Cross Education from Unilateral Resistance Training Occurs Without Changes in Muscle Size or Activation

Abstract

Cross education is the phenomenon by which an untrained limb will experience a gain in strength following a unilateral resistance training (URT) program. The underlying mechanisms occurring during cross education are relatively unknown. PURPOSE: To examine the effect of URT on strength, muscle size and activation of the trained and untrained legs. METHODS: Seventeen untrained men were randomized into an exercise group (URT) (n = 9; 22.9±3.1y, 174.1±8.2cm, 76.8±14.4kg) or control group (CON) (n=8; 24.0±4.6y, 184.5±5.0cm, 94.2±16.1kg) groups. The URT group completed a four week URT program. Unilateral isometric force (PKF), leg press (LP) and leg extension (LE) 1 repetition maximums were used to assess strength. Ultrasonography and electromyography (EMG) were utilized to assess changes in muscle morphology (muscle thickness (MT) and cross sectional area (CSA)) and activation (RMS) of the vastus lateralis (VL) and rectus femoris (RF) muscles. The URT program was conducted on the dominant leg (DOM) and cross education was measured in the non-dominant leg (NON). The URT group trained three days per week for four weeks. Exercises included were unilateral leg press, leg extension, bilateral chest press and low row. All data were analyzed using repeated measures ANOVA. Post hoc Bonferroni adjusted dependent t-tests were conducted to determine the changes in each group. RESULTS: Comparisons between URT and CON revealed group × time interactions (p<0.05) for PKF, LP, LE, MT RF, CSA RF and CSA VL in the DOM leg and for LP in the NON leg with URT being significantly higher than CON for all interactions. There were no significant differences between groups for any EMG measures. Post hoc tests revealed a significant increase in URT for strength in DOM (PKF: 9.3%; LE: 36.4%; LP: 55.5%) and NON (LP: 39.5%) and muscle morphology in DOM (MT RF: 16.0%; CSA RF: 15.0%; CSA VL: 15.7%). CON experienced a strength increase in DOM (LP: 6.9%) only. CONCLUSION: Four weeks of URT may result in the cross education of strength, specifically in exercises which utilize larger muscle mass, but had no effect on muscle size or activation of the untrained leg. URT could potentially be a useful training modality to improve the strength of the trained and untrained musculature. Acknowledgements: This study was funded by iSatori, Inc.