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

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