Inter-limb differences in quadriceps strength and volitional activation

In this crossover study, we wished to determine if normalized inter-limb differences in strength differed from inter-limb differences in voluntary activation at 30°, 70°, and 90° of knee flexion. We also assessed the relationship between inter-limb differences in torque with the inter-limb differences in activation. Twenty-five healthy volunteers were used for final data analyses; the order of leg tested, joint angle, and measurement technique (isokinetic strength, voluntary activation) were randomly assigned. Quadriceps strength was measured isokinetically at 1.05 rad · s(-1), while quadriceps voluntary activation was assessed via the central activation ratio. Absolute values of inter-limb differences for both measures were calculated by subtracting the non-dominant leg values from those of the dominant leg. Inter-limb isokinetic strength differences were greater than inter-limb central activation ratio differences at all joint angles (P = 0.003). Interestingly, inter-limb deficits between measures were not strongly correlated, suggesting that these measurements may be evaluating completely different phenomena within the neuromuscular system. These measurement techniques may provide unique information regarding neuromuscular function, suggesting that researchers and clinicians must utilize information from both techniques to determine the true clinical nature of inter-limb deficits.