Journal of Sport Rehabilitation, 2014, 23, 319-329 http://dx.doi.org/10.1123/JSR.2013-0065 © 2014 Human Kinetics, Inc.

Lower-Extremity Biomechanics and Maintenance of Vertical-Jump Height During Prolonged Intermittent Exercise Randy J. Schmitz, John C. Cone, Timothy J. Copple, Robert A. Henson, and Sandra J. Shultz Context: Potential biom echanical com pensations allow ing for m aintenance o f m axim al explosive perfor­ m ance during prolonged interm ittent exercise, with respect to the corresponding rise in injury rates during the later stages o f exercise or com petition, are relatively unknown. Objective: To identify low er-extrem ity counterm ovem ent-jum p (CM J) biom echanical factors using a principal-com ponents approach and then examine how these factors changed during a 90-m in interm ittent-exercise protocol (IEP) w hile m aintaining maximal ju m p height. Design: M ixed-m odel design. Setting: Laboratory. Participants: Fifty-nine interm ittent-sport athletes (30 m ale, 29 fem ale) participated in experim ental and control conditions. Interventions: Before and after a dynam ic w arm -up and every 15 min during the 1st and 2nd halves o f an individually prescribed 90-m in IEP, participants w ere assessed on rating o f perceived exertion, sprint/cut speed, and 3-dim ensional CM J bio­ m echanics (experim ental). On a separate day, the same m easures were obtained every 15 min during 90 min o f quiet rest (control). Main Outcome Measures: U nivariate piecewise growth m odels analyzed progressive changes in CM J perform ance and biom echanical factors extracted from a principal-com ponents analysis of the individual biom echanical dependent variables. Results: W hile C M J height was m aintained during the 1st and 2nd halves, the body descended less and knee kinetic and energetic m agnitudes decreased as the IEP progressed. Conclusions: T he results indicate that vertical-jum p perform ance is m aintained along w ith pro­ gressive biom echanical changes com m only associated with decreased perform ance. A better understanding o f low er-extrem ity biom echanics during explosive actions in response to IEP allow s us to further develop and individualize perform ance training program s.

Keywords: counterm ovem ent jum p, joint biom echanics, principal-com ponent analysis In interm ittent endurance sports such as soccer, fatigue-related decreases in physical perform ance in high-intensity running are observed toward the end o f com petitive m atch es1-4 w ith no associated change in ju m p height.56 However, despite the m aintenance o f explosive counterm ovem ent-jum p (CM J) perform ance w ith in c re a sin g p h y sic al ac tiv ity d u ra tio n , low erextrem ity injury rates are reported to rise,7-9 suggesting that protective m ovement strategies may be altered while m aintaining physical perform ance. Given that maximal ju m p perform ance does not necessarily represent m axi­ mal isolated pow er o f individual jo in ts,10 understanding how biom echanical strategies o f the low er extrem ity are m odulated during interm ittent exercise and allow one to m aintain perform ance may be beneficial in enhancing perform ance, as well as understanding and preventing injury.

The authors are with the Dept of Kinesiology, University of North Carolina at Greensboro, Greensboro, NC. Address author correspondence to Randy Schmitz at [email protected].

Investigations o f exercise-related changes in m axi­ mal jum ping biom echanics have often used relatively short-term , exhaustive fatigue protocols with exercises consisting o f hopping,11squatting,12 and vertical jum ping com bined with sprinting.13 Biom echanical exam ination o f jum ping after maximal jum ping exercise dem onstrated decreased center-of-m ass displacem ent in the downward phase o f the CM J that was largely explained by decreased knee-flexion excursion14 with a corresponding loss of knee peak p ow er.14 A lthough not directly supported, that previous study hypothesized that during fatiguing exercise there may changes in patterns o f segm ental contributions.14 However, that previous work was lim ­ ited in its ability to adequately replicate the dem ands o f dynam ic sport activity, as fatigue seldom occurs in the relatively rapid, exhaustive m anner com m only found in laboratory studies. Little is known about changes in low er-extrem ity jo in t b io m e c h a n ic s d u rin g p ro lo n g ed su b m a x im a l interm ittent exercise that is characteristic o f field and court sports. C om petitive-soccer-m atch analysis dem ­ onstrated decreased interm ittent endurance capacity and

