Mid Sweden University

Resistance exercise using weights typically offers constant external load during coupled shortening (concentric) and lengthening (eccentric) muscle actions in sets of consecutive repetitions until failure. However, the constant external load and the inherent capability of skeletal muscle to produce greater force in the eccentric compared with the concentric action, would infer that most actions are executed with incomplete motor unit involvement. In contrast, use of the inertia of flywheels to generate resistance allows for maximal voluntary force to be produced throughout the concentric action, and for brief episodes of greater eccentric than concentric loading, i.e. “eccentric overload”. Thus, it was hypothesized that acute flywheel resistance exercise would induce greater motor unit and muscle use, and subsequent fatigue, compared with traditional weight stack/free weight resistance exercise. Furthermore, it was hypothesized that flywheel training would induce more robust neuromuscular adaptations compared with training using weights. A total of 43 trained and untrained men were investigated in these studies.

Knee extensor muscle activation, fatigue response and muscle use were assessed during exercises by recording electromyographic signals and by means of functional magnetic resonance imaging, respectively. Flywheel resistance exercise provoked maximal or near maximal muscle activation from the first repetition, induced robust fatigue, and prompted more substantial motor unit and muscle use than weight stack/free weight resistance exercise in both novice and resistance trained men. Both prior to and following five weeks of unilateral knee extension training, the eccentric muscle activation was greater with flywheel than weight stack training. Furthermore, weight stack training generated greater increases of dynamic strength and neural adaptations, while flywheel training generated more prominent hypertrophy of individual quadriceps muscles and greater improvement of maximal isometric strength. Hence, due to the preferential metabolic cost of the concentric rather than eccentric actions, the maximal activation through the entire range of the concentric action within each repetition of a set during flywheel resistance exercise probably evoked the marked fatigue, and prompted more substantial muscle use than resistance exercise using weights. Furthermore, while any cause‐effect relationship remains to be determined, results of the pesent study suggest that brief episodes of “eccentric overload” amplify muscular adaptations following concentriceccentric resistance training.