Original Research ARTICLE
Neuromuscular properties of the human wrist flexors as a function of the wrist joint angle
- 1Institute of Sport Science, University of Rostock, Germany
- 2School of Human Movement and Nutrition Sciences, University of Queensland, Australia
The joint angle dependence of voluntary activation and twitch properties has been investigated for several human skeletal muscles. However, although they play a key role for hand function and possess a unique neural control compared to muscles surrounding other joint complexes, little is known about the wrist flexors innervated by the median nerve. Therefore, isometric voluntary and electrically evoked contractions of the wrist flexors were analyzed at three wrist joint angles (extension: − 30°, neutral: 0°, flexion: 30°) to quantify the joint angle dependence of (i) voluntary activation (assessed via peripheral nerve stimulation and electromyography [EMG]), (ii) unpotentiated twitch torques, and (iii) potentiated twitch torques.
Maximum voluntary torque was lower in extension compared to neutral and flexion. Although voluntary activation was generally high, data indicate that voluntary activation of the wrist flexors innervated by the median nerve was lower and the antagonist·agonist–1 EMG ratio was higher with the wrist joint in flexion compared to extension. Peak twitch torque, rate of twitch torque development, and twitch half-relaxation time increased, whereas electromechanical delay decreased from flexion to extension for the unpotentiated twitch torques. Activity-induced potentiation partly abolished these differences and was higher in short than long wrist flexors.
Different angle-dependent excitatory and inhibitory inputs to spinal and supraspinal centers might be responsible for the altered activation of the investigated wrist muscles. Potential mechanisms were discussed and might have operated conjointly to increase stiffness of the flexed wrist joint. Differences in twitch torque properties were probably related to angle-dependent alterations in series elastic properties, actin-myosin interaction, Ca2+ sensitivity, and phosphorylation of myosin regulatory light chains. The results of the present study provide valuable information about the contribution of neural and muscular properties to changes in strength capabilities of the wrist flexors at different wrist joint angles. These data could help to understand normal wrist function, which is a first step in determining mechanisms underlying musculoskeletal disorders and in giving recommendations for the restoration of musculoskeletal function after traumatic or overuse injuries.
Keywords: muscle length, voluntary activation, Electrical Stimulation, Flexor Carpi Radialis, Median Nerve, Twitch, Post-activation potentiation, activity-dependent potentiation
Received: 08 Apr 2019;
Accepted: 11 Jul 2019.
Edited by:Yih-Kuen Jan, University of Illinois at Urbana-Champaign, United States
Reviewed by:Chi-Wen Lung, Asia University, Taiwan
Ben-Yi Liau, Hungkuang University, Taiwan
Fu-Lien Wu, National Yang-Ming University, Taiwan
Copyright: © 2019 Behrens, Husmann, Mau-Moeller, Schlegel, Reuter and Zschorlich. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Dr. Martin Behrens, Institute of Sport Science, University of Rostock, Rostock, 18051, Mecklenburg-Vorpommern, Germany, firstname.lastname@example.org