Abstract
The synchronized firings of active motor units (MUs) increase the oscillations of muscle force, observed as physiological tremor. This study aimed to investigate the effects of synchronizing the firings within three types of MUs (slow—S, fast resistant to fatigue–FR, and fast fatigable–FF) on the muscle force production using a mathematical model of the rat medial gastrocnemius muscle. The model was designed based on the actual proportion and physiological properties of MUs and motoneurons innervating the muscle. The isometric muscle and MU forces were simulated by a model predicting non-synchronized firing of a pool of 57 MUs (including 8 S, 23 FR, and 26 FF) to ascertain a maximum excitatory signal when all MUs were recruited into the contraction. The mean firing frequency of each MU depended upon the twitch contraction time, whereas the recruitment order was determined according to increasing forces (the size principle). The synchronization of firings of individual MUs was simulated using four different modes and inducing the synchronization of firings within three time windows (± 2, ± 4, and ± 6 ms) for four different combinations of MUs. The synchronization was estimated using two parameters, the correlation coefficient and the cross-interval synchronization index. The four scenarios of synchronization increased the values of the root-mean-square, range, and maximum force in correlation with the increase of the time window. Greater synchronization index values resulted in higher root-mean-square, range, and maximum of force outcomes for all MU types as well as for the whole muscle output; however, the mean spectral frequency of the forces decreased, whereas the mean force remained nearly unchanged. The range of variability and the root-mean-square of forces were higher for fast MUs than for slow MUs; meanwhile, the relative values of these parameters were highest for slow MUs, indicating their important contribution to muscle tremor, especially during weak contractions.
Highlights
Most studies of motor unit (MU) firings have revealed the existence of a certain level of synchronization between the firings of motoneurons innervating the same muscle [1,2,3,4]
The results based on the two used coefficients corMU and cross-interval synchronization index (CISI) showed that the range, the maximum, and the root-mean-square of the forces rose with increased synchronization, while the mean forces remained nearly unchanged
This increase was stronger for fast MUs; notably, these units are mostly responsible for the force instability in the context of moderate or strong muscle contractions, wherein fast MUs are recruited into activity
Summary
Most studies of motor unit (MU) firings have revealed the existence of a certain level of synchronization between the firings of motoneurons innervating the same muscle [1,2,3,4]. Two concepts for long- and short-term synchronization can be found in the literature. Short-term synchronization is attributed to last-order projections that provide common, nearly simultaneous, excitatory synaptic input across motoneurons [3,8,9,10], generating a narrow peak around the origin of the cross-correlogram of MU discharges [1,8,11,12]. The narrow synchronous peak principally reflects shared, monosynaptic projections to motor neurons from corticomotoneuronal cells via the lateral corticospinal tract [13]
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