# Muscle length: A new feature to investigate neural control of lengthening contractions

Marc Jubeau

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Valentin Doguet

University of Nantes, Laboratory 'Movement, Interactions, Performance', MIP, EA 4334, Nantes, F-44000, France

**Correspondence**

M. Jubeau: University of Nantes, Faculty of Sport Sciences, Laboratory 'Movement, Interactions, Performance', 25 bis Boulevard Guy Mollet, 44322 Nantes, France.

Email: marc.jubeau@univ-nantes.fr

Edited by: Michael Tipton

## INTRODUCTION

Connections link a sequence of three related research papers. The central article which links the other two papers has been published in *Experimental Physiology*. In a Connections article, an author (or authors) of the central article outlines its principal novel findings, tracing how they were influenced by the first article and how the central article has contributed to the developments made in the third article. The author(s) may also speculate on the direction of future research in the field. Connections articles aim to set the research in a wide context.

The central nervous system regulates skeletal muscles in all motor tasks, such as walking, jumping and running, and adopts unique strategies during lengthening muscle contractions compared with static or shortening muscle contractions (for a detailed review, see Duchateau & Enoka, 2016). This unique neural strategy has been suggested because of the reduced EMG activity or lower voluntary activation level during maximal voluntary lengthening contractions compared with either static or shortening contractions (Duchateau & Enoka, 2016). Results from a battery of neurophysiological techniques (e.g. electrical nerve stimulation, transcranial magnetic stimulation) indicate that the spinal level of the central nervous system, rather than cortical motor regions, is responsible for the reduced neural activation during lengthening contractions (Duclay, Pasquet, Martin, & Duchateau, 2011, 2014). However, the neurophysiological mechanisms responsible are unclear.

In our work (Doguet et al., 2017), we investigated the influence of muscle length on the neural control of lengthening, shortening and static voluntary contractions. Changes in the motor evoked potential (MEP) amplitude, indicative of corticospinal excitability, and changes in the duration of the cortical silent period (CSP), indicative of intracortical inhibition, were compared at intermediate (75 deg knee joint

angle; 0 deg is full extension) and long (100 deg) muscle lengths during maximal lengthening, shortening and static voluntary contractions of the knee extensors. The main finding by Doguet et al. (2017) was that there was no difference in the MEP amplitude between contraction modes at a long muscle length, whereas differences were observed at an intermediate muscle length. However, the CSP duration was longer during lengthening contractions at a long muscle length and shorter at an intermediate muscle length compared with other contraction modes. Thus, our work suggested that the neural control of lengthening contractions is unique, and its specificity appears to be dependent on muscle length.

Our findings about neurophysiological responses at an intermediate muscle length extended previous studies of a plantar flexor muscle group (Duclay et al., 2011, 2014). Indeed, Duclay and colleagues clearly highlighted the specific modulation of corticospinal mechanisms during lengthening contractions. These authors have investigated spinal and cortical excitability during submaximal and maximal lengthening, shortening and static contractions. They reported that cortical and spinal excitability were different during lengthening contractions compared with shortening and static contractions and, unexpectedly, similar whether submaximal or maximal contractions were considered. The duration of the CSP was also specifically modulated during lengthening contractions compared with shortening and static contractions in their studies. Although Duclay et al. (2011, 2014) reported new findings, they did not investigate the influence of muscle length on the neural control of lengthening muscle actions.

Building on these findings, our work on the influence of muscle length on the neural control of different muscle contraction modes (Doguet et al., 2017) added to our understanding of the unique neural control of lengthening contractions. However, it remains to elucidate how this specific neural control during eccentric contraction can
influence neurophysiological responses associated with muscle fatigue or damage.

Recently, Garnier, Paizis, and Lepers (2019) have investigated this question. They compared corticospinal responses (i.e. MEP and CSP) associated with a series of lengthening and shortening contractions of the knee extensors at a short muscle length. They devised an original experimental design, i.e. lengthening and shortening contractions performed at a similar torque level and total impulse. After both protocols (lengthening and shortening), a significant and similar reduction of the maximal voluntary torque was observed, suggesting that muscle fatigue was induced. Importantly, no changes in MEP amplitude were observed during and after both protocols. The duration of the CSP was increased for one quadriceps head during and after the shortening protocol, whereas it was only reduced after the lengthening protocol. Based on these findings, Garnier et al. (2019) suggested that when torque level and total impulse are matched, the contraction mode does not affect corticospinal excitability but modulates supraspinal regions. However, the authors conceded that the lack of consideration of different muscle lengths while testing neurophysiological indexes (i.e. MEP and CSP) might have hidden possible changes in corticospinal excitability that could have occurred at longer muscle lengths that the one used for their testing (i.e. short muscle length, 45 deg knee angle; 0 deg is full extension).

