The gravity-independent flywheel exercise device was first developed for use in space to counteract muscle loss in astronauts duringlong-duration space flights (Berg and Berg, 1993, Berg and Tesch, 1994), and has been used in many training programs and research studies ever since. The flywheel exercise device allows for varying loading throughout the repetitions, and this talk will cover some ofthe differences between flywheel resistance exercise and traditional weight training.

Traditional weight training commonly provides constant load and therefore often call for submaximal muscle activation during the eccentric actions, and maximal or near maximal muscle activation around the so called “sticking point” in the concentric actions. During flywheel resistance exercise, the force applied to the strap in the concentric phase is transformed to kinetic energy of therotating flywheel, and by decelerating the flywheel rotation over a short time in the last part of the eccentric action, it is possible to produce eccentric overload (Fernandez-Gonzalo et al., 2014). The subsequent high muscle activation during the concentric and parts of the eccentric action of flywheel resistance exercise (Pozzo et al., 2006) may promote a more efficient resistance training, rather than being restricted at a submaximal level as in traditional weight training (Norrbrand et al., 2008, Maroto-Izquierdo et al., 2017).

Previous flywheel studies have demonstrated substantial eccentric overload during open-loop knee extensions, and consequently high muscle activation during the eccentric action (Norrbrand et al., 2010). However, eccentric overload was not always apparent in closed-loop leg-extension exercises (Alkner and Tesch, 2004, Norrbrand et al., 2011, Sjöberg et al., 2021), possibly due to that the concentric peak force commonly occurs where muscle mechanics are favourable, i.e. at relatively extended knee and hip angles, whereas the peak eccentric force is produced in the unfavourable flexed position close to the turning point (Sjöberg et al., 2021).Thus, depending on which type of workout that is performed, e.g. body positioning, lever arms, and muscle length will affect the force output throughout the flywheel repetitions.

Furthermore, flywheel resistance training was proven effective counteracting muscle atrophy during 90 days of bed rest (Alkner andTesch, 2004), induced substantial hypertrophy following only 5-7 weeks of resistance exercise (Seyennes et al., 2007, Norrbrand etal., 2008, Lundberg et al., 2013), and was reported to induce an overall greater hypertrophy response than traditional weight-stackresistance training (Maroto-Izquierdo et al., 2017). In summary, the present talk will cover the muscle activation and the biomechanical loading during flywheel resistance exercise, and the resulting hypertrophy response following flywheel resistance training.