Reflex and muscular adaptations in rat soleus muscle after hindlimb suspension

Reflex, mechanical and histochemical adaptations of the soleus muscle follo wing 3 weeks of hindlimb suspension (HS) were measured in the rat. HS trans formed the soleus muscle fibre type composition from predominantly slow, ty pe I, to approximately equal proportions of fast, type II and slow fibres, Consistent with this transformation was an increase in the maximum shorteni ng velocity, Vmax, and a decrease in the stiffness of the series elastic co mponent. Disuse also produced muscle atrophy and a resultant decrease in tw itch and tetanic force. Reflex responses of the ankle extensors were also o btained at 5 and 9 weeks of age for six control rats (C group) and six rats subjected to HS for 3 weeks (HS group). The soleus reflexes to a mechanica l tap applied to the Achilles tendon (T reflex) and to an electrical stimul ation of the sciatic nerve (H reflex) were measured. The maximal amplitude of these reflexes (T-max and H-max) were normalised to the maximal direct m otor response (M-max) and the T-max/H-max ratio was also calculated to give an index of the relative adaptations of the peripheral and central compone nts of the reflex pathway. The HS group showed significantly higher H refle x gains than the C group, possibly due to changes in synaptic efficiency af ter HS, Conversely, the HS group presented strongly inhibited T reflexes an d negative gains for the T-max/H-max ratios. This result indicated a reduce d spindle solicitation after HS, which may reflect changes in the spindle s ensitivity itself, but it could also be due to the decrease in stiffness of the musculo-tendinous elements in series with the muscle spindles. Such me chanical changes may play an important part in the decreased T reflex respo nses.