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Acute effects of static and dynamic stretching on hamstrings’ response times a

b

c

d

Francisco Ayala , Mark De Ste Croix , Pilar Sainz de Baranda & Fernando Santonja a

Department of Education, ISEN University Formation, University of Murcia, Murcia, Spain

b

School of Sport and Exercise, University of Gloucestershire, Gloucester, UK

c

Faculty of Sport and Physical Education of Toledo, University of Castilla La Mancha, Toledo, Spain d

Department of Traumatology, V. de la Arrixaca University Hospital, University of Murcia, Murcia, Spain Published online: 10 Jan 2014.

To cite this article: Francisco Ayala, Mark De Ste Croix, Pilar Sainz de Baranda & Fernando Santonja , Journal of Sports Sciences (2014): Acute effects of static and dynamic stretching on hamstrings’ response times, Journal of Sports Sciences, DOI: 10.1080/02640414.2013.861606 To link to this article: http://dx.doi.org/10.1080/02640414.2013.861606

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Journal of Sports Sciences, 2013 http://dx.doi.org/10.1080/02640414.2013.861606

Acute effects of static and dynamic stretching on hamstrings’ response times

FRANCISCO AYALA1, MARK DE STE CROIX2, PILAR SAINZ DE BARANDA3, & FERNANDO SANTONJA4 1

Department of Education, ISEN University Formation, University of Murcia, Murcia, Spain, 2School of Sport and Exercise, University of Gloucestershire, Gloucester, UK, 3Faculty of Sport and Physical Education of Toledo, University of Castilla La Mancha, Toledo, Spain, and 4Department of Traumatology, V. de la Arrixaca University Hospital, University of Murcia, Murcia, Spain

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(Accepted 29 October 2013)

Abstract The main purposes of this study were to (a) investigate acute effects of static and dynamic lower limb stretching routines on total response time, pre-motor time and motor time of the medial and lateral hamstrings during maximal eccentric isokinetic knee flexion; and (b) determine whether static and dynamic routines elicit similar responses. A total of 38 active adults completed the following intervention protocols in a randomised order on separate days: (a) non-stretching (control condition), (b) static stretching and (c) dynamic stretching. After the stretching or control intervention, total response time, pre-motor time and motor time of the medial and lateral hamstrings were assessed during eccentric knee flexion movements with participants prone. Measures were compared via a mixed-design factorial ANOVA. There were no main effects for total response time, pre-motor time and motor time. The results suggest that dynamic and static stretching has no influence on hamstrings response times (total response time, pre-motor time and motor time) and hence neither form of stretching reduces this primary risk factor for anterior cruciate ligament injury. Keywords: anterior cruciate ligament tears, injury prevention, injury, isokinetic, knee

Introduction Stretching protocols have been extensively recommended as part of a typical pre-exercise warm-up because it is thought that they improve sport performance and temporarily reduce the risk of injury (Shellock & Prentice, 1985). However, recent literature has stated that the role of pre-exercise stretching as a preventive measure to reduce the relative risk of injury has not been well verified (McHugh & Cosgrave, 2010; Woods, Bishop, & Jones, 2007). To elucidate the role of pre-exercise stretching on injury prevention, McHugh and Cosgrave (2010) stated that the effects of pre-exercise stretching on the likelihood of sustaining an injury should be analysed in relation to the specific type of injury (i.e. muscle strains, ligament tears); focus on a particular primary risk factor associated with that injury; and the stretching protocols performed should represent the typical

warm-ups used by athletes and recreationally active people to prepare for exercise or competition. Anterior cruciate ligament (ACL) injury is a common and potentially traumatic sport-related injury, presenting with substantial short- and long-term morbidities (Griffin et al., 2006). ACL tears tend to occur during activities with a high intensity of stretch-shortening cycles, including sudden acceleration and deceleration, rapid changes of directions, jumping and landing tasks, where rapid and unanticipated movement responses of the medial and lateral hamstring muscles (act as synergistic to the ACL) are necessary to stabilise the knee joint and successfully counteract the extreme load forces generated (McLean, Huang, & van den Bogert, 2008; Smith et al., 2012). It has therefore been postulated that the hamstrings’ response time is one of the most important primary risk factors for ACL tears

