reported any current or ongoing neuromuscular diseases or musculoskeletal injuries specific to the ankle, knee, or hip joints. At the time of testing, all referees were involved in a weekly training program that consisted of 3 h$wk21 of resistance exercise aimed towards hypertrophy and strength (i.e., 60–95% 1 repetition maximum) and 5 h$wk21 of conditioning exercise aimed towards endurance, speed, agility, and power. Testing for all referees was completed immediately after the conclusion of the 2012 MLS competitive season. This study was approved by the University’s Institutional Review Board for human subjects research, and before any testing, each participant completed an informed consent document and health history questionnaire. Procedures
All testing was conducted at the Cooper Aerobics Center in Dallas, Texas. Each participant visited the center on 2 occasions separated by 2–3 days. During the first visit, anthropometric measurements of circumference and skinfolds were performed on the right thigh and participants were familiarized with the testing procedures by performing several vertical jumps and isometric strength assessment trials. During the second visit, participants completed 3 countermovement vertical jumps (CMJ) followed by 2 maximal isometric strength assessments of the hip extensors. Assessment of Maximal and Rapid Isometric Torque Characteristics
Maximal and rapid isometric torque characteristics were quantified using a manual isometric strength assessment technique, which consisted of the participant extending the thigh towards the floor against a load cell (LCHD-250; Omega Engineering, Inc., Bridgeport, NJ, USA) attached immediately posterior to the heel (Figure 1). Participants laid supine during each isometric strength assessment with the knee braced in full extension and the ankle immobilized in a neutral 908 position (between foot and leg) with a custommade cast that was fixed around the foot and held with straps above the ankle and over the toes and metatarsals. Because assessments were conducted in a field-based setting, stabilizing the hips with restraining straps was not feasible in the present
study. However, to provide a measure of control during the testing, the investigator instructed the participants to maintain the hips in a stable position with a verbal instruction to “keep the hips in constant contact with the floor” throughout the entire duration of the maximal contraction for each maximal voluntary contraction (MVC) assessment. All assessments were performed on the right leg at a hip joint angle of 208 above the horizontal plane, which was consistent with the hip joint angle for the isometric hip extensor strength assessment previously reported by Perry et al. (25). Before the maximal isometric strength testing, participants performed 3 submaximal isometric hip extension muscle actions at approximately 75% of their perceived maximal effort. After the submaximal contractions, each participant performed 2 isometric MVCs of the posterior muscles of the hip and thigh with 1 minute of recovery between each contraction. For all isometric MVCs, participants were verbally instructed to extend the thigh “as hard and fast as possible” for a total of 3–4 seconds, and strong verbal encouragement was given throughout the duration of the MVC (34). Estimated Muscle Cross-Sectional Area
Estimated muscle cross-sectional area (eCSA) of the right thigh was determined using thigh circumference and corrected for subcutaneous fat with thigh skinfold measurements (23). Briefly, the radius was calculated from the circumference and eCSA was quantified by subtracting the mean of the 4 skinfolds taken at the anterior, lateral, posterior, and medial thigh positions (23). Circumference was measured with a tension-gauged measuring tape (Gullick II; Country Technology, Inc., Gays Mills, WI, USA) and skinfolds were measured with a Lange caliper (Santa Cruz, CA, USA). All anthropometric measurements were taken to the nearest millimeter at the midpoint of the thigh between the inguinal crease and the proximal border of the patella with the participant standing upright with approximately equal distribution of body mass on each leg and the feet placed shoulder width apart (41). Muscle eCSA (in square centimeters) was calculated using the equation by Moritani and deVries (23):
TABLE 1. Mean (SE) values for age, height, body mass, and estimated muscle eCSA.* Full-time (n = 7) Age (y) Height (cm) Mass (kg) Muscle eCSA (cm2)
36.00 178.53 82.14 156.30
(2.38) (2.97) (3.64) (5.88)
Part-time (n = 9) 34.11 181.19 84.28 162.85
(2.06) (1.80) (2.47) (8.28)
*eCSA = estimated cross-sectional area.
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Figure 1. An example of the manual isometric strength assessment. Participants were verbally instructed to extend the thigh towards the floor as hard as possible against a load cell attached immediately posterior to the heel for 3–4 seconds.
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procedures and equation by Moritani and deVries (23) provided reliable (intraclass correlation coefficient [ICC] = 0.98; standard error of measurement [SEM] expressed as a percentage of the mean = 2.7%) and valid estimates of muscle eCSA. Assessment of Lower-Body Power Output
Lower-body power output was measured during the CMJs using a linear velocity transducer (Tendo weightlifting analyzer, Tendo Sports Machines, Slovak Republic) that was attached to the posterior portion of a belt fastened around the participants’ waistFigure 2. An example of a torque-time curve taken from a participant during an isometric maximal voluntary line (27). During the CMJs, contraction (MVC) of the posterior muscles of the hip and thigh. Peak torque (PT; in Newton meters) was determined from the highest 500-millisecond epoch. Rate of torque development (RTD; in Newton meters per participants stood on the floor second) was determined from the linear slope of the torque-time curve over the time interval of 0–200 milliseconds with feet shoulder width apart from onset. and hands positioned on the hips. Participants were not allowed to take any steps before " #2 performing the CMJ and a quick descending quarter-squat P4 Circumference skfi countermovement was allowed before the ascending take-off 2 i ¼1 CSA ¼ p ; 2p 4 phase. The participants were instructed to jump as explosively as possible with both feet at the same time and land on where Circumference was defined as the circumference of the the floor in the starting position. To calculate power output thigh, and skf was defined as the sum of the thigh skinfold in watts, each participants’ body mass was measured with thicknesses from each of the 4 sites. A recent study by a stadium scale (Detecto, Webb City, MO, USA) and entered DeFreitas et al. (13) demonstrated that the anthropometric into the linear velocity transducer microcomputer. After the
TABLE 2. Mean (SE), 95% CI, and effect size values for PT, RTD, relative RTD at 1/6, 1/2, and 2/3 MVC, and vertical jump Pmax.*
PT (N$m) 95% CI RTD (N$m$s21) 95% CI Relative RTD1/6 (% MVC$s21) 95% CI Relative RTD1/2 (%MVC$s21) 95% CI Relative RTD2/3 (%MVC$s21) 95% CI Pmax (W) 95% CI
Full-time (n = 7)
Part-time (n = 9)
Effect size
109.49 (11.67) 80.93–138.06 373.23 (33.62)† 290.96–455.50 385.84 (58.41) 242.93–528.76 488.49 (108.46)† 223.11–753.87 474.85 (120.43)† 180.17–769.53 4058.29 (285.64) 3359.35–4757.23
103.57 (7.29) 86.76–120.38 223.20 (36.95) 137.98–308.41 297.97 (41.61) 202.02–393.93 221.34 (36.30) 137.64–305.05 217.30 (25.84) 157.72–276.89 3420.33 (293.06) 2744.53–4096.13
0.23 1.20 0.62 1.11 1.04 0.74
*PT = peak torque; CI = confidence interval; RTD = rate of torque development; MVC = maximal voluntary contraction; Pmax = peak power output. †Significantly greater RTD and relative RTD at 1/2 and 2/3 MVC for the full-time referees than the part-time referees (p # 0.05).
