Why are our muscles limited to 7 Hertz?
According to the book, Principles of Animal Locomotion by R. McNeil Alexander, oscillatory movements in animals are limited by an organism's nature in general to minimize metabolic energy costs. The energy capacity, inertial, compliant, and dissipative forces involved in the muscles, tendons, skeleton, and non-actuating load all enter into this cost equation.

Strictly speaking these structures are very difficult to model with any degree of fidelity, and so are often approximated by simple mechanical lumped parameter systems of masses, springs and dashpots. From these simple systems, differential equations can be written, and the frequency that minimizes cost turns out to be the natural frequency - since at the natural frequency the system has the greatest ability to trap, store and re-use energy for the movement.

Size and compliance are therefore the two parameters that matter most. Highest oscillatory speeds are able to occur in the smallest, stiffest muscles, assuming dissipative forces are small and input energy is available.