The Effects of 5 Minutes of Static Stretching on Joint Flexibility and Muscle Strength Are Comparable Between Ballet Dancers and Non-Dancers

Abstract

Introduction: Ballet dancers have a special morphology, such as a large muscle thickness that affects passive torque. Ballet dancers also possess specialized mechanical, and neural properties of muscles and tendons. These characteristics may produce different static stretching effects than non-dancers. Therefore, this study aimed to determine the differences in the effects of static stretching on joint range of motion, passive torque, and muscle strength between ballet dancers and non-dancers. Methods: This study included 13 ballet dancers and 13 college students. The muscle and tendon thicknesses were assessed using ultrasonography. In the right lower extremity, torque-angle data and muscle-tendon junction displacement measurements were obtained during isokinetic passive dorsiflexion before and after a 5-minute static stretch against the right plantar flexors. The relative stretching intensity was calculated by dividing the stretching angle by the maximal dorsiflexion angle pre-stretch. Additionally, the isometric maximal voluntary plantar flexion torque on the left ankle was measured before and after 5 minutes of static stretching against the left plantar flexors. Results: Ballet dancers had significantly greater muscle thickness than non-dancers (22.4 ± 2.2 vs 18.1 ± 1.7 mm), whereas no significant difference was observed in the Achilles tendon thickness. No significant difference was observed in the stretching angle; however, the relative stretching intensity was higher in the control group (65.9 ± 19.8 vs 127.5 ± 63.8%). Static stretching increased the maximal dorsiflexion angle (dancer: 30.4° ± 9.6° to 33.9° ± 9.5°, non-dancer: 18.4° ± 8.6° to 20.5° ± 9.5°) and maximal passive torque in both groups, whereas the maximal isometric plantar flexion torque and submaximal passive torque decreased. However, no significant differences were observed in the changes between the groups. Conclusion: These results indicate that despite having a lower relative stretching intensity, ballet dancers experienced similar changes as non-dancers after 5 minutes of static stretching.

Keywords

stiffness plantar flexors ultrasonography passive torque muscle displacement Achilles tendon thickness

Key Points

The relative stretching intensity was higher in the non-dancers, although absolute stretching intensity was comparable between ballet dancers and non-dancers.

Stretching-induced changes in passive flexibility properties were comparable between ballet dancers and non-dancers.

Stretching-induced changes in muscle strength were comparable between ballet dancers and non-dancers.

Introduction

Joint mobility is an important element in physical fitness for ballet dancers.¹ Ballet dancers conduct high-intensity stretching over extended periods to improve their joint range of motion (ROM).² Ballet dancers who have trained for more than 3 years have higher hip, knee, and ankle joint ROM than amateur ballet dancers who have trained for less than 1 year or non-dancers.³ In addition, ballet dancers have different mechanical and neural factors related to flexibility, such as passive torque (PT), joint stiffness, and stretch tolerance, compared with non-dancers.²

Previous studies have reported an increase in joint ROM following static stretching (SS).^4
-11 The mechanism has been explained by changes in mechanical factors, such as decreased PT of submaximal angle and decreased stiffness, and neurological factors, such as increased stretch tolerance.^7,12 However, ballet dancers have lower submaximal PT² and stiffness² and higher stretch tolerance than non-dancers¹³ as a result of their adaptation to ballet training. Thus, in order for ballet dancers to increase their ROM, they must further decrease their initially low values of submaximal PT and stiffness and further increase their initially high values of stretch tolerance, compared to non-dancers. For instance, Moltubakk et al² showed that the passive torque at 0° of the ankle joint for ballet dancers was 12 ± 3 Nm, compared to 17 ± 3 Nm for non-dancers. On the other hand, 5 minutes of SS increased maximal dorsiflexion angle 5.1° ± 3° and decreased passive torque at 0° of ankle joint 0.7 Nm (change rate was 10%) in non-dancers.¹⁴ To increase the maximal dorsiflexion angle, if the same amount (or percentage) of passive torque was to be reduced in both groups, a 5.8% (or 1.2 Nm) reduction in passive torque would be required in the ballet dancers and 4.1% (or 1.7 Nm) in the non-dancers. However, in practical terms, it would be difficult for a ballet dancer with a low initial value to make a larger percentage change than a non-dancer with a high initial value, or an absolute change of approximately the same magnitude. Instead, assuming that PT of submaximal angle, stiffness, and stretch tolerance have a maximum or minimum convergence point for physiological, mechanical, and morphological reasons, it is not easy to change the indicators of ballet dancers closer to the convergence point, and different amounts of change to stretching would be expected. Therefore, ballet dancers are less likely to experience a decrease in PT and stiffness and an increase in stretch tolerance, which could result in a smaller increase in joint ROM.

