Cross-Education Strength and Activation After Eccentric Exercise
Lepley LK, Palmieri-Smith RM. J Athl Train. 2014 Sep-Oct; 49(5):582-9. DOI: 10.4085/1062-6050-49.3.24.
Take Home Message: Eccentric strength training improves eccentric quadriceps muscle strength in the unexercised limb.
Many studies have reported on the phenomenon of cross-education, where unilateral strength training induces strength gains not only in the trained limb, but also in the corresponding muscles of the contralateral limb. It has been suggested that neural adaptation is the driving force behind this, as no significant structural changes within the muscles have been found in association with this gain through dynamometer readings. To date, very few studies have evaluated whether cross-education training could improve quadriceps strength or activation. Hence, the authors aimed to place cross-education into a rehabilitation setting by first testing it on healthy individuals. In doing so, they sought to determine if eccentric exercises improved quadriceps strength and activation in the unexercised limb of healthy individuals. Eighteen healthy individuals were randomly placed into either an eccentric training group or a control group. The authors followed participants for eight weeks. The eccentric training group completed supervised training sessions (3 times per week). During the sessions the participants completed a warm-up and 4 sets of 10 maximal eccentric isokinetic movements of their dominant limb with a dynamometer at 60º/s. The training group increased eccentric strength in the unexercised limb between pre- and mid-intervention as well as pre- to post-intervention, but the control group had no changes. Additionally, there was some evidence that the training group may had increased muscle activation in the unexercised leg between the pre-intervention and post-intervention measurements. The authors noted no concentric strength gains at any time point in either the training or control groups.
In this study, eccentric quadriceps training improved eccentric strength in the unexercised limb of healthy individuals. The authors believe the gains found in the unexercised limb of the training group may have occurred because of heightened neural activity, as evidenced by the trend toward increased muscle activation. For these reasons, cross-education via eccentric training could become a useful tool in rehabilitation programs to improve quadriceps strength following an injury or surgery. It will be helpful to see if these protocols work as well, if not better, among individuals with a recent knee injury or surgery. The researchers claimed that isotonic exercises would only result in a small strength gain because isokinetic exercise maximally loads throughout the entire range of motion. However, it would be interesting to see if using large resistance with isotonic exercises could display close to the same results as isokinetic exercises. Furthermore, it would be helpful to determine if isotonic exercises are enough to elicit the cross-education effect among injured individuals. Clinically, this is an important topic, because after an injury, especially in the lower extremities, most clinicians do not want to start with eccentric exercises, as mentioned. Therefore, exercising the uninjured (contralateral) limb to produce strength gains in the injured limb could be critical to the rehabilitation process. While more research may be warranted the potential benefits of trying these exercises with our patients probably outweigh the small risks.
Questions for Discussion: What types of contralateral exercises can help increase strength in the injured limb? Do you think eccentric training the contralateral limb has the potential to improve strength around an injured joint at other regions of the body?
Written by: Blake Crosby & Jennifer Payne
Reviewed by: Kim Pritchard
Lepley, L., & Palmieri-Smith, R. (2014). Cross-Education Strength and Activation After Eccentric Exercise Journal of Athletic Training, 49 (5), 582-589 DOI: 10.4085/1062-6050-49.3.24