Correspondence: Francisco Ayala, ISEN University Formation, Center Affiliate to the University of Murcia, C/Huerto Manú, Nº5 3ºE, CP: 30009, Murcia, Spain. E-mail: Franciscoayalarodrí[email protected] Present affiliation for Francisco Ayala: Sports Research Centre, Miguel Hernández University of Elche, Alicante, Spain Present affiliation for Pilar Sainz de Baranda: Faculty of Sport Sciences and Physical Activity, University of Murcia, Murcia, Spain © 2013 Taylor & Francis

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(Hughes & Watkins, 2006). Specifically, longer hamstrings’ response times could adversely influence muscle´s ability to quickly stabilise the knee against the large external loads generated during sporting tasks and subsequently might increase the risk of tear (Besier, Lloyd, & Ackland, 2003; Blackburn, Bell, Norcross, Hudson, & Engstrom, 2009; McLean, Borotikar, & Lucey, 2010). Response time comprises two critical phases: pre-motor time and motor time also known as electromechanical delay (Botwinick & Thompson, 1966). Specifically, a notable portion of pre-motor time phase is dictated by neural (intensity of perception, speed of processing and transmitting information) (Ozyemisci-Taskiran, Gunendi, Bolukbasi, & Beyazova, 2008) and psychological (past experiences, current state of mind and fatigue condition) (Fontani et al., 2007) factors. It is also thought that there is an inverse relationship between motor time and musculotendinous stiffness (Grosset, Piscione, Lambertz, & Pérot, 2009). It seems reasonable to hypothesise that preexercise stretching-induced impairments in hamstrings’ response times predispose the athlete to be more prone to ACL tears. However, this hypothesis has not been well validated since few studies have determined acute effects of pre-exercise static stretching on total response time (Alpkaya & Koceja, 2007; Behm, Bambury, Cahill, & Power, 2004) and motor time (Costa et al., 2010; Esposito, Limonta, & Cè, 2011; Herda et al., 2010) especially during eccentric muscle actions. In addition, the applicability of these data in training is questionable because the extensive overall stretch duration per muscle used in stretching protocols (ranging from 225 s to 480 s) are related more to clinical than sporting contexts. Finally, to the author´s knowledge, no studies have investigated either acute effects of pre-exercise static stretching on premotor time or acute effects of pre-exercise dynamic stretching on total response time, pre-motor time and motor time. Therefore, the purposes of this study were to (a) investigate acute effects of static and dynamic lower limb stretching routines on total response time, pre-motor time and motor time of the medial and lateral hamstrings during maximal eccentric isokinetic knee flexion; and (b) determine whether static and dynamic routines elicit similar responses.

Methods Participants Forty-nine participants, consisting of 25 men and 24 women who were recreationally active adults (engaging in 2–5 h of moderate physical activity 3–5 days per week), took part in the current study. Although all participants reported engaging in recreational sports at intramural or competitive university standard, none

was involved in a systematic and specific strengthtraining programme. Participants were instructed to avoid their regular training throughout the experimental period and refrain from vigorous physical activity 48 h before each testing day. Other exclusion criteria were (1) histories of orthopaedic problems, such as episodes of hamstrings injury, fractures, surgery or pain in the spine or hamstring muscles over the past six months; (2) missing one testing session during the data collection phase; (3) presence of self-reported delayed onset muscle soreness at any testing session and (4) the women were not in the ovulation phase (days 10–14) of their menstrual cycle during testing as fluctuating concentrations of oestrogen throughout the menstrual cycle affect musculotendonous stiffness and joint laxity (Bell et al., 2009; Eiling, Bryant, Petersen, Murphy, & Hohmann, 2007). The participants were verbally informed about the study procedures before testing and provided written informed consent. This study was approved by the University of Gloucestershire Research Ethics Committee (United Kingdom). Twenty men (age = 21.3 ± 2.5 years; stature = 176.3 ± 8.4 cm; body mass = 74.4 ± 10.8 kg) and 18 women (age = 20.4 ± 1.8 years; stature = 164.7 ± 7.6 cm; body mass = 62.9 ± 8.6 kg) classified as recreationally active adults completed this study. Five men and six women were excluded from the study because they missed one or more of the testing sessions. Research design A crossover-study design, in which participants performed all experimental conditions, was used. Participants visited the laboratory on four occasions, with 72–96 h rest between testing sessions. The first visit was a practice/habituation session to the isokinetic testing procedure and stretching exercises, and the following three visits were the experimental sessions. During each experimental session, participants began by completing a 5-min standardised warm-up (cycling at 90 W for men and 60 W for women at 60– 70 rpm). The stretching (static or dynamic) or nonstretching (control) intervention was performed immediately after the standardised warm-up. The order of stretching (static and dynamic) and non-stretching conditions was randomised. After the stretching and non-stretching conditions, the participants performed a specific isokinetic warm-up consisting on four submaximal (self-perceived 50% effort) and two maximal eccentric knee-flexion actions. The rationale of using this warm up structure (standardised warm-up + stretching or non-stretching + specific warm-up) was to replicate the typical warm-up structure that is usually performed by athletes and recreationally active participants (Behm & Chaouachi, 2011).

