Effets aigus de 15min d'étirements sur la performance

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Original paper

Acute effects of 15 min static or contract-relax stretching modalities on plantar flexors neuromuscular properties Nicolas Babault a,b,∗ , Blah Y.L. Kouassi b , Kevin Desbrosses c a

Centre d’Expertise de la Performance, Université de Bourgogne, Dijon, France b UFR STAPS, Université Marc Bloch, Strasbourg, France c INRS, Laboratoire de physiologie du travail, Nancy, France

Received 8 July 2008; received in revised form 17 December 2008; accepted 18 December 2008

Abstract The present study aimed to investigate the immediate effects of 15 min static or sub-maximal contract-relax stretching modalities on the neuromuscular properties of plantar flexor muscles. Ten male volunteers were tested before and immediately after 15 min static or contractrelax stretching programs of plantar flexor muscles (20 stretches). Static stretching consisted in 30 s stretches to the point of discomfort. For the contract-relax stretching modality, subjects performed 6 s sub-maximal isometric plantar flexion before 24 s static stretches. Measurements included maximal voluntary isometric torque (MVT) and the corresponding electromyographic activity of soleus (SOL) and medial gastrocnemius (MG) muscles (RMS values), as well as maximal peak torque (Pt) elicited at rest by single supramaximal electrical stimulation of the tibial nerve. After 15 min stretching, significant MVT and SOL RMS decreases were obtained (−6.9 ± 11.6% and −6.5 ± 15.4%, respectively). No difference was obtained between stretching modalities. Pt remained unchanged after stretching. MG RMS changes were significantly different between stretching modalities (−9.4 ± 18.3% and +3.5 ± 11.6% after static and contract-relax stretching modalities, respectively). These findings indicated that performing 15 min static or contract-relax stretching had detrimental effects on the torque production capacity of plantar flexor muscles and should be precluded before competition. Mechanisms explaining this alteration seemed to be stretch modality dependent. © 2009 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.

Keywords: Electromyography; Evoked contractions; Maximal voluntary contractions; Medial gastrocnemius; Soleus

1. Introduction Stretching exercises could be used before, during and after training for injury prevention, performance improvement and recovery optimisation. At this moment, stretching efficacy for injury prevention1 and recovery2 is not well demonstrated. The usefulness of stretching for improving performance during warm-up remains also debated partly due to stretching procedures. Recently, numerous studies have reviewed the detrimental acute effects of stretching on muscular performance.3,4 Deleterious effects have been demonstrated on maximal voluntary strength,5–10 vertical jump ability11,12 and force endurance.13 Conclusions remain contradictory with regards to running speed.14,15 ∗

Corresponding author.

E-mail address: nicolas.babault@u-bourgogne.fr (N. Babault).

Various mechanisms may explain performance reductions subsequent to stretching. Neural factors (decreased activation partly due to afferent feedbacks) may play a major role.5 This neural drive reduction has been shown to be maintained 1 h after stretching.9 Simultaneously, an alteration of muscular mechanical properties may be involved and may override neural factors beyond 15 min post-stretching.16 Mechanical impairments might originate from musculotendinous stiffness reductions11 and a shift of the optimal length toward longer muscle lengths.10 Various stretching modalities, such as static, dynamic or contract-relax stretches, are generally included into warmup. For example, contract-relax stretches promote muscle relaxation and subsequently increase muscle compliance. It is one proprioceptive neuromuscular facilitation (PNF) technique that involves maximal voluntary contraction (MVC) of agonistic muscles before a static stretch.

