Stretching is commonly performed before exercises, especially on those demanding an important neuromuscular component (e.g., strength and sprinting) (11). It has been proposed that on increasing the range of motion (ROM), stretching may also improve performance and reduce the incidence of muscle-tendinous injuries (31). However, some studies have demonstrated that maximal isometric torque (i.e., maximal voluntary contraction [MVC]) (24), maximal isokinetic concentric torque (9), and vertical jump height (35) are compromised after stretching. The impaired MVC values after stretching have been attributed to reduced neural drive (3,10) and musculotendinous stiffness (23), which may alter the muscle length-tension relationship and sarcomere shortening velocity (25).
The explosive muscle strength is highly dependent on the rate of increase in contractile force at the onset of the contraction (i.e., rate of force development [RFD]), with its maximal values being attained in 80–120 milliseconds (26). The RFD is obtained from the slope of the torque-time curve (Δtorque/Δtime) and can be determined during isometric (8) and isokinetic contractions (26). Andersen and Aagaard (2) have shown that the RFD is influenced by different factors at the early (<100 milliseconds) and late phases (>100 milliseconds) of isometric contraction. The early phase is mainly influenced by neural drive (1) and intrinsic muscle contractile properties (2). During the late phase, muscle cross-sectional area (30), neural drive (14), and stiffness of tendon-aponeurosis complex (7) are more important. It is noteworthy that the research is equivocal regarding the effect of stretching on the explosive movements. Some investigators have found that the vertical jumping height can be reduced (4–7%) after stretching (5,35). However, other studies have observed no effects of static stretching on the foot speed while kicking a football (34) or performing a tennis serve (20). These contradictory results can be explained, at least in part by the different time spans involved in the explosive movements (e.g., unloaded kick ∼80 milliseconds vs. vertical jump ∼250 milliseconds), which can be affected by different physiological mechanisms. Therefore, the RFD measured at different time intervals from the onset of contraction can provide relevant information regarding the effects of stretching on explosive muscle strength, which is an important component of many athletic events.
Thus, the objective of this study was to analyze the effect of active static stretching on MVC and RFD measured at different time intervals from the onset of muscle contraction. Based on the studies cited above (1–3,7,10,14,18,30), it was hypothesized that only MVC and consequently RFD obtained during the late phase of contraction are affected by active static stretching.
Experimental Approach to the Problem
The hypothesis that MVC and consequently only RFD obtained during the late phase of contraction are affected by active static stretching was tested. This study had a crossover design in which the participants underwent a control and an experimental condition in a balanced fashion. In the control condition, each participant completed a 5-minute warm-up at 50 W on a stationary cycle ergometer (Excalibur Sport, Lode BV, Groningen, Holland) followed by 3 minutes of rest and then performed tests to determine the MVC and RFD (dependent variables). In the experimental condition, the participants followed the same procedures but performed active static stretching exercises (independent variable) between warm-up and neuromuscular tests. The total duration of the active static stretching session (10 minutes) was based on data obtained by Ryan et al. (28). In this study, the authors suggested that there may be a threshold stretching duration between 8 and 10 minutes that may distinguish between significant and nonsignificant decreases in muscle strength in untrained individuals. Because the RFD can be influenced by different mechanisms during the early (<100 milliseconds) and late phase (>100 milliseconds) of contraction, its values were determined at different time intervals from the onset of contraction (i.e., 0–30, 0–50, 0–100, 0–150, and 0–200 milliseconds), instead of its maximal values.
Fifteen men (age = 21.3 ± 2.4 years; stature = 173.0 ± 21.4 cm; and body mass = 78.3 ± 11.2 kg) volunteered for the study. They were physical education students involved in recreational sports (soccer, basketball, volleyball), but they had not participated in regular strength training for at least 6 months before the start of the study. All the subjects were informed about the procedures and experimental risks and signed an informed consent document before the beginning of this investigation. This study was approved by the Institutional Review Board of the university.
The participants visited the laboratory on 3 occasions each separated by 3–5 days. Initially, each participant was required to attend a laboratory familiarization session to lessen any effect of learning during subsequent strength testing. During this session, each participant performed 5 maximal isometric contractions for knee extensors in the isokinetic dynamometer. In the subsequent 2 visits, the subjects performed, in random order, the following procedures: (a) 2 maximal isometric contractions for knee extensors in the isokinetic dynamometer to determine the MVC and RFD (control) and (b) 2 active static stretching exercises for the dominant leg extensors (10 × 30 seconds for each exercise with 20-second rest interval between bouts). Immediately after stretching (∼3 minutes), the isokinetic test was repeated (poststretching). Conditions 1 and 2 were performed in random order. The subjects were instructed to be fully rested and hydrated, at least 3 hours postprandial when reporting to the laboratory and to refrain from using caffeine-containing food or beverages, drugs, alcohol, cigarette smoking, or any form of nicotine intake 24 hours before testing. All the experimental trials were performed at the same time of the day (±2 hours) for each subject.
