Muscle architecture (MA) is an important factor in functional task performance (6,8), because it relates to muscle force-length and force-velocity properties (13). Previous studies have observed that MA parameters can be changed in consequence of strength training (1); however, previous studies have found no changes on MA in consequence of static stretching (4,5,10). For instance, Nakamura et al. (10) performed a 4-week static stretching to the plantarflexors (self-stretching; volume: 2 × 60 seconds daily; intensity: “largest stretch that participants were willing to tolerate”) and found no changes in fascicle length (FL), fascicle angle (FA), or muscle thickness (MT). Lima et al. (5) also found no differences in the MA parameters after an 8-week static stretching program (assisted stretching; volume: 3 × 30 seconds with 30-second rest between repetitions, 3 times a week; intensity: “within the physiological limit and preceding the pain threshold”). Lima et al. (5) suggested that the lack of change in MA could be due to the low intensity and duration of the static stretching intervention, since adaptations on MA after static stretching has been observed in studies with animal models (12,15). In addition, there is evidence of a greater maximal range of motion (ROM) increase when performing static stretching with higher stretching intensity (14), duration (7), repetitions (2), and stretching session frequency (6). Thus, a change on MA parameters after a static stretching may depend on the stretching intensity; however, this has never been tested.
Recently, it was demonstrated that a stretching with no rest interval between the repetitions induces a greater ROM increase during the stretching compared with a conventional rest interval protocol (3). The purpose of this pilot study was to determine whether 8 weeks of high-intensity static stretching by using a nonrest interval protocol would change the MA architecture.
Experimental Approach to the Problem
A randomized controlled trial was conducted to determine the effects of a knee flexor stretching (Figure 1A) intervention on the biceps femoris long head (BF) architecture and passive knee extension maximal ROM (Figure 1B).
Ten healthy university students [mean ± SD: all men; physically active; age = 21.2 ± 0.8 years; body mass = 72.7 ± 11.6 kg; height = 175.9 ± 4.9 cm (age range from 18 to 23)] volunteered to this study. During the study, participants performed normal daily living activities but were not involved in specific exercise program (i.e., strength or stretching). The local University Ethics Council approved this study (#1/2013), and informed consent was obtained from all participants.
Participants were allocated in 2 groups: a control (CG, n = 5) and a high-intensity stretching training (SG, n = 5). The CG were not involved in any type of stretching program during the intervention period. The SG performed a stretching at a ROM that corresponded to the highest tolerable torque before the onset of pain for 450 seconds (Figure 1C). To assure that the maximum passive torque was obtained, the ROM was increased every 90 seconds to a new maximal ROM, and thus not resting between repetitions (3). When participants reported that they could not stretch further, the knee was held statically until the end of the 450 seconds. We previously observed that this type of protocol achieves a greater ROM and passive peak torque during the stretching than a conventional rest interval protocol, and consequently a higher intensity (3). The SG group was monitored for maximal ROM (Lafayette Gollehon Extendable, Model 01135, IN, USA) every training session at the beginning and during the stretching session. Experienced exercise professionals assisted the stretching maneuvers (Figure 1A). Participants were asked to participate in 5 sessions per week.
Both groups were assessed for BF architecture parameters before and after the training period by an experienced researcher (Figure 1B), using a 6-cm 10-MHz linear probe (EUB-7500; Hitachi Medical Corporation, Chiyoda-ku, Tokyo, Japan). A blinded researcher digitized the sonograms (1.47v; ImageJ software, Bethesda, MD, NIH, USA). A classic linear extrapolation method was used to calculate the BF architecture parameters (11). The FL was calculated using the equation: FL = L +(h/sina), where L is the observable FL from the midmuscle aponeurosis to the most visible end point, h is the distance between the superficial aponeurosis and the fascicle visible distal end point, and β is the angle between the fascicle (drawn linearly) and the superficial aponeurosis (Figure 1B).
Data were analyzed using the SPSS software (Version 20.0, IBM Corp., Armonk, NY). Interday assessment reliability was previously confirmed for FL (Intraclass correlation coeficient (ICC) = 0.79 [0.55–0.91]), FA (ICC = 0.80 [0.56–0.91]), and MT (ICC = 0.94 [0.86–0.98]) in a previously study with 20 participants (data not published); and the minimal detectable difference determined was 8.4 mm, 1.5°, and 1.6 mm for FL, FA, and MT, respectively. Wilcoxon tests were used to determine pre- to post-effects on FL, FA, MT, and maximal ROM. Cohen's d coefficient was calculated to determine the magnitude of the MA and ROM changes. Statistical significance was set to p ≤ 0.05. Data are presented as mean ± SD.
The SG participants performed a total of 25.0 ± 6.4 training sessions (i.e., 3.1 ± 0.8 sessions per week). The knee extension maximal ROM before, during, and after the stretching program are shown in Figure 2A. The BF architecture parameters before and after the stretching program are shown in Figure 2B.
The main finding of this pilot study was that the BF architecture and the knee extension maximal ROM were changed in vivo in consequence of an 8-week high-intensity stretching program. FL increased 13.6% and the FA decreased 15.1% (p = 0.13) in the SG. Previous studies have reported changes of up to +33% for FL and +20% for FA as a consequence of resistance training (1). However, this is the first study showing changes in consequence of a stretching intervention. The previous studies have reported no significant changes in MA after a stretching program (4,5,10). However, studies in animal models suggest that static stretching can change MA (12,15). We think that this might be due to the duration and intensity of the stretching intervention. Participants have stretched for 450 seconds in each session and had a frequency of 3.1 ± 0.8 sessions per week (∼1406 seconds of stretching for week). This duration is much higher than those used in previous studies (5,6,10). We also used a method that led to greater maximal ROM and torque during the stretching, and this may have been higher than in previous studies.
Another observation was the magnitude of ROM increase after the intervention. The total knee extension maximal ROM increase was 14.3 ± 10.7° (+11.2 ± 9.5%) at the end of the program. This is much higher compared with the results of previous studies examining the knee extension flexibility (5,6). This was also probably due to the intensity and duration of the stretching (6,7,9,13,14). In this study, the ROM was increased every 90 seconds during the 450-second stretching maneuver until the maximal ROM, until the participant report that he could not stretch further without felling pain, and stretching was superior to 300 seconds (7).
A major study limitation was the small sample size in both groups. However, it must be noted that the change in both FL and FA was higher than the minimal detectable change using ultrasonography to assess BF MA (i.e., 8.4 mm and 1.5°, for FL and FA, respectively). A future study should be conducted with a higher sample size and should assess other mechanical variables (e.g., joint passive torque-angle).
In conclusion, a high-intensity stretching program of 8 weeks was observed to efficiently increase the FL and decrease the FA of the BF, as well to increase the knee extension maximal ROM. These findings are important to those who seek MA changes through physical training. A larger sample size is wanted in a future study to confirm these results.
Muscle architecture is recognized as an important physical performance variable. In this study, we observed a BF architecture change in consequence of knee flexors static stretching intervention. Approximately, 3 stretching sessions per week, for 8-week duration, with a high stretching intensity (i.e., by using a nonrest interval protocol) and a duration of 450 seconds (i.e., considerable superior to conventional practice) was seen to increase the FL and passive knee extension maximal ROM, without affecting the MT. It is possible that stretching intensity and duration are a key variable to induce changes in MA.
The authors acknowledge the support of the Portuguese Scientific Foundation, and the kind contribution of João Marmeleira for their co-operation in data digitalization, and all the participants for their effort. All the authors declare that they have no conflicts of interest regarding this article.
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