Experimental studies on mouse slow and fast twitch muscles

Abstract: This thesis deals with physiological and mechanical properties of fast and slow twitch mouse muscles. It discusses isometric, concentric and eccentric contractions of mouse extensor digitorum longus ('EDL') and soleus ('SOL') muscles. This project primarily investigated the behaviors of muscles, to give better understanding and improved descriptions for the human system, when subjected to impact or sustained high loading conditions.Muscle force has been shown to be length and activation dependent. The effect of passive or active length changes on muscle force production was studied. Isometric activation showed a maximal force at optimum length for each individual muscle, to which all experiments were related. This optimum length was stimulation frequency dependent and maximum produced force shifted towards shorter length with increasing frequency.Active shortening of maximally stimulated muscle was shown to produce reduced force, but also a reduced isometric force (force depression) following shortening, regardless of the shortening conditions and the method of muscle stimulation.Steady state force depression (ΔFstdep) was correlated with the instant force depression ΔFidep, pre-activation time, t0-t1 and the work (WS), performed by the muscle during shortening. It was positively correlated with the ΔFidep when the shortening magnitude was varied ('VSM'). But in varying the shortening velocity ('VST'), it was negatively correlated with the instant force depression.In active stretch, force was analogously enhanced during stretch, and this effect remained after stretch. Steady state force enhancement (ΔFstenh)following muscle stretch was correlated with the instant force enhancement,ΔFienh, pre-activation time, t0-t1 and the work (WL), done on the muscle during stretch. It was positively correlated with ΔFienh when the stretch magnitude was varied ('VLM'). But in varying the stretch velocity ('VLT'), ΔFstenh was negatively correlated with ΔFienh.Furthermore, the rise time constant, (Tr) of redeveloped isometric forces following theshortening and the fall time constant, (Tf) of the relaxed isometric force following muscle stretch were calculated. In VSM, Tr and Fstdep were positively correlated with each other, while in VST, they were negatively correlated. Tf and Fstenh were negatively correlated in both VLM and VLT.