Effects of stretching on hamstrings’ response time

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Figure 1. Stretching exercises (left to right: gluteus, quadriceps, hamstrings, psoas and adductors).

The hamstrings total response time, pre-motor time and motor time assessment was carried out 2–3 min (post-test) after the entire warm-up was completed.

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Stretching protocols In each stretching session, participants performed five un-assisted stretching exercises designed to stretch the major muscle groups used during running (gluteus, psoas, adductors, hamstrings and quadriceps) and reflect the stretching typically performed by athletes and recreationally active people (Figure 1). A multiple-muscle stretching protocol (in which participants stretched the major lower-limb muscles) investigated the aims of this study instead of a single-muscle protocol (in which participants stretched only the muscle studied) because an acute bout of static stretching may reduce muscle activation via peripheral (autogenic inhibition of the Golgi tendon reflex, mechanoreceptor and nociceptor afferent inhibition) and central nervous system (supraspinal fatigue) mechanisms (Avela, Finni, Liikavainio, Niemela, & Komi, 2004; Cramer et al., 2004). Avela et al. (2004) and Cramer et al. (2004) found that an acute bout of static stretching caused a decrease in muscle activation not only in the stretched muscle but also in the un-stretched contralateral muscle (via central nervous system mechanism). However, the degree of contribution of each mechanism (peripheral and central) on the reduction in muscle activation is still unclear. Therefore, effects of stretching before exercise and sport events should be investigated using multiplemuscle stretching protocols that reflect the stretching stimuli that athletes and recreationally active people usually apply both to the peripheral and central nervous system during a typical warm-up to make evidence-based recommendations. The static and dynamic stretching sessions differed only in the stretch technique used, whereas the other stretching load characteristics (duration, intensity, repetition and exercise positions) were identical. The stretching exercises were performed twice in a randomised order under the direct supervision and guidance of the investigators. Each

stretching exercise was completed on the right and left limb before another exercise was performed. Norest interval was allowed between limbs, although a 20-s rest period was allowed between stretch repetitions and exercises (once the leg was returned to a neutral position). The intensity of stretching was self-determined but set to the threshold of mild discomfort, not pain, as acknowledged by the participant. During the static stretching session, participants were asked to hold actively each stretch position for 30 s. During the dynamic stretching session, participants were instructed to perform 15 continuous controlled dynamic movements from the neutral stance to the end of the range of movement. A rate of one stretch cycle every 2 s was set, and the movements were at a controlled speed throughout the range of movements. Testing procedure The post-intervention assessments of the hamstrings total response time, pre-motor time and motor time were performed using a Biodex System-3 isokinetic dynamometer (Biodex Corp., Shirley, NY, USA) and a wireless eight-channel DelSys electromyography telemetry system (DelSys Myomonitor III, DelSys Inc., Boston, MA, USA). The dynamometer and EMG data were interfaced by feeding the analogue data directly from the dynamometer in to the Universal Input Unit via a trigger box and were displayed online on a computer using dedicated software (Delsys). This system allowed the conversion of dynamometer data to digital signals in parallel with the EMG signals. Thus, both data sets were collected in synchrony before processing by the EMG software (EMG Works 2, Delsys). Hence, this method allowed data from the EMG and dynamometer to be time–aligned, thus making it possible to determine the onset of surface EMG activity in relation to the onset of torque production. Before testing procedure commenced, the dynamometer and the EMG devices were calibrated according to their respective manufacturer’s instructions. Immediately after each testing procedure, a

