Hamstring fascicle behavior during leg curl exercises performed at varying knee joint angles and loading intensities...

Shorter resting fascicle length of the biceps femoris long head (BFlh) has been associated with a history and risk of hamstring strain injury (HSI). However, acute mid-region BFlh fascicle operating length under different knee joint angles and contraction intensities remain unclear.

To examine the effects of knee joint angle and contraction intensity on acute mid-region BFlh fascicle operating length and to determine whether these responses differ between previously injured and uninjured limbs.

Fourteen male university athletes with a history of unilateral HSI participated in a randomized crossover study. Mid-region BFlh fascicle length was assessed using ultrasonography during isometric prone leg curl exercise at three knee joint angles (30°, 60°, and 90°) and four contraction intensities (25%, 50%, 75%, and 100% of maximal voluntary isometric contraction [MVIC]). Measurements were obtained at 50% of thigh length using B-mode ultrasonography. Knee joint angle and force were measured using a manual goniometer and a hand-held dynamometer, respectively. The effects of limb, knee joint angle, and contraction intensity were examined using a three-way within-subject repeated-measures ANOVA.

Significant main effects of knee joint angle (p = 0.002) and contraction intensity (p < 0.01) were observed for mid-region BFlh fascicle length. However, neither the main effect of limb nor any interaction involving limb was statistically significant (p > 0.05). Exploratory condition-specific comparisons indicated longer fascicle operating lengths at 30° than at 60° or 90° under some loading conditions. At 30° in the uninjured limb, fascicle operating length was longer at 25% than at 100% MVIC (p = 0.004); however, the corresponding difference was not significant in the previously injured limb. Because the interactions involving the limb were not significant, this finding does not demonstrate a difference in response between limbs. The absolute normalized between-limb difference in fascicle length was approximately 10%, with no significant effects of knee joint angle or contraction intensity.

Acute mid-region BFlh fascicle operating length varied according to knee joint angle and contraction intensity during isometric prone leg curl exercise. Longer fascicle operating lengths were observed at 30° under some loading conditions. However, the present findings do not demonstrate that the previously injured and uninjured limbs responded differently. Knee flexion at 30° may represent a mechanically interesting condition for future longitudinal intervention studies, but the present acute findings do not establish chronic fascicle-length adaptation or a reduction in HSI risk.

This trial was retrospectively registered at Thai Clinical Trials Registry (TCTR) (Identifier: TCTR20260508002) on May 8, 2026. https://www.thaiclinicaltrials.org/show/TCTR20260508002 .