Effects of Different Amounts of Dynamic Stretching on Musculotendinous Extensibility and Muscle Strength

Abstract

Dynamic stretching (DS) is performed as a warm-up to improve the range of motion and athletic performance. However, the effect of different amounts of DS on muscle performance remains unclear. This study investigated the effects of DS repetitions with one or four sets of 30 s on musculotendinous extensibility and muscle strength. Fourteen healthy men (23.6 ± 1.5 years) underwent DS to ankle plantar flexors for one set (fifteen repetitions) or four sets after warm-up. The maximal ankle dorsiflexion angle, musculotendinous stiffness (MTS), passive torque, peak plantarflexion torque during maximal isometric contraction, and muscle temperature were measured before and after stretching. A significant effect of time was observed on the maximal ankle dorsiflexion angle, MTS, passive torque, and muscle temperature (p < 0.001). However, no interactions or effects between the conditions were observed. After DS, the maximal ankle dorsiflexion angle and muscle temperature significantly increased (p < 0.01), while the MTS and passive torque significantly decreased (p < 0.01). The maximal muscle strength showed no significant effects or interactions (p = 0.198−0.439). These results indicated that one and four sets of DS effectively increased musculotendinous extensibility. Thus, one set of DS may have similar effects as a warm-up before four sets of DS.

1. Introduction

Dynamic stretching (DS) involves repeated movements throughout the full range of motion (ROM) by contracting the antagonist muscles of the target muscles, such that the target muscles are stretched by reciprocal inhibition [1]. DS is widely performed as a routine warm-up before sports [2,3]. Previous studies have reported that DS improves the ROM and sports performance [2,4,5,6,7,8,9]. A recent study reported that static stretching (SS) reduces the incidence of musculotendinous injuries during explosive contraction activities such as sprint activities and sports [10]. DS may decrease muscle–tendon unit (MTU) injury rates by increasing muscle strength and MTU compliance, thereby increasing the ability to absorb greater forces and torque [10,11]. Therefore, DS should be conducted as part of a warm-up before sports [2].

Static stretching (SS) is a type of stretching in which a muscle is held in a stretched position for certain time [12]. Regarding the dose–response relationship between stretching time and ROM, previous studies have found that a longer SS is associated with a wider ROM and lower athletic performance [12,13]. A recent systematic review by Warneke et al. also reported that SS reduces the maximal muscle strength but does not affect jumping or sprinting performance [14]. Short-term SS had little effect on athletic performance [12,13]. Another study investigated the dose–response relationship between the amount of DS and musculotendinous extensibility [15]. A positive dose–response relationship was observed between the number of repetitions of DS and ROM, although the musculotendinous stiffness (MTS) did not change after one, four, or seven sets of DS [15]. Regarding athletic performance, vertical jump height increased regardless of the amounts of DS when comparing DS for 6 min to DS for 12 min [8]. Furthermore, after one, two, and three sets of DS, sprint times decreased after the first and second sets but increased after the third set [16]. Notably, large amounts of DS may induce fatigue and impair athletic performance. The effects of different amounts of DS on jumping and sprinting performances have been investigated. However, no study has measured muscle strength and temperature simultaneously with the ROM and MTS. Therefore, clarifying the effects of different DS amounts on musculotendinous extensibility and muscle strength and temperature may help determine the optimal DS protocol for improving musculotendinous extensibility and athletic performance within a limited warm-up time.

An increase in muscle and/or body temperature induced by repeated DS contractions was positively correlated with athletic performance [17]. Five types of DS with two sets of ten repetitions and six types of DS with two sets of twelve repetitions performed on multiple muscle groups increased core temperature, jump height, and isokinetic knee flexion muscle strength [17,18]. These findings imply that a larger number of muscle contractions during DS may lead to an increase in muscle temperature, core temperature, and better athletic performance. Moreover, a reduction in MTS and thixotropic effects that increase the ROM are observed with an increase in the muscle temperature. The changes in the ROM and MTS are speculated to be affected by increased muscle and/or core temperatures [19].

