The muscle myosin family comprises ~ 12 major isoforms each expressed from a different gene and each used for a distinct physiological role(Schiaffino & Reggiani, 2011)(Golomb et al., 2004). In a striated skeletal muscle, for example, the maximum velocity of contraction is a property of the myosin isoform expressed in a muscle fibre(Pellegrino et al., 2003). Therefore the distinct mechanical properties of each myosin isoform and how they are suited to specific tasks is of broad interest from a structure-function viewpoint. Similarly it is of interest how naturally occurring non-lethal mutations in the myosin motor alter the biochemical and mechanical properties of the motor to cause a range of human myopathies. Using mouse C2C12 cell lines we have been able to express the motor domain of most of the human muscle isoforms and complete a biochemical kinetic analysis of the actin-myosin cross bridge cycle(Deacon, Bloemink, Rezavandi, Geeves, & Leinwand, 2012)(Bloemink et al., 2014). This study has revealed that for individual myosin isoforms the basic cross bridge cycle is adjusted to alter the speed of contraction (by altering the rate of ADP release), the load sensitivity of the cycle and economy of ATP usage while maintaining a low duty ratio (the fraction of the cycle time spent strongly attached to actin) and the intrinsic force/step size of individual myosin motors(Mijailovich et al., 2017). The results emphasise the importance of balancing each primary event in the cross bridge cycle to maintaining an efficient cycle. Our data on has been used to model the complete cross bridge cycle for four major muscle myosin isoforms; fast muscle-2a, slow or β-cardiac, α-cardiac and embryonic myosin. This reveal how speed and power maybe increase but at the cost of a greater rate of ATP usage. Exactly how the amino acid sequence changes between isoforms bring about the changes in behaviour is a current interest. Our biochemical analysis is useful when studying mutations in myosin associated with human disease. For example, mutations associated with major heart disease such as hypertrophic or dilated cardiomyopathy are relatively mild since the heart carrying such mutations can often continue to function well for 30 or 40 years. Understanding how the mutation alters the overall cycle is important to understand changes in the mechanics and economy of contraction. Recent data will be presented utilising human β-cardiac myosin carrying one of 10 distinct mutations; 5 associated with hypertrophic and 5 dilated cardiomyopathies.