Benjamin Pasquet

Specific modulation of motor unit discharge for a similar change in fascicle length during shortening and lengthening contractions in humans

This study examines the effect of a change in fascicle length on motor unit recruitment and discharge rate in the human tibialis anterior during shortening and lengthening contractions that involved a similar change in torque. The dorsiflexor torque and the surface and intramuscular electromyograms (EMGs) from the tibialis anterior were recorded in eight subjects. The behaviour of the same motor unit (n= 63) was compared during submaximal shortening and lengthening contractions performed at a constant velocity (10 deg s−1) with the dorsiflexor muscles over a 20 deg range of motion around the ankle neutral position. Muscle fascicle length was measured non‐invasively using ultrasonography. Motor units that were active during a shortening contraction were always active during the subsequent lengthening contraction. Furthermore, additional motor units (n= 18) of higher force threshold that were recruited during the shortening contraction to maintain the required torque were derecruited first during the following lengthening contraction. Although the change in fascicle length was linear (r2 > 0.99), and similar for both shortening and lengthening contractions, modulation of discharge rate differed during the two contractions. Compared with an initial isometric contraction at short (11.9 ± 2.4 Hz) or long (11.7 ± 2.2 Hz) muscle length, discharge rate increased only slightly and stayed nearly constant throughout the lengthening contraction (12.6 ± 2.0 Hz; P < 0.05) whereas it augmented progressively and more substantially during the shortening contraction, reaching 14.5 ± 2.5 Hz (P < 0.001) at the end of the movement. In conclusion, these observations indicate a clear difference in motor unit discharge rate modulation with no change in their recruitment order between shortening and lengthening contractions when performed with a similar change in muscle fascicle length and torque.

Specific modulation of corticospinal and spinal excitabilities during maximal voluntary isometric, shortening and lengthening contractions in synergist muscles

Non‐technical summary  The neural control of muscle activity differs during voluntary shortening and lengthening contractions. In this paper, we show that the relative contribution of both cortical and spinal mechanisms to the modulation of neural activation is specific during lengthening contraction and differs between synergist muscles. Knowledge of spinal and corticospinal excitabilities modulations during shortening and lengthening muscle contraction improves our understanding of the processes that underlies the neural control of muscles during dynamic contractions.

Specific modulation of spinal and cortical excitabilities during lengthening and shortening submaximal and maximal contractions in plantar flexor muscles

This study investigated the influence of the torque produced by plantar flexor muscles on cortical and spinal excitability during lengthening and shortening voluntary contractions. To that purpose, modulations of motor-evoked potential (MEP) and Hoffmann (H) reflex were compared in the soleus (SOL) and medial gastrocnemius (MG) during anisometric submaximal and maximal voluntary contraction (MVC) of the plantar flexor muscles. For the submaximal shortening and lengthening contractions, the target torque was set at 50% of their respective MVC force. The results indicate that the amplitudes of both MEP and H-reflex responses, normalized to the maximal M wave, were significantly (P < 0.05) lower during lengthening compared with shortening submaximal contraction. For these two parameters, the reduction reached, respectively, 22.1 and 31.9% for the SOL and 34.5 and 29.3% for the MG. During MVC, normalized MEP and H reflex of the SOL were both reduced significantly by 19.9% (P < 0.05) and 29.9% (P < 0.001) during lengthening and shortening contraction, respectively, whereas no significant change (P > 0.05) was observed for MG. In addition, the silent period in the ongoing electromyogram (EMG) activity following the MEP was significantly (P < 0.01) briefer during lengthening than shortening contractions but did not differ (P > 0.05) between contraction intensities and muscles. Together, these results indicate that cortical and spinal mechanisms involved in the modulation of muscle activation during shortening and lengthening contractions differ between synergistic muscles according to the torque produced. Data further document previous studies reporting that the specific modulation of muscle activation during lengthening contraction is not torque dependent.

Change in motor unit recruitment pattern at different fascicle lengths in the human tibialis anterior

Conflicting results have been reported regarding motor units (MU) recruitment patterns during isometric contractions at different muscle lengths. Although, some authors have observed an earlier recruitment of MUs at short compared with long muscle length (Vander Linden et al. 1991), others have demonstrated opposite results (Kennedy and Cresswell 2001). The purpose of this work was to examine the effect of muscle length change on MU recruitment and behaviour in the human tibialis anterior (TA) during submaximal isometric ankle dorsiflexions. The fascicles length (Lf) of the TA muscle were measured non-invasively using ultrasonography in order to associate change in muscle architecture and MU discharge characteristics.