Hamstrings Volumetric Adaptations to a Four-Week Nordic Hamstring Exercise Protocol in Individuals with ACL Reconstruction: A Preliminary Case Series

Background Hamstrings atrophy is widely reported following anterior cruciate ligament reconstruction (ACLR) with hamstrings tendon (HT) autograft. Nordic hamstring exercise (NHE) has yielded improvements in hamstrings volumes among uninjured individuals, yet this has not been investigated following ACLR. Purpose To describe changes in hamstrings volumes following an isolated NHE protocol among individuals with a history of ACLR via HT. Study Design Prospective case series. Methods Individuals (age 18-35) with primary, unilateral ACLR via HT was recruited from an a larger clinical trial. Participants were not engaged in formal rehabilitation at enrollment. Outcomes were assessed in a university research laboratory and medical center. Involved limb muscle volumes were quantified for the total hamstrings, biceps femoris long head, biceps femoris short head, semimembranosus, and semitendinosus via magnetic resonance imaging before and after a standardized, progressive four-week (10-session) NHE protocol. Average peak eccentric force was quantified using a NordBord dynamometer during the first and last exercise sessions. Change scores were reported descriptively as percentages (%) and their magnitudes were quantified using effect sizes (Cohen’s d). Results Seven individuals participated (age: 22.0±2.3 years, time from surgery: 53.6±29.5 months). Volumetric changes ranged -6.0 to 17.6% across participants and muscles. On average, trivial-to-moderate magnitude increases in hamstrings volumes (3.3-7.2%, d=0.12-0.53) were observed, except for semimembranosus (0.0%, d=0.01). Changes in eccentric force ranged -3.2 to 19.4% across participants. On average, a moderate magnitude increase in force (11.0%, d=0.53) was observed. Conclusion The majority of participants in this case series demonstrated trivial-to-moderate increases in hamstrings muscle volumes in parallel with a moderate increase in eccentric force after four weeks of isolated NHE. Variable responses to exercise appear to support the need for individualized exercise prescription in this population. Level of Evidence 4