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verification process of the dynamometer and the EMG devices was carried out to ensure that no changes occurred in the stability and sensitivity of the torque values and EMG signal, respectively. The verification process of the isokinetic dynamometer was conducted using known weights to assess the reliability of torque, velocity and position measurement (Valovich-mcLeod, Shultz, Gansneder, Perrin, & Drouin, 2004). Furthermore, the quality of the EMG signal was checked visually on the screen during the measurements and confirmed by evaluation of the spectral analysis (Kramer et al., 2001).

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Participant and dynamometer orientation Participants were secured prone on the dynamometer with the hip passively flexed at 10–20° (using a cylinder-shaped foam rubber cushion placed under the hip and standardised by measuring hip angle with a goniometer) and the head was maintained erect. The axis of rotation of the dynamometer lever arm was aligned with the lateral epicondyle of the knee. The force pad was placed approximately 3 cm superior to the medial malleolus, with the foot in a relaxed position. Adjustable strapping across the pelvis, posterior thigh proximal to the knee and foot localised the action of the musculature involved. The range of movement was set from 70° knee flexion (starting position) to 0° (0° was determined as maximal voluntary knee extension for each participant). Surface electromyography Surface electromyography was obtained from the dominant limb medial and lateral hamstring muscles represented by semitendinosus and biceps femoris using bipolar and pre-amplified electrodes with a fixed inter-electrode spacing of 10 mm (DE-02, Delsys, Bagnoli-8). The electrodes were attached parallel to the muscle fibres and over the dorsomedial muscle bulge at two thirds of the proximodistal thigh length for the semitendinosus, and at the dorsolateral side of the thigh at one half of the proximodistal thigh length for the biceps femoris (Merletti & Parker, 2004). The visually largest area of muscle belly was selected using a muscle action against manual resistance. The ground electrode was placed on the lateral malleolus of the ankle. Each electrode placement was marked with semi-permanent ink during the practise session and re-marked at the end of each testing session to ensure consistent placement on subsequent testing days. Electrodes and cables were secured with surgical tape to avoid movement artefacts. Before the placement of the electrodes, the hair of the area was shaved, and the

skin was cleaned using alcohol swabs and abraded lightly with sandpaper to reduce impedance below 5 kΩ. Data acquisition Before the assessments of the hamstrings response time, pre-motor time and motor time, all participants performed a “zero offset” function to establish a zero baseline from each of the EMG channels during 10 s of stationary lying. The EMG signals were acquired at a sampling rate of 1000 Hz. The root-mean-square amplitude for each muscle burst was calculated as follows: the raw EMG signals were measured in a band of 20–450 Hz, full-wave rectified, high-pass filtered with a Butterworth filter to remove movement artefacts with a cut-off frequency of 20 Hz, and smoothed with a 100-ms RMS algorithm. After this baseline calculation process, participants were instructed to resist as hard and quickly as possible, knee extension movement generated by the arm of the dynamometer by eccentric action of the hamstrings throughout the full range of motion immediately after receipt of a simultaneous auditory (dynamometer) and visual (trigger box) signal. Both signals, which were given randomly within 1–4 s, defined the beginning of data acquisition. Six maximal voluntary eccentric hamstrings muscle actions were performed with 30-s rest between each action. The speed of the isokinetic level arm throughout each repetition was preset at 240° s−1 so that the onset of torque was developed during the acceleration phase to replicate the high speeds that might be an underlying mechanism in non-contact knee injury (Boden, Dean, Feagin, Bishop, & Garrett, 2000). Response time was defined as the time interval from the application of the auditory and visual stimulus to the development of torque (9.6 Nm) (Winter & Brookes, 1991). The pre-motor time was determined as the time between the initial stimuli and the associated muscle activation onset defined by the EMG activity (change from the EMG mean baseline level to ±15 μV deviation) (Zhou, Lawson, Morrison, & Fairweather, 1995). Visual inspection of the EMG signal was also used to confirm that there was no EMG activity before movement of the lever arm. Motor time was calculated as the time interval between the onset of EMG activity and torque development (time taken [milliseconds] to generate 9.6 Nm torque) (Zhou et al., 1995). The two trials with the longest and shortest total response time, pre-motor time and motor time for each participant were discarded, and mean values for each neuromechanical variable were calculated across the remaining four trials.

