An improved power transfer mechanism for use on a human-powered machine, adapted to be drivingly connected to a power take-off device of said machines, includes: a housing adapted in use, to be disposed within a frame of said human-powered machine; at least one crankshaft adapted to be disposed within said housing; respective left-side and right-side crank arms each adapted to be drivingly associated with an end of said at least one crankshaft; and drive transfer means adapted to co-operate with said at least one crankshaft and each associated crank arm, the arrangement being such that a first member of the group comprising and left-side and right-side crank arms and said power take-off device is adapted to be attached to said at least one crankshaft, such that each of the second and third of said members is adapted to be rotatable relative to said first of said members, and each other, only to a limited and controlled degree, whilst not be movable axially or in a longitudinal direction relative to said at least one crankshaft.