1440-2440/$ – see front matter © 2009 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jsams.2008.12.633

Please cite this article in press as: Babault N, et al. Acute effects of 15 min static or contract-relax stretching modalities on plantar flexors neuromuscular properties. J Sci Med Sport (2009), doi:10.1016/j.jsams.2008.12.633

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Stretching procedures comparison for immediate effects on neuromuscular properties remains, however, conflicting.14,17,18 Indeed, while no difference has been observed between static and contract-relax stretches,19,20 larger performance decreases with contract-relax stretches were also obtained.21 These contradictory results could partly be ascribed to stretch procedures such as stretching duration or intensity. Feland and Marin22 have recently shown that sub-maximal voluntary contractions (<50% of MVC) are as well as efficient than MVCs for contract-relax stretching with a reduced injury risk. However, to the best of our knowledge, no study has attempted to determine immediate effects of sub-maximal contract-relax stretches on subsequent neuromuscular properties. Therefore, the present study compared the acute effects of 15 min static and sub-maximal contract-relax stretching modalities on the neuromuscular system.

2. Methods Ten healthy men, recreationally active, volunteered for the experiment. Their mean (±SD) age, height and body mass were 23 ± 1 years, 181 ± 6 cm and 75 ± 6 kg. All subjects agreed to participate in the study and signed an informed consent form. The study was conducted according to the declaration of Helsinki and approval was obtained from the local committee on human research. Subjects were asked to refrain from strenuous activity at least 48 h preceding all testing sessions.

Each subject performed two tests sessions (randomly presented) in order to study the effects of static or contract-relax stretching on the neuromuscular properties of the right plantar flexors. At least 2 weeks separated each session. As compared with real training conditions, stretching interventions were lengthened so as to emphasise stretching effects. Sessions were therefore composed of 15 min static or contract-relax stretching and were preceded and followed by evaluations of the neuromuscular system. Tests included measurements of plantar flexor maximal torque produced voluntarily or electrically evoked. The resultant electromyographic activity (EMG) was also registered from soleus (SOL) and medial gastrocnemius (MG) muscles.

For the static stretching procedure, subjects performed 20 stretches. Each stretch was 30 s long as recommended23 and 15 s rest was presented between stretches. Twenty stretches were also realised for the contract-relax stretching modality. Stretches were 24 s long and were preceded by a 6 s sub-maximal plantar flexion (<50% MVC).22 Subjects were therefore asked to exert a small plantar flexion contraction before stretching. This sub-maximal contraction intensity was controlled using a handheld dynamometer (Lafayette Instrument, USA) applied on tiptoes. This intensity was controlled with respect to a maximal voluntary contraction only during the assisted contract-relax stretch.

During each session, stretches were maintained at the point of discomfort. As previously used during acute stretching effects studies,6 different stretch exercises were used. For both static and contract-relax stretches, one assisted and three unassisted stretch exercises were randomly alternated during 15 min. (1) Subjects remained in the supine position with knees fully extended. The experimenter, positioned laterally, then dorsiflexed the right ankle joint until the point of discomfort. (2) Wall push up exercise: subjects leaned with both hands against a wall and the right leg back several feet from the wall with the heel firmly positioned on the floor. The left leg was flexed about halfway between the back leg and the wall. Both feet are moved back to stretch the calf muscles.

(3) Subjects stood on a raised platform on the balls of their right foot, then dropping the heel down toward the floor. (4) Subjects stood on tiptoes with flexed legs and both hands positioned forward flat on the floor. Then subjects extended both leg while putting the right heel down to the floor.

Neuromuscular properties of plantar flexor muscles were evaluated before and immediately after (∼1 min) stretching interventions. Voluntary and electrically evoked contractions were studied. During all tests, subjects were comfortably seated with the trunk inclined 10◦ with reference to the vertical, the hip and knee joints were flexed at 90◦ and 50◦ , respectively (0◦ = full extension). The right foot was fixed with Velcro straps on a custom-made device composed of a pedal equipped with a strength gauge (Interface Inc., Scottsdale, USA). The plantar flexion isometric torque was calculated as the product of strength and lever arm at a 0◦ ankle angular position (tibia perpendicular to the sole of the foot). After a short warm-up, composed of 10 isometric contractions at increasing intensities, subjects performed two 5 s MVC with at least 30 s rest between MVCs. A series of five electrical stimulations was also delivered over plantar flexors for mechanical properties evaluation. Voluntary and electrically evoked contractions were presented in a random order.

