Real-time Mechanical Performance Of Old And Young Skeletal Muscle During Chronic Exposure To Stretchshortening Contractions

Abstract

While many studies conducted to date have investigated the effect of age on the performance response and morphological changes after a single injurious exposure of eccentric contractions, there are very few studies that have investigated the effect of age on muscle injury susceptibility and adaptation due to chronic, repetitive loading. PURPOSE To investigate if age affects the change in real-time mechanical response of skeletal muscle during a chronic administration of stretch-shortening cycles. METHODS Dorsiflexor muscles of old (30 months, N= 5) and young (12 weeks, N = 6) Fischer 344 × Brown Norway rats were exposed 3 times per week for 4.5 weeks to 80 maximal stretch-shortening contractions (SSCs) in vivo. The SSCs were administered in 8 sets of 10 repetitions at 2 minute intervals. Within each set, there was 2 seconds rest between each SSC. Both the eccentric and concentric components of the SSC were conducted at an angular velocity of 60 deg/s. Changes in real-time performance during each exposure were quantified and it was determined that sets 1, 3, 5 and 8 could accurately represent the changes in mechanical behavior during the protocol. From the force and position data, peak force, minimum force, cyclic force, negative work, positive work, and net work were quantified for the first oscillation in each set to determine changes between sets. These force and work values were calculated for all 14 exposure sessions. RESULTS The peak force, minimum force, negative work, and positive work showed an increasing trend in the young group over the 4.5 week protocol, whereas the old age group showed a decreasing trend. Peak force (p=0.0065), minimum force (p=0.0067), negative work (p=0.0011), and positive work (p=0.0018) showed a consistent difference between the old and young age groups beginning the second week of exposure. Cyclic force (p=0.5575) and net work (p=0.4567) showed no age by time interaction over the 4.5 week exposure period. CONCLUSIONS The young age group was able to adapt to the chronic exercise protocol and showed a steady increase in real-time performance during each exposure period. However, the old age group showed a clear inability to adapt to the chronic exercise protocol with a steady decrease in real-time performance measures.