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Statistical analysis Descriptive statistics including means and standard deviations were calculated for each measure for men and women separately. Distributions of raw data sets were examined using the KolomogorovSmirnov test and confirmed as normal (P > 0.05). Mean effects of stretching (static and dynamic) and their 90% confidence limits were estimated using a spreadsheet designed by Hopkins (2007) via the unequal-variances t statistic computed for change scores between paired sessions (control vs static; control vs dynamic; static vs dynamic) for each variable. Alpha was P < 0.05. Each participant’s change score was expressed as a percentage of baseline score via analysis of log-transformed values, to reduce bias arising from non-uniformity of error. Errors of measurement and individual responses expressed as coefficients of variation were also estimated. In addition, the analysis determines the chances that the true effects are substantial or trivial when a value for the smallest worthwhile change is entered. Inter-session technical errors of measurement determined the smallest substantial/worthwhile change for each of the variables. To the authors’ knowledge, no studies have determined the intersession technical errors of measurement for hamstrings total response time, pre-motor time and motor time during either concentric or eccentric knee flexion movements, so a pilot study addressed this issue. Thus, the same testing procedure that was carried out in the current study was repeated twice at 3–5-day intervals in 20 healthy men (n = 10) and women (n = 10) recreational athletes, who were not included in the current study. Inter-session coefficients of variations (expressed as percentage technical errors of measurement) were calculated using the method previously described by Perini, de Oliveira, Ornellas, and de Oliveira (2005), and these are illustrated in Table II and Table III for men and women, respectively. The qualitative descriptors proposed by Hopkins (2002) were used to interpret the probabilities (clinical inferences based on threshold chances of harm and benefit of 0.5% and 25%) that the true affects are harmful, trivial or beneficial: 99%, almost certainly. A mixed-design factorial ANOVA was used to compare men’s and women’s responses for each of the three experimental conditions (control, static stretching and dynamic stretching). Effect sizes were also calculated to determine the magnitude of differences between the groups or experimental conditions for each variable using the method previously described by Cohen (1988).

Cohen (1988) assigned descriptors to the effect sizes (d) such that an effect size of 0.4 or less represented a small magnitude of change, while 0.41–0.7 and greater than 0.7 represented moderate and large magnitudes of change, respectively. Results Table I shows the mean and standard deviation for medial (biceps femoris) and lateral (semitendinous) hamstrings total response time, pre-motor time and motor time values among experimental sessions and separated by sex. The statistical analysis reported no sex-related differences (P > 0.05 trivial effect with a probability of 75–94%; d < 0.4) in medial (biceps femoris) and lateral (semitendinous) hamstrings total response time pre-motor time and motor time values in any of the conditions (except for the total reaction time value reported in the control condition [P = 0.018; d = 0.35]). Effects of stretching on total response time, premotor time and motor time are displayed in Table II and Table III for men and women, respectively. As presented in Table II and Table III, there were no main effects (P > 0.05; trivial effect with a probability of 75–94%; d < 0.4) in total response time (mean change ranged from −4.9 to 5.8%), pre-motor time (mean change ranged from −8.1 to 9.8%) and motor time (mean change ranged from −3.7 to 6.9%) between paired treatments for both men and women.

