Who are some the more successful tennis players that have been more heavily muscled? Especially in the legs and glutes?
Not a very well informed view of the muscle cell but -
In terms of the stretch shortening cycle, the Hill Muscle Model and the microscopic description of muscle cell function, the squat involves mostly actin & myocin active muscle shortening. There may be some stretch used in some squatting techniques at the bottom.
The actin-myosin active shortening appears to be slow in this animation.
Illustrations - Actin, Myosin & Titin
Besides the actin & myosin in each muscle sacomere there is a giant protein molecule, Titin, that stretches. Titin has been researched since the 1990s(?) for its passive shortening properties from a pre-stretched state. I believe the current belief is that stretch occurs within each muscle cell because of Titin and is not a property of an overall muscle-tendon spring. For muscle shortening biomechanical researchers study Force vs Velocity of both the active & passive components of muscle shortening. My interpretation of references is that the active shortening provided by the actin & myosin may fail to provide force at high velocities while the passive shortening provided by Titin can still provide force.
Squatting probably more trains the Actin & Myosin for slower movements. Plyometrics probably more trains the Titin for faster movements.
Maybe since the legs in tennis are used mostly for slower motions, such as leg thrust on the serve, maybe squats are useful.
For the faster arm motions maybe the Actin & Myosin training is less important.
Resistance bands can be used at higher velocities where, for example, the mass of dumbbells makes them difficult to stop at higher velocities.