Differences in the effects of SS on individuals with varying flexibility levels are unclear. Pessali-Marques et al¹³ reported that the increase in maximal knee extension angle was greater in ballet dancers than non-dancers after six 30-second SS on hamstrings. Another previous study reported that the rate of increase in maximal hip flexion angle was greater in team sports athletes than rhythmic gymnasts after six 30-second SS on hip extensors.¹⁵ Thus, previous studies have shown inconsistent results regarding the differences in the effects of SS on individuals with different flexibility levels,^13,15 making it unclear which group, the more flexible (such as ballet dancers and rhythmic gymnasts) or less flexible (such as non-dancers and team sports athletes), would show a greater change. Moreover, previous studies have primarily focused on measuring joint ROM and have not adequately examined the effects of mechanical factors or neurological factors.^13,15,16

It is possible that the effect of SS on maximal muscle strength may also differ between ballet dancers and non-dancers. Muscular strength is one of the important components of physical fitness for ballet dancers.¹ However, immediately after SS, a decrease in maximal muscle strength^17
-20 and the rate of force development¹⁰ (RFD) has been reported. This reduction has been explained by decreased stiffness and reduced motor neuron excitability.^{10,17,18,21
-24} It has been reported that the stiffness of ballet dancers at common joint angle is less than half that of non-dancers.² Therefore, considering the trainability of stiffness to SS, one would expect that the decrease in stiffness to the same SS would be smaller for ballet dancers. Consequently, ballet dancers may experience a different effect of SS on maximal muscle strength reduction than non-dancers.

Therefore, this study aimed to determine the differences in the effects of SS on joint ROM, PT, stretch tolerance, stiffness, and muscle strength between the ballet dancer and control groups. We hypothesized that the effects of SS on these measures would differ between the ballet dancer and control groups, with the ballet dancer group showing fewer changes than the control group.

Methods

Participants

The number of participants required was calculated using the following parameters (power = 0.80, alpha = 0.05, and effect size = 0.46 [Calculated using data from ROM, which is one of the indicators for which we specifically want to compare group differences in this study and whose data are presented in a previous study by Donti et al¹⁵ with an experimental design similar to that of this study]), and the results indicated that a minimum of 10 participants were required in each group (G*Power 3.1). The participants included 13 female ballet dancers and 13 female control participants (sedentary college students). Descriptive data are shown in Table 1. The participants were not undergoing treatment for cardiovascular, respiratory, or orthopedic diseases at the time of the experiment. In the control group, verbal confirmation that the participant was not exercising regularly. The participants were fully informed of the content of the study, and they provided written consent to participate. This study was conducted in accordance with the principles of the Declaration of Helsinki and was approved by the local ethics committee (Approval No. 21-06).

Table 1.

Physical description, morphometry, passive torque, and stretching intensity.

	Ballet dancers	Controls	P
Age (years)	20.7 ± 2.6	21.5 ± 1.1	.064
Height (cm)	157.5 ± 4.5	160.0 ± 5.2	.198
Weight (kg)	50.2 ± 7.2	52.4 ± 6.1	.400
Training history of the dancers (years)	15.7 ± 3.0
Current training or exercise volume (min/week)	1093.8 ± 981.6	12.7 ± 41.5
Muscle thickness (mm)	22.4 ± 2.2	18.1 ± 1.7	<.001*
Tendon thickness (mm)	4.9 ± 1.1	4.5 ± 0.7	.185
Passive torque at 0° (Nm)	5.1 ± 0.9	4.3 ± 0.6	.027*
Passive torque at 5° (Nm)	6.8 ± 1.4	5.9 ± 1.0	.051
Absolute stretching intensity of the right foot (°)	18.8 ± 3.9	19.6 ± 3.8	.623
Absolute stretching intensity of the left foot (°)	18.2 ± 3.2	18.7 ± 3.2	.687
Relative stretching intensity (%)	65.9 ± 19.8	127.5 ± 63.8	.005*

Mean ± standard deviation.

Significant difference between groups.

Study Design

Figure 1 shows the experimental flow. The measurement familiarization included practice with passive dorsiflexion flexibility and plantar flexion muscle strength measurements, and determination of stretch intensity in SS. Subsequently, the participants performed bicycle pedaling (50 W) for 5 minutes as a warm-up, and the morphology of the right lower limb was measured. Passive dorsiflexion flexibility measurements were obtained before and after 5 minutes of SS of the right lower leg. After a 15-minute rest period, ankle plantar flexion muscle strength was measured before and after 5 minutes of SS of the left lower leg.

Figure 1.

Schematic representation of the study protocol.

Procedures

Morphometry

Muscle thickness of the medial gastrocnemius muscle (MG) and tendon thickness of the Achilles tendon were measured using an ultrasound imaging device (LOGIQ 5; GE Healthcare, CT, USA). MG thickness was measured by acquiring a long-axis image at the proximal 30% of the length of the lower leg (distance from the popliteal fossa to the lateral malleolus of the fibula). Achilles tendon thickness was measured by acquiring a short-axis image at the height of the lateral malleolus of the fibula. The muscle and tendon thicknesses were analyzed using the open-license software Kinovea (version 0.8.27).