Discussion The primary findings of this study were that static and dynamic lower-limb stretching exercises with an Table I. Biceps femoris (BF) and semitendinous (ST) total response time (RT), pre-motor time (PMT) and motor time (MT) data collected after non-stretching and stretching (static and dynamic) conditions. Values are mean ± standard deviation expressed in ms. Experimental treatment

Variables

Nonstretching

Static stretching

Dynamic stretching

Men (n = 20) 218.4 ± 31.8 BFRT 139.0 ± 26.9 BFPMT 79.4 ± 14.8 BFMT 226.6 ± 36.4 STRT 137.3 ± 27.4 STPMT 89.3 ± 26.5 STMT

211.4 127.9 83.5 214.7 129.2 85.5

± ± ± ± ± ±

29.1 25.9 20.1 27.6 26.1 19.3

224.2 140.5 83.7 224.3 139.6 84.7

± ± ± ± ± ±

36.2 27.9 17.1 28.8 26.9 17.7

Women (n = 18) 248.0 ± 34.0 BFRT 159.1 ± 25.8 BFPMT 88.9 ± 17.2 BFMT 246.0 ± 32.9 STRT 153.6 ± 25.7 STPMT 92.4 ± 20.9 STMT

237.5 149.9 87.6 243.6 152.7 90.9

± ± ± ± ± ±

39.6 35.6 17.1 31.7 36.1 15.3

252.2 158.4 93.8 250.5 151.4 99.1

± ± ± ± ± ±

38.2 48.6 19.8 37.1 38.8 21.4

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Table II. Men (n = 20) biceps femoris (BF) and semitendinous (ST) total response time (RT), pre-motor time (PMT) and motor time (MT) mean percentage changes among treatment sessions (paired comparisons). Confidence limits (CL), chances that the true effects were substantial and practical assessments of the effects are also shown. Chances that the true effects were substantiala (%)

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Paired comparison BFRT Static vs control Dynamic vs control Dynamic vs static STRT Static vs control Dynamic vs control Dynamic vs static BFPMT Static vs control Dynamic vs control Dynamic vs static STPMF Static vs control Dynamic vs control Dynamic vs static BFMT Static vs control Dynamic vs control Dynamic vs static STMT Static vs control Dynamic vs control Dynamic vs static

Mean change (%)

±90% CL

Effect size (d)

Positive

Trivial

Negative

Qualitative inferenceb

−3.1 1.7 5.2

−9.0 to 3.1 −7.4 to 11.8 −0.6 to 11.0

−0.2 0.1 0.3

0 9 6

97 89 94

3 2 0

Likely trivial Likely trivial Likely trivial

−4.9 −0.5 4.6

−11.2 to 1.8 −8.0 to 7.5 −2.4 to 12.0

−0.3 −0.1 0.3

0 0 11

89 94 89

11 6 0

Likely trivial Likely trivial Likely trivial

−8.1 0.9 9.8

−16.6 to 1.2 −9.4 to 12.3 −1.3 to 22.2

−0.4 0.1 0.4

0 6 39

70 84 55

30 10 6

Possible trivial Likely trivial Possible harmful

−6.1 1.7 8.1

−15.3 to 4.4 −9.7 to 14.4 −2.5 to 19.9

−0.3 0.1 0.4

0 10 26

68 82 74

31 8 0

Possible trivial Likely trivial Possible trivial

4.2 5.1 0.9

−6.7 to 16.3 −4.7 to 16.1 −9.1 to 12.1

0.2 0.2 0.1

19 9 10

79 80 90

2 11 0

Likely trivial Likely trivial Likely trivial

−3.7 −4.0 −0.3

−13.5 to 7.2 −13.4 to 6.7 −10.2 to 10.6

−0.2 −0.2 −0.1

0 0 1

81 79 92

19 21 7

Likely trivial Likely trivial Likely trivial

Notes: ±90% CL: add and subtract this number to the mean effect to obtain the 90% confidence limits for the true difference. a Substantial is an absolute change in performance of >11.3%, 10.2% and 9.9% for measures of biceps femoris and semitendinous total response time, motor time and pre-motor time respectively for passing accuracy (see Methods). b If chance of benefit and harm both >5%, true effect was assessed as unclear (could be beneficial or harmful). Otherwise, chances of benefit or harm were assessed as follows: 5–25%, unlikely; >25–75%, possible; >75–95%, likely; >95–99%, very likely; >99%, almost certain.

