Adjustments in Motor Unit Properties during Fatiguing Contractions after Training

VILA-CHÃ, CAROLINA1,2; FALLA, DEBORAH3,4; CORREIA, MIGUEL VELHOTE2; FARINA, DARIO3

Medicine & Science in Sports & Exercise: April 2012 - Volume 44 - Issue 4 - p 616–624
doi: 10.1249/MSS.0b013e318235d81d
Basic Sciences

Objective: The objective of the study was to investigate the effect of strength and endurance training on muscle fiber membrane properties and discharge rates of low-threshold motor units of the vasti muscles during fatiguing contractions.

Methods: Twenty-five sedentary healthy men (age (mean ± SD) = 26.3 ± 3.9 yr) were randomly assigned to one of three groups: strength training, endurance training, or a control group. Conventional endurance and strength training was performed 3 d·wk−1, during a period of 6 wk. Motor unit conduction velocity and EMG amplitude of the vastus medialis obliquus and lateralis muscles and biceps femoris were measured during sustained isometric knee extensions at 10% and 30% of the maximum voluntary contraction before and immediately after training.

Results: After 6 wk of training, the reduction in motor unit conduction velocity during the sustained contractions at 30% of the maximum voluntary force occurred at slower rates compared with baseline (P < 0.05). However, the rate of decrease was lower after endurance training compared with strength training (P < 0.01). For all groups, motor unit discharge rates declined during the sustained contraction (P < 0.001), and their trend was not altered by training. In addition, the biceps femoris–vasti coactivation ratio declined after the endurance training.

Conclusions: Short-term strength and endurance training induces alterations of the electrophysiological membrane properties of the muscle fiber. In particular, endurance training lowers the rate of decline of motor unit conduction velocity during sustained contractions more than strength training.

1Polytechnic Institute of Bragança, Bragança, PORTUGAL; 2Faculdade de Engenharia, Universidade do Porto, Porto, PORTUGAL; 3Department of Neurorehabilitation Engineering, Bernstein Center for Computational Neuroscience, University Medical Center Göttingen, Georg-August University, Göttingen, GERMANY; and 4Pain Clinic, Center for Anesthesiology, Emergency and Intensive Care Medicine, University Hospital Göttingen, Göttingen, GERMANY

Address for correspondence: Dario Farina, Ph.D., Department of Neurorehabilitation Engineering, Bernstein Center for Computational Neuroscience, University Medical Center Göttingen, Georg-August University, Von-Siebold-Str. 4, 37075 Göttingen, Germany; E-mail: dario.farina@bccn.uni-goettingen.de.

Submitted for publication March 2011.

Accepted for publication September 2011.

©2012The American College of Sports Medicine