Is the spring quality of muscle plastic?

During locomotion, major muscle groups are often activated cyclically. This alternate stretch-shorten pattern of activity could enable muscle to funct ion as a spring, storing and recovering elastic recoil potential energy. Be cause the ability to store and recover elastic recoil energy could profound ly affect the energetics of locomotion, one might expect this to be an adap table feature of skeletal muscle. This study tests the hypothesis that chro nic eccentric (Ecc) training results in a change in the spring properties o f skeletal muscle. Nine female Sprague-Dawley rats underwent chronic Ecc tr aining for 8 wk on a motorized treadmill. The spring properties of muscle w ere characterized by both active and passive lengthening force productions. A single "spring constant'' (Delta force/Delta length) from the passive le ngth-tension curves was calculated for each muscle. Results from measuremen ts on long heads of triceps brachii muscle indicate that the trained group produced significantly more passive lengthening force (P = 0.0001) as well as more active lengthening force (P = 0.0001) at all lengths of muscle stre tch. In addition, the spring constants were significantly different between the Ecc (1.71 N/mm) and the control (1.31 N/mm) groups. A stiffer spring i s capable of storing more energy per unit length stretched, which is of fun ctional importance during locomotion.