It’s quite simple really, muscle fibre contractile force is in effect limited by the rate of hydrolysis of ATP at the actin to myosin interface, bottle necking the actin myosin cycle, this is what causes the force production differences between slow and fast twitch muscle fibre, the myosin atpase variant of fast twitch is more catalytic than the slow twitch variant, to produce muscle fibres that produce peak forces multiples stronger than natural muscle, simply replace the last phosphate group with a more reactive chemically analogous group, vanadate or arsenate both would work, modify the myosin head to only bind to a specific unnatural nucleoside triphosphate molecule rather than atp and modify some of the mitochondria within cells to attach vanadate to the nucleoside diphosphate, this results in faster stronger muscles and the energy transport molecules are specific to ONLY that one biological function all other functions remain unaffected.
5-10x specific force production would be a result