Cross-Education Strength and Activation After Eccentric Exercise

Context: After injury, eccentric exercise of the injured limb is often contraindicated. Cross-education training, whereby the uninvolved limb is exercised, is an alternative that may improve quadriceps muscle strength and activation in the unexercised limb. Objective: To determine the effect of eccentric exercise on quadriceps strength and activation gains in the unexercised limb. Patients or Other Participants: Eighteen healthy individuals were randomly assigned to an eccentric training group or a control group. Intervention(s): Quadriceps strength and activation measures were collected at preintervention, midintervention, and postintervention. Eccentric training participants exercised their dominant limb with a dynamometer in eccentric mode at 60 degrees/s, 3 times per week for 8 weeks. Main Outcome Measure(s): Quadriceps strength was quantified at 30 degrees and 60 degrees/s in concentric and eccentric modes. Quadriceps activation was assessed using the burst superimposition technique and quantified via the central activation ratio. A 2 x 3 repeated-measures analysis of variance was used to detect the effects of group and testing session on quadriceps strength and activation. Where appropriate, post hoc Bonferroni multiple-comparisons procedures were used. Results: We found greater eccentric strength in the unexercised limbs of eccentric training participants between preintervention and midintervention and between preintervention and postintervention (preintervention to midintervention: 30 degrees/s P = .05; preintervention to postintervention: 30 degrees/s P = .02, 60 degrees/s P = .02). No differences were noted in concentric strength (P>.05). An overall trend toward greater quadriceps activation in the unexercised knee was detected between preintervention and postintervention (P = .063), with the eccentric training group demonstrating a strong effect (Cohen d = 0.83). Control strength did not change (P>.05). Conclusions: Exercising with eccentric actions resulted in mode-specific and velocity-specific gains in quadriceps strength in the unexercised limb. A trend toward greater quadriceps activation in the unexercised knee was noted, suggesting that strength gains may have occurred because of enhanced neural activity. This type of therapy may be a useful addition to rehabilitation programs designed to improve quadriceps strength.