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30-m-sprint performance postmatch with no correspond­ ing change in maximal vertical-jump height.6 Similarly, a laboratory-based analysis using an individualized inter­ mittent-exercise protocol (IEP) simulating a soccer match demonstrated decreased sprint and agility performance with no change in squat and stretch-shortening-cycle jum p heights.5 In the latter study, general lower-extremity coordination was suggested to be unchanged during jumping as evidenced by no change in leg-spring vertical stiffness.5 However, that study was limited to examina­ tion of vertical ground-reaction force. A more detailed analysis of changes in lower-extremity joint mechanics may better explain how explosive performance such as vertical jum ping is maintained during intermittent exer­ cise, where decreases in sprinting performance along with cognitively perceived fatigue are observed. Moreover, it is important to conduct these analyses during exercise that most effectively simulates actual demands of sport.5’6 Given the large number of potential kinematic and kinetic biomechanical variables that could be potentially studied to explain coordinative changes across the joints of the lower extremity, principal-component analysis (PCA) provides a way to reduce a relatively large number of correlated biom echanical variables into a smaller number of independent factors15 and consequently effec­ tively characterize exercise-related changes in coordina­ tion of lower-extremity biomechanics. Thus, the purpose of this study was to use PCA to extract lower-extremity biomechanical principal components from biomechani­ cal testing of serial CMJs across an individualized IEP and then examine changes in these principal components during the course of the IEP. Our hypotheses were that CMJ height would remain unchanged during the IEP, and decreases in principal components related to knee function would occur during the IEP, but their effect on CMJ height would be offset by contributions of principal components related to hip and/or ankle function.

Methods Participants included 30 male and 30 female intercolle­ giate and club-sport athletes who participated in a larger study examining performance and biomechanical and knee-laxity changes during an IEP.1617 All participants were consistently engaged in competitive sport activities that include running, cutting, and landing maneuvers for the past 5 years; were currently active a minimum of 6 h/wk; were injury-free for the past 6 months; and had no known coexisting medical conditions affecting physical performance. In addition, the left leg (test leg for biome­ chanical measures) had no prior history of knee injury. Before participation, subjects were informed of risks and signed a university institutional review board-approved consent form. For 48 hours before testing days, subjects were instructed to avoid moderate to strenuous activity and maintain common dietary habits on testing days.

T e s tin g P ro tocol Participants first attended a fam iliarization session, completing a 15-minute dynamic-flexibility warm-up followed by the Yo-Yo Intermittent Recovery Test Level 1 (Y Y IRl).3 l8_20The YYIR1 assessed fitness level and was then used to prescribe submaximal IEP running speeds. After theY Y IR l, participants were familiarized with all procedures. They were then scheduled for control and experimental test sessions (order counterbalanced equally among men and women) spaced 3 or 4 days apart. On the experimental day, participants were first instrumented using a cluster technique with 3 optical markers (PhaseSpace, San Leandro, CA) on the left foot, shank, thigh, and sacrum. Knee- and ankle-joint centers were determined using centroid methods, whereas the hip center was calculated using the rotational method.21 Once instrumented, participants completed the same dynamic warm-up as the familiarization day, followed by the IEP. The IEP was designed to simulate physiological and biomechanical demands of intermittent field or court sports, with two 45-minute halves and a 20-minute half­ time.5 The IEP was performed as an intermittent shuttle run consisting of alternating 6 seconds of submaximal running at varying intensities (standing, walking, jog­ ging, low-intensity running, moderate-intensity running, high-intensity running, and sprinting), followed by 6 seconds of walking and standing. Each 15-minute exer­ cise segment included 2 consecutive sets of a 6-minute submaximal down-and-back shuttle run plus 2 maximal 505 agility sprint trials (1 off each leg),22 23 which were followed by 2 CMJs, 3 drop-jump landings,21 4 single-leg perturbation trials,24 and laxity testing. Before the IEP and after each 15-minute testing bout, ratings of perceived exertion (RPE) were acquired. All performance and biomechanical measures were integrated into the IEP to mimic the stretch-shortening-cycle work of match play. Performance assessment included RPE, the 505 agil­ ity test, and CMJ height. The 505 agility test consists of a 15-m down-and-back sprint, with a single cut performed on the left limb. Timing gates (Brower Timing Systems, Salt Lake City, UT, USA) quantified total 505 perfor­ mance as running speed (m/s) over the entire 30 m. No more than 30 seconds elapsed before 2 CMJ trials were then performed using the step-close technique25 in which participants placed their arms across their chest, stepped forward with their right leg followed by their left leg onto a force plate (Type 4060-130, Bertec Corp, Columbus, OH, USA), and then performed a maximal CMJ. Kine­ matics during the CMJ were measured at 240 Hz with an 8-camera optical system (PhaseSpace, San Leandro, CA, USA) while force-plate data were obtained at 1000 Hz. A total of 8 testing time points of the performance and CMJ biomechanics were examined: before and after the dynamic warm-up (PreWm, PostWm) and every 15 minutes during the IEP (15Min, 30Min, 45Min, 60Min, 75Min, 90Min).