Warm-ups before sports are commonly performed using aerobic exercises, stretching, and sport-specific movements [20,21]. However, the factors contributing to impaired athletic performance after stretching may be caused not only by stretching but also by activities that cause fatigue, such as pre-aerobic exercises [14]. In previous studies that examined the efficacy of DS on athletic performance, aerobic exercise was not performed before DS [22,23,24]. Without pre-aerobic exercise, isokinetic knee flexion torque decreased after DS [24]. In contrast, isokinetic knee flexion and extension torque increased when DS was performed followed by aerobic exercise [9]. Thus, combining DS with aerobic exercise and clarifying the effects of DS with similar warm-up protocols before sports are necessary.

Therefore, this study aimed to investigate the effects of the number of DS repetitions with one set for 30 s (DS1, fifteen repetitions) and four sets (DS4, fifteen repetitions of four sets) on musculotendinous extensibility and muscle strength. The number of DS repetitions was determined from a previous study [15]. Based on a previous review article that showed greater force and power improvement with DS > 90 s than with DS < 90 s, the following hypotheses were proposed: musculotendinous extensibility would not change after DS1 and DS4, and muscle strength would improve after DS4 but would not change after DS1 [25].

2. Materials and Methods

2.1. Participants

Fourteen healthy young men (age: 23.6 ± 1.5 years; height: 173.2 ± 5.5 cm; weight: 63.0 ± 6.5 kg) volunteered in this study. The sample size was calculated using a repeated measures ANOVA and within–between interaction model in the G*Power 3.1 software (Heinrich Heine University Düsseldorf, Düsseldorf, Germany), with a large effect size (f = 0.55), an α error of 0.05, and a power of 0.80 [15]. The effect size was determined based on MTS outcomes from a previous study that compared different amounts of DS. The required sample size was calculated to be 10. Participants had no current neuromuscular or musculoskeletal disorders of the lower limbs. They were asked to refrain from intensive exercise and alcohol consumption for 24 h before each experiment. This study was approved by the Institutional Review Board of the affiliated university (approval number: 23–18). Participants were fully informed of the procedures to be utilised as well as the purpose of this study. Written informed consent was obtained from all participants.

2.2. Study Design

The participants visited the laboratory thrice. On the first visit, the study protocol was explained to the participants, they practiced DS, and muscle strength and the maximum tolerable torque threshold for ankle dorsiflexion were measured. During the second and third visits, they performed one set of fifteen DS repetitions or four sets of fifteen DS repetitions at 60 beats/min. The experimental sessions were conducted in randomised order. Furthermore, each visit was separated by more than 48 h, and the experimental sessions were performed at the same time of the day (±2 h). The participants were instructed to sit and rest for 20 min before the experiments to adapt to the laboratory environment. Second, aerobic exercise was performed using a cycling ergometer (POWER MAX VIII, Konami Sports, Inc., Tokyo, Japan) for 5 min at a rate of 60 repetitions/min. Musculotendinous extensibility and strength were measured before and after stretching (Figure 1). The room temperature was set to 25 °C for all sessions.

2.3. Maximal Ankle Dorsiflexion Angle and MTS Measurement

The maximal ankle dorsiflexion angle and passive torque were measured using a Biodex System 3 dynamometer (Biodex Medical Systems, Inc., Shirley, NY, USA) and MyoSystem 1200 (Noraxon USA, Inc., Scottsdale, AZ, USA), respectively, to assess muscle activity during the measurement. The participants were placed in the supine position with their knees fully extended and strapping belts secured around the pelvis and right distal thigh [26]. The right foot was placed in the mid-position of the plantar dorsiflexion of the ankle joint, with the lateral malleolus aligned with the axis of the dynamometer and secured to the footplate with strapping belts. A 0° angle of the ankle joint was defined as the position at which the footplate was perpendicular to the ground. The maximum ankle dorsiflexion angle and passive torque were measured when the ankle was passively dorsiflexed at 2°/s from 20° plantar flexion until the maximum passive torque threshold was reached. The maximum passive torque threshold was recorded on the first day. The participants were instructed to press the safety button when they felt maximal stretching without pain and to relax during the measurements. In the pre-measurements, the ankle joint was dorsiflexed twice to minimise the influence of the change in the passive torque value due to dorsiflexion/plantarflexion [27,28].