Determination of Maximal Voluntary Contraction and Rate of Force Development
A System 3 Biodex isokinetic dynamometer (Biodex Medical Systems Inc., Shirley, NY, USA) and computer software were used to measure the MVC. The subjects were placed in a sitting position and securely strapped into the test chair. Extraneous movement of the upper body was limited by 2 crossover shoulder harnesses and an abdominal belt. The trunk-thigh angle was 85°. The axis of the dynamometer was lined up with the right knee flexion-extension axis, and the lever arm was attached to the shank by a strap. The subject was asked to relax his leg so that the passive determination of the effects of gravity on the limb and lever arm could be carried out. The MVC were determined for the knee extensors (m. quadriceps femoris) at a static knee joint angle of 60° (0° = full extension), that is, the angle at which maximal isometric torque is attained (33). Two maximal isometric attempts were performed with rest periods of 60 seconds in between. The subjects were instructed to extend their knee “as fast and as hard as possible,” (6,29) and each maximal contraction was sustained for approximately 3 seconds. Strong verbal encouragement was given during each trial. The intraclass correlation coefficient for the test-retest reliability for the MVC and RFD of quadriceps were 0.97 and 0.87, respectively (21).
Torque curves were smoothed by using a 10-Hz Butterworth fourth-order zero-lag filter. The MVC was calculated as the average torque over a 1-second period around the torque-plateau level. The highest torque achieved between the 2 isometric contractions was considered as the MVC. The RFD (newtons per meter per second) was defined as the slope of the torque-time curve (i.e., ΔTorque/ΔTime) in incrementing time periods of 0–30, 0–50, 0–100, 0–150, and 0–200 milliseconds from the onset of contraction (2). The onset of muscle contraction was considered when the torque level reached 8 N·m (Figure 1).
Static Stretching Exercises
Each subject underwent 2 static stretching exercises designed to stretch the leg extensor muscles of the dominant limb. Ten repetitions of each stretching exercise were held for 30 seconds at a point of mild discomfort, but not pain, as acknowledged by the subject. Between each stretching repetition, the leg was returned to a neutral position for a 20-second rest period. The total stretching time was 10 minutes. For the first stretching exercise, the subject stood upright with one hand against a wall for balance. The subject then flexed the dominant leg to a knee joint angle of 90o. The ankle of the flexed leg was grasped by the ipsilateral hand, and the foot was raised so that the heel of the dominant foot approached the buttocks. For the second stretching exercise, the subjects laid down on their side and flexed their superior knee by pulling the foot of that leg toward their buttocks with one hand.
Data are reported as mean ± SD unless stated otherwise. The normal distribution of all dependent variables was examined by the Shapiro–Wilk test. A dependent Student's t-test was used to compare any variable between the 2 conditions. Significance was set at p ≤ 0.05.
Figure 2 presents the mean and individual values of the MVC obtained with and without previous stretching exercise. The MVC (285 ± 59 vs. 271 ± 56 N·m) was significantly reduced after stretching exercise (p < 0.01).
The mean ± SD values of the RFD obtained with and without previous stretching exercise are presented in Figure 3. The RFD determined within time intervals of 30, 50, and 100 milliseconds relative to onset of contraction was unchanged after stretching (p > 0.05). However, the RFD measured at intervals of 0–150 and 0–200 milliseconds was significantly reduced after stretching (p < 0.01).
The objective of this study was to analyze the influence of active static stretching on MVC and RFD measured at different time intervals from the onset of muscle contraction. Our main findings were that (a) similar to that observed in other studies (15,22), the MVC measured at 60° of knee flexion decreased after the stretching session and (b) the RFD was significantly reduced only during the late phase (>100 milliseconds), suggesting that the factors associated with maximal muscle strength (neural drive and stiffness of tendon-aponeurosis complex) may be responsible for the changes in mechanical muscle properties.
Studies have demonstrated that acute static stretching impairs muscle force and torque (15,22,24). Similar to that found previously (15,22,24), the MVC was reduced by 5% in this study. The main factors affecting the percentage of force-torque loss are the duration of the stretching session (28) and the length (tension-length relationship) at which the muscle is analyzed (22). Consistent with the findings of previous studies (15,32), Ryan et al. (28) have demonstrated that there may be a threshold stretching duration between 8 and 10 minutes (as used in this study) that may distinguish between significant and nonsignificant decreases in the MVC. For the tension-length relationship, it has been shown that the reduction in muscle torque is greater in joint positions where the muscle is shortened when compared with that in lengthened positions (22).