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Table 1 Biomechanical Dependent Variables Calculated During the Countermovement Jump Eccentric phase (15 variables)

Concentric phase (12 variables)

Initial hip, knee, and ankle angle Peak hip, knee, and ankle angle

Peak hip, knee, and ankle angle Peak hip, knee, and ankle moment

Peak hip, knee, and ankle moment Peak hip, knee, and ankle energy power

Peak hip, knee, and ankle energy power Peak hip, knee, and ankle energy production

Peak hip, knee, and ankle energy absorption

The control day consisted of the same instrumenta­ tion and biomechanical methods as the experimental session. During each 13-minute segment where intermit­ tent running took place during the experimental session, participants instead were seated at a table. Otherwise, all testing procedures with the exception of 505 testing (due to potential injury risk in maximal sprinting with no prior warm-up) were identical to those of the experimental testing session.

Data Reduction Sprint speed from left-limb trials was quantified as over­ all speed (m/s) from the 505 agility test. RPE data were recorded by hand at each testing interval. As the current investigation is part of a larger study, 505 sprint trials data are also reported in a previous manuscript.26 CMJ jump height (m) was calculated as pelvis center-of-mass flight height.27 Biomechanical data were modeled with Motion Monitor (Innovative Sports Training, Chicago, IL, USA). Three-dimensional hip-, knee-, and ankleflexion angles were calculated using Euler definitions with a rotational sequence of z (sagittal), y (frontal), x (transverse). Kinematic and kinetic data were processed using a fourth-order, zero-lag low-pass Butterworth filter at 12 Hz. Extension was defined as the positive direction. Biomechanical CMJ data were averaged over the trials and then calculated for the eccentric phase (beginning of descent to peak center-of-mass displacement) and the concentric phase (peak center-of-mass displacement to flight). Initial-to-peak and peak sagittal-joint angles were obtained for eccentric and concentric phases, respectively. Sagittal hip, knee, and ankle moments (Nm • body weight [BW]-1 • height-1) were calculated using the inverse dynamics method. Peak negative and positive powers (W • BW-1 • height-1) were calculated during the descent and ascent phases, respectively. Energy absorption and production (J • BW-1 • height-1) were calculated as the area under the negative curve during the descent phase and positive power curve during the ascent phase, respec­ tively. For all analyses, moments, powers, and energy absorption or production were normalized to body weight (N) x height (m). A total of 27 biomechanical variables were calculated (Table 1).

Statistical Analyses Fifty-nine participants were included in analyses as 1 woman was unable to complete the protocol due to health issues (men 20.3 ± 2.0 y, 1.79 ± 0.05 m, 75.2 ± 7.2 kg; women 20.5 ± 2.3 y, 1.67 ± 0.08 m, 61.8 ± 9.0 kg). Descriptive statistics for discrete biomechanical depen­ dent variables are reported in Table 2. A PCA using the covariance matrix was performed (SPSS Statistics 20) on the discrete biomechanical dependent variables obtained from the CMJs across all time points on the experimental test day. Using an Oblimin rotation, the original loadingpattern matrix was rotated to aid in interpretation. The number of principal components in the pattern matrix extracted by the PCA was chosen based on the cumulative variance, while also considering the interpretation of each component. To help identify coherent factor structure and allow of the most substantively accurate factor interpreta­ tion while not giving low-loading individual biomechani­ cal variables excessive emphasis in factor interpretation, only the loadings of individual biomechanical variables that were relatively larger for each factor (>.370) were interpreted. For comparative purposes, the same factors were extracted from CMJ biomechanical data collected during the control session. A univariate piecewise growth model analyzed the progressive changes in all dependent variables (fatiguerelated variables: jump height, RPE, and sprint speed; and CMJ biomechanical factors extracted from the PCA) across 4 separate intervals including (1) PreWm to PostWm that ascertained changes from a dynamic warm-up, (2) PostWm to 45Min that ascertained changes during the first half after a warm-up, (3) 45Min to 60Min that ascertained changes from halftime recovery period to 15 minutes into the second half, and (4) 60Min to 90Min that ascertained changes during the second half. The advantage of the piecewise growth model is that changes within each distinct interval are modeled using separate parameters. For example, it was not expected that the change during the warm-up (interval 1) would be the same as change in the second interval. We decided to not include the first 15 minutes of the second half in our defined second-half interval (60Min to 90Min) as the IEP protocol did not allow for a warm-up period to begin the