2.4. DS Protocol

DS was performed using an isokinetic dynamometer with the right ankle plantar flexor in the same position as for the maximum ankle dorsiflexion angle and passive torque measurements. Participants were instructed to actively move their right ankle from the maximum angle of ankle plantar flexion to the maximal angle of ankle dorsiflexion (Figure 2). DS velocity was set using a metronome (60 beats/min). Against the 0.5 N resistance of the isokinetic dynamometer, the patient was instructed to pull their foot to the maximum dorsiflexion without bouncing and to push to the maximum plantar flexion synchronised with a metronome. The participants performed the DS in one or four sets of fifteen repetitions [15]. Between each set, rest was provided for 30 s at 0° to the ankle joint [29].

2.5. Isometric Muscle Strength

The maximal muscle strength was measured as a performance index by using a Biodex System 3 dynamometer. To determine the peak torque of ankle plantar flexion, the participants performed two maximum voluntary isometric contractions (MVICs) of the right ankle plantar flexor muscles, pre- and post-stretching. Each MVIC was maintained for 3 s with a 2 min rest period between sets [30]. The peak torque was determined as the highest value of the two trials, and this value was normalised to body weight.

2.6. Muscle Temperature

Muscle temperature was measured using an electronic thermometer (Coretemp CM-210; Terumo, Japan) [31,32,33,34]. The probe was placed on the skin surface of the belly of the lateral head of the gastrocnemius muscle 20 min before the start of the experiment [31]. Muscle temperature was measured from the start of the aerobic exercise until the end of the experiment.

2.7. Data Analysis

The maximal ankle dorsiflexion angle, passive torque, and peak torque of the ankle plantar flexion signals were processed and analysed using MATLAB (R2022a, MathWorks, Natick, MA, USA). The torque data were filtered using a fourth Butterworth filter at 10 Hz and collected to eliminate the effects of gravity on the starting limb position [35]. Therefore, the processed torque data were 0 Nm at 20° plantar flexion, only indicating resistance to stretching from the musculotendinous complex. The maximal ankle dorsiflexion angle was defined as the dorsiflexion angle at the point where the maximum passive torque threshold was reached. The MTS was determined as the slope of the second-order polynomial passive torque–dorsiflexion angle regression curve at each of the four measurement points (1°, 5°, 9°, and 13°) during the final 13° of the ankle dorsiflexion angles, as described in previous studies [15,36]. The average of the four points was used to calculate the MTS. Passive torque was determined at the maximum ankle dorsiflexion angle. Thus, the passive torque was calculated at the same ankle angle for the pre- and post-measurements.

2.8. Statical Analysis

SPSS software (version 26.0; IBM Japan Co., Tokyo, Japan) was used for all the statistical analyses. The Shapiro–Wilk test was used to confirm the normality of all data. A two-way repeated measures analysis of variance (condition [DS1 and DS4] × time [pre- and post-measurement]) was used to analyse the maximal ankle dorsiflexion angle, MTS, passive torque, peak torque of ankle plantar flexion, and muscle temperature. The Bonferroni correction was applied when significant effects were identified. The effect size was determined using partial eta-squared values (η_p²) for the ANOVA. Cohen’s d_z was used for post hoc comparisons as small (d_z = 0.2), moderate (d_z = 0.5), or large (d_z = 0.8) [37]. The intraclass correlation coefficient was calculated as the reliability of the maximal ankle dorsiflexion angle and MTS using ICC _(1,2) with the pre-measurement values for each condition. The pre-stretching measurements under both conditions were used to calculate the ICC. The ICC _(1,2) of the maximal ankle dorsiflexion angle was 0.927 (95% confidence interval = 0.788−0.976, standard error of measurement = 1.15). The ICC _(1,2) of the MTS was 0.962 (95% confidence interval = 0.887–0.988, standard error of measurement = 0.05). The significance level was set at p < 0.05. All data are presented as means.