Two hypotheses have been pointed to explain the acute deficit on the MVC. Reduction in the amplitude of the electromyography (EMG) signal (3,9), voluntary activation (%VA) (4), and torque of the unstretched contralateral limb (10) have been used to support a neural component. The impairment on muscle strength has been explained by the reduced reflex sensibility and muscular activation resulting from the inhibition of the central neural system. Recently, Herda et al. (16) hypothesized that stretching may provide a temporary inhibition of the function of efferent γ fibers. It has been proposed that the feedback of the afferent 1a fibers from the muscle spindles and the efferent motor response (i.e., γ loop) is important for the facilitation and recruitment of type 2 fibers during an MVC (17). A second hypothesis has been based on mechanical alterations occurring after the static stretching (18). The main contributing factors for this are reduced muscle-tendon stiffness, which modifies the muscular tension-length relationship and the sarcomere shortening velocity (24,25). Moreover, stretching may create a plastic deformation of connective tissues that may also impair strength (13).
Studies that have employed cross-sectional and longitudinal experimental designs (e.g., training effects) have shown equivocal results regarding the relationship between maximal strength and explosive strength. Moderate to high correlation levels have been found between both maximal strength and explosive strength (27) and maximal strength and RFD measured during isometric contraction (12). However, other studies failed to verify significant relationship between maximal strength and functional measurements of explosive strength (i.e., jumps and sprinting performance) (19). As demonstrated by Andersen and Aagaard (2), these contradictory data may be explained, at least in part, by the different time intervals from the onset of contraction at which RFD was determined. In this study (2), the maximal strength explained 52–81% of the variance in the RFD measured at time intervals >90 milliseconds from the onset of muscle contraction. Moreover, the RFD was less dependent on intrinsic muscle properties, as the time from the onset of contraction increased. Data from this study seem to confirm that the relationship between explosive strength (i.e., RFD) and maximal strength (MVC) is dependent on the time interval in which the RFD is determined.
To our knowledge, this is the first study that has analyzed the effects of stretching on the RFD measured at different time intervals from the onset of contraction in untrained individuals. The RFD was significantly reduced (5–7%) only in the late phase (>100 milliseconds) of the muscle contraction. Thus, the main determining factors of maximal muscle strength (neural drive and muscle cross-sectional area) could explain, at least in part, the effects of stretching on the RFD. As discussed above, studies have demonstrated a reduction of EMG and %VA after stretching, suggesting a decreased neural drive in these conditions. Additionally, Bojsen-Moller et al. (7) have found that the stiffness of muscle-tendon unit may explain for up to 30% of the variance in the RFD measured during the late phase of muscle contraction. Thus, similar to maximal strength, the lower RFD after stretching may also be explained by the reduction of muscle-tendon stiffness.
Data of this study may help to explain the different effects of stretching on the explosive movements. Some studies have verified that the vertical jumping height can be reduced (4–7%) after the stretching, which did not seem to be dependent on the type of jump performed (i.e., squat × countermovement) (5). However, Young et al. (34) did not verify the effect of static stretching on the foot velocity during the kick in football. These explosive movements involve different time spans (e.g., unloaded kick ∼80 milliseconds vs. vertical jump ∼250 milliseconds) and can be affected by different physiological mechanisms. Therefore, our study seems to confirm the results described above, which suggested that the explosive strength may be more affected during explosive movement lasting >100 milliseconds.
Based on these data, it can be concluded that the active static stretching, performed for 10 minutes reduces the MVC and RFD measured during the late phase (>100 milliseconds) of contraction. Thus, explosive muscle activities of a very short duration seem to be less affected by static muscle stretching, when compared with activities using the maximum muscle strength.
Our findings are in agreement with those of other studies and confirm that active static stretching determines the reduction in maximal muscle strength. Moreover, explosive movements lasting >100 milliseconds are negatively influenced by static stretching, also confirming previous results referring to muscle power (i.e., reduced vertical jump height). Therefore, the use of active static stretching during warm-up before training sessions or competitions using maximal strength and muscle power must be avoided. However, it may be used before training sessions involving explosive movements of a very short duration (e.g., kicking, throwing), particularly that involving a wide ROM, which can be increased after static stretching exercises.
The authors thank Fundação de Amparo à Pesquisa do Estado de São Paulo and Conselho Nacional de Desenvolvimento Científico e Tecnológico for financial support.
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