Electrical stimulations were delivered on resting subjects using a Compex 2 electrical stimulator (Compex Medical SA, Ecublens, Switzerland). The tibial nerve was stimulated using single rectangular impulse (impulse duration = 1 ms, 100 mA maximal intensity). The cathode (∼5 mm diameter handheld ball probe) was pressed in the popliteal fossa and moved to the position giving the greatest visible plantar flexors contraction. The anode (10 × 5 cm adhesive electrode) was positioned over the patella. Subjects were familiarised with the stimulation procedure with sub-maximal stimulations.

The stimulation intensity was then progressively increased in order to determine each subject’s maximal stimulation intensity (plateau in the evoked twitch torque). The maximal intensity was further increased (maximal intensity + 10%) to apply supramaximal stimulations. Force signal and the associated EMG responses were recorded and averaged for each series of five stimulations. From the force signal was determined the maximal amplitude of the evoked mechanical response (Pt). Moreover, peak-to-peak amplitudes of the

Please cite this article in press as: Babault N, et al. Acute effects of 15 min static or contract-relax stretching modalities on plantar flexors neuromuscular properties. J Sci Med Sport (2009), doi:10.1016/j.jsams.2008.12.633

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associated M-waves were determined for SOL and MG EMG signals.

Surface EMG signals were recorded during all voluntary and electrically evoked contractions using two pairs of silverchloride electrodes (10 mm diameter, 20 mm interelectrode distance). For SOL, electrodes were placed over the middorsal line of the leg, 5 cm distal where the two heads of the gastrocnemius join the Achilles tendon. MG electrodes were pasted longitudinally over the muscle belly. A reference electrode was fixed to the patella of the opposite leg. Low impedance of the skin–electrode interface (<2000 ) was obtained by shaving, abrading with sand paper and cleansing the skin with alcohol. EMG signals were amplified with a bandwidth frequency ranging from 10 to 2000 Hz (gain: 1000). EMG and mechanical signals were digitised on-line at 2000 Hz and stored for further analyses. The best MVC was only retained. Root mean square values (RMS) were then calculated over a 500 ms period during the plateau in maximal voluntary torque (MVT). RMS values were subsequently normalised with respect to the M-wave peak-to-peak amplitudes.

Results are presented as mean values (±SD). Maximal voluntary torque, RMS values for SOL and MG, Pt and M-waves were determined before and immediately after stretching (dependent variables). The effects of the different stretching modalities (independent variables) were studied using a two-way (stretching modality × time) analysis of variance (ANOVA) with repeated measures. Subsequent Student–Newman–Keuls (SNK) post hoc tests were performed if significant main effects or interactions were obtained. Percent changes between stretching modalities were also tested using a Student’s t-test. P < 0.05 was taken as the level of statistical significance. Statistical power for the various comparisons ranged from 0.35 to 0.46.

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Table 1 Neuromuscular parameters obtained pre- and post-static and contract-relax stretching.

Static

Contract-relax MVT (N m)

Pre Post*

132.1 ± 23.9 118.8 ± 23.9

Pre Post*

3.45 ± 1.38 3.10 ± 0.83

Pre Post

4.88 ± 2.60 4.59 ± 2.72

Pre Post

14.7 ± 4.0 14.5 ± 2.8

Pre Post

10.0 ± 2.7 10.3 ± 2.6

Pre Post

7.5 ± 3.5 7.3 ± 3.9

129.3 ± 32.2 120.6 ± 18.7 SOL RMS 3.25 ± 1.20 2.93 ± 0.85 MG RMS 4.76 ± 1.63 4.98 ± 0.92 Pt (N m) 14.9 ± 3.9 14.0 ± 2.8 SOL M-wave (mV) 9.4 ± 3.8 9.3 ± 4.2 MG M-wave (mV) 7.6 ± 3.8 6.7 ± 4.1

Mean values (±SD). MG, medial gastrocnemius; MVT, maximal voluntary torque; Pt, electrically evoked torque; SOL, soleus. Significant reductions in MVT and SOL RMS were revealed (*, time effect P < 0.05) with no stretching modality effect nor interaction.

