Central contributions to torque depression: an antagonist perspective
Torque depression (TD) is the reduction in steady-state isometric torque following active muscle shortening when compared to an isometric reference contraction at the same muscle length and activation level. Central nervous system excitability differs in the TD state. While torque production about a joint is influenced by both agonist and antagonist muscle activation, investigations of corticospinal excitability have focused on agonist muscle groups. Hence, it is unknown how the TD state affects spinal and supraspinal excitability of an antagonist muscle. Eight participants (~ 24y, three female) performed 14 submaximal dorsiflexion contractions at the intensity needed to maintain a level of integrated electromyographic activity in the soleus equivalent to 15% of that recorded during a maximum plantar flexion contraction. The seven contractions of the TD protocol included a 2 s isometric phase at an ankle angle of 140°, a 1 s shortening phase at 40°/s, and a 7 s isometric phase at an angle of 100°. The seven isometric reference contractions were performed at an ankle angle of 100° for 10 s. Motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and maximal M-waves (Mmax) were recorded from the soleus in both conditions. In the TD compared to isometric reference state, a 13% reduction in dorsiflexor torque was accompanied by 10% lower spinal excitability (normalized CMEP amplitude; CMEP/Mmax), and 17% greater supraspinal excitability (normalized MEP amplitude; MEP/CMEP) for the soleus muscle. These findings demonstrate a neuromechanical coupling following active muscle shortening and indicate that the underlying mechanisms of TD influence antagonist activation during voluntary force production.
KeywordsIntegrated electromyography iEMG Transcranial magnetic stimulation TMS Motor evoked potential MEP Cervicomedullary motor evoked potential CMEP
We would like to thank all of the participants in this study.
CTS, LRB, CJM, and GAP contributed to the conception and design of the study; CTS, VC, and GAP performed experiments; CTS, CJM, and GAP analyzed and interpreted the data; CTS, CJM, and GAP prepared figures and drafted the manuscript; CTS, VC, LRB, CJM, and GAP edited and revised the manuscript.
This project was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). Infrastructure was provided by the University of Guelph start-up funding.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. This article does not contain any studies with animals performed by any of the authors.
Informed consent was obtained from all individual participants included in the study.
Individual values of all supporting data are accessible as supplementary material.
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