INFLUENCE OF SIMULATED BED REST AND INTERMITTENT WEIGHT-BEARING ON SINGLE SKELETAL-MUSCLE FIBER FUNCTION IN AGED RATS

Objective: To characterize specific musculoskeletal contractile proper ty changes that occur during inactivity and intermittent weight bearin g in aged muscle. Design: Randomized control trial. Setting: A control led laboratory environment. Subjects: Fifteen aged rats were randomly assigned to control (CON), hindlimb unweighted (HU), and hindlimb unwe ighted 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-beari ng.Main Outcome Measures: Muscle weights, muscle fiber diameter, peak absolute force, peak specific tension (P-o), and maximal shortening ve locity (V-o). Results: In comparison to CON animals, the soleus (SOL) wet weight was significantly (p less than or equal to .05) reduced by 19% and 6% in HU and HU-IWB animals, respectively, SOL single fiber an alysis showed no difference in fiber diameter between the three groups . However, peak absolute force and P-o of SOL type I fibers were signi ficantly (p less than or equal to .05) reduced in the HU group compare d to CON values. V-o of SOL fibers increased with HU. In comparison to CON animals, the gastrocnemius (GAS) wet weight was significantly red uced 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 b y inactivity in the SOL. However, this weight-bearing protocol did not attenuate the inactivity-induced alterations in aged mammalian GAS sk eletal muscle. (C) 1997 by the American Congress of Rehabilitation Med icine ann the American Academy of Physical Medicine and Rehabilitation