A Comparison of Muscle Unit Activation during Biceps Curl Exercise at 40, 60, 80 and 100% of 1 Repetition Maximum

Abstract

The optimal intensity of resistance training exercise is subject to ongoing debate. There is universal agreement that electromyography (EMG) is an excellent index of motor unit (MU) activation or recruitment. The amount of EMG activity can infer the relative intensity of the exercise as compared to a maximal voluntary contraction (MVC) of a specific muscle, performing a specific exercise within one specific individual. PURPOSE: To determine the magnitude of muscle fiber recruitment (activation) of the Bicep Brachii during various loading of the muscle (40, 60, 80, and 100% of 1 RM) during bicep curl (BC) exercise performed to momentary muscular fatigue (MMF), 12 subjects (age 22.5±1.0 yr, ht. 169.4 cm±11.1 cm, body mass 75.3 kg±17.5 kg, 7♀) with resistance training experience, volunteered. METHODS: A familiarization trial on a selectorized BC machine was performed and the 1 RM was titrated for each subject. In addition, a maximal voluntary contraction (MVC) was obtained for the BC exercise against an immovable load on the same selectorized machine. MU recruitment was estimated by EMG. All subjects adhered to a 3-1-3-1 sec repetition duration during all trials. All trials were randomly assigned and terminated at MMF. RESULTS: Statistical analysis by ANOVA (p<.05) with repeated measures was applied to these data. Percent MVC for the concentric portion of the final repetition were 62.5±27.8, 71.7±24.0, 72.8±18.4, and 79.6 ±18.0 at 40, 60, 80, and 100% 1RM, respectively, with NSD among 60, 80 and 100%1RM. With the exception of the 40% trial, all trials revealed similar peak MU recruitment, despite different loading. The recruitment of ~70% of MVC at MMF may, in part, be attributed to the level of resistance training of the subject’s and their ability to activate MU’s. CONCLUSION: The size principle of MU recruitment is supported, in that peak recruitment of motor units can be accomplished at any workload greater than 60% 1 RM, provided MMF is achieved. Intensity is best defined by the final repetition, as opposed to the initial repetition (i.e. % of 1RM).