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Isometric Testing and Professional Referee Status completion of each jump, estimated peak power output (Pmax, Watts) was calculated and displayed by the microcomputer. A total of 3 jumps were performed with 1 minute of rest between trials, and the average of the 3 trials for Pmax was used for all subsequent data analyses. Previous reliability statistics for these procedures from our laboratory have revealed an ICC of 0.89 and a SEM value expressed as a percentage of the mean of 4.9% (31). Signal Processing
During each isometric strength assessment, the force (Newton) signal was sampled from the load cell at 1 kHz (MP100WSW; Biopac Systems, Inc., Santa Barbara, CA, USA), stored on a personal computer (Dell Inspiron 8200; Dell, Inc., Round Rock, TX, USA), and processed off-line using custom-written software (LabVIEW, Version 11.0; National Instruments, Austin, TX, USA). The scaled force signal was low-pass filtered with a 10-Hz cutoff (zero-phase lag, fourth-order Butterworth filter). A torque (Newton meters) signal was subsequently derived off-line by multiplying the force signal
from the load cell by the limb length (meters) for each participant. Limb length was measured as the distance from the greater trochanter to the heel (positioned over the load cell), and was measured once before the familiarization trial. The passive baseline torque value was considered the limb weight and subtracted from the signal so that the new baseline value was 0 Nm. All subsequent analyses were conducted on the scaled, filtered, and gravity-corrected torque signal. Isometric MVC PT was determined as the highest mean 500-millisecond epoch during the entire 3–4 second MVC plateau (Figure 2). The absolute RTD (Newton meters per second) was calculated as the linear slope of the torque-time curve (Dtorque/Dtime) over the time interval of 0–200 milliseconds (Figure 2) (20). The absolute torque-time curve was also subsequently normalized to PT, from which relative RTD values (percentage of MVC per second) were calculated from the normalized torque-time curve at 1/6, 1/2, and 2/3 of the MVC per the procedures of Aagaard et al. (1,2). The onset of contraction was determined as the point when the torque signal reached a threshold of 4 Nm, which was selected based on the torque threshold for the leg flexors previously reported by Thompson et al. (34). Test-retest reliability from our laboratory was examined during manual isometric hip extension MVC’s from 10 participants measured 48–72 hours apart. For all of the maximal and rapid isometric torque variables reported in this study, the ICC and SEM values expressed as a percentage of the mean ranged from 0.63 to 0.89 and 9.1 to 21.0%, respectively. Statistical Analyses
Independent samples t-tests were used to analyze differences in demographic characteristics, PT, absolute and normalized RTD variables, and Pmax between the full-time and parttime referees. Type I error rates (P) and effect sizes were reported for each t-test. Effect size statistics were calculated using the following equation (17): Figure 3. A) Isometric peak torque (PT) and (B) absolute rate of torque development (RTD) values for the full-time and part-time referees. Values are mean 6 SE. *Significant difference for RTD between groups. Full-time . parttime (p , 0.05).
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Effect Size ¼ jx1 2x2 j=σ; where x represents the mean and σ is the pooled SD.
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Figure 4. Relative rate of torque development (RTD) values at 1/6, 1/2, and 2/3 MVC for the full-time and part-time referees. Values are mean 6 SE. *Significant difference between groups for RTD at 1/2 and 2/3 MVC. Full-time . part-time (p # 0.05).
Using the pooled data of all participants (full-time and part-time referees, n = 16), the relationships between absolute RTD, PT, and Pmax were determined by Pearson product moment correlation coefficients (r). Statistical analyses were performed using IBM SPSS Statistics v. 20.0 (SPSS Inc., Chicago, IL, USA), and an alpha level of P # 0.05 was used to determine statistical significance.
RESULTS There were no differences (p = 0.434–0.623) between the full-time and part-time referees for age, height, mass, and muscle eCSA (Table 1). Table 2 shows the means, standard errors (SE), confidence intervals, and effect sizes for PT, absolute and relative RTD values, and Pmax. The full-time referees exhibited greater absolute RTD (p = 0.011; Figure 3) and relative RTD at 1/2 and 2/3 MVC (p = 0.022 and 0.033; Figure 4) compared with the part-time referees. There were no differences between the full-time and part-time referees for PT (p = 0.660; Figure 3), relative RTD at 1/6 MVC (p = 0.228; Figure 4), or Pmax (p = 0.149). Significant relationships were observed between absolute RTD and Pmax and PT (r = 0.527 and 0.621; p = 0.036 and 0.010); however, there was no relationship observed between PT and Pmax (r = 0.188; p = 0.486).
DISCUSSION The primary findings of this study revealed that the absolute and relative rapid torque characteristics were significantly greater for the full-time referees than the part-time referees, despite no differences between groups for maximal torque and power characteristics (i.e., PT and Pmax) or anthropometric measurements (Tables 1 and 2). In addition, significant positive relationships were observed between absolute RTD and Pmax and PT based on the pooled data of all referees.
