43.1 Thursday, Jan. 5 Does extended training alter the operating length of leg extensor muscles? KONOW, N*; ROBERTS, TJ; Brown University; Brown University firstname.lastname@example.org
Muscles are susceptible to damage during active lengthening. If a muscle operates at short fascicle lengths, on the ascending limb of its length-tension (LT) curve, the risk of damage may be reduced. It has been argued that adaptive changes in response to training may allow a muscle to shift its operating length to the left (i.e., relatively shorter) on the LT curve. We tested this hypothesis by determining relative operating length of the lateral gastrocnemius (LG) muscle. Two cohorts of wild turkeys (six animals each) were trained to run either uphill only (UH) or downhill only (DH) on a treadmill for 30 minutes per day for 10 weeks. After training, the right LG muscle of each bird was fitted with straingages on the calcified tendon to directly measure muscle force, while sonomicrometry was used to measure muscle fascicle length. During experiments, all birds ran a set trial sequence of 8 incremental slopes, from +20 to -25. A tetanic length-tension curve was then measured for each muscle in situ. By superimposing the in-vivo work loops and the LT curve for each muscle we could determine if training-mediated changes in the active operating range had occurred. Our prediction was that during downhill running, the DH trained muscles would operate at relatively shorter lengths compared with UH trained muscles. We found no consistent pattern in the active operating length of DH trained muscles during eccentric loading. Contrary to our expectations, we found that UH trained muscles operated at relatively short fascicle lengths. Our data do not support the hypothesis that safe operating range of a muscle results from adaptive changes acquired with training. This implies that the mechanisms of training-mediated resistance to muscle damage have yet to be fully explored. Supported by NIH (AR055295) to TJR.