Recruitment of faster motor units is associated with greater rates of fascicle strain and rapid changes in muscle force during locomotion

Abstract

Animals modulate the power output needed for different locomotor tasks by changing muscle forces and fascicle strain rates. To generate the necessary forces, appropriate motor units must be recruited. Faster motor units have faster activation-deactivation rates than slower motor units, and they contract at higher strain rates; therefore, recruitment of faster motor units may be advantageous for tasks that involve rapid movements or high rates of work. This study identified motor unit recruitment patterns in the gastrocnemii muscles of goats and examined whether faster motor units are recruited when locomotor speed is increased. The study also examined whether locomotor tasks that elicit faster (or slower) motor units are associated with increased (or decreased) in vivo tendon forces, force rise and relaxation rates, fascicle strains and/or strain rates. Electromyography (EMG), sonomicrometry and muscle-tendon force data were collected from the lateral and medial gastrocnemius muscles of goats during level walking, trotting and galloping and during inclined walking and trotting. EMG signals were analyzed using wavelet and principal component analyses to quantify changes in the EMG frequency spectra across the different locomotor conditions. Fascicle strain and strain rate were calculated from the sonomicrometric data, and force rise and relaxation rates were determined from the tendon force data. The results of this study showed that faster motor units were recruited as goats increased their locomotor speeds from level walking to galloping. Slow inclined walking elicited EMG intensities similar to those of fast level galloping but different EMG frequency spectra, indicating that recruitment of the different motor unit types depended, in part, on characteristics of the task. For the locomotor tasks and muscles analyzed here, recruitment patterns were generally associated with in vivo fascicle strain rates, EMG intensity and tendon force. Together, these data provide new evidence that changes in motor unit recruitment have an underlying mechanical basis, at least for certain locomotor tasks.

Fig. 1.

Lateral (left) and posterior (right) view of the goat lower hindlimb. The approximate locations of the electromyographic (EMG) electrodes and sonomicrometry crystals (proximal, mid-belly and distal) in the lateral and medial gastrocnemius (LG and MG) muscles, and the common gastrocnemius and MG tendon-buckle force transducers are shown (adapted from Lee et al., 2011).

Fig. 2.

Representative LG force, fascicle length and EMG signals from fine-wire EMG of a goat during different locomotor tasks. Shaded bars indicate the stance phase. The dashed line indicates fascicle length during standing.

Fig. 3.

Mean values of LG (A) total EMG intensity, (B) force, (C) force rate, (D) fascicle strain and (E) strain rate for different locomotor tasks over a stride (level walking, trotting and galloping, incline walking and trotting).

Fig. 4.

Mean values of MG (A) total EMG intensity, (B) force, (C) force rate, (D) fascicle strain and (E) strain rate for different locomotor tasks over a stride (level walking, trotting and galloping, incline walking and trotting).

Fig. 5.

Principal component analysis (PCA) of the EMG signals. (A) The first two PCs defined from the EMG spectra of LG and MG explained ~76% of the signal; the percentage values shown reflect the percentage of the signal explained by each PC. (B) PCI and PCII loading scores for MG (solid) and LG (dashed) during the stance phase for different locomotor tasks (diamond, level walking; triangle, level trotting; star, level galloping; circle, incline walking; square, incline trotting). Angle θ describes the contribution of high- and low-frequency content in the EMG signal; smaller values of θ indicate higher-frequency content.