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Jum p B iom echanics and Prolonged Exercise

in greater CMJ height along with changes in biomechani­ cal principal-component scores that reflect increased hip, knee, and ankle utilization; increased initial flexion with lesser resultant hip work absorption; and less knee and ankle flexion and work production. Control-day CMJ bio­ mechanical changes PreWm to PostWm were consistent with the exception of the greater knee- and ankle-flexion and work-production factor, which exhibited no change on control day. Whereas other investigations have com­ monly reported increases in jump height as a result of dynamic warm-up,3031 we have been unable to locate any studies investigating how individual joint biomechanics may change as the result of a dynamic warm-up. Collec­ tively, it appears that a dynamic warm-up is critical for assessment of true maximal explosive function. While CMJ height was maintained during the first and second halves of the IEP, the mechanical solution to obtain the jump height changed during the course of the IEP. Specifically, during the first half, there was a significant decrease in concentric and eccentric knee utilization (PC 1) along with decreasing contribution of coupled knee and ankle flexion with concentric work production (PC 5). The knee kinetic and energetic contributions continued to decrease from the end of the first half (45Min) to 15 minutes into the second half (60Min). However, as the IEP progressed into the second half (60Min-90Min), participants adopted a more extended position at the start of the CMJ descent phase. Control-day findings demonstrated that factor scores that significantly changed as a result of the dynamic warm-up then returned toward the pre-warm-up values (Figure 4). Experimental-day findings can be collectively interpreted as indicating that the body does not descend as interiorly and decreases knee kinetics and energetics magnitudes as exercise progresses. In addition, in the later phases of the IEP, the body starts the CMJ in a more upright position. Whereas from a standing position it would be thought that the CMJ would always start from a completely extended position, for instrumentation reasons the current study adopted a step-close protocol in which the participant started behind the force plate and then stepped forward onto it. This allowed for varying flexion angles when starting the CMJ. Previous investigations of the effect of intermittent exercise on CMJ height have been quite limited with respect to biomechanical outcomes. We located only 1 study examining lower-extremity jumping biomechanics during an IEP.5An intermittent assessment of a stretch­ shortening jump during an identical IEP demonstrated no changes in jump height or lower-extremity stiffness.5 Previous work has assessed coordination among the joints of the lower extremity after fatiguing repetitive CMJ exercise.14 Although discrete biomechanical variables were reported, a finding of a more extended knee at the end of the negative phase14 was similar to our observed decrease in PC 5. This suggests that whether exercise is intermittent or continuous, the body adopts a less flexed strategy. It has been hypothesized that this may be a mechanism to help sustain jump height during periods

327

of continued exertion by maximizing stiffness during the eccentric phase.14 However, direct comparison with this previous work is quite limited due to the unique IEP used in the current study that is more aligned with the demands of a soccer match. It was unexpected that CMJ height was maintained during the IEP when knee mechanical contributions decreased with no corresponding increase of the hip or ankle mechanics. Thus, it stands to reason that some other variable not represented in the current biomechanical model also contributed to CMJ height. Previous work has suggested that inclusion of arm swing in a CMJ improves performance by ~9%.32While we attempted to control for arm swing through detailed instruction and visual moni­ toring, it is possible that participants may have begun to use their upper extremities to a greater extent, which in turn helps optimize trunk mechanics,32 thus increasing total lower-extremity work in jumping.33 The inclusion of upper-extremity and trunk data in future investigations would aid in understanding the maintenance of CMJ height during intermittent exercise. Although PCA allows for the reduction of multiple dependent variables, the dimensionality of each extracted component is not clinically interpretable. While this affects comparisons of measures across studies, the relative directional change in the extracted components allows for a generalized description of mechanical behavior. While previous work has reported sex differ­ ences in CMJ biomechanics,25 we chose to include both sexes in our biomechanical analyses, as performance changes were not different between sexes and our main purpose was to see how biomechanical factors changed as a result of fatiguing exercise. Furthermore, lack of electromyographic data limits our ability to determine potential alterations in neural strategies affecting the CMJ mechanism. Finally, the investigation was limited to 27 variables, listed in Table 1, but we acknowledge that a multitude of other biomechanical variables (ie, joint angular velocity, rate of force development, eccentricand concentric-phase durations, etc) may also explain CMJ performance.

C o n c lu s io n s

This study demonstrates that lower-extremity joint bio­ mechanics during a CMJ, as described through 5 distinct factors that each describe a collection of correlated variables, are altered during a 90-minute IEP designed to simulate a soccer match. As exercise progressed, CMJ performance was maintained, yet participants did not descend as interiorly, and the magnitude of knee kinetics and energetics decreased. In addition, in the later phases of the IEP participants started the CMJ in a more upright position. These findings may be useful for researchers and practitioners in understanding explosive lower-extremity biomechanical function in the latter stages of intermit­ tent exercise. However, none of the current extracted biomechanical factors changed in a direction that would

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S ch m itz e t al

be expected to counter the observed altered joint param­ eters associated with decreased performance. While the current study demonstrates the effect of an IEP on the lower-extremity joint function during a CMJ, future work should look at potential alterations in the trunk and upper extremities that would give further insight to the total-body biomechanics associated with explosive performance during functional game scenarios.

Acknowledgments

12.

13.

This work was supported by a grant from the National Football League Charities. 14.

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