Training and Detraining Effects of Eccentric Exercise and Static Stretching on Hamstring Aponeurosis Dimensions

This study examined the effects of 10 weeks of eccentric knee-flexor training or matched-duration static stretching on hamstring aponeurosis dimensions and muscle size, and explored the changes after 4 weeks of detraining. Eleven healthy untrained men completed a within-participant contralateral intervention (3 sessions·week^-1, 10 weeks) followed by 4 weeks of detraining. One leg performed eccentric knee-flexion exercise on an isokinetic dynamometer (3 × 10 reps at 5°·s^-1; 50% maximal eccentric torque), while the contralateral leg performed static stretching (3 holds matched to eccentric time under tension). Hamstring outer aponeurosis volumes, inner aponeurosis volumes and widths, and muscle volumes were quantified using magnetic resonance imaging. Outer aponeurosis volume increased after both training protocols across all muscles (p < 0.05) and was maintained during the 4 week detraining period, with comparable changes observed between stretching (~19.0%-31.8%, d = 0.91-1.69) and eccentric training (~15.8%-22.8%, d = 0.85-1.13). Inner-aponeurosis volume and width of biceps femoris long head and semimembranosus also increased after both training protocols (~7%-23%; p < 0.05; d = 0.33-0.86) and were preserved during detraining. Muscle hypertrophy occurred only after eccentric training (~22%-42%; p < 0.05; d = 0.98-1.65) in biarticular hamstrings and remained above baseline in the detraining period. Stretching exercise produced minimal changes in muscle volume (p > 0.05). Changes in muscle volume were not significantly associated with changes in aponeurosis dimensions in almost all cases (p > 0.05). These findings suggest that hamstring aponeurosis dimensions can increase after both eccentric loading and static stretching, whereas muscle hypertrophy is largely eccentric-specific and more sensitive to short-term detraining.