Passive dorsiflexion flexibility measurement

The ankle joint angle, PT, and displacement of the muscle-tendon junction were measured during passive dorsiflexion of the right ankle joint using an isokinetic machine (S-15177, Takei Scientific Instruments, Niigata, Japan). The participants were instructed to relax during the measurements. Measurements were obtained with the knee joint in full extension and the right foot^12,14 fixed to the footplate. The ankle joint was set at 0° when the footplate was vertical to the floor, and the rotation angle of the footplate was used as the ankle joint angle. In addition, the dorsiflexion angle was considered positive. The ankle joints of the participants were passively dorsiflexed by rotating the footplate at an angular velocity of 1° per second from 30° of plantar flexion. When the participants first experienced the onset of pain in the lower leg, they were instructed to press a button to stop the footplate rotation. The angle of the footplate at that time was used as the maximal dorsiflexion angle.¹⁴ Passive dorsiflexion flexibility measurements were performed twice in pre- and post-stretching exercises. The measurement with the highest value of the maximal dorsiflexion angle was analyzed for the other measures described below and used in subsequent analyses.

PT during passive dorsiflexion was measured using an isokinetic machine (S-15177, Takei Scientific Instruments, Niigata, Japan) at 0° and 5° pre-stretching and submaximal and maximal dorsiflexion positions, pre-and post-stretching. PT in the submaximal dorsiflexion position was evaluated every 4° during the final 13° (that is, at final 1°, 5°, 9°, and 13°) in pre-and post-stretching for each participant. A second-order regression curve was plotted using the values of the ankle dorsiflexion angle and PT at the 4 points during the final 13°. The slope of the tangent line at the final 1°, 5°, 9°, and 13° was determined as the stiffness. PT at the maximal dorsiflexion position was evaluated as stretch tolerance.

In addition, muscle-tendon junction displacement of the MG was measured using an ultrasound imaging device (LOGIQ5; GE Healthcare) during passive dorsiflexion flexibility measurement. The relative displacement between the reflective marker affixed to the skin and the muscle-tendon junction was measured, and the amount of change was calculated as the muscle displacement. Muscle displacement was measured as the amount of displacement from the final 1° to the final 5°, 9°, and 13° and at the maximal dorsiflexion position.

Electromyography (EMG)

Muscle activation during passive dorsiflexion in passive dorsiflexion flexibility measurement was measured using an EMG machine (DL-140; S&ME, Tokyo, Japan) to confirm that the subject was not contracting muscles during passive dorsiflexion. Disposable surface electrodes (F-150s; Nihon Kohden, Tokyo, Japan) were affixed to the MG and tibialis anterior muscle at 20 mm. The electrode location was the most prominent bulge of the MG and one-third of the distance from the tip of the fibula to the medial malleolus.²⁵ The EMG signals were transmitted to a digital data recorder at a sampling rate of 1.0 kHz and recorded at a 5 to 500 Hz bandwidth. The root mean square was calculated for the first 10° interval (Initial 10°) from the dorsiflexion starting position (−30°) in the passive dorsiflexion flexibility measurement and the last 5° interval (Last 5°) from an angle of 5° less than the maximal dorsiflexion position to the maximal dorsiflexion position.¹⁴

Plantar flexion muscle strength measurement

The maximal isometric voluntary contraction strength was measured with the knee joint in full extension and the left foot fixed to the footplate (S-17199, Takei Scientific Instruments, Niigata, Japan). The participants were instructed to exert maximal plantar flexion force as quickly as possible using the left foot. The measurements were obtained twice, and the result with the highest maximal torque value was used in the subsequent analysis. The slope of the time-torque curve from the start of muscle exertion to each elapsed time (0-30 ms, 0-50 ms, 0-100 ms, and 0-200 ms) was calculated as the RFD.²⁶ The point at which the exerted muscle strength exceeded 2 SD at rest was used as the force onset point.

SS

Stretching was performed using a stretching board (ASAHI Corp., Gifu, Japan), using one lower leg for 1 minute per set, for 5 sets with a 30-second rest between sets. The duration of SS and number of sets were determined based on the methods of previous studies^12,14 that reported increased ROM and decreased stiffness after SS for the plantar flexors. Rest periods were provided to allow time to adjust the stretching angles for each set. The stretching intensity was set at the angle at which the participant felt maximal stretch sensation in the lower leg without pain. The stretching intensity was checked with the participant between each rest period, and the angle of the stretching board was adjusted accordingly. The average angle of the 5 sets of stretching boards was used for subsequent analysis and defined as the absolute stretching intensity. In addition, the relative stretching intensity was defined as the absolute stretching intensity divided by the maximal dorsiflexion angle before the stretch.²⁷ The participants were instructed to maintain a straight body posture without bending their knees or hips during stretching. The examiner confirmed that there was no collapse of this posture during stretching.

Data reliability

Test-retest reliability was calculated using data from 2 measurements each for passive dorsiflexion flexibility measurement and plantar flexion muscle strength measurement. The intraclass correlation coefficient values for ankle ROM, passive torque at maximal dorsiflexion angle, and maximal plantar flexion torque were .987, .975, and .973, respectively. In addition, there were no significant differences between data from 2 measurements each for ankle ROM, passive torque at maximal dorsiflexion angle, and maximal plantar flexion torque.