isolated muscle stretch duration of 2 × 30 s (static) or 2 × 15 rhythmic movements (dynamic) had no stretching-related impairment on total response time (change: from −4.9% to 1.7% [likely trivial]), premotor time (change: from −8.1% to 9.8% [likely trivial]) and motor time (change: from −4.0% to 6.9% [likely trivial]) of the medial and lateral hamstring muscles. Our findings do not support the results reported by previous studies that a bout of static stretching lengthened total response (Behm et al., 2004) and motor times (Costa et al., 2010; Esposito et al., 2011), although there are conflicting studies with which our data agree (Alpkaya & Koceja, 2007). A possible explanation for these conflicting results between the current study and previous findings could be attributed to the different testing procedure and duration of stretching. The current study determined total response time, pre-motor time and motor time using eccentric isokinetic actions, whereas previous studies determined the response time and motor time throughout

concentric (Costa et al., 2010) and isometric (Behm et al., 2004; Esposito et al., 2011) isokinetic actions. During dynamic sports activities where the ACL is overload, the hamstrings are eccentrically activated to act as synergist to improve the knee stability and successfully absorb the external forces generated (McLean et al., 2008; Smith et al., 2012). Based on this, the current study determined hamstrings’ total response time, pre-motor time and motor time under eccentric actions because it improves simulation of mechanisms of ACL injury (Boden et al., 2000). It should also be noted that testing in this study occurred with participants prone instead of the widely used seated position, because the former is more representative of hip joint angle during most athletic activities, especially during sprinting and rapid changes of directions where ACL injury is most likely to occur (Worrell, Denegar, Armstrong, & Perrin, 1990). A possible reason for the lack of any static stretching-induced changes on the eccentric total response time and motor time reported in this study could be based

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Table III. Women (n = 18) biceps femoris (BF) and semitendinous (ST) total response time (RT), pre-motor time (PMT) and motor time (MT) mean percentage changes among treatment sessions (paired comparisons). Confidence limits (CL), chances that the true effects were substantial and practical assessments of the effects are also shown. Chances that the true effects were substantiala (%)

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Paired comparison BFRT Static vs control Dynamic vs control Dynamic vs static STRT Static vs control Dynamic vs control Dynamic vs static BFPMT Static vs control Dynamic vs control Dynamic vs static STPMF Static vs control Dynamic vs control Dynamic vs static BFMT Static vs control Dynamic vs control Dynamic vs static STMT Static vs control Dynamic vs control Dynamic vs static

Qualitative inferenceb

Mean change (%)

±90% CL

Effect size (d)

Positive

Trivial

Negative

−4.6 1.0 5.8

−9.7 to 0.8 −7.1 to 9.7 0.8 to 11.1

−0.3 0.1 0.3

0 3 10

94 94 90

6 3 0

Likely trivial Likely trivial Likely trivial

−3.3 0.6 4.1

−9.0 to 2.6 −6.1 to 7.8 −2.0 to 10.6

−0.2 0.1 0.3

0 0 7

96 98 93

4 2 0

Likely trivial Likely trivial Likely trivial

−6.9 −3.4 3.7

−14.5 to 1.4 −12.1 to 6.2 −5.6 to 14.1

−0.3 −0.2 0.2

0 0 14

76 88 86

24 12 0

Likely trivial Likely trivial Likely trivial

−1.7 −4.4 −2.8

−10.3 to 7.8 −13.9 to 6.1 11.3 to 6.5

−0.1 −0.2 −0.1

0 0 0

92 80 91

8 20 9

Likely trivial Likely trivial Likely trivial

−1.6 5.2 6.9

−10.7 to 8.4 −3.5 to 14.7 −2.5 to 17.3

−0.1 0.2 0.3

0 15 29

93 85 71

7 0 0

Likely trivial Likely trivial Possible trivial

−0.6 2.3 2.9

−7.9 to 7.2 −5.8 to 11.0 −4.4 to 10.8

−0.3 −0.3 0.1

0 8 9

98 92 91

2 0 0

Likely trivial Likely trivial Likely trivial

Notes: ±90% CL: add and subtract this number to the mean effect to obtain the 90% confidence limits for the true difference. Substantial is an absolute change in performance of >9.9, 10.7 and 10.3% for measures of biceps femoris and semitendinous total response time, motor time and pre-motor time respectively for passing accuracy (see Methods). b If chance of benefit and harm both >5%, true effect was assessed as unclear (could be beneficial or harmful). Otherwise, chances of benefit or harm were assessed as follows: 5–25%, unlikely; >25–75%, possible; >75–95%, likely; >95–99%, very likely; >99%, almost certain. a

