No adaptations in muscle architecture after 12 weeks of static stretching in young healthy participants

Introduction Muscle architecture is one of the main determinant of muscle function and performance. Adequate mechanical stimuli (e.g., strength training) can induce modifications in muscle architecture. However, whether long-term stretching provides a sufficient stimulus for bringing architectural changes is still controversial. Therefore, the aim of the present study was to assess possible changes in muscle architecture after 12 weeks of static stretching (SS) in young healthy participants. Methods Twenty-eight participants were randomly divided into 2 groups: stretching (STR, n=16, 7 females and 9 males; mean±SD: age=23.5±1.0 yrs; stature=1.75±0.08 m; body mass=69.8±8.9 kg) and controls (CTR, n=12, 6 females and 6 males; mean±SD: age=24.2±2.4 yrs; stature=1.70±0.06 m; body mass=65.5±7.2 kg). STR underwent 12 weeks of SS for both pantarflexors (PF) and knee extensors (KE) muscles. CTR did not undergo any training intervention. SS was performed five times per week. One session consisted in 5 sets of 45-s SS each (15-s rest in-between). A total of four exercises were executed: two for PF and two for KE of the right lower limb. All participants kept a file record of the training sessions performed. Ultrasound images of the medial head of the right gastrocnemius (GM) and vastus lateralis (VL) muscles were obtained at 50% muscle length before, at six weeks, and the end of the 12 weeks in both groups. Fascicle length (Lf), pennation angle (PA) and muscle thickness (MT) were measured offline by using an open source software. A 3x2 mixed-model ANOVA was used to assess possible differences over time and between groups in all architectural parameters for each muscle group. Significance was set with p<0.05. Results Lf, PA and MT did not change significantly in STR over time (p>0.05 for all parameters) in both PF and KE. No significant differences were found between STR and CTR over time (p>0.05 for all parameters at any time point) in both PF and KE. No time x group interactions were found (p>0.05 for all parameters) in both PF and KE. Discussion 12 weeks of SS did not induce architectural changes in the mid portion of the GM and VL muscles. These results suggest that common SS exercises for the PF and KE do not provide a sufficient mechanical stimulus for structural adaptations of GM and VL, respectively, despite the protocol duration (12 weeks). However, regional adaptations cannot be excluded. References Simpson CL, Kim BDH, Bourcet MR, Jones GR, Jakobi JM. Stretch training induces unequal adaptation in muscle fascicles and thickness in medial and lateral gastrocnemii. Scand J Med Sci Sports. 2017 Dec;27(12):1597-1604. doi: 10.1111/sms.12822