Data analyses

The Shapiro–Wilk test was used to assess normal distribution. Age was analyzed using the Mann–Whitney U test. Height, weight, MG thickness, Achilles tendon thickness, absolute stretching intensity of the right and left feet, relative stretching intensity of the right foot, and PT at 0° and 5° of pre-stretching were analyzed using an unpaired t-test. The maximal dorsiflexion angle and maximal plantar flexion torque were analyzed using a 2-way analysis of variance (ANOVA) (group [ballet dancers vs controls] × time [pre- vs post-stretching]). Muscle displacement and PT during the final 13° and at the maximal dorsiflexed positions were analyzed using a 3-way ANOVA (group [ballet dancers vs controls] × time [pre- vs post-stretching] × angle [final 1° vs final 5° vs final 9° vs final 13° vs maximal dorsiflexed position]). Stiffness was analyzed using a 3-way ANOVA (group [ballet dancers vs controls] × time [pre- vs post-stretching] × angle [final 1° vs final 5° vs final 9° vs final 13°]. RFD was analyzed using a 3-way ANOVA (group [ballet dancers vs controls] × time [pre- vs post-stretching] × interval [0-30 ms vs 0-50 ms vs 0-100 ms vs 0-150 ms vs 0-200 ms]). The EMG of the MG and tibialis anterior muscles were analyzed using a 3-way ANOVA (group [ballet dancers vs. controls] × time [pre- vs post-stretching] × portion [Initial 10° vs Last 5°]). Follow-up analyses were performed using lower-order ANOVA and t-tests with Bonferroni correction. Statistical significance was set at P ≤ .05. All data are reported as means ± standard deviations. SPSS (version 22.0; SPSS, Inc., Chicago, IL, USA) was used to conduct statistical analyses.

Results

Physical Description

No significant differences were observed between the 2 groups in age (P = .064), height (P = .198), or weight (P = .400) (Table 1).

Morphometry

The muscle thickness of the MG was greater in the ballet dancers than in the controls (P < .001). However, no significant difference was observed in Achilles tendon thickness between the 2 groups (P = .185) (Table 1).

Absolute and Relative Stretching Intensity

No significant differences were observed between the absolute stretching intensities of the right (P = .623) and left (P = .687) feet for both groups. However, the controls had significantly greater relative stretching intensity than the ballet dancers (P = .005) (Table 1).

Maximal Dorsiflexion Angle

No significant 2-way interaction was observed (F (1, 24) = 1.342, P = .258); however, significant main effects for time (F (1, 24) = 20.849, P < .001) and group (F (1, 24) = 12.568, P = .002) were observed. Post-hoc testing revealed that the ballet dancers had a greater maximal dorsiflexion angle than the controls. Additionally, the maximal dorsiflexion angle increased after SS (Figure 2).

Figure 2.

Changes in maximal dorsiflexion angle before and after static stretching in the ballet dancers and controls.

PT

Regarding PT at 0° and 5° during pre-stretch, ballet dancers had greater PT at 0° than the controls (P = .027). However, no significant difference was observed in PT at 5° (P = .051) (Table 1).

Regarding PT during the final 13° and maximal dorsiflexed positions, no significant 3-way interaction was observed (F (1.065, 25.565) = 2.262, P = .144), and no significant 2-way interaction was observed between time and group (F (1, 24) = 0.006, P = .941). However, significant 2-way interactions were observed between the angle and group (F (1.180, 28.321) = 12.295, P = .001) and time and angle (F (1.065, 25.565) = 10.737, P = .003). Post-hoc testing revealed that the ballet dancers had greater PT than the controls during the final 13° and at the maximal dorsiflexed position (final 1°: P = .005; final5°: P = .004; final9°: P = .002; final 13°: P = .002; maximal dorsiflexed position: P = .002). Additionally, the PT during the final 13° decreased (all P < .001), whereas the PT at the maximal dorsiflexed position increased after stretching (P = .044). PT increased with an increasing joint angle in both groups (all P < .05), except between the final 13° and maximal dorsiflexed position of the controls. Similarly, the PT increased with an increased joint angle before and after stretching (all P < .05), except between the final 13° and the maximal dorsiflexed position before stretching (Table 2).

Table 2.

Changes in passive torque (Nm) after static stretching in ballet dancers and controls.

	Final 1°^b,d	Final 5°^b,d	Final 9°^b,d	Final 13°^b,d	Maximal dorsiflexed position^b,d
Ballet dancers^a
Pre	15.9 ± 7.7	20.0 ± 9.0	25.2 ± 10.7	31.1 ± 12.8	31.1 ± 12.8
Post	15.0 ± 7.6^c	18.7 ± 8.9^c	23.5 ± 11.0^c	29.2 ± 13.7^c	34.3 ± 16.1^c
Controls
Pre	7.5 ± 5.9	9.5 ± 7.4	12.0 ± 9.0	15.0 ± 10.6	15.2 ± 10.6
Post	7.0 ± 5.4^c	8.9 ± 6.7^c	11.0 ± 7.8^c	13.7 ± 8.9^c	16.1 ± 12.6^c

Mean ± standard deviation.

Passive torque of the ballet dancers was greater than that of the controls.

Passive torque increased with an increasing joint angle in both groups, except between the final 13° and maximal dorsiflexed position of the controls.

Passive torque decreased during the final 13° and increased in the maximal dorsiflexed position after static stretching.