In a nutshell, it appears that the central nervous system adopts a unique strategy while a muscle performs lengthening contractions, compared with shortening and static contractions, and this unique strategy is dependent of the muscle length. Although this specific neural control occurs at both cortical and spinal levels (Doguet et al., 2017; Duclay et al., 2011, 2014), spinal inhibitory processes appear to bear the main responsibility for the reduced muscle activation observed in the literature (Duclay et al., 2011, 2014).

We believe that cortical and spinal mechanisms associated with lengthening contractions should be investigated more at different muscle lengths to cast light onto neural control of lengthening contractions. In parallel, it remains to be elucidated whether the 'specific' neural control of lengthening contractions is influenced by intrinsic muscle functional impairments, such as fatigue or muscle damage, that can follow lengthening exercise. Thus, some stimulating and challenging research questions still remain, which will lead to better understanding of the neurophysiological mechanisms modulating neural control of muscle contractions, especially during lengthening contractions.

## ORCID

Marc Jubeau https://orcid.org/0000-0002-4878-7813

## REFERENCES

* Doguet, V., Nosaka, K., Guével, A., Thickbroom, G., Ishimura, K., & Jubeau, M. (2017). Muscle length effect on corticospinal excitability during maximal concentric, isometric and eccentric contractions of the knee extensors. *Experimental Physiology, 102*, 1513-1523.

## Connected Articles

* Duclay, J., Pasquet, B., Martin, A., & Duchateau, J. (2011). Specific modulation of corticospinal and spinal excitabilities during maximal voluntary isometric, shortening and lengthening contractions in synergist muscles. *The Journal of Physiology, 589*(Pt 11), 2901-2916.
https://doi.org/10.1113/jphysiol.2011.207472
Duclay, J., Pasquet, B., Martin, A., & Duchateau, J. (2014). Specific modulation of spinal and cortical excitabilities during lengthening and shortening submaximal and maximal contractions in plantar flexor muscles. *Journal of Applied Physiology (Bethesda, Md.: 1985)*, 117(12), 1440-1450. https://doi.org/10.1152/japplphysiol.00489.2014

* Doguet, V., Nosaka, K., Guével, A., Thickbroom, G., Ishimura, K., & Jubeau, M. (2017). Muscle length effect on corticospinal excitability during maximal concentric, isometric and eccentric contractions of the knee extensors. *Experimental Physiology, 102*(11), 1513-1523. https://doi.org/10.1113/EP086480

* Garnier, Y. M., Paizis, C., & Lepers, R. (2019). Corticospinal changes induced by fatiguing eccentric versus concentric exercise. *European Journal of Sport Science, 19*(2), 166-176.
https://doi.org/10.1080/17461391.2018.1497090

* Duchateau, J., & Enoka, R. M. (2016). Neural control of lengthening contractions. *Journal of Experimental Biology, 219*, 197-204.

* Duclay, J., Pasquet, B., Martin, A., & Duchateau, J. (2011). Specific modulation of corticospinal and spinal excitabilities during maximal voluntary isometric, shortening and lengthening contractions in synergist muscles. *The Journal of Physiology, 589*, 2901-2916.

* Duclay, J., Pasquet, B., Martin, A., & Duchateau, J. (2014). Specific modulation of spinal and cortical excitabilities during lengthening and shortening submaximal and maximal contractions in plantar flexor muscles. *Journal of Applied Physiology, 117*, 1440-1450.

* Garnier, Y. M., Paizis, C., & Lepers, R. (2019). Corticospinal changes induced by fatiguing eccentric versus concentric exercise. *European Journal of Sport Science, 19*, 166-176.

**How to cite this article:** Jubeau M, Doguet V. Muscle length: A new feature to investigate neural control of lengthening contractions. *Experimental Physiology*. 2020;105:930-931.
https://doi.org/10.1113/EP088455