3. Results

3.1. Maximal Ankle Dorsiflexion Angle

A significant main effect of time was observed (p < 0.001; η_p² = 0.714). However, no significant two-way interaction (condition × time) or main effect of condition was observed (p = 0.990; η_p² = 0.000; and p = 0.866; η_p² = 0.002). The maximal ankle dorsiflexion angle significantly increased after DS1 and DS4 (4.8%; p < 0.001; d_z = 1.46 and 4.8%; p = 0.001; d_z = 1.10, respectively) (Table 1).

3.2. MTS

A significant main effect of time was also observed (p < 0.001; η_p² = 0.664). However, no significant two-way interaction (condition × time) or main effect of condition was observed (p = 0.909; η_p² = 0.001, and p = 0.647; η_p² = 0.017). MTS significantly decreased after DS1 and DS4 (−5.9%; p < 0.001; d_z = 1.59 and −5.9%; p = 0.003; d_z = 0.98, respectively) (Table 1).

3.3. Passive Torque

A significant main effect of time was observed (p < 0.001; η_p² = 0.695). However, no significant two-way interaction (condition × time) or main effect of condition was observed (p = 0.924; η_p² = 0.001, and p = 0.170; η_p² = 0.140). Passive torque decreased significantly after both DS1 and DS4 (−7.1%; p < 0.001; d_z = 1.71 and −7.0%; p = 0.002; d_z = 1.07, respectively) (Table 1).

Table 1. Mean (±standard deviation) musculotendinous extensibility before and after stretching (n = 14).

	One Set of DS		Four Sets of DS		p-Value
Outcomes	Pre	Post	Pre	Post
Maximal ankle dorsiflexion angle (°)	37.12 ± 6.15	38.91 ± 6.52 a	37.23 ± 6.46	39.01 ± 6.78 a	0.990
MTS (Nm/°)	1.18 ± 0.29	1.11 ± 0.26 b	1.19 ± 0.31	1.12 ± 0.28 b	0.909
Passive torque (Nm)	36.41 ± 8.68	33.84 ± 8.38 b	37.32 ± 8.88	34.69 ± 8.26 b	0.924

Mean ± standard deviation. DS: dynamic stretching; MTS: musculotendinous stiffness. ^a indicates a significant increase from pre-stretching (p < 0.05). ^b indicates a significant decrease from pre-stretching (p < 0.05).

Table 1. Mean (±standard deviation) musculotendinous extensibility before and after stretching (n = 14).

	One Set of DS		Four Sets of DS		p-Value
Outcomes	Pre	Post	Pre	Post
Maximal ankle dorsiflexion angle (°)	37.12 ± 6.15	38.91 ± 6.52 a	37.23 ± 6.46	39.01 ± 6.78 a	0.990
MTS (Nm/°)	1.18 ± 0.29	1.11 ± 0.26 b	1.19 ± 0.31	1.12 ± 0.28 b	0.909
Passive torque (Nm)	36.41 ± 8.68	33.84 ± 8.38 b	37.32 ± 8.88	34.69 ± 8.26 b	0.924

Mean ± standard deviation. DS: dynamic stretching; MTS: musculotendinous stiffness. ^a indicates a significant increase from pre-stretching (p < 0.05). ^b indicates a significant decrease from pre-stretching (p < 0.05).

3.4. Maximal Muscle Strength

For the maximal muscle strength, no significant two-way interaction (condition × time), main effect of condition, or main effect of time was observed (p = 0.198; η_p² = 0.097, p = 0.259; η_p² = 0.047, and p = 0.439; η_p² = 0.124, respectively) (Table 2).

3.5. Muscle Temperature

Time had a significant effect (p < 0.001; η_p² = 0.828). However, there was no significant two-way interaction (p = 0.145; η_p² = 0.003) or main effect of condition (p = 0.845; η_p² = 0.156). Post hoc tests revealed that the muscle temperature increased after both the DS1 and DS4 sessions (2.2%; p < 0.001; d_z = 1.54, and 2.1%; p < 0.001; d_z = 1.61, respectively) (Table 2).

Table 2. Mean (±standard deviation) peak torque and muscle temperature before and after stretching (n = 14).

