Author(s): BLAZEVICH, A., Institution: EDITH COWAN UNIVERSITY, Country: AUSTRALIA, Abstract-ID: 241

During dynamic muscle contractions, whole-muscle length change results from both fibre length change and rotation. Therefore, muscles can work in a gear >1.0, where muscle length change exceeds fibre length change. The reduced need for fibre length change should profoundly affect fibre, and thus whole muscle, function in accordance with the muscle's force-velocity, force-length, and history dependent properties. In eccentric contractions in particular, the reduced fibre strain for a given muscle length change is speculated to reduce fibre injury risk as well as influence chronic adaptations to repeated exposures (e.g. hypertrophy). Nonetheless, studies of muscle gearing in vivo in humans are still relatively scarce, and little information exists in relation to gearing during eccentric contractions in particular. In one experiment, vastus intermedius fascicle length changes (measured using ultrasonography) were greater during singular (non-consecutive) eccentric than concentric knee extensions, although length changes in vastus lateralis were similar between contraction modes. Such fascicle behaviours may partly explain the greater muscle damage observed in vastus lateralis after bouts of eccentric contractions. However, it is unclear whether such behaviours are observed in other muscles or when consecutive concentric-eccentric contractions are performed. The first aim of this presentation is to present new data from experiments in which we have studied both biceps femoris (long head) and gastrocnemius medialis during dynamic contractions. Data show that fascicle behaviours, and thus gearing, during concentric and eccentric phases were similar under the study conditions, so eccentric contractions appeared to be 'mirror images' of the concentric contractions. Nonetheless, data from research currently underway in which gearing is studied under conditions of different muscle forces and contraction speeds will be presented.