Changes of the force-frequency relationship in the rat medial gastrocnemius muscle after total transection and hemisection of the spinal cord

Abstract

The relationships between the stimulation frequency and the force developed by motor units (MUs) of the medial gastrocnemius muscle were compared between intact rats and animals after total transection or hemisection of the spinal cord at the low thoracic level. The experiments on functionally isolated MUs were carried out 14, 30, 90, and 180 days after the spinal cord injury. Axons of investigated MUs were stimulated with trains of pulses at 10 progressively increased frequencies (from 1 to 150 Hz), and the force-frequency curves were plotted. Spinal cord hemisection resulted in a considerable leftward shift of force-frequency curves in all types of MUs. After the total transection, a leftward shift of the curve was observed in fast MUs, whereas there was a rightward shift in slow MUs. These changes coincided with a decrease of stimulation frequencies necessary to evoke 60% of maximal force. Moreover, the linear correlation between these stimulation frequencies and the twitch contraction time observed in intact rats was disrupted in all groups of animals with spinal cord injury. The majority of the observed changes reached the maximum 1 mo after injury, whereas the effects evoked by spinal cord hemisection were significantly smaller and nearly constant in the studied period. The results of this study can be important for the prediction of changes in force regulation in human muscles after various extends of spinal cord injury and in evaluation of the frequency of functional electrical stimulation used for training of impaired muscles.

Force regulation and electrical properties of motor units in overloaded muscle

“…A strategy of force regulation realized by individual MUs during muscle activity can be defined by analysis of a course of the force–frequency curve . In our study, for all MU types, the effects of overload are reflected in the rightward shifts of the steep parts of the force–frequency curves and in significantly higher stimulation frequencies for development of 60% of maximal force.…”

Changes in motor unit properties in SOD1 (G93A) rats

“…A significant correlation between the stimulation frequency needed to reach 60% of maximum force and contraction time, despite a progression of disease, was maintained in all groups of transgenic animals. Such correlation was observed in healthy animals, but disappeared after spinal cord injury . Perhaps stability of the correlation is an effect of gradual disease progression enabling progressive adaptation of MUs to new, altered conditions.…”

Estimating the minimum stimulation frequency necessary to evoke tetanic progression based on muscle twitch parameters

“…In particular, unfused tetanus has muscular relaxation between individual/successive stimulations, whereas fused tetanus has no muscular relaxation (Raikova et al 2007). The minimum stimulation frequency necessary to evoke unfused/fused tetanus is related to twitch (Cooper 1930, Kernell 1979, Celichowski and Grottel 1995, Yoshitake et al 2002, Mrowczynski et al 2006.…”

“…Celichowski et al proposed the fusion index (FI) to evaluate tetanic progression (Celichowski and Grottel 1995, Celichowski et al 1996, 2014, Raikova et al 2010, Mrowczynski et al 2011, Dobrzynska and Celichowski 2016. The FI can be easily used to evaluate tetanic progression in skeletal muscles, because the FI at 0% shows muscle twitch, and that at 100% shows fused tetanus.…”