	One Set of DS		Four Sets of DS		p-Value
Outcomes	Pre	Post	Pre	Post
Peak torque during MVIC (Nm/kg)	1.81 ± 0.23	1.81 ± 0.25	1.83 ± 0.26	1.87 ± 0.27	0.198
Muscle temperature (°C)	33.81 ± 0.62	34.55 ± 0.59 a	33.54 ± 0.95	34.26 ± 0.64 a	0.145

Mean ± standard deviation. DS: dynamic stretching; MVIC: maximal voluntary isometric contraction. ^a indicates a significant increase from pre-stretching (p < 0.05).

Table 2. Mean (±standard deviation) peak torque and muscle temperature before and after stretching (n = 14).

	One Set of DS		Four Sets of DS		p-Value
Outcomes	Pre	Post	Pre	Post
Peak torque during MVIC (Nm/kg)	1.81 ± 0.23	1.81 ± 0.25	1.83 ± 0.26	1.87 ± 0.27	0.198
Muscle temperature (°C)	33.81 ± 0.62	34.55 ± 0.59 a	33.54 ± 0.95	34.26 ± 0.64 a	0.145

Mean ± standard deviation. DS: dynamic stretching; MVIC: maximal voluntary isometric contraction. ^a indicates a significant increase from pre-stretching (p < 0.05).

4. Discussion

This study compared the effects of one and four sets of DS repetitions for 30 s on musculotendinous extensibility and muscle strength and temperature. A significant increase in the maximal ankle dorsiflexion angle and muscle temperature and a significant decrease in MTS and passive torque were observed after the DS1 and DS4 sessions, regardless of the number of DS repetitions. In contrast, the isometric maximal ankle plantar flexion torque did not change after DS and did not differ between DS conditions.

A significant increase and decrease in the ankle dorsiflexion angle and in the MTS and passive torque was observed, regardless of the number of DS repetitions. An increased maximal ankle dorsiflexion angle and decreased MTS have been reported after DS of four sets or ten sets of 30 s [7,24], and the present results support these studies. The ROM is influenced by mechanical properties, such as the reduction in MTS and increase in pain tolerance [15,24,38]. As the MTS reflects changes in the musculotendinous complex, it does not indicate which structures, such as muscles or tendons, have changed. The changes in MTS properties after DS showed that 30 s sets of DS had the effect of lengthening tendon tissues but not muscle tissues [39]. In this study, the same changes may have occurred, and the MTS decreased after one set of DS for 30 s. Further research using ultrasonography may be required to clarify the musculotendinous complex in detail.

A cycling ergometer was used for aerobic exercise before DS. In a previous study that did not find significant changes in the MTS after DS, aerobic exercise was not performed before DS [6,15]. This may have resulted in a greater increase in muscle temperature after DS than reported in previous studies [6,15]. Therefore, aerobic exercise before DS was considered an important factor that contributed to the different results from those of a previous study.

Although DS improved musculotendinous extensibility, there was no change in the maximal isometric muscle strength. Many studies have reported improvements in athletic performance after DS [4,5,8,9,16,17,18]. However, in some studies, DS did not change isometric muscle strength [23,40]. An increase in isokinetic knee flexion and extension torque was observed after DS for two sets of 30 s [9]. In contrast, no change in the maximal plantar flexion torque was observed after two 20 s sets of DS for the plantar flexor muscles [40]. A possible explanation for these discrepancies is the difference in the muscles and the repetitions of DS. Sekir et al. performed four different DS targeting the quadriceps and hamstrings for two sets of 30 s each, taking 6 ± 1 min [9]. In the present study, only one set of 30 s and four sets of DS were performed on the plantar flexors, and there was no change in the maximal plantar flexion torque with either set of DS.

The present study was conducted to determine the least effective durations for musculotendinous extensibility and muscle strength by comparing 30 s (one set of 30 s) and 2 min (four sets of 30 s). Although a short DS was effective in changing musculotendinous extensibility in this study, it had no significant effect on the maximal muscle strength. In addition to the stretching volume, two other factors that can influence the DS effect are the stretching velocity and amplitude [2,17,29]. Different stretching velocities and amplitudes may change musculotendinous extensibility and maximal muscle strength. Therefore, future studies need to consider DS velocity and amplitude when comparing the effects of different amounts of DS on both musculotendinous extensibility and maximal muscle strength.

