The proposed project will evaluate the musculoskeletal outcomes of quadriceps and hamstring muscle size and function following orthopedic knee surgery involving anterior cruciate ligament (ACL) repair or reconstruction. Currently, the research team collaborates with a team of orthopedic specialists at the University of Kansas Health System and monitor muscle size post-knee repair and follow the standard of care (SOC) practices of the licensed physical therapists (PT). The proposed project will include a randomized clinical trial to observe the muscular outcomes following the current SOC plus supplementation of calcium-β-hydroxy-β-methylbutyrate (caHMB) or placebo. CaHMB has been shown to improve rates of muscle protein synthesis while suppressing muscle protein breakdown in healthy adults. The use of caHMB has also provided evidence of muscular protection from atrophy during prolonged bed rest. This evidence supports the utility in clinically injured athletes that are subjected to disuse atrophy from the inability to bear weight or participate in typical daily physical activity. Additionally, matched for activity-related knee injuries, female athletes are more susceptible to incurring a significant injury due to a variety of genetic, hormonal, biological, anatomical, and biomechanical predispositions. Therefore, the proposed study will recruit approximately 30 females over the age of 18 that have sustained an injury to the ACL and will plan to undergo reconstructive knee surgery involving the ACL. Subjects will be monitored and measured prior to their surgical date (T0), at 2-weeks post operative (T1), and every 6-weeks until they are cleared to return to sport (T2-TRTS). Participants will be randomly assigned 1:1 in a double-blind manner to either an experimental (EXPHMB) or placebo (CONPLA) group. Doses will be provided to the participants in coded containers and will complete their dosing and a record log of intake for the duration of their rehabilitation. Three 3-day food, exercise, and health record logs will be collected to monitor nutritional intake, activity, and menstrual patterns at T0, T3, and TRTS. Participant's assessments will include body composition analysis via bioelectrical impedance analysis for total and segmental muscle and fat mass, skeletal muscle mass, and body fat percent. We will collect ultrasound images of the quadriceps and hamstrings of the operative-involved limb (OPIL) and non-operative limb (NOPL) limbs for muscle cross-sectional area (mCSA), thickness (mT), subcutaneous fat thickness (TFAT), and corrected echo intensity (EICOR) at all time points. Strength and functional assessments will occur upon entrance to the study (T0), and after loaded exercise is indicated by the practitioner (T3-TRTS) to the tolerance of the athlete. These assessments include maximal voluntary isometric contractions (MVIC) for leg extension and leg curl, standing balance tests, single-leg and double-leg jump assessment, and drop landing deviation, all on dual force plates. Data will be analyzed using multiple three-way analyses of variance \[surgical leg (OPIL vs. NOPL) x treatment (EXPHMB vs. CONPLA) x time (T0 vs. T1 vs. T2 vs. T3 vs. T4 vs. T5 vs. TRTS) for the dependent variables. Significance is established at p≤0.05 and follow-up ANOVAS, T-tests, and post-hoc analyses will be conducted when significance is present. The evidence from this study will support the practitioners and coaches' abilities to maximize recovery and training outcomes, respectively, in previously injured female athletes.