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The present finding that the absolute and relative isometric rapid torque characteristics were greater in the fulltime compared with the part-time referees demonstrated the effectiveness of these characteristics at discriminating between PRS (Figures 3 and 4) in an elite professional referee population. Previous studies have also reported the capacity of performance tests to discriminate among referees, demonstrating that dynamic measures of performance such as 50- and 200-m sprints (7,28), 12-minute endurance runs (7), and intermittent recovery tests (6,7,11,39) may be used as discriminators among soccer referees of different professional levels. However, we are aware of no previous studies that have used strength-related parameters to differentiate between PRS in this population. Thus, the present findings may yield some novel insight, highlighting the potential importance of rapid torque capacities for professional soccer referees; however, because of a lack of strength-related research in this population, the ability to make comparisons with previous studies among these variables is limited. Nevertheless, the finding that rapid torque characteristics may be a more sensitive discriminating parameter when compared with maximal strength aligns with those previously reported in athletes of other sports. For example, Thompson et al. (34) demonstrated that rapid isometric torque characteristics of the leg flexors were able to discriminate between playing status in collegiate American football players, such that starters had greater rapid torque production than nonstarters, despite no differences in maximal strength. Furthermore, Sleivert et al. (30) showed that power athletes who performed more explosive-type activities during games and practices were capable of generating greater relative RTD values of the leg extensors and plantar flexors than endurance athletes who performed only aerobic-type activities. It was hypothesized that because RTD was measured relative to isometric PT, game participation for the power athletes resulted in a qualitative change of the contractile characteristics of the muscle (30). For elite-level soccer referees, the physiological demands of game participation often require the performance of a relatively large amount (i.e., 150–200 per game) of explosive-type activities (19). Therefore, it is possible that full-time professional soccer referees, such as those in the present study, who officiate more games, and consequently perform more explosive-type activities over the course of a competitive season may exhibit higher relative RTD values, as a result of game-induced qualitative factors (i.e., motor unit recruitment and firing rates, fiber type, tendon stiffness, pennation angle, etc.) (2,30). However, given the scope of the present study and the limited data available regarding these variables and findings, it is not presently feasible to ascertain the cause and effect associations, consequences, or patterns resulting in the higher rapid torque variables for the full-time vs. part-time referees. Thus, future research is necessary to elucidate more specifically the mechanisms that may be responsible for the RTD differences displayed between groups of referees with varying professional statuses as well VOLUME 28 | NUMBER 5 | MAY 2014 |
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Isometric Testing and Professional Referee Status as the magnitude of the importance of rapid lower-body strength characteristics and the association pertaining to on the field job performance abilities in higher level soccer referees. In addition to PT being similar between referee groups, Pmax was also an ineffective PRS discriminating variable. These findings were consistent with those previously reported among young tennis athletes of varying athletic ability (8); however, in other sports, such as weightlifting and sprint cycling, lower-body muscle power and strength have been shown to be important determinants of athletic success and elite-level status (10,32). These conflicting results may be because of differences in the activity requirements of the sports that were tested. For example, weightlifting and sprint cycling involve activity patterns that require higher anaerobic intensities than soccer and tennis (21) and therefore, measurements of maximal strength and vertical jump power may be more important to the success of individuals who participate in those types of sports. Furthermore, previous studies have indicated that because the intensity of a soccer game is dependent on the level of competition (22), higher level soccer referees may be required to perform more sprints and quick changes of direction throughout the course of a game to better position themselves to observe the actions of the players (11). Therefore, it is possible that because these referee-related activities (i.e., sprinting, accelerating, and cutting) involve relatively short foot strike contact times (#100 milliseconds) (4,34), rapid torque production (i.e., RTD #200 milliseconds) of the posterior muscles of the hip and thigh may be more functionally relevant than vertical jump and maximal strength characteristics, which involve relatively long torque development durations in excess of 200 milliseconds (4,36). However, it is also possible that the limited number of participants may have influenced the maximal strength and vertical jump data in the present study and thus, more research with larger sample sizes is needed to further examine the effects of referee status on the muscle power output and strength characteristics of the posterior muscles of the hip and thigh. Previous authors who have investigated the relationships between isometric strength-related variables and vertical jump power have reported contrasting findings (15,16,24,38). For example, Haff et al. (16) reported that peak force and rapid force development as measured during an isometric midthigh pull test were unrelated to vertical jump power, whereas Nuzzo et al. (24) reported that both maximal and rapid isometric force variables from the midthigh pull and isometric squat tests were effective measures at predicting vertical jump power capacities. Our findings add to these conflicting results by demonstrating a significant positive relationship between absolute RTD and Pmax with no relationship between PT and Pmax. The inconsistencies between these findings and those reported by previous studies (15,16,24,38) may be a result of discrepancies in vertical jump techniques (squat vs. CMJ), isometric testing modalities (i.e., midthigh pull, squat, and hip
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extension), the joint angle of testing, and body mass variability among participants (35). To our knowledge, this was the first study to use a supine hip extensor test to investigate the relationships between maximal and rapid isometric strength characteristics of the posterior muscles of the hip and thigh and vertical jump power. Although the posterior muscles of the hip and thigh have been identified as being important contributors to vertical jumping movements (18), these muscles and their ability to extend the thigh at the hip have largely been overlooked (25), particularly as they pertain to functional performance relationships. A recent study by Thompson et al. (35) demonstrated a significant relationship between the hamstrings (which are included within the posterior muscles of the hip and thigh), and vertical jump performance, which supports the present findings regarding the relationships of the posterior muscles of the hip and thigh and vertical jump performance. However, as in the example of the previous study, the hamstrings muscle group has typically been assessed for their actions as leg flexors rather than hip extensors (3,5,35). Because the explosive strength exhibited by the hamstrings during an isometric leg flexion test does not take into account the explosive strength contributions of the other hip extensor muscles (i.e., gluteus maximus, adductor magnus) (37), the hip extension test may be more functionally meaningful in terms of explosive strengthrelated performances. Thus, the present findings highlight the important relationship between rapid torque capacities of the posterior muscles of the hip and thigh and vertical jump performance, and suggest that isometric rapid torque testing may share similar performance qualities with dynamic actions such as the vertical jump. In addition, the manual technique proposed in the present study may provide researchers with a reliable and portable measurement tool capable of enhancing the practicality and utility of these (typically lab based) measurements when examining the lower-body performance capacities of athletes in the field.
PRACTICAL APPLICATIONS Given the absence of strength-related research investigating professional referees, this study aimed to examine strengthrelated parameters in an elite professional referee population to evaluate the characteristics and discriminating abilities of these measures based on level of referee status. The results showed that the rapid isometric torque characteristics of the posterior muscles of the hip and thigh were the only variables to effectively discriminate between the full-time and part-time referees. These findings provide support for assessing rapid torque capacities in professional soccer referees and perhaps demonstrate the functional utility of a manual isometric hip extension assessment to discriminate between PRS in this population. Moreover, given the higher rapid torque characteristics of the full-time referees in the present study and the positive relationships observed between rapid torque production and measures of muscle power output and strength, coaches and employers may use these findings as
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Journal of Strength and Conditioning Research performance evaluation tools to help in identifying professional referees with high lower-body explosive strengthrelated capacities (i.e., RTD) and possibly overall refereeing ability. These findings highlight a potentially important role of strength and explosive strength-related parameters in elite referees and provide support for the need to further investigate these performance measures in this population. In addition, training programs aimed at increasing rapid torque production of the posterior muscles of the hip and thigh may be beneficial for both increasing referee status and simultaneously enhancing muscle power output and strength capacities.
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27. Rhea, MR, Kenn, JG, and Dermody, BM. Alterations in speed of squat movement and the use of accommodated resistance among college athletes training for power. J Strength Cond Res 23: 2645– 2650, 2009.
