Research into skeletal muscle's response to hindlimb unloading (HU) of the rodent has focused on that of the markedly affected slow-twitch anti-gravity muscles (e.g., soleus). However, the ability of the animal to locomote following HU should be best determined by the in vivo functional properties of the muscle groups involved and, to our knowledge, this has not been investigated. Our objective was to determine how the in vivo functional properties of the rat ankle plantarflexor group change after 28 days of HU and during a subsequent 28-day recovery. Rats ( n=48) were unloaded for 28 days after which they were either tested immediately or allowed to recover for 7, 14, or 28 days before being tested. Control rats ( n=61) were tested at comparable times. In vivo functional properties of the ankle plantarflexors were assessed under anesthesia using an isokinetic dynamometer and included determination of the isometric torque-frequency relationship, the concentric torque-ankle angular velocity relationship, and fatigability. Immediately after HU, plantarflexor muscle weight was reduced by 24% but isometric torque production was reduced by 7-9% only at > or =100 Hz and concentric torque production was not significantly affected. However, after 7 days of recovery, in vivo function was more adversely affected; isometric and concentric torques were reduced by 12-33% and 16-36%, respectively, relative to control levels. In vivo plantarflexor function was recovered by 14 days. In conclusion, 28 days of HU has minor adverse effects on the in vivo function of the rat ankle plantarflexors. During the first week of recovery from HU, injury apparently occurs to the plantarflexors resulting in a transient impairment of functional capacity.

The purpose of this research was to characterize the strains in the rat tibia induced by controlled muscle contractions of the lower limb. The muscle contractions were generated unilaterally by electrical stimulation of the sciatic nerve in anesthetized animals. The technique was developed by muscle biologists as a way of simulating resistive exercise, and it was used in the current research as a countermeasure for rats undergoing hindlimb unloading to simulate musculoskeletal effects of space flight. The intensity of muscle contraction is quantified by measuring the torque generated at the ankle joint. A wide range of torques can be generated by varying the frequency of the electrical stimulating signal. Three specific questions were addressed in the current study. First, what is the relationship between strain and torque? Second, does the relationship between strain and torque change during the period of hindlimb unloading with exercise? Third, are the strain magnitudes in the range generally regarded to be osteogenic? This project is part of a larger effort aimed at assessing the efficacy of the muscle stimulation method as a countermeasure for the negative musculoskeletal effects of mechanical unloading.