Passive torque increased with an increasing joint angle before and after stretching, except between the final 13° and maximal dorsiflexed position before stretching.

Stiffness

No significant 3-way (F (1, 24) = 2.789, P = .108) and 2-way interactions were observed between time and group (F (1, 24) < 0.001, P = 1.000) and time and angle (F (1, 24) = 0.093, P = .763). However, a significant 2-way interaction was observed between the angles and groups (F (1, 24) = 14.662, P = .001). Post-hoc testing revealed that the ballet dancers experienced greater stiffness than the controls at all angles during the final 13° (final 1°: P = .004; final5°: P = .001; final9°: P = .001; final 13°: P < .001). In addition, stiffness increased with an increasing joint angle in both groups (all P < .05) (Table 3).

Table 3.

Changes in stiffness (Nm/°) after static stretching in ballet dancers and controls.

	Final 1°^b	Final 5°^b	Final 9°^b	Final 13°^b
Ballet dancers^a
Pre	0.91 ± 0.38	1.15 ± 0.46	1.39 ± 0.55	1.63 ± 0.64
Post	0.80 ± 0.30	1.06 ± 0.40	1.32 ± 0.60	1.58 ± 0.70
Controls
Pre	0.44 ± 0.37	0.57 ± 0.38	0.69 ± 0.40	0.81 ± 0.43
Post	0.40 ± 0.32	0.50 ± 0.3	0.60 ± 0.28	0.70 ± 0.28

Mean ± standard deviation.

The stiffness of the ballet dancers was greater than that of the controls.

Stiffness increased with an increasing joint angle in both groups.

Muscle Displacement

No significant 3-way (F (2.557, 61.363) = 2.643, P = .066) and 2-way interactions were observed between time and group (F (1, 24) = 1.216, P = .281) and angle and group (F (1.570, 37.673) = 0.494, P = .569. However, a significant 2-way interaction was observed between time and angle (F (2.557, 61.363) = 10.635, P < .001)). Post-hoc testing revealed that muscle displacement increased after stretching in the maximal dorsiflexed position (P < .001). In addition, muscle displacement increased with increasing joint angle before and after stretching (all P < .001), except between the final 13° and the maximal dorsiflexed position before stretching (P = .931) (Table 4).

Table 4.

Changes in muscle displacement (mm) after static stretching in ballet dancers and controls.

	Final 1°^a	Final 5°^a	Final 9°^a	Final 13°^a	Maximal dorsiflexed position^a
Ballet dancers
Pre	0.0 ± 0.0	1.2 ± 0.5	2.4 ± 0.6	3.6 ± 0.9	3.6 ± 0.9
Post	0.0 ± 0.0	1.3 ± 0.6	2.6 ± 0.9	3.8 ± 1.2	5.0 ± 1.3^b
Controls
Pre	0.0 ± 0.0	1.1 ± 0.7	2.3 ± 0.9	3.7 ± 1.2	3.8 ± 1.2
Post	0.0 ± 0.0	1.2 ± 0.6	2.5 ± 1.0	3.8 ± 1.1	4.3 ± 1.4^b

Mean ± standard deviation.

Muscle displacement increased with increasing joint angle before and after stretching, except between the final 13° and maximal dorsiflexed position before stretching.

Muscle displacement increases after stretching in the maximal dorsiflexed position.

Maximal Plantar Flexion Torque

No significant 2-way interaction (F (1, 24) = 0.035, P = .853) and main effects of the group (F (1, 24) = 2.323, P = .141) were observed; however, a significant main effect of time was observed (F (1, 24) = 13.985, P = .001). Post-hoc testing revealed that the maximal plantar flexion torque decreased after stretching (Figure 3).

Figure 3.

Changes in maximal plantar flexion torque before and after static stretching in the ballet dancers and controls.

RFD

No significant 3-way (F (1.288, 30.906) = 0.239, P = .689) and 2-way interactions (time and period: F (1.288, 30.906) = 1.409, P = .252; period and group: F (1.342, 32.219) = 1.018, P = .344; time and group: F (1, 24) = 2.612, P = .119) were observed. In addition, no significant main effect of time was observed (F (1, 24) = 0.134, P = .717); however, significant main effects of the period (F (1.342, 32.219) = 43.829, P < .001) and group (F (1, 24) = 5.539, P = .027) were observed. Post-hoc testing revealed that ballet dancers had greater RFD than the controls. In addition, the RFD increased with increasing periods (all P < .005), except between 0-150 ms and 0-200 ms.

EMG

Regarding the EMG of the MG, a significant 3-way interaction was observed (F (1, 24) = 5.781, P = .024). Post-hoc testing revealed that the ballet dancers had less EMG during the initial 10° pre-stretch than the controls (P = .038). The EMG of the ballet dancers in the pre-stretch increased from the initial 10° to the final 5° (P = .004), whereas the EMG of controls in the post-stretch increased from the initial 10° to the final 5° (P = .013) (Table 5).

Table 5.

Changes in electromyography (mV) after static stretching in ballet dancers and controls.

