Region-specific hamstring activation and its dissociation from subjective contraction sensation during leg curl exercise at different hip flexion angles

Abstract Background Hamstring activation during resistance training may vary according to muscle region and joint position. However, the relationship between region-specific hamstring activation and subjective contraction sensation during leg curl exercise remains unclear. The purpose of this study was to investigate region-specific and longitudinally compartmentalized muscle dynamics of the hamstrings during leg curl (LC) exercise performed at different hip flexion angles (HFA), and to examine the relationship between subjective contraction sensation and objective muscle activity. Methods Nine male bodybuilders performed unilateral LC at three hip flexion angles (0°, 40°, and 80°), with knee flexion ranging from 0° to 130°. Pre- and post-exercise transverse relaxation times (T2) were measured using magnetic resonance imaging. Each LC session consisted of 5 sets of 10 repetitions at 70% of one-repetition maximum, with 1-minute rest intervals. The hamstrings were divided into proximal, middle, and distal regions for the biceps femoris long (BF-long) and short heads, semitendinosus (ST), and semimembranosus. T2 activation rates were calculated for each region. The subjective sensation of hamstrings contraction during LC exercise was evaluated using a numerical rating scale (NRS) and compared with the T2 value as an objective index. Results Across all conditions, the ST exhibited greater activation than the other hamstring muscles (p < 0.01, p < 0.05). In particular, the distal region showed higher activation than the proximal region at HFA 0° (p < 0.05). For the BF-long, greater activation was observed in the proximal region compared with the distal region at HFA 40° (p < 0.01). The NRS scores were inconsistent with the objective index. Conclusion The ST is preferentially activated during LC exercise. Hamstring activation is both region-specific and longitudinally compartmentalized, depending on muscle length as influenced by HFA. Because subjective contraction sensation was dissociated from objective muscle activity, hamstring resistance training should not rely solely on perceived muscle contraction, but should instead consider objective muscle activation characteristics when selecting exercise positions and training strategies. Optimal hip positioning should therefore be considered when designing resistance training programs to effectively target specific hamstring regions.