8. Bencke, J, Damsgaard, R, Saekmose, A, Jørgensen, P, Jørgensen, K, and Klausen, K. Anaerobic power and muscle strength characteristics of 11 years old elite and non-elite boys and girls from gymnastics, team handball, tennis and swimming. Scand J Med Sci Sports 12: 171– 178, 2002. 9. Burnfield, JM, Josephson, KR, Powers, CM, and Rubenstein, LZ. The influence of lower extremity joint torque on gait characteristics in elderly men. Arch Phys Med Rehabil 81: 1153–1157, 2000. 10. Carlock, JM, Smith, SL, Hartman, MJ, Morris, RT, Ciroslan, DA, Pierce, KC, Newton, RU, Harman, EA, Sands, WA, and Stone, MH. The relationship between vertical jump power estimates and weightlifting ability: A field-test approach. J Strength Cond Res 18: 534–539, 2004. 11. Castagna, C, Abt, G, and D’Ottavio, S. Competitive-level differences in Yo-Yo intermittent recovery and twelve minute run test performance in soccer referees. J Strength Cond Res 19: 805–809, 2005. 12. Castagna, C, Abt, G, and D’Ottavio, S. Physiological aspects of soccer refereeing performance and training. Sports Med 37: 625–646, 2007. 13. DeFreitas, JM, Beck, TW, Stock, MS, Dillon, MA, Sherk, VD, Stout, JR, and Cramer, JT. A comparison of techniques for estimating training-induced changes in muscle cross-sectional area. J Strength Cond Res 24: 2383–2389, 2010. 14. Delecluse, C. Influence of strength training on sprint running performance. Current findings and implications for training. Sports Med 24: 147–156, 1997. 15. Haff, GG, Carlock, JM, Hartman, MJ, Kilgore, JL, Kawamori, N, Jackson, JR, Morris, RT, Sands, WA, and Stone, MH. Force-time
28. Rontoyannis, G, Stalikas, A, Sarros, G, and Vlastaris, A. Medical, morphological and functional aspects of Greek football referees. J Sports Med Phys Fitness 38: 208–214, 1998. 29. Sheppard, JM, Cormack, S, Taylor, KL, McGuigan, MR, and Newton, RU. Assessing the force-velocity characteristics of the leg extensors in well-trained athletes: The incremental load power profile. J Strength Cond Res 22: 1320–1326, 2008. 30. Sleivert, GG, Backus, RD, and Wenger, HA. Neuromuscular differences between volleyball players, middle distance runners and untrained controls. Int J Sports Med 16: 390–398, 1995. 31. Sobolewski, EJ, Smith, DB, Thompson, BJ, Everett, KL, Fiddler, RE, and Ryan, ED. Test-retest reliability of peak power, average power, and vertical jump height using portable commercial measurement systems. J Strength Cond Res 26: S44, 2012. 32. Stone, MH, Sands, WA, Carlock, J, Callan, S, Dickie, D, Daigle, K, Cotton, J, Smith, SL, and Hartman, M. The importance of isometric maximum strength and peak rate-of-force development in sprint cycling. J Strength Cond Res 18: 878–884, 2004. 33. Tessitore, A, Cortis, C, Meeusen, R, and Capranica, L. Power performance of soccer referees before, during, and after official matches. J Strength Cond Res 21: 1183–1187, 2007. 34. Thompson, BJ, Ryan, ED, Sobolewski, EJ, Smith, DB, Conchola, EC, Akehi, K, and Buckminster, T. Can maximal and rapid isometric torque characteristics predict playing level in division I american collegiate football players? J Strength Cond Res 27: 655–661, 2013. VOLUME 28 | NUMBER 5 | MAY 2014 |
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Isometric Testing and Professional Referee Status 35. Thompson, BJ, Ryan, ED, Sobolewski, EJ, Smith, DB, Akehi, K, Conchola, EC, and Buckminster, T. Relationships between rapid isometric torque characteristics and vertical jump performance in division I collegiate American football players: Influence of body mass normalization. J Strength Cond Res 27: 2737–2742, 2013. 36. Thorstensson, A, Karlsson, J, Viitasalo, JHT, Luhtanen, P, and Komi, PV. Effect of strength training on EMG of human skeletal muscle. Acta Physiol Scand 98: 232–236, 1976. 37. Waters, R, Perry, J, McDaniels, J, and House, K. The relative strength of the hamstrings during hip extension. J Bone Joint Surg Am 56: 1592–1597, 1974. 38. West, DJ, Owen, NJ, Jones, MR, Bracken, RM, Cook, CJ, Cunningham, DA, Shearer, DA, Finn, CV, Newton, RU, Crewther, BT, and Kilduff, LP. Relationships between forcetime characteristics of the isometric midthigh pull and dynamic
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performance in professional rugby league players. J Strength Cond Res 25: 3070–3075, 2011. 39. Weston, M, Helsen, W, MacMahon, C, and Kirkendall, D. The impact of specific high-intensity training sessions on football referees’ fitness levels. Am J Sports Med 32: 54S–61S, 2004. 40. Weyand, PG, Sternlight, DB, Bellizzi, MJ, and Wright, S. Faster top running speeds are achieved with greater ground forces not more rapid leg movements. J Appl Physiol (1985) 89: 1991–1999, 2000. 41. Whaley, MH, Brubaker, PH, and Otto, RM. ACSM’s Guidelines for Exercise Testing and Prescription. Philadelphia, PA: Lippincott Williams & Wilkins, 2006. 42. Wilson, GJ and Murphy, AJ. The use of isometric tests of muscular function in athletic assessment. Sports Med 22: 19–37, 1996.
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AND
BRENNAN J. THOMPSON2
1
Applied Musculoskeletal and Human Physiology Research Laboratory, Department of Health and Human Performance, Oklahoma State University, Stillwater, Oklahoma; and 2Human Performance Laboratory, Department of Health, Exercise, and Sports Sciences, Texas Tech University, Lubbock, Texas ABSTRACT
INTRODUCTION
Palmer, TB, Hawkey, MJ, Smith, DB, and Thompson, BJ. The influence of professional status on maximal and rapid isometric torque characteristics in elite soccer referees. J Strength Cond Res 28(5): 1310–1318, 2014—The purpose of this study was to examine the effectiveness of maximal and rapid isometric torque characteristics of the posterior muscles of the hip and thigh and lower-body power to discriminate between professional status in full-time and part-time professional soccer referees. Seven full-time (mean 6 SE: age = 36 6 2 years; mass = 82 6 4 kg; and height = 179 6 3 cm) and 9 part-time (age = 34 6 2 years; mass = 84 6 2 kg; and height = 181 6 2 cm) professional soccer referees performed 2 isometric maximal voluntary contractions (MVCs) of the posterior muscles of the hip and thigh. Peak torque (PT) and absolute and relative rate of torque development (RTD) were calculated from a torque-time curve that was recorded during each MVC. Lower-body power output was assessed through a vertical jump test. Results indicated that the rapid torque characteristics were greater in the full-time compared with the part-time referees for absolute RTD (p = 0.011) and relative RTD at 1/2 (p = 0.022) and 2/3 (p = 0.033) of the normalized torque-time curve. However, no differences were observed for PT (p = 0.660) or peak power (Pmax, p = 0.149) between groups. These findings suggest that rapid torque characteristics of the posterior muscles of the hip and thigh may be sensitive and effective measures for discriminating between full-time and part-time professional soccer referees. Strength and conditioning coaches may use these findings to help identify professional soccer referees with high explosive strengthrelated capacities and possibly overall refereeing ability.
