In Situ Force-Sarcomere Length Relationship in Old Rats

Abstract

With adult aging, there is a loss of muscle mass and alterations to the structural components of the human neuromuscular system resulting in impaired contractile function. A potential source of impairment could be aged muscle operating at a restricted force-length relationship (i.e., smaller plateau region owing to shorter fascicle lengths and greater sarcomere length (SL) changes for a given excursion). PURPOSE: To investigate the force-fascicle length (F-Lf) relationship as related to SL during electrically evoked tetanic contractions in the rat medial gastrocnemius (MG). METHODS: Two groups of Fisher344×BN rats were used; young (7-8 mo.) (n = 9) and old (30-35 mo.) (n = 10). The MG was isolated surgically, attached in series to a muscle puller and force transducer. The tibial nerve was electrically stimulated via a nerve cuff. Contractions were evoked at 200 Hz for 250 ms separated by 2 min rest across muscle lengths corresponding to the: ascending, plateau, and descending limb of the F-L relationship. The rats were then sacrificed and the hind limb was placed immediately in 10% Formalin (fixative) at the muscle length corresponding to peak force (Lo). After 1 h of fixation, the MG was secured to a stick and allowed to fix for 2 weeks. The muscle was then dissected into 4 lengthwise sections medial and lateral to the longitudinal axis of the muscle. After a 4 hour, 30% nitric acid digestion process, 5 individual fascicles from each muscle section were isolated and placed on slides for SL measurement at 5 locations along the fascicle by laser diffraction. RESULTS: At Lo, muscle fascicles were ~14% shorter in old compared to young, with no difference in average SL, resulting in ~10% fewer sarcomeres in series in the old. Peak tetanic force (N) was ~18% lower in the old as compared with young. Upon activation, SL was not different between young and old on the ascending limb or plateau region of the F-L relationship but was ~20% longer in old on the descending limb. Upon accounting for passive force, the old had a smaller plateau region as compared with young. CONCLUSION: The functional consequences of these age related changes could be a reduced range of motion for older adults and less force production capacity throughout that range of motion (particularly long muscle lengths) to perform activities of daily living. Supported by: NSERC and CIHR