Date of Award


Publication Type

Master Thesis

Degree Name




First Advisor

Matthew P. Krause


Cardiotoxin, Endurance, Exercise, Injury, Mouse, Muscle




Skeletal muscle fibre types are distinct in their physiological, structural, and metabolic properties and are each associated with a specific myosin heavy chain (MyHC). They exist on a continuum of pure and hybrid fibres (I↔I/IIa↔IIa↔IIa/x↔IIx↔IIx/b↔IIb) which allows for a vast versatility of function. Each muscle has a unique fibre type composition that by adulthood is well established, though it has been shown that endurance exercise can induce a shift from type IIb MyHC toward type I MyHC expression. Skeletal muscle also has the capacity to regenerate and restore its structure and function in response to injury. Interestingly, fibre type composition can change dramatically following severe injury in skeletal muscle. The interplay between endurance training and the regeneration process on fibre type composition is largely unknown. Thus, this study aimed to elucidate the effect of endurance exercise training closely following injury on fibre type composition. 12-week-old C57BL/6J mice were endurance exercise trained, via treadmill running, for 40 minutes, 4 times per week, over 2 weeks. Fibre type specific MyHC (type I, IIa, IIx, and IIb) were compared between sedentary and exercised mice at 14, and 50 days following cardiotoxin-induced injury. Immunohistochemical analyses were preformed to assess fibre type specific MyHC positive area to determine changes in fibre type composition. Results indicated that 14 days of endurance exercise training did not affect the fibre type composition of the newly regenerated fibres in the predominately type II tibialis anterior or plantaris. However, there was a significant increase in hybrid type IIa/x fibre positive area in the soleus at the end of the 14 days of exercise training. Though, the observed changes in the soleus returned to baseline after 36 days of sedentary behaviour. Thus, it appears that the interplay of injury and endurance exercise on fibre type composition is dependent on the muscle group.