KEY WORDS peak torque, rate of torque development, hamstrings, hip extension, vertical jump Address correspondence to Dr. Brennan J. Thompson, brennan. [email protected]. 28(5)/1310–1318 Journal of Strength and Conditioning Research Ó 2014 National Strength and Conditioning Association
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eferees play an important role in the quality and development of the sport of soccer (7). During competition, soccer referees are required to regulate the actions of players by keeping pace with the intensity of play (12,26). Over recent years, the intensity of soccer play has increased (33), which has subjected referees to greater physiological demands in keeping pace with player movements and monitoring playing actions from optimal viewing positions (39). In light of these increasing demands, previous studies (6,7,11,28,39) have suggested that the fitness level of referees may be an important component for providing high-quality officiating and, therefore, may be used to discriminate between referees of varying professional and competitive statuses. For example, Bartha et al. (7) showed that the aerobic fitness level demonstrated in a 12-minute run was significantly higher for elite-level soccer referees than lower-level referees, and thus was an effective measure at determining referee status. Furthermore, in a study of Greek referees of different competitive levels, anaerobic sprint performance was shown to be significantly related to refereeing ability (28). Previous authors (6,11,39) have indicated that refereeing ability is influenced by the capacity to perform intense intermittent exercise. Consequently, performance attributes resulting in a greater capacity to perform high-intensity sprints, including rapid acceleration and quick changes of direction may be an important aspect of refereeing performance ability and could help referees in achieving higher professional status. Taken together, the previous studies suggest that both aerobic and anaerobic dynamic fitness measures are important predictors of refereeing performance; however, considering the potential influence of explosive strength on these parameters (40), it may be important to investigate the use of maximal and explosive strength measures to differentiate between referee status in an elite professional referee population. Maximal and explosive strength are considered essential physiological performance parameters for successful performance in many sports (4,32) and are typically measured in vivo from an isometric torque-time curve that is recorded during a voluntary
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Journal of Strength and Conditioning Research isometric maximal contraction (42). Maximal torque production (i.e., peak torque [PT]) has been shown to be an important predictor of performance in power-type sports, such as weightlifting (10) and sprint cycling (32); however, because maximal torque requires .300 milliseconds to achieve (36), it may not be functionally relevant for intermittent-type sports (i.e., soccer, tennis) that involve movement durations that last ,250 milliseconds (i.e., sprinting, accelerating, and cutting) (4,34). Alternatively, rapid torque characteristics, which include rate of torque development (RTD), have functional significance in fast (i.e., #250 milliseconds) and forceful muscle contractions (2), and thus, may be better discriminators of performance in sports that include high-intensity sprints and quick changes of direction. To obtain an ideal viewing position, which involves being in a close proximity to the playing actions, elite-level soccer referees are often required to perform a large number of sprint-type movements during competition (19,33). Consequently, given the association with explosive type tasks, rapid torque characteristics may be particularly effective for determining refereeing status, in comparison with maximal strength. Although maximal and explosive strength have been examined extensively in athletes (15,24,30,32,34,38), previous authors have acknowledged an important research gap regarding strength-related characteristics in referees. For example, Castagna et al. (12) previously stated that “no research has been published on the strength performance of soccer referees. [which] would be of importance for the development of proper training strategies.” (p. 640). Thus, future studies are needed to examine the characteristics of strength-related measures in professional soccer referees and to evaluate the ability of these parameters to discriminate among referee status, which could provide insight regarding implications in the development of proper training programs for this population. In addition to isometric strength-related measures being potentially useful performance characteristics, dynamic muscle power has also been suggested as being an important variable for explosive-type activities. Previous authors have reported that lower-body muscle power was effective at discriminating among athletic performances in explosive-type sports including volleyball (29), weightlifting (10), and sprint cycling (32). Given its dynamic relevance, muscle power may be an important performance-related variable and assessment tool when examining referee performance abilities. Because the hamstrings are more commonly recognized for their actions as leg flexors rather than hip extensors (37), the contribution of the hamstrings in extending the thigh at the hip is often overlooked (25). Moreover, previous studies (3,5,35) examining the strength capacities of the hamstrings have traditionally used an isometric leg flexion test; however, leg flexion does not take into consideration the strength contributions of the other hip extensor muscles surrounding the posterior hip and thigh (i.e., gluteus maximus, adductor magnus) (37). Hip extensor strength has been reported to be a large contributor to important functional-related tasks
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including walking (9), running (14), and jumping (18). Therefore, from a functional standpoint, using an isometric hip extension assessment may provide a highly relevant, safe, and effective method for examining the maximal and explosive strength characteristics of the posterior muscles of the hip and thigh, which may offer the advantages of encompassing a more complete functional aspect of these muscles with the practical utility of a relatively simple and yet portable (potential for field-based testing) strength-related performance test. Thus, the purpose of this study was to perform a strength-based profiling assessment of an elite professional referee population and to examine the effectiveness of maximal and rapid isometric torque characteristics during an isometric hip extension test to discriminate between referee status in full-time and part-time professional soccer referees. A secondary aim of this study was to compare vertical jump power between the 2 referee groups and to examine the relationships between rapid torque development and lower-body muscle power and strength.
