Muscle electrical activity during force modulation exercise

Several forms of exercise are adopted to increase muscular strength and power. The amplitude of the electromyogram (EMG) increases with the training loads and is maximal for levels near maximal voluntary contraction. Therefore, strength training requires use of heavy loads. Lately, an alternative form of exercise characterized by the superimposition of vibration has been suggested as an effective methodology to improve muscle strength and power performance. In this paper, a prototype for muscular conditioning is presented and validated. It uses a controlled electrical motor to generate a loading force that can be modulated at any frequency up to 40 Hz. We refer to this training modality as force modulation (FM). Modulation amplitude and frequency can be independently determined by computer. We evaluated the activation and co-activation of biceps and triceps muscles during isometric exercise with and without 28-Hz FM. Twelve healthy volunteers performed isometric elbow flexion and extension trials at different force levels in two conditions: force modulation [FM] and normal loading [C]. EMG activity of biceps and triceps muscles was simultaneously measured and assessed by estimation of the EMG root mean square (RMS). The RMS was estimated by power spectrum integration excluding the harmonics generated by movement artifacts due to FM. The results revealed a significant RMS increase especially when FM was applied at higher force levels. During elbow flexion, the EMG activity of biceps and triceps was respectively 38% and 26% higher than C, while during elbow extension, their activity was 28% and 98% higher than C, respectively. This study shows that FM exercise produces an increase of the activation and coactivation of biceps and triceps. This training modality seems therefore suitable for several applications, ranging from sports to rehabilitation.