Growth induced by incremental static stretch in adult rabbit latissimus dorsi muscle

Incrementally applied static stretch over 3 weeks resulted in a 72% increas e in the weight of the in situ latissimus dorsi muscle in rabbits. True gro wth rather than tissue oedema was confirmed by increases in the protein con tent (130%), the cross-sectional area of the type I fibres (30%) and the mu scle length (i.e. number of sarcomeres in series increased 25%). Despite an increase in the proportion of fibres staining positive for the enzyme succ inate dehydrogenase (SDH), the myosin ATPase stain showed no appreciable fi bre type transformation. While total power output in the stretched muscle w as unchanged, its maximum mass specific power output, as determined by osci llatory work loops, was decreased by 50%. The cross-sectional area that was occupied by connective tissue increased from 15 to 19% in the stretched mu scles, with a concomitant increase in passive energy dissipation. Some incr ementally stretched muscles were then allowed an additional 3 weeks of main tained stretch to determine whether the adaptive changes would be preserved or reversed. Previous gains in muscle weight, length and area of type I fi bres all remained. In contrast, the connective tissue content and the passi ve properties returned to control values during this period.