METHODS Experimental Approach to the Problem
A cross-sectional research design was used to investigate the ability of maximal and rapid isometric torque characteristics and vertical jump performance to discriminate between professional referee status (PRS) in full-time and part-time professional soccer referees. Previous studies (6,7,11,28,39) have demonstrated that dynamic measures of performance (i.e., speed, endurance, and agility) may discriminate between soccer referees of different professional levels. However, we are aware of no studies that have examined maximal and rapid isometric torque characteristics to differentiate between PRS in this population. Thus, full-time and parttime professional soccer referees performed isometric strength testing of the hip extensors and a vertical jump test to assess maximal and rapid isometric torque characteristics and lower-body power output, respectively. It was hypothesized that the field-based rapid torque characteristics would discriminate between the full-time and part-time referees and that rapid torque development would also be related to lower-body power output and strength. Subjects
Sixteen healthy, professional men soccer referees employed by the Professional Referee Organization volunteered for this investigation. All demographic data are presented in Table 1. Each referee had at least 7 years of officiating experience, of which 3 were spent in major league soccer (MLS). Of the 16 participants, 7 were identified as full-time professional referees and 9 as part-time professional referees. Classification of PRS (“full-time” vs. “part-time”) was made qualitatively based on a review of the biographical sketches of each referee in accordance with the number of games officiated. Specifically, the full-time referees officiated an average of 2 games per month, from March to December (MLS competitive season), whereas the part-time referees only officiated an average of 1 game per VOLUME 28 | NUMBER 5 | MAY 2014 |
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Isometric Testing and Professional Referee Status month. None of the referees reported any current or ongoing neuromuscular diseases or musculoskeletal injuries specific to the ankle, knee, or hip joints. At the time of testing, all referees were involved in a weekly training program that consisted of 3 h$wk21 of resistance exercise aimed towards hypertrophy and strength (i.e., 60–95% 1 repetition maximum) and 5 h$wk21 of conditioning exercise aimed towards endurance, speed, agility, and power. Testing for all referees was completed immediately after the conclusion of the 2012 MLS competitive season. This study was approved by the University’s Institutional Review Board for human subjects research, and before any testing, each participant completed an informed consent document and health history questionnaire. Procedures
All testing was conducted at the Cooper Aerobics Center in Dallas, Texas. Each participant visited the center on 2 occasions separated by 2–3 days. During the first visit, anthropometric measurements of circumference and skinfolds were performed on the right thigh and participants were familiarized with the testing procedures by performing several vertical jumps and isometric strength assessment trials. During the second visit, participants completed 3 countermovement vertical jumps (CMJ) followed by 2 maximal isometric strength assessments of the hip extensors. Assessment of Maximal and Rapid Isometric Torque Characteristics
Maximal and rapid isometric torque characteristics were quantified using a manual isometric strength assessment technique, which consisted of the participant extending the thigh towards the floor against a load cell (LCHD-250; Omega Engineering, Inc., Bridgeport, NJ, USA) attached immediately posterior to the heel (Figure 1). Participants laid supine during each isometric strength assessment with the knee braced in full extension and the ankle immobilized in a neutral 908 position (between foot and leg) with a custommade cast that was fixed around the foot and held with straps above the ankle and over the toes and metatarsals. Because assessments were conducted in a field-based setting, stabilizing the hips with restraining straps was not feasible in the present
study. However, to provide a measure of control during the testing, the investigator instructed the participants to maintain the hips in a stable position with a verbal instruction to “keep the hips in constant contact with the floor” throughout the entire duration of the maximal contraction for each maximal voluntary contraction (MVC) assessment. All assessments were performed on the right leg at a hip joint angle of 208 above the horizontal plane, which was consistent with the hip joint angle for the isometric hip extensor strength assessment previously reported by Perry et al. (25). Before the maximal isometric strength testing, participants performed 3 submaximal isometric hip extension muscle actions at approximately 75% of their perceived maximal effort. After the submaximal contractions, each participant performed 2 isometric MVCs of the posterior muscles of the hip and thigh with 1 minute of recovery between each contraction. For all isometric MVCs, participants were verbally instructed to extend the thigh “as hard and fast as possible” for a total of 3–4 seconds, and strong verbal encouragement was given throughout the duration of the MVC (34). Estimated Muscle Cross-Sectional Area
Estimated muscle cross-sectional area (eCSA) of the right thigh was determined using thigh circumference and corrected for subcutaneous fat with thigh skinfold measurements (23). Briefly, the radius was calculated from the circumference and eCSA was quantified by subtracting the mean of the 4 skinfolds taken at the anterior, lateral, posterior, and medial thigh positions (23). Circumference was measured with a tension-gauged measuring tape (Gullick II; Country Technology, Inc., Gays Mills, WI, USA) and skinfolds were measured with a Lange caliper (Santa Cruz, CA, USA). All anthropometric measurements were taken to the nearest millimeter at the midpoint of the thigh between the inguinal crease and the proximal border of the patella with the participant standing upright with approximately equal distribution of body mass on each leg and the feet placed shoulder width apart (41). Muscle eCSA (in square centimeters) was calculated using the equation by Moritani and deVries (23):
TABLE 1. Mean (SE) values for age, height, body mass, and estimated muscle eCSA.* Full-time (n = 7) Age (y) Height (cm) Mass (kg) Muscle eCSA (cm2)
36.00 178.53 82.14 156.30
(2.38) (2.97) (3.64) (5.88)
Part-time (n = 9) 34.11 181.19 84.28 162.85
(2.06) (1.80) (2.47) (8.28)
*eCSA = estimated cross-sectional area.
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Figure 1. An example of the manual isometric strength assessment. Participants were verbally instructed to extend the thigh towards the floor as hard as possible against a load cell attached immediately posterior to the heel for 3–4 seconds.
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procedures and equation by Moritani and deVries (23) provided reliable (intraclass correlation coefficient [ICC] = 0.98; standard error of measurement [SEM] expressed as a percentage of the mean = 2.7%) and valid estimates of muscle eCSA. Assessment of Lower-Body Power Output
Lower-body power output was measured during the CMJs using a linear velocity transducer (Tendo weightlifting analyzer, Tendo Sports Machines, Slovak Republic) that was attached to the posterior portion of a belt fastened around the participants’ waistFigure 2. An example of a torque-time curve taken from a participant during an isometric maximal voluntary line (27). During the CMJs, contraction (MVC) of the posterior muscles of the hip and thigh. Peak torque (PT; in Newton meters) was determined from the highest 500-millisecond epoch. Rate of torque development (RTD; in Newton meters per participants stood on the floor second) was determined from the linear slope of the torque-time curve over the time interval of 0–200 milliseconds with feet shoulder width apart from onset. and hands positioned on the hips. Participants were not allowed to take any steps before " #2 performing the CMJ and a quick descending quarter-squat P4 Circumference skfi countermovement was allowed before the ascending take-off 2 i ¼1 CSA ¼ p ; 2p 4 phase. The participants were instructed to jump as explosively as possible with both feet at the same time and land on where Circumference was defined as the circumference of the the floor in the starting position. To calculate power output thigh, and skf was defined as the sum of the thigh skinfold in watts, each participants’ body mass was measured with thicknesses from each of the 4 sites. A recent study by a stadium scale (Detecto, Webb City, MO, USA) and entered DeFreitas et al. (13) demonstrated that the anthropometric into the linear velocity transducer microcomputer. After the
TABLE 2. Mean (SE), 95% CI, and effect size values for PT, RTD, relative RTD at 1/6, 1/2, and 2/3 MVC, and vertical jump Pmax.*
PT (N$m) 95% CI RTD (N$m$s21) 95% CI Relative RTD1/6 (% MVC$s21) 95% CI Relative RTD1/2 (%MVC$s21) 95% CI Relative RTD2/3 (%MVC$s21) 95% CI Pmax (W) 95% CI
Full-time (n = 7)
Part-time (n = 9)
Effect size
109.49 (11.67) 80.93–138.06 373.23 (33.62)† 290.96–455.50 385.84 (58.41) 242.93–528.76 488.49 (108.46)† 223.11–753.87 474.85 (120.43)† 180.17–769.53 4058.29 (285.64) 3359.35–4757.23
103.57 (7.29) 86.76–120.38 223.20 (36.95) 137.98–308.41 297.97 (41.61) 202.02–393.93 221.34 (36.30) 137.64–305.05 217.30 (25.84) 157.72–276.89 3420.33 (293.06) 2744.53–4096.13
0.23 1.20 0.62 1.11 1.04 0.74
*PT = peak torque; CI = confidence interval; RTD = rate of torque development; MVC = maximal voluntary contraction; Pmax = peak power output. †Significantly greater RTD and relative RTD at 1/2 and 2/3 MVC for the full-time referees than the part-time referees (p # 0.05).