	Medial gastrocnemius muscle		Tibialis anterior muscles
	Initial 10°	Final 5°	Initial 10°	Final 5°
Ballet dancers
Pre	0.043 ± 0.002	0.046 ± 0.005^b	0.018 ± 0.003	0.018 ± 0.006
Post	0.044 ± 0.003^a	0.044 ± 0.004	0.017 ± 0.006	0.018 ± 0.006
Controls
Pre	0.045 ± 0.003	0.046 ± 0.002	0.019 ± 0.003	0.019 ± 0.003
Post	0.045 ± 0.002	0.046 ± 0.002^c	0.021 ± 0.004	0.021 ± 0.005

Mean ± standard deviation.

The EMG of the ballet dancers during the initial 10° at pre was less than that of controls.

The EMG of the ballet dancers at pre increased from initial 10° to final 5°.

The EMG of the controls at post increased from initial 10° to final 5°.

Regarding the EMG of the tibialis anterior muscle, no significant 3-way (F (1, 24) = 0.027, P = .870) and 2-way interactions (time and portion: F (1, 24) = 0.873, P = .359; portion and group: F (1, 24) = 0.508, P = .483; time and group: F (1, 24) = 2.698, P = .114) and main effects (time: F (1, 24) = 1.271, P = .271; portion: F (1, 24) = 0.508, P = .483; group: F (1, 24) = 1.834, P = .188) were observed (Table 5).

Discussion

This study aimed to determine the differences in the effects of SS on joint ROM, PT, stretch tolerance, stiffness, and muscle strength between the ballet dancer and control groups. The main findings of this study were that maximal dorsiflexion angle, PT, muscle displacement, and maximal plantar flexion torque changed in both groups after SS; however, no differences were observed between the groups.

No difference was observed in the change in the maximal dorsiflexion angle after SS between the ballet dancers and controls. Previous studies have reported that the increase in joint ROM after SS in ballet dancers and rhythmic gymnasts, who have a larger joint ROM, differed from that in the controls.^13,15 In contrast, no difference was observed in the increase in the maximal dorsiflexion angle between the ballet dancers and controls in this study. One reason for the different results between previous studies and this study may be the difference in the stretching intensity. Although one previous study does not clearly state whether the angle or torque was maintained during stretching,¹⁵ but the other study states that the PT was maintained during stretching (ie, constant torque stretching).¹³ Constant torque stretching requires increasing the joint angle to maintain constant torque during the stretching.^28,29 Stretching angles have been used as a parameter of stretching intensity,³⁰ and it has been reported that greater stretching angles are associated with greater increases in ROM.³¹ Therefore, constant torque stretching provides a greater increase in ROM than constant angle stretching.³² On the other hand, constant angle stretching was performed in this study. In constant angle stretching, passive torque during stretching is reduced due to stress-relaxation.³¹ Therefore, the stretching load³³ calculated by the passive torque during stretching and stretching time is smaller in constant angle stretching than that in constant torque stretching, so the relative increase in ROM would be also smaller. Thus, it is possible that the lower stretching intensity in this study compared to the previous study¹³ did not elicit the maximal increase in ROM. This effect may have been particularly large in the ballet dancers, who usually stretch and have a large joint range of motion. We speculate that this may be the reason why there was no difference in ROM increase between the groups, but further research is needed to clarify the truth of this speculation.

Other possible reason would include characteristics of the participants. Since previous study² had shown that submaximal passive torque was lower in ballet dancers than in controls, we hypothesized that it would be more difficult to further reduce the originally low passive torque of ballet dancers with SS than the originally high passive torque of controls with. However, in this study, ballet dancers had higher passive torque than the control group at 0°, and there was no significant difference at 5°. Therefore, these characteristics of the ballet dancers in this study, which were different from those of the ballet dancers in the previous study,² may have contributed to the fact that no difference in stretching effect was observed. A possible reason for the discrepancy in results between this study and previous studies could be differences in ballet dance history. The average training history of dancers was 24 years in the previous study and 15.7 years in this study. Thus, differences in years of adaptation to dance training may have influenced differences in passive torque across studies.

The group differences in relative stretching intensity may be attributed to differences in the methods of maximal dorsiflexion angle between stretching and passive dorsiflexion flexibility measurement. It has been reported that the maximal dorsiflexion angle of the ankle joint measured in the non-weight-bearing position correlates with ankle stiffness, as in the passive dorsiflexion flexibility measurement in the present study, but the maximum dorsiflexion angle of the ankle joint measured in the weight-bearing position, as in the SS in the present study, does not correlate with ankle stiffness.³⁴ This suggests that factors related to the maximal dorsiflexion angle differ depending on whether it was measured in the non-weight-bearing or weight-bearing position. Stiffness at the same angle was lower in ballet dancers than in controls.² Therefore, it is possible that the dancers showed a higher maximal dorsiflexion angle in the non-weight-bearing position, where stiffness and maximal dorsiflexion angle were negatively correlated, but no difference between groups was observed in the weight-bearing position, where stiffness and maximum dorsiflexion angle were not correlated.³⁴ It is possible that there are other factors besides stiffness that may or may not be related to the maximal dorsiflexion angle as a result of differences in measurement posture. Such factors may contribute to the low relative stretching intensity in ballet dancers.

