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Background <p>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.</p> Objective <p>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.</p> Methods <p>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.</p> Results <p>Significant main effects of knee joint angle (<i>p</i> = 0.002) and contraction intensity (<i>p</i> &lt; 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 (<i>p</i> &gt; 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 (<i>p</i> = 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.</p> Conclusion <p>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.</p> Trial registration <p>This trial was retrospectively registered at Thai Clinical Trials Registry (TCTR) (Identifier: TCTR20260508002) on May 8, 2026. <a href="https://www.thaiclinicaltrials.org/show/TCTR20260508002">https://www.thaiclinicaltrials.org/show/TCTR20260508002</a>.</p>

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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

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  • 发表日期：

    2026年7月15日

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  • Taspol Keerasomboon,
  • Toshiaki Soga,
  • Pasawee Rachasena,
  • Pipatpong Intasri,
  • Parunchaya Jamkrajang,
  • Norikazu Hirose

摘要

Background

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.

Objective

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.

Methods

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.

Results

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.

Conclusion

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.

Trial registration

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