The effects of passive dehydration on motor unit firing rates of the vastus lateralis in males

Purpose The purpose of the present study was to examine the effects of dehydration on strength and motor unit (MU) firing rates of the vastus lateralis (VL) during submaximal contractions. Methods Eight resistance trained, healthy males (mean +/- SD; age = 27.6 +/- 3.2 years, mass = 98.7 +/- 15.1 kg, stature = 181.0 +/- 3.8 cm) performed submaximal leg extension contractions at 70% of maximal voluntary contraction (MVC) before and after a passive dehydration protocol to a 5% loss in body weight. Surface EMG signals of the VL were collected and decomposed into their constituent MU action potential trains and linear mean firing rate (pps) vs. recruitment threshold [expressed as a percentage of MVC (%MVC)] relationships were analyzed for each subject and hydration condition. Results The MVC torque was higher during euhydration (269.3 +/- 58.9 Nm) than during hypohydration (242.8 +/- 60.8 Nm) while there were no differences between the slopes (euhydrated = -0.43 +/- 0.23 pps/%MVC; hypohydrated = -0.50 +/- 0.28 pps/%MVC; p = 0.23) or the y-intercepts (euhydrated = 30.43 +/- 10.47 pps; hypohydrated = 35.22 +/- 14.58 pps; p = 0.078) between conditions. Conclusion The 9.8% decrease in strength during hypohydration might be caused by a decrease in the contractile force of the active muscle fibers rather than decreased activation of the MU pool.