Changes in motor unit recruitment thresholds of the human anconeus muscle during torque 2 development preceding shortening elbow extensions

24 Rate of torque development and the subsequent movement velocity are modulated by 25 motor unit (MU) properties, primarily MU discharge rate and MU recruitment threshold 26 (MURT). In isometric conditions, MURTs have been shown to decrease with increased rates of 27 torque development. It is unclear whether this relationship is similar in the production of 28 dynamic shortening contractions. Using fast joint velocities to drive the system, we aimed to 29 determine how anconeus MURTs recorded during the torque production phase preceding 30 movement were affected in relation to the resultant peak elbow extension velocity. 31 Recruitment thresholds of seventeen MUs from nine young men were tracked throughout non32 isokinetic dynamic elbow extensions with velocities ranging from 64° s to 500 ° s at a 33 constant resistance of 25% of maximal voluntary isometric contraction, and during isometric 34 elbow extensions (0° s). Relative MURTs decreased ~50% from the slowest (<25% of maximal 35 velocity) to the fastest (>75% of maximal velocity) resultant velocity ranges (p<0.05). Although 36 a significant (p<0.001) but weak (r=-0.27, R=0.08) relationship was observed between MURT 37 and resultant peak elbow extension velocity for the group, only seven of the seventeen MUs 38 displayed significant moderate (r=-0.40, R=0.17) to strong (r=-0.85, R=0.73) negative MURT39 velocity relationships. These data indicate variable responses of MURTs with increasing 40 resultant peak velocity, which may be related to the intrinsic properties of individual MUs. 4