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Isometric Testing and Professional Referee Status completion of each jump, estimated peak power output (Pmax, Watts) was calculated and displayed by the microcomputer. A total of 3 jumps were performed with 1 minute of rest between trials, and the average of the 3 trials for Pmax was used for all subsequent data analyses. Previous reliability statistics for these procedures from our laboratory have revealed an ICC of 0.89 and a SEM value expressed as a percentage of the mean of 4.9% (31). Signal Processing
During each isometric strength assessment, the force (Newton) signal was sampled from the load cell at 1 kHz (MP100WSW; Biopac Systems, Inc., Santa Barbara, CA, USA), stored on a personal computer (Dell Inspiron 8200; Dell, Inc., Round Rock, TX, USA), and processed off-line using custom-written software (LabVIEW, Version 11.0; National Instruments, Austin, TX, USA). The scaled force signal was low-pass filtered with a 10-Hz cutoff (zero-phase lag, fourth-order Butterworth filter). A torque (Newton meters) signal was subsequently derived off-line by multiplying the force signal
from the load cell by the limb length (meters) for each participant. Limb length was measured as the distance from the greater trochanter to the heel (positioned over the load cell), and was measured once before the familiarization trial. The passive baseline torque value was considered the limb weight and subtracted from the signal so that the new baseline value was 0 Nm. All subsequent analyses were conducted on the scaled, filtered, and gravity-corrected torque signal. Isometric MVC PT was determined as the highest mean 500-millisecond epoch during the entire 3–4 second MVC plateau (Figure 2). The absolute RTD (Newton meters per second) was calculated as the linear slope of the torque-time curve (Dtorque/Dtime) over the time interval of 0–200 milliseconds (Figure 2) (20). The absolute torque-time curve was also subsequently normalized to PT, from which relative RTD values (percentage of MVC per second) were calculated from the normalized torque-time curve at 1/6, 1/2, and 2/3 of the MVC per the procedures of Aagaard et al. (1,2). The onset of contraction was determined as the point when the torque signal reached a threshold of 4 Nm, which was selected based on the torque threshold for the leg flexors previously reported by Thompson et al. (34). Test-retest reliability from our laboratory was examined during manual isometric hip extension MVC’s from 10 participants measured 48–72 hours apart. For all of the maximal and rapid isometric torque variables reported in this study, the ICC and SEM values expressed as a percentage of the mean ranged from 0.63 to 0.89 and 9.1 to 21.0%, respectively. Statistical Analyses
Independent samples t-tests were used to analyze differences in demographic characteristics, PT, absolute and normalized RTD variables, and Pmax between the full-time and parttime referees. Type I error rates (P) and effect sizes were reported for each t-test. Effect size statistics were calculated using the following equation (17): Figure 3. A) Isometric peak torque (PT) and (B) absolute rate of torque development (RTD) values for the full-time and part-time referees. Values are mean 6 SE. *Significant difference for RTD between groups. Full-time . parttime (p , 0.05).
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Effect Size ¼ jx1 2x2 j=σ; where x represents the mean and σ is the pooled SD.
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Figure 4. Relative rate of torque development (RTD) values at 1/6, 1/2, and 2/3 MVC for the full-time and part-time referees. Values are mean 6 SE. *Significant difference between groups for RTD at 1/2 and 2/3 MVC. Full-time . part-time (p # 0.05).
Using the pooled data of all participants (full-time and part-time referees, n = 16), the relationships between absolute RTD, PT, and Pmax were determined by Pearson product moment correlation coefficients (r). Statistical analyses were performed using IBM SPSS Statistics v. 20.0 (SPSS Inc., Chicago, IL, USA), and an alpha level of P # 0.05 was used to determine statistical significance.
RESULTS There were no differences (p = 0.434–0.623) between the full-time and part-time referees for age, height, mass, and muscle eCSA (Table 1). Table 2 shows the means, standard errors (SE), confidence intervals, and effect sizes for PT, absolute and relative RTD values, and Pmax. The full-time referees exhibited greater absolute RTD (p = 0.011; Figure 3) and relative RTD at 1/2 and 2/3 MVC (p = 0.022 and 0.033; Figure 4) compared with the part-time referees. There were no differences between the full-time and part-time referees for PT (p = 0.660; Figure 3), relative RTD at 1/6 MVC (p = 0.228; Figure 4), or Pmax (p = 0.149). Significant relationships were observed between absolute RTD and Pmax and PT (r = 0.527 and 0.621; p = 0.036 and 0.010); however, there was no relationship observed between PT and Pmax (r = 0.188; p = 0.486).
DISCUSSION The primary findings of this study revealed that the absolute and relative rapid torque characteristics were significantly greater for the full-time referees than the part-time referees, despite no differences between groups for maximal torque and power characteristics (i.e., PT and Pmax) or anthropometric measurements (Tables 1 and 2). In addition, significant positive relationships were observed between absolute RTD and Pmax and PT based on the pooled data of all referees.
