Influence of Simulated Bed Rest and Intermittent Weight Bearing on Single Skeletal Muscle Fiber Function in Aged Rats

Objective: To characterize specific musculoskeletal contractile property changes that occur during inactivity and intermittent weight bearing in aged muscle.

Design: Randomized control trial.

Setting: A controlled laboratory environment.

Subjects: Fifteen aged rats were randomly assigned to control (CON), hindlimb unweighted (HU), and hindlimb unweighted with intermittent weight bearing (HU-IWB) groups.

Interventions: The HU and HU-IWB rats were suspended for 1 week. The HU-IWB animals were unsuspended four times daily allowing 15 minutes of weight-bearing.

Main Outcome Measures: Muscle weights, muscle fiber diameter, peak absolute force, peak specific tension (P0), and maximal shortening velocity (V0).

Results: In comparison to CON animals, the soleus (SOL) wet weight was significantly (p < or = .05) reduced by 19% and 6% in HU and HU-IWB animals, respectively. SOL single fiber analysis showed no difference in fiber diameter between the three groups. However, peak absolute force and P0 of SOL type I fibers were significantly (p < or = .05) reduced in the HU group compared to CON values. V0 of SOL fibers increased with HU. In comparison to CON animals, the gastrocnemius (GAS) wet weight was significantly reduced by 9% and 8% in HU and HU-IWB animals, respectively.

Conclusions: Inactivity significantly altered the contractile properties of single fibers isolated from aged mammalian SOL skeletal muscle. Furthermore, minimal weight bearing attenuated these detrimental effects induced by inactivity in the SOL. However, this weight-bearing protocol did not attenuate the inactivity-induced alterations in aged mammalian GAS skeletal muscle.