Changes in Passive Muscle Stiffness Following Biceps Femoris Strain Injury in Track-and-Field Athletes: Cross-Sectional and Longitudinal Analyses.

A history of muscle strain injury has consistently been identified as one of the strongest risk factors for future muscle strain injuries. However, information on the characteristics associated with a history of muscle strain injury is limited. This study aimed to examine whether passive muscle stiffness changes following biceps femoris long head (BFlh) strain injury in track-and-field athletes through cross-sectional and longitudinal analyses. In Study 1 (cross-sectional), 13 male track-and-field athletes with unilateral BFlh strain injury history underwent passive BFlh stiffness assessment using ultrasound shear wave elastography of previously injured and uninjured limbs. In Study 2 (longitudinal), seven athletes who sustained BFlh strain injury during the competitive season were followed from pre-injury baseline through return-to-sport. Passive BFlh stiffness was assessed in both limbs at baseline, acute phase, rehabilitation phase, and return-to-sport phase. In Study 1, passive BFlh stiffness was significantly greater in the previously injured limb than in the uninjured limb (p = 0.001). In Study 2, a significant limb × time interaction was observed (p = 0.001). In the injured limb, passive BFlh stiffness at the rehabilitation phase was significantly greater than at baseline and remained elevated at return-to-sport, whereas it returned to baseline in the uninjured limb. Passive BFlh stiffness increases following strain injury and persists through return-to-sport. These findings reveal that strain injury leads to lasting increases in passive muscle stiffness, which may have implications for post-injury muscle function.