Author(s): THOMASON, H., FOLLAND, J., ŠKARABOT, J., Institution: LOUGHBOROUGH UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 1010

INTRODUCTION:
Maximal rate of force development (RFD) is determined by neural (motor unit [MU] recruitment speed and discharge rate) and contractile properties (1,2). Chronic resistance training may confer neural adaptations including greater spinal cord output that facilitates faster RFD, whereas endurance trained (ET) individuals may have slower muscle contractile properties, resulting in limited increases in spinal cord output (3). Here, we assessed MU activity and intrinsic muscle contractile properties within RT, ET and UT individuals during maximal rapid contractions.
METHODS:
Sixty-six RT, ET and UT (22 per group, 6 females) individuals produced maximal and rapid voluntary isometric dorsiflexion force to determine maximal voluntary force (MVF) and RFD, respectively. Supramaximal percutaneous nerve stimulation (300 Hz) was administered to the common peroneal nerve at rest to record maximal evoked force (MEF) and RFD. A 64-channel grid electrode was placed on the tibialis anterior muscle of the dominant leg to assess myoelectrical activity (EMG). The EMG signals were decomposed into individual MU spike trains using Convolution Kernel Compensation algorithm to calculate MU recruitment speed, and discharge rate (DR) in the initial and plateau period of rapid contractions.
RESULTS:
MVF, MEF and maximal evoked RFD were significantly larger in RT (408.2 [358.0, 458.4] N, 151.3 [137.3, 165.3] N, 1998 [1776, 2220] N/s) compared to ET (346.6 [312.1, 381.0] N, 93.0 [78.2, 107.8] N, 1317 [1084, 1551] N/s, p<0.04) and to UT (301.4 [267.8, 335.0] N, 110.4 [96.7, 124.0] N, 1492 [1276, 1708] N/s, p<0.005), respectively. When maximal evoked RFD was normalised to MVF, both RT (482 [434, 529]%MVF/s) and UT (494 [448, 540]%MVF/s) were greater than ET (383 [333, 433]%MVF/s, p<0.0157). Greater absolute voluntary RFD was seen in RT compared to UT (1754 [1542, 1996] N/s vs. 1219 [1007, 1431] N/s, p=0.0031) but not compared to ET (1469 [1258, 1681] N/s, p=0.1486). However, when normalised to MVF, there were no differences between RT, ET or UT (437 [410, 464], 419 [392, 446], 403 [376, 430] %MVF/s, p=0.1984). Initial DR was greatest in RT (71.7 [67.3, 76.2] pps) compared to UT (63.9 [59.4, 68.5] pps, p = 0.0454), but not compared to ET (64.8 [60.4, 69.2] pps, p = 0.0773). DR during the plateau phase was the highest in RT (33.1 [30.5, 35.6] pps) vs. ET (27.8 [25.3, 30.4] pps, p=0.0136), but not compared to UT (31.0 [28.4, 33.6] pps, p=0.4949). No differences were noted in recruitment speed between RT, ET or UT (290 [224, 355], 316 [250, 381], 259 [184, 333] MUs/ms, p=0.767), respectively.
CONCLUSION:
RT individuals exhibited greater maximal strength and absolute RFD. However, despite greater initial MU discharge rate, relative RFD was similar between groups, likely due to between-group similarity in MU recruitment speed, a key determinant of RFD.

1) Del Vecchio et al 2019, J Physiol
2) Folland et al 2014, Scand J Med Sci Sports
3) Vila-Cha et al 2010, J Appl Physiol