Percent changes were significantly different depending on stretching modality (P < 0.05; Fig. 1). After static stretching, MG RMS alterations were −9.4 ± 18.3%, whereas, after contract-relax stretching, MG RMS changes were +3.5 ± 11.6%.

4. Discussion 3. Results Before stretching, evoked contractions were not significantly different among sessions. Whatever the stretching modality, the ANOVA did not point out any significant main effect or interaction (time × stretching modality) on Pt, SOL and MG M-wave peak-to-peak amplitudes (Table 1). Evoked twitches were unaffected by 15 min static or contract-relax stretching.

Before stretching, MVT and EMG were not significantly different between the two experimental sessions. Performing 15 min stretching significantly decreased (P < 0.05) plantar flexor MVT (significant time effect; Table 1). The mean decrease for the two stretch interventions was −6.9 ± 11.6%.

No stretching modalities difference was obtained (Fig. 1).

With time, SOL RMS significantly decreased after 15 min stretching (mean decrease for the two stretching techniques: −6.5 ± 15.4%, P < 0.05) with no significant stretching modalities effect and interaction (Fig. 1). MG RMS exhibited a trend toward alterations following stretching (P = 0.089).

This study’s most important finding was that 15 min plantar flexor stretching was detrimental for immediate torque production. This impairment in strength production capacity was independent on the stretching procedure applied. However, it appeared differently modulated with various central contributions depending on the stretching modality.

Fifteen min stretching interventions produce ∼7% reduction of the MVT. This impairment of the strength production capacity is in accordance with previous reports.5–10,16,20 However, contradictory results are often obtained 18,19,24 as a result of various tests (e.g., angular velocity for isokinetic tests) or stretching protocols. For example, stretch duration is of great importance. Indeed, while unchanged muscular performance were obtained after 30 s stretching,24 1 h stretch resulted in 13.8% MVT decreases.25 The 15 min stretching, used here, is therefore long enough to produce strength decreases but is too long as compared with stretching applied during training. Although not investigated here, these acute stretching effects were generally persistent and remained beyond 1 h post-stretch.9,16 While neural activation returned

Please cite this article in press as: Babault N, et al. Acute effects of 15 min static or contract-relax stretching modalities on plantar flexors neuromuscular properties. J Sci Med Sport (2009), doi:10.1016/j.jsams.2008.12.633

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Fig. 1. Changes in electrically evoked torque (Pt) and maximal voluntary torque (MVT) (upper graph) and EMG activity changes for soleus and medial gastrocnemius muscles (SOL RMS and MG RMS, respectively) (lower graph) immediately after 15 min static or contract-relax stretch interventions.

Mean values (ﾂｱSE) are presented and expressed as percent changes of prestretching values. *: difference between static and contract-relax stretching modalities (P < 0.05).

to pre-stretch values after 15 min, mechanical impairment recovered slowly.16 The acute effects of stretching on MVT could also depend on stretching modalities. For example, dynamic stretching appeared less detrimental14 or even beneficial24 for vertical jumps and muscle power compared with static stretching.

Contrarily, similar vertical jump decreases were observed after static or ballistic stretching.18 In the present study, although more pronounced changes were obtained after static stretching, no significant stretching modality effect was observed on MVT. Inclusion of sub-maximal voluntary contractions before stretches did not significantly modify MVT decreases. The lack of difference between static and contractrelax procedures is in agreement with previous experiments using MVC before stretches12,20,26,27 but is in contradiction

with others.21 Bradley et al.26 registered 4.0% and 5.1% jump height decreases after 10 min static and PNF stretching, respectively. Contrarily, larger decreases were obtained in vertical jump after contract-relax compared with static stretching.21 Differences between these studies are unclear but may partly be attributed to the stretched muscles or MVC duration before stretches (5 s vs. 10 s, respectively, for Refs.