A decrease was observed in the maximal plantar flexion torque in both groups after SS; however, no difference was observed between the groups. Previous studies have reported a muscle force deficit immediately after SS^17,20,21,35 caused by the nocebo effect, decreased muscle stiffness, and decreased signaling ability in motoneuron dendrites.^17,18 Recently, central neurological factors, such as decreased information transmission in motoneuron dendrites, have been considered the main cause of muscle weakness after SS.^18,23,36 However, in this study, no differences were observed in the decrease in maximal plantar flexion torque between the groups, suggesting no differences in the effects of central neurological factors between the ballet dancer and control groups for the stretching method used in this study.

This study has some limitation. It included only young women as the ballet dancers and controls. Previous studies have reported sex- and age-related differences in flexibility, with women having greater joint ROM and muscle displacement than men and younger participants having greater joint ROM and stretch tolerance than older participants.^37,38 Therefore, future experiments that include men and middle-aged to older adults or developing ballet dancers may yield different results. Second, the intensity of the stretching was insufficient and may not have elicited adequate changes due to SS. Therefore, a comparison of the effects of constant torque stretching is needed. Third, this study focused on the ankle joint; effects at other sites, such as the hip joint, are unknown. Fourth, although the control group in this study was sedentary college students, comparisons with active controls and athletes from other sports disciplines will further clarify whether there are ballet dancer-specific effects on SS. Finally, SS was implemented in this study, but the use of dynamic stretching is recommended,³⁹ especially in the warm-up. Therefore, future studies should examine whether there is a ballet dancer-specific influence on the effectiveness of dynamic stretching.

Practical and Clinical Applications and Implications

In the case of the stretching method used in this study, there was no difference in the effects of static stretching on flexibility and muscle strength between ballet dancers and the general population. In general, people with greater flexibility tend to be asked to perform SS with greater intensity and for longer periods of time than those who are not as flexible. Ballerinas and their instructors should understand that a 5-minute constant angle stretch in the weight-bearing position for the ankle joint can be as effective as the general population (which is less flexible than the ballet dancers) without longer or more intense stretching (ie, a less flexible group than the ballet dancers). However, it should be noted that different stretching target areas and different stretching methods (eg, intensity, duration, position, etc.) may produce different results from those in this study.

Conclusion

SS increased the maximal dorsiflexion angle, PT at the maximal dorsiflexed position, muscle displacement at the maximal dorsiflexed position, and decreased PT during the final 13° and maximal plantar flexion torque. However, no group differences were observed in the changes in these measures. Thus, SS with constant angle produce similar change regardless of the individual’s flexibility.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the JSPS KAKENHI Grant-in-Aid for Scientific Research (C) [Grant Number 22K11525].

ORCID iD

Takamasa Mizuno

References

Koutedakis

Jamurtas

The dancer as a performing athlete: physiological considerations. Sports Med. 2004;34(10):651-661.

Moltubakk

Magulas

Villars

Seynnes

Bojsen-Møller

Specialized properties of the triceps surae muscle-tendon unit in professional ballet dancers. Scand J Med Sci Sports. 2018;28(9):2023-2034.

Sharma

Nuhmani

Wardhan

Muaidi

QI.

Comparison of lower extremity muscle flexibility in amateur and trained bharatanatyam dancers and nondancers. Med Probl Perform Art. 2018;33(1):20-25.

Kokkonen

Nelson

Eldredge

Winchester

JB.

Chronic static stretching improves exercise performance. Med Sci Sports Exerc. 2007;39(10):1825-1831.

Konrad

Tilp

Increased range of motion after static stretching is not due to changes in muscle and tendon structures. Clin Biomech. 2014;29(6):636-642.

Magnusson

. Passive properties of human skeletal muscle during stretch maneuvers. A review. Scand J Med Sci Sports. 1998;8(2):65-77.

Magnusson

Simonsen

Aagaard

Kjaer

Biomechanical responses to repeated stretches in human hamstring muscle in vivo. Am J Sports Med. 1996;24(5):622-628.

Magnusson

Simonsen

Aagaard

Sørensen

Kjaer

A mechanism for altered flexibility in human skeletal muscle. J Physiol. 1996;497(Pt 1):291-298.

Mahieu

McNair

De Muynck

, et al. Effect of static and ballistic stretching on the muscle-tendon tissue properties. Med Sci Sports Exerc. 2007;39(3):494-501.

10.

Nakamura

Suzuki

Yoshida

, et al. The time-course changes in knee flexion range of motion, muscle strength, and rate of force development after static stretching. Front Physiol. 2022;13:917661.

11.

Oba

Samukawa

Nakamura

, et al. Influence of constant torque stretching at different stretching intensities on flexibility and mechanical properties of plantar flexors. J Strength Cond Res. 2021;35(3):709-714.

12.

Morse

Degens

Seynnes

Maganaris

Jones

DA.

The acute effect of stretching on the passive stiffness of the human gastrocnemius muscle tendon unit. J Physiol. 2008;586(1):97-106.

13.