on the hypothesis proposed by Wilson, Murphy, and Pryor (1994). They suggested that musculotendinous stiffness is related to isometric and concentric muscle performance and that there is no relationship between muscle-tendon unit stiffness and eccentric force production. Another difference between the current study and others is the muscle group tested. We tested the hamstring muscles, while Behm et al. (2004) tested the quadriceps, and Esposito et al. (2011) and Herda et al. (2010) tested the plantar flexors muscles. Stretching-induced changes in muscle neuromechanical factors could be muscle specific, although studies are required to address this issue. Another possible explanation for the discrepancy between the results of the current study that showed no static stretching-induced impairments on the muscle total response time, pre-motor time and motor time, in contrast with the results reported in previous studies (Costa et al., 2010; Esposito et al., 2011; Herda et al., 2010), is the different stretch duration used. Previous studies have designed

stretching routines with an extensive overall stretch duration per muscle, ranging from 225 s to 480 s (Costa et al., 2010; Esposito et al., 2011; Herda et al., 2010), while the current study stretched the major muscle groups of the lower limb using an overall stretch duration of 60 s per muscle group, which is more representative of a typical warm-up used by athletes to prepare for exercise or competition (Young & Behm, 2002). It has been suggested that there is a dose-dependent threshold of static stretching necessary to reflect any detectable change on the neural and mechanical properties of the musculotendinous unit (Ryan et al., 2008). Perhaps, the short static stretching stimuli used in the current study (60 s per muscle group) were not enough to elicit changes on muscletendon unit neuromechanical properties and/or that their effects were short duration. Conversely, we could not contrast the lack of dynamic stretching-induced changes on hamstrings total response time (change: from −0.5% to 1.7% [likely trivial]), pre-motor time (change: from −4.4%

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F. Ayala et al.

to 1.7% [likely trivial]) and motor time (change: from −4.4% to 5.2% [likely trivial]) results demonstrated in this research with other studies because to our knowledge the current study is the first to have explored this issue. Another important finding of the current study is that there were no meaningful differences between the total response time (change: from 4.0% to 5.8% [likely trivial]), pre-motor time (change: from −2.8% to 9.8% [likely trivial]) and motor time (change: from −0.3% to 6.9% [likely trivial]) results obtained during both static and dynamic stretching sessions. Furthermore, the statistical analysis revealed that static and dynamic routines elicit similar responses in both men and women. The results of this study also revealed that there were no sex-related differences in medial and lateral hamstrings total reaction time, pre-motor rime and motor time values. These results are in agreement with the only other study (to the author´s knowledge) that has compared hamstring motor time (measured during isometric contractions) between men and women (Blackburn et al., 2009). However, it should be highlighted that, although not significant, women reported consistently longer total reaction time (≈29 ms), pre-motor time (≈20 ms) and motor time (≈8 ms) values than men, which might be attributable to greater joint laxity and lower musculotendonous stiffness in women. Although the current study is novel in several aspects (testing procedure, statistical analyses and stretching protocol design), some limitations should be noted. The first is that this study did not directly evaluate changes in the range of motion or changes in resistance and tolerance to stretch attributable to the experimental stretching treatments. Therefore it is not known if the stretching interventions were effective in increasing flexibility or in decreasing muscle stiffness, although previous studies from our laboratory that have used identical stretching doses have reported increases in flexibility (Ayala & Sainz de Baranda, 2010). Another possible limitation of the current study is the sampling frame. The age distribution of participants and their physical activity status were narrow, so the generalisability cannot be ascertained. Evaluation of response times during dynamic testing in high-demand activities should occur. Conclusions The results of the present study indicate that sports contextualised pre-exercise static (2 × 30 s) and dynamic (2 × 15 rhythmic movements) lower-limb stretching routines have no adverse effects on hamstrings total response time, pre-motor time and motor time. Although it is well established that

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