26,21).

Neural5,9,16,20,25,28 as well as mechanical factors8,10,11,28 are the two main mechanisms that may account for impairments in strength production capacity. Our results partly agreed with the hypothesis of neural drive alterations (e.g., decreased motor unit activation). Whatever the stretching modality, 15 min stretching resulted in decreased SOL RMS. However, these small modifications seemed to be stretching modality and muscle dependent. Indeed, MG RMS decreases were obtained after static stretching whereas increases were obtained for contract-relax stretching. Such muscle specific responses after stretching have previously been reported on quadriceps muscles using mechanomyography.20 This general and persisting decrease in muscle activation5,9,16,25,28,29 is generally ascribed to reductions in motoneuron excitation due to sensory afferents. Pre- and post-synaptic inhibitions have been suggested.19 Regardless of the mechanisms, MG behavior surprisingly differs from SOL muscle since MG RMS increases were measured for contract-relax stretching modality. This small effect in MG RMS measured after contract-relax stretches contradicts previous studies.20 As proposed previously, contract-relax stretching should have produced greater decrease in muscle activation due to additive effects of autogenic and reciprocal inhibition.26 The reasons for our unexpected result are unclear but could be attributed, for example, to stretching parameters such as contraction duration. Some studies performing PNF stretch with maximal contractions attributed neural drive decreases to neuromuscular fatigue.20 Therefore, it can be speculated that sub-maximal voluntary contractions, used here, might counteract the possible stretching-induced fatigue. Moreover, the different muscle behavior might be explained by the fact that contract-relax stretching effects could be more pronounced on gastrocnemii muscles due to their higher relative contribution with extended compared with flexed legs.30 Because stretching was primarily performed with straight legs, the effects of sub-maximal torque produced during contract-relax stretching could therefore be more apparent on MG than SOL. Mechanical factors generally responsible for alterations in strength production likely originate from changes in viscous and elastic properties of the musculotendinous system.31,32 Decreases in muscle stiffness have been thought to modify the length/tension relationship.16 Nevertheless, the hypothesis of a rightward shift in optimal muscle length needs to be confirmed.28 The lack of change in Pt, observed in the present experiment, disagrees with possible stretchinginduced mechanical alterations5,16,25 but is in line with other reports.10 At first sight, our results contradict musculotendinous stiffness changes but they could confirm Cramer et

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al.29 hypothesis suggesting a flattening of the torque–angle relationship. Indeed, this flattening reduces peak torque but allows for greater torque at other joint angles.29 The ankle angle, used here, did not coincide with peak torque and could therefore partly explain the absence of Pt change. While modifications of the torque–angle relationship could not be excluded, differences between our results and others could also originate from methodological aspects. Indeed, stretching, performed here, was primarily achieved with straight legs whereas tests were performed with a more flexed leg (50◦ knee flexion). Therefore, specific muscle contributions according to muscle length during stretching or tests would likely influence our findings.

In conclusion, sub-maximal voluntary contractions were performed here to reduce potential injury risks during contract-relax stretching.22 However, even performed using sub-maximal voluntary contractions, 15 min contract-relax as well as static stretching appeared detrimental for plantar flexion torque production. After stretching, different central effects were obtained for MG and SOL that can partly be attributed to the relative mechanical contribution of each muscle depending on joint angles. However, our findings should be tempered by the fact that the 15 min stretching duration, used in the present experiment, was longer as compared with stretches used during training and by our small sample size with large inter-subjects variability. Further researches are clearly needed so as to identify potential origins of stretch interventions acute effects with shorter stretch durations, with varying angular positions during tests and with a larger sample size.

Practical implications • 15 min stretching has deleterious effects on subsequent maximal strength production capacity and is precluded before competition.

• Static and sub-maximal contract-relax stretching have similar effects on muscle strength.

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