Duration-dependent effects of static stretching on neuromuscular outcomes

Static stretching (SS) is a popular warm-up activity prior to exercise as it increases range of motion and has evidence supporting injury risk reduction. However, SS >60s has been suggested to be detrimental to torque production during a subsequent exercise bout, with altered neural excitability having a possible role. To investigate if total SS duration influences maximal strength, voluntary activation (VA), and motor pathway excitability, participants underwent separate sessions of either three (short-duration; SDSS) or six (long-duration SS; LDSS) 15-s maximally-tolerable passive stretches of the knee extensors via an isokinetic dynamometer. Prior to as well as ~30 s, 5 min, and 10 min following SS (post0, post5, and post10, respectively), single stimuli were delivered to the motor cortex, thoracic spine, and femoral nerve during 7-s isometric quadriceps contractions at 15% maximal voluntary contraction (MVC) torque to permit evaluation of excitability at cortical, spinal, and peripheral levels, respectively. To measure maximal strength and VA before and ~40s after SS, participants performed 2-s MVCs with femoral nerve stimulation. Repeated-measures ANOVAs found a protocol by time interaction (p = 0.018) and main effect of time (p = 0.001) for corticospinal excitability. Follow-up one-way ANOVAs identified an increase of 8.1 ± 8.9% at Post0 for SDSS (p = 0.020), but not LDSS (p = 0.845). Despite a large effect size (η²ₚ = 0.210), a repeated-measures ANOVA found no significant effects for cortical excitability, with the nearest significant effect being a main effect of time (p = 0.059). With data pooled for both protocols, MVC torque was reduced 8.2% (p < 0.001), whereas VA remained unchanged (p = 0.117). The findings lead to the conclusion that corticospinal excitability was increased after SDSS but not LDSS, with the enhancement likely mediated at the cortical level. Additionally, MVC torque decreased irrespective of the SS durations used in this study, and was not due to reduced VA.