Hamstring fascicle behavior during leg curl exercises performed at varying knee joint angles and loading intensities in individuals with a history of unilateral hamstring injury: a randomized cross-over design study.

backgroundShorter 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.

objectiveTo 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.

methodsFourteen 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.

resultsSignificant 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.

conclusionAcute 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.

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