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The present finding that the absolute and relative isometric rapid torque characteristics were greater in the fulltime compared with the part-time referees demonstrated the effectiveness of these characteristics at discriminating between PRS (Figures 3 and 4) in an elite professional referee population. Previous studies have also reported the capacity of performance tests to discriminate among referees, demonstrating that dynamic measures of performance such as 50- and 200-m sprints (7,28), 12-minute endurance runs (7), and intermittent recovery tests (6,7,11,39) may be used as discriminators among soccer referees of different professional levels. However, we are aware of no previous studies that have used strength-related parameters to differentiate between PRS in this population. Thus, the present findings may yield some novel insight, highlighting the potential importance of rapid torque capacities for professional soccer referees; however, because of a lack of strength-related research in this population, the ability to make comparisons with previous studies among these variables is limited. Nevertheless, the finding that rapid torque characteristics may be a more sensitive discriminating parameter when compared with maximal strength aligns with those previously reported in athletes of other sports. For example, Thompson et al. (34) demonstrated that rapid isometric torque characteristics of the leg flexors were able to discriminate between playing status in collegiate American football players, such that starters had greater rapid torque production than nonstarters, despite no differences in maximal strength. Furthermore, Sleivert et al. (30) showed that power athletes who performed more explosive-type activities during games and practices were capable of generating greater relative RTD values of the leg extensors and plantar flexors than endurance athletes who performed only aerobic-type activities. It was hypothesized that because RTD was measured relative to isometric PT, game participation for the power athletes resulted in a qualitative change of the contractile characteristics of the muscle (30). For elite-level soccer referees, the physiological demands of game participation often require the performance of a relatively large amount (i.e., 150–200 per game) of explosive-type activities (19). Therefore, it is possible that full-time professional soccer referees, such as those in the present study, who officiate more games, and consequently perform more explosive-type activities over the course of a competitive season may exhibit higher relative RTD values, as a result of game-induced qualitative factors (i.e., motor unit recruitment and firing rates, fiber type, tendon stiffness, pennation angle, etc.) (2,30). However, given the scope of the present study and the limited data available regarding these variables and findings, it is not presently feasible to ascertain the cause and effect associations, consequences, or patterns resulting in the higher rapid torque variables for the full-time vs. part-time referees. Thus, future research is necessary to elucidate more specifically the mechanisms that may be responsible for the RTD differences displayed between groups of referees with varying professional statuses as well VOLUME 28 | NUMBER 5 | MAY 2014 |
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Isometric Testing and Professional Referee Status as the magnitude of the importance of rapid lower-body strength characteristics and the association pertaining to on the field job performance abilities in higher level soccer referees. In addition to PT being similar between referee groups, Pmax was also an ineffective PRS discriminating variable. These findings were consistent with those previously reported among young tennis athletes of varying athletic ability (8); however, in other sports, such as weightlifting and sprint cycling, lower-body muscle power and strength have been shown to be important determinants of athletic success and elite-level status (10,32). These conflicting results may be because of differences in the activity requirements of the sports that were tested. For example, weightlifting and sprint cycling involve activity patterns that require higher anaerobic intensities than soccer and tennis (21) and therefore, measurements of maximal strength and vertical jump power may be more important to the success of individuals who participate in those types of sports. Furthermore, previous studies have indicated that because the intensity of a soccer game is dependent on the level of competition (22), higher level soccer referees may be required to perform more sprints and quick changes of direction throughout the course of a game to better position themselves to observe the actions of the players (11). Therefore, it is possible that because these referee-related activities (i.e., sprinting, accelerating, and cutting) involve relatively short foot strike contact times (#100 milliseconds) (4,34), rapid torque production (i.e., RTD #200 milliseconds) of the posterior muscles of the hip and thigh may be more functionally relevant than vertical jump and maximal strength characteristics, which involve relatively long torque development durations in excess of 200 milliseconds (4,36). However, it is also possible that the limited number of participants may have influenced the maximal strength and vertical jump data in the present study and thus, more research with larger sample sizes is needed to further examine the effects of referee status on the muscle power output and strength characteristics of the posterior muscles of the hip and thigh. Previous authors who have investigated the relationships between isometric strength-related variables and vertical jump power have reported contrasting findings (15,16,24,38). For example, Haff et al. (16) reported that peak force and rapid force development as measured during an isometric midthigh pull test were unrelated to vertical jump power, whereas Nuzzo et al. (24) reported that both maximal and rapid isometric force variables from the midthigh pull and isometric squat tests were effective measures at predicting vertical jump power capacities. Our findings add to these conflicting results by demonstrating a significant positive relationship between absolute RTD and Pmax with no relationship between PT and Pmax. The inconsistencies between these findings and those reported by previous studies (15,16,24,38) may be a result of discrepancies in vertical jump techniques (squat vs. CMJ), isometric testing modalities (i.e., midthigh pull, squat, and hip
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extension), the joint angle of testing, and body mass variability among participants (35). To our knowledge, this was the first study to use a supine hip extensor test to investigate the relationships between maximal and rapid isometric strength characteristics of the posterior muscles of the hip and thigh and vertical jump power. Although the posterior muscles of the hip and thigh have been identified as being important contributors to vertical jumping movements (18), these muscles and their ability to extend the thigh at the hip have largely been overlooked (25), particularly as they pertain to functional performance relationships. A recent study by Thompson et al. (35) demonstrated a significant relationship between the hamstrings (which are included within the posterior muscles of the hip and thigh), and vertical jump performance, which supports the present findings regarding the relationships of the posterior muscles of the hip and thigh and vertical jump performance. However, as in the example of the previous study, the hamstrings muscle group has typically been assessed for their actions as leg flexors rather than hip extensors (3,5,35). Because the explosive strength exhibited by the hamstrings during an isometric leg flexion test does not take into account the explosive strength contributions of the other hip extensor muscles (i.e., gluteus maximus, adductor magnus) (37), the hip extension test may be more functionally meaningful in terms of explosive strengthrelated performances. Thus, the present findings highlight the important relationship between rapid torque capacities of the posterior muscles of the hip and thigh and vertical jump performance, and suggest that isometric rapid torque testing may share similar performance qualities with dynamic actions such as the vertical jump. In addition, the manual technique proposed in the present study may provide researchers with a reliable and portable measurement tool capable of enhancing the practicality and utility of these (typically lab based) measurements when examining the lower-body performance capacities of athletes in the field.
PRACTICAL APPLICATIONS Given the absence of strength-related research investigating professional referees, this study aimed to examine strengthrelated parameters in an elite professional referee population to evaluate the characteristics and discriminating abilities of these measures based on level of referee status. The results showed that the rapid isometric torque characteristics of the posterior muscles of the hip and thigh were the only variables to effectively discriminate between the full-time and part-time referees. These findings provide support for assessing rapid torque capacities in professional soccer referees and perhaps demonstrate the functional utility of a manual isometric hip extension assessment to discriminate between PRS in this population. Moreover, given the higher rapid torque characteristics of the full-time referees in the present study and the positive relationships observed between rapid torque production and measures of muscle power output and strength, coaches and employers may use these findings as
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Journal of Strength and Conditioning Research performance evaluation tools to help in identifying professional referees with high lower-body explosive strengthrelated capacities (i.e., RTD) and possibly overall refereeing ability. These findings highlight a potentially important role of strength and explosive strength-related parameters in elite referees and provide support for the need to further investigate these performance measures in this population. In addition, training programs aimed at increasing rapid torque production of the posterior muscles of the hip and thigh may be beneficial for both increasing referee status and simultaneously enhancing muscle power output and strength capacities.
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