Pessali-Marques

Peixoto

GHC

Cabido

CET

, et al. Biomechanical response to acute stretching in dancers and non-dancers. J Dance Med Sci. 2020;24(1):12-18.

14.

Mizuno

Matsumoto

Umemura

Viscoelasticity of the muscle-tendon unit is returned more rapidly than range of motion after stretching. Scand J Med Sci Sports. 2013;23(1):23-30.

15.

Donti

Gaspari

Papia

, et al. Acute effects of intermittent and continuous static stretching on hip flexion angle in athletes with varying flexibility training background. Sports. 2020;8(3):28.

16.

Rubini

Souza

Mello

, et al. Immediate effect of static and proprioceptive neuromuscular facilitation stretching on hip adductor flexibility in female ballet dancers. J Dance Med Sci. 2011;15(4):177-181.

17.

Behm

Chaouachi

A review of the acute effects of static and dynamic stretching on performance. Eur J Appl Physiol. 2011;111(11):2633-2651.

18.

Behm

Kay

Trajano

Blazevich

AJ.

Mechanisms underlying performance impairments following prolonged static stretching without a comprehensive warm-up. Eur J Appl Physiol. 2021;121(1):67-94.

19.

Kay

Blazevich

AJ.

Effect of acute static stretch on maximal muscle performance: a systematic review. Med Sci Sports Exerc. 2012;44(1):154-164.

20.

Kokkonen

Nelson

Cornwell

Acute muscle stretching inhibits maximal strength performance. Res Q Exerc Sport. 1998;69(4):411-415.

21.

Mizuno

Matsumoto

Umemura

Stretching-induced deficit of maximal isometric torque is restored within 10 minutes. J Strength Cond Res. 2014;28(1):147-153.

22.

Costa

Ryan

Herda

, et al. Acute effects of passive stretching on the electromechanical delay and evoked twitch properties. Eur J Appl Physiol. 2010;108(2):301-310.

23.

Trajano

Blazevich

AJ.

Static stretching reduces motoneuron excitability: the potential role of neuromodulation. Exerc Sport Sci Rev. 2021;49(2):126-132.

24.

Trajano

Seitz

Nosaka

Blazevich

AJ.

Passive muscle stretching impairs rapid force production and neuromuscular function in human plantar flexors. Eur J Appl Physiol. 2019;119(11-12):2673-2684.

25.

Mizuno

Matsumoto

Umemura

Decrements in stiffness are restored within 10 min. Int J Sports Med. 2013;34(6):484-490.

26.

Aagaard

Simonsen

Andersen

Magnusson

Dyhre-Poulsen

Increased rate of force development and neural drive of human skeletal muscle following resistance training. J Appl Physiol. 2002;93(4):1318-1326.

27.

Fukaya

Nakamura

Sato

, et al. The relationship between stretching intensity and changes in passive properties of gastrocnemius muscle-tendon unit after static stretching. Sports. 2020;8(11):140.

28.

Ryan

Herda

Costa

, et al. Viscoelastic creep in the human skeletal muscle-tendon unit. Eur J Appl Physiol. 2010;108:207-211.

29.

Ryan

Herda

Costa

Walter

Cramer

JT.

Dynamics of viscoelastic creep during repeated stretches. Scand J Med Sci Sports. 2012;22(2):179-184.

30.

Young

Elias

Power

Effects of static stretching volume and intensity on plantar flexor explosive force production and range of motion. J Sports Med Phys Fitness. 2006;46(3):403-411.

31.

Fukaya

Matsuo

Iwata

, et al. Acute and chronic effects of static stretching at 100% versus 120% intensity on flexibility. Eur J Appl Physiol. 2021;121(2):513-523.

32.

Cabido

Bergamini

Andrade

, et al. Acute effect of constant torque and angle stretching on range of motion, muscle passive properties, and stretch discomfort perception. J Strength Cond Res. 2014;28(4):1050-1057.

33.

Takeuchi

Akizuki

Nakamura

Association between static stretching load and changes in the flexibility of the hamstrings. Sci Rep. 2021;11(1):21778-21830.

34.

Whitting

Steele

McGhee

Munro

BJ.

Passive dorsiflexion stiffness is poorly correlated with passive dorsiflexion range of motion. J Sci Med Sport. 2013;16(2):157-161.

35.

Ryan

Beck

Herda

, et al. Do practical durations of stretching alter muscle strength? A dose-response study. Med Sci Sports Exerc. 2008;40(8):1529-1537.

36.

Trajano

Seitz

Nosaka

Blazevich

. Contribution of central vs. peripheral factors to the force loss induced by passive stretch of the human plantar flexors. J Appl Physiol. 2013;115(2):212-218.

37.

Gajdosik

Vander Linden

Williams

AK.

Influence of age on length and passive elastic stiffness characteristics of the calf muscle-tendon unit of women. Phys Ther. 1999;79(9):827-838.

38.

Morse

CI.

Gender differences in the passive stiffness of the human gastrocnemius muscle during stretch. Eur J Appl Physiol. 2011;111(9):2149-2154.

39.

Opplert

Babault

Acute effects of dynamic stretching on muscle flexibility and performance: an Analysis of the current literature. Sports Med. 2018;48(2):299-325.