Time course and regional heterogeneity of hamstring muscle thickness after maximal concentric exercise in men and women

This study examined the early time course of hamstring thickness changes after acute maximal concentric knee-flexions across multiple measurement sites in men and women. Twenty-two young adults (11 men, 11 women) performed 50 maximal unilateral concentric knee-flexions at 120°·s⁻¹. Muscle thickness of the biceps femoris (BF) and semitendinosus (ST) was assessed using ultrasound at proximal, middle, and distal sites at baseline and repeatedly during 30-min of recovery. Linear mixed-effects models showed that thickness increased rapidly, peaking immediately post-exercise or at 5-min (mean relative change: 10.4 ± 4.9%), and returned toward baseline by 15–30-min. Absolute thickness increases differed between muscles and sites ( p < .05), with larger responses observed in BF than ST ( d = 0.61–3.23) and the greatest change at the middle site ( d = 1.00–4.04). In contrast, relative thickness changes were largely comparable across muscles, sites, and sexes ( p > .05). However, considerable inter-individual variability was found in relative thickness changes among measurement sites with the distal regions most often contributed the largest share of total swelling (BF 45.5%, ST 54.5%). Maximal concentric exercise induces transient hamstring thickness changes. Although relative thickness responses were similar across sexes, muscles, and regions, substantial inter-individual variability in regional patterns suggests that the hamstrings may not respond as a uniform unit acutely following exercise.