BACKGROUND: Risk for lower extremity musculoskeletal injury increases after sport-related concussion (SRC) and may result from unresolved motor control deficits. Muscle weakness is a deficit that could contribute to musculoskeletal injury risk. HYPOTHESIS: Athletes with SRC will demonstrate quadriceps and hamstring muscle weakness at the time of return to sport and 30 days later compared with controls. STUDY DESIGN: Prospective matched cohort. LEVEL OF EVIDENCE: Level 3. METHODS: A total of 31 athletes with SRC (CONCUSSION) were matched by sex, age, and activity level to controls (CONTROL). Testing was conducted at initial assessment and 30 days later; initial assessment in CONCUSSION occurred when cleared for return to play. Isokinetic testing assessed quadriceps and hamstring strength of the dominant and nondominant legs at 60 and 180 deg/s. Peak torque values were normalized to body mass (N-m/kg). Data were analyzed with repeated measures general linear models (group × time), and effect sizes were calculated. RESULTS: Analysis at 60 deg/s included 26 matched pairs (15 male per group) and at 180 deg/s included 30 matched pairs (17 males per group). Time from concussion to initial assessment was 21.3 (7.8) mean (standard deviation) days. No significant interactions or main effects were detected (P > 0.05). Across muscle groups, legs, and testing speeds, effect sizes at initial assessment were small (d = 0.117 to 0.353), equating to a strength deficit in CONCUSSION of 0.04 to 0.18 N-m/kg, and effect sizes were further reduced at 30-day follow-up (d = -0.191 to 0.252). CONCLUSION: In athletes with SRC, quadriceps and hamstring strength were decreased only minimally at return to play compared with controls and the difference lessened over 30 days. CLINICAL RELEVANCE: Strength deficits may not be a major contributor to increased lower extremity musculoskeletal injury risk after SRC. Strength training could be implemented before return to play after SRC to mitigate any strength deficits.