Abstract
Background: Cross-training is the process whereby training of one limb gives rise to enhancements in the performance of the opposite, untrained limb, most likely due to neural adaptations at the level of the motor cortex and/or spinal cord. Objective: To investigate whether unilateral resistance training can affect the ability of untrained homologous muscle in the contralateral limb to maintain force output over a sustained contraction. Methods: Fifteen healthy subjects completed 12 weeks of resistance training consisting of 36 sessions, using the unilateral leg press exercise. Sustained isometric knee extension performed at 50% of maximal force until task failure on both the ipsilateral trained leg and contralateral untrained leg. Surface electromyography (EMG) signals were recorded from the trained and contralateral untrained quadriceps (vastus medialis, rectus femoris, and vastus lateralis). Results: Time to task failure of the contralateral untrained leg was significantly increased after 12 weeks of unilateral resistance training (p < .05). Moreover, the EMG amplitude of the contralateral untrained quadriceps was significantly higher during the post training sustained contractions with respect to pre training condition (p < .05). Conclusions: Unilateral resistance training of the quadriceps can increase the ability of homologous muscle in the contralateral limb to maintain force output.
Highlights
Muscular endurance is defined as the ability of muscle to sustain a continuous contraction over an extended period of time
The number of participants was based on previous studies examining the effects of exercise on neuromuscular activity of same muscle, which showed that 10 subjects were sufficient to show a difference (Hedayatpour, Arendt-Nielsen, & Farina, 2008; Hedayatpour, Falla, Arendt-Nielsen, & Farina, 2008)
In both the trained and untrained leg Average rectified value (ARV) of EMG significantly increased over time during sustained contraction at 50% maximal voluntary contractions (MVC) (F = 9.6; p = .001; η2 = .59)
Summary
Muscular endurance is defined as the ability of muscle to sustain a continuous contraction over an extended period of time. It has been reported that the cross-training process is primarily caused by adaptations in the central nervous system for the untrained limb following exercise training (Bezerra, Zhou, Crowley, Brooks, & Hooper, 2009; Dragert & Zehr, 2011; Everaert, Thompson, Chong, & Stein, 2010) and this may affect the ability of homologous muscle in the untrained limb to sustain a continues contraction over the extend period of time. Objective: To investigate whether unilateral resistance training can affect the ability of untrained homologous muscle in the contralateral limb to maintain force output over a sustained contraction. Conclusions: Unilateral resistance training of the quadriceps can increase the ability of homologous muscle in the contralateral limb to maintain force output
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