Author(s): NARA, G., NAKAO, G., ADACHI, R., ISHIYAMA, K., KOZAWA, K., SEKIGUCHI, K., TANIGUCHI, K., Institution: GRADUATE SCHOOL OF HEALTH SCIENCES, Country: JAPAN, Abstract-ID: 985

INTRODUCTION:
Understanding the passive mechanical properties of the biceps femoris long head (BFlh) will contribute to the development of accurate musculoskeletal models that can provide valuable insights into the mechanisms underlying strain injuries. The BFlh has intramuscular variability in fascicle and aponeurosis morphology and architecture (1), suggesting regional heterogeneity in passive mechanical properties during muscle elongation. However, whether passive stretch responses, such as the slack angle, differ regionally within the BFlh, in vivo, remains unclear. Therefore, this study investigated the region-specific stretch response within the BFlh during passive knee extension using ultrasound shear wave elastography.
METHODS:
Nine healthy males (mean, 21.7 ± 2.3 years) participated. The shear modulus was measured using shear wave elastography at knee flexion angles ranging from 90° to 0°, in 5° decrements. Measurements were taken at three regions of the right BFlh: proximal (25%), middle (50%), and distal (75%) of the distance between the musculotendinous junctions (0% proximal, 100% distal). A piecewise exponential model was applied to the shear modulus–knee angle data to calculate the passive stretch response characteristics, including the slack angle, slack shear modulus, and the slope of the increase in shear modulus (2). The fit quality of the model was evaluated using the coefficient of determination (R²), and data with R² ≥ 0.5 were included in the statistical analysis. For passive stretch response characteristics, a one-way analysis of variance (sites: proximal, middle, and distal), followed by Tukey’s honestly significant difference post hoc test (p < 0.05), was performed.
RESULTS:
All data had an R² value ≥ 0.5 (distal: 0.93 ± 0.05, middle: 0.91 ± 0.03, and proximal: 0.86 ± 0.11). A significant main effect of the site (p < 0.05) was observed for slack angle (distal: 68.02 ± 15.58°, middle: 59.80 ± 12.18°, and proximal: 50.96 ± 10.83°), with post hoc analysis revealing a significantly larger slack angle at the distal region compared to the middle region (p < 0.05). There were no significant main effects of the site for slack shear modulus (distal: 2.91 ± 0.39 kPa, middle: 2.96 ± 0.31 kPa, and proximal: 2.97 ± 0.17 kPa) or the slope of the increase in shear modulus (distal: 0.0110 ± 0.0045, middle: 0.0081 ± 0.0021, and proximal: 0.0087 ± 0.0061).
CONCLUSION:
This study investigated regional variability in the passive stretch response within the BFlh, as quantified by the shear modulus–knee angle relationship. The distal region had a significantly larger slack angle than the proximal regions, indicating earlier development of passive tension during knee extension. These findings provide novel insights into the regional passive mechanical properties of the BFlh with implications for developing musculoskeletal models and understanding mechanisms underlying strain injuries.
REFERENCE:
1) Kellis, E et al., Sports Med. (2018)
2) Kositsky, A et al., J Appl Physiol. (2022)