Does It Matter? Isometric or Isokinetic Assessment of Quadriceps Strength Symmetry 9 Months After ACLR in Collegiate Athletes

Background: Greater quadriceps strength symmetry is associated with better outcomes after anterior cruciate ligament reconstruction (ACLR). Isometric and isokinetic assessments of quadriceps strength inform therapeutic exercise prescription and return-to-sport decisions. It is unclear whether isometric and isokinetic measures provide similar information post-ACLR. Hypothesis: Quadriceps strength symmetry is similar between isometric and isokinetic assessments. Isokinetic and isometric strength symmetries have similar associations to functional knee kinetics and self-reported knee function. Study Design: Cross-sectional study. Level of Evidence: Level 3. Methods: NCAA Division I athletes (N = 35), 8.9 ± 2.5 months post-ACLR completed isometric and isokinetic quadriceps strength assessments, countermovement jumps (CMJs), and treadmill running. Self-reported knee function was assessed using the International Knee Documentation Committee Subjective Knee Form (IKDC). Agreement between isometric and isokinetic strength symmetry was assessed using Bland-Altman analysis, with associations to functional knee kinetics and IKDC assessed using Pearson correlations and linear regressions. Results: Mean difference in quadriceps strength symmetry between isokinetic and isometric assessments was 1.0% (95% limits of agreement of -25.1% to 23.0%). Functional knee kinetics during running and CMJ were moderately to strongly associated with isometric strength symmetry ( r = 0.64-0.80, P < 0.01) and moderately associated with isokinetic strength symmetry ( r = 0.41-0.58, P < 0.01). IKDC scores were weakly to moderately associated with isometric ( r = 0.39, P = 0.02) and isokinetic ( r = 0.49, P < 0.01) strength symmetry. Conclusion: Isokinetic and isometric assessments of quadriceps strength symmetry in collegiate athletes 9 months post-ACLR demonstrated strong agreement. Quadriceps strength symmetry is associated with functional knee kinetic symmetry post-ACLR. Clinical Relevance: Considerable individual variation suggests mode of contraction should be consistent throughout postoperative assessment. Isometric strength symmetry may be a better indicator of functional knee kinetic symmetry, while isokinetic strength symmetry may be associated more closely with patient-reported outcomes.