Neuromuscular function is thought to improve because of increased core and muscle temperatures and increased muscle activity during the maximum voluntary contraction [9]. As a stretch that benefits from active and rhythmic muscle contraction, DS helps warm up and increases heart rate, core temperature, and muscle temperature [2]. Additional warm-up time increases the muscle temperature [41]. Previous studies have reported that an increase in muscle temperature increases nerve conduction velocity and improves athletic performance [41] and that an increase in muscle temperature of 1 °C increases athletic performance by 2–5% [42]. Therefore, an increase in the neuromuscular function and muscle activity may have occurred. Consequently, an increase in the muscle temperature may positively affect athletic performance. The present study suggests that an increase in the muscle temperature counteracted the negative effects of the decreased MTS on athletic performance, such that maximal muscle strength did not change.

In a survey study on stretching in sports fields, it was reported that a DS of 30 s or less per set was generally widely used and that the time required for stretching the full body was approximately nine minutes [3]. Because the warm-up and practice times before a match are limited, a shorter and more effective stretching method is required. In this study, one set of 30 s and four sets of DS decreased MTS and increased flexibility but did not decrease athletic performance. This suggests that 30 s of DS is useful as a warm-up before sports. DS may be effective for warming up several body parts within a limited time. Therefore, the results of the present study may be useful in planning DS protocols for warm-up exercises in sports.

As DS is used in warm-ups before sports, its effects must be sustained and exerted at the start of a match or training session. Therefore, it is necessary to clarify the sustained effects of DS. Several studies have reported the long-term effects of DS; however, the evidence is insufficient. One study showed that the effects of DS on the ROM and passive stiffness lasted for >90 min, whereas the peak passive torque returned to baseline within 30 min [7]. Another study showed that the ROM and peak passive torque persisted for 60 min after DS [43]. Future studies should investigate the sustained effects of varying amounts of DS.

This study had some limitations. First, we did not set control conditions that included either aerobic or dynamic stretching. Pre-stretching aerobic exercise may have affected our results; however, it is unclear whether this effect was due to aerobic exercise or DS. Musculotendinous extensibility may also be affected by aerobic exercise. Second, we did not investigate physiological outcomes, such as muscle blood flow or nerve conduction velocity. Although DS improves performance by improving neuromuscular function, the details of this mechanism remain unclear. Third, we investigated only the acute effects of DS. Examining the sustained effects will help to clarify the appropriate timing for DS. Fourth, we performed MVICs before the post-measurements of musculotendinous extensibility and muscle strength. A previous study reported that MVICs altered the mechanical properties of musculotendinous complexes [44]. Therefore, MVICs pre-stretching may affect musculotendinous extensibility and maximal muscle strength. Fifth, since it is not clear whether a 48 h washout between DS conditions is sufficient, it cannot be said that there was no carry-over effect. Therefore, the participants were required to visit the lab once more to exclude testing effects. Finally, participants were limited to healthy males. Sex-related differences exist in the effects of stretching caused by sex hormones [45]. Therefore, further studies are required to investigate the effects of DS in participants of different ages and sexes.

5. Conclusions

This study investigated the effects of different amounts of DS on the ankle plantar flexors, ankle dorsiflexion range of motion, MTS, passive torque, maximal muscle strength, and muscle temperature. Except for muscle strength, these variables changed significantly after the intervention. However, there were no significant differences in any variables between the DS 1-set and 4-set conditions. These findings indicate that there are no differences in the immediate effects on musculotendinous extensibility and strength between one and four sets of 30 s DS of a single joint targeting only the plantar flexor muscles. Therefore, one set of DS (fifteen repetitions) may be more efficient as part of the warm-up prior to playing sports. However, the practical relevance of these results needs to be carefully considered as isolated isometric muscle contractions have limited applications in complex athletic performance.