The effects of fatigue on the torque-frequency curve of the human paralysed soleus muscle.

An advanced understanding of the torque-generating properties of the chronically paralysed soleus muscle may be instrumental in developing improved methods to activate human paralysed muscle. We established the shape of the torque-frequency curve before and after fatigue of the human paralysed soleus muscle. After fatigue, the normalized torque-frequency curve was shifted to the right, suggesting a higher frequency was required to generate the same relative torque. Low frequency fatigue (LFF) consisting of reduced torques at low frequencies and normal torques at higher frequencies was demonstrated. Conversely, the acutely paralysed soleus muscle was found to be fatigue-resistant and showed no shift in the torque-frequency curve. The muscle activation history (potentiation), LFF, and changing contractile speeds may affect the torque-frequency curve after fatigue. These factors may also play an important role in the development of optimal methods to activate paralysed muscle to attenuate fatigue.