• Full Name: TASPOL KEERASOMBOON
• Affiliation/Institution: College of Sports Science and Technology, Mahidol University, Nakhon Pathom, Thailand
• Country: Thailand

• Full Name: TOSHIAKI SOGA
• Affiliation/Institution: Faculty of Human Sciences, University of East Asia, Yamaguchi, Japan
• Country: Japan

• Full Name: PASAWEE RACHASENA3
• Affiliation/Institution: Faculty of Sport and Health Science, Kasetsart University, Nakhon Pathom, Thailand
• Country: Thailand

• Full Name: PIPATPONG INTASRI
• Affiliation/Institution: College of Sports Science and Technology, Mahidol University, Nakhon Pathom, Thailand
• Country: Thailand

• Full Name: PARUNCHAYA JAMKRAJANG
• Affiliation/Institution: College of Sports Science and Technology, Mahidol University, Nakhon Pathom, Thailand
• Country: Thailand

• Full Name: NORIKAZU HIROSE
• Affiliation/Institution: Faculty of Sport Sciences, Waseda University, Tokyo, Japan
• Country: Japan

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Background
    A history of hamstring strain injury (HSI) is a strong risk factor for future HSI injury [1]. After an HSI, the fascicle length of the biceps femoris long head (BFlh FL) is diminished in the previously injured limb compared to the uninjured limb [2]. A decrease in BFlh FL is a significant risk factor for HSI, and this shortening may increase the probability of recurrence. [3]. Although many studies have investigated BFlh FL under resting conditions [1,2], hamstring exercise performed at varying knee joint angles and load intensities remains unknown.
Objectives
    Thus, this study aimed to investigate how muscle architectural characteristics differ between injured and uninjured limbs under different biomechanical conditions, with the goal of generating novel evidence relevant to injury prevention, rehabilitation, and sports
performance. We hypothesize that hamstring injury history may influence BFlh fascicle behavior across different muscle lengths and loading conditions.

STUDY DESIGN AND METHODS
    Fourteen male volunteers with previous HSI injury performed unilateral isometric leg curls for 5 seconds at three knee flexion angles (30°, 60°, and 90°) and four percentages of maximal effort (25%, 50%, 75%, and 100%). The BFlh fascicle length was measured at 50% of the total thigh length by a B-mode ultrasound system (Philips Lumify, Bothell, WA, USA) with a L12–4 linear probe (frequency, 4–12 MHz; width, 3.4 cm; depth, 8.0 cm). Knee flexion angles and force were measured using a manual goniometer and a handheld dynamometer (Lafayette, USA). The percentage of maximal effort was determined by force during maximal voluntary isometric contraction. All ultrasonograms were analyzed using Tracker software. The injured–uninjured fascicle length difference was calculated across conditions. A repeated-measures analysis of variance was used to compare BFlh fascicle length asymmetry across different conditions. Significance was set at p < 0.05.
Result
   No significant difference in the main within-factor effect in both degree (F2,24 = 0.622, p = 0.545) and the percentage of maximal effort factor (F3,36 = 1.342, p = 0.276), consistent with the result of a previous study showed that no significant fascicle-length difference between injured and contralateral legs, though injured legs tended to have slightly shorter fascicles[4]. 

CONCLUSION
These findings indicate that, compared to the contralateral limb, the injured limb exhibits a consistent fascicle length asymmetry of roughly 10% across all loading conditions and knee angles, possibly indicating ongoing structural and/or functional remodeling post-injury.