Loading consists of the engagement of the major muscle groups to create the most efficient kinetic chain movement while keeping the dynamic balance. What does this mean?
THE MORE YOU CONTROL YOUR BODY, MORE YOU CONTROL YOUR SHOTS.
Definition of balance is the ability to stay in control of body movements. Dynamic balance is the ability to stay in control of body movements while the body is in a constant movement. The base of a efficient tennis game is a maintenance of the body control while moving, positioning to the ball, accelerating towards the ball, decelerating while getting close to the ball, loading, following through the stroke and recovering.
Two very important elements of the body control through the dynamic balance are the posture and a wide base of stability (support), which I wrote about in the previous blogs.
Efficient loading relies on the posture and base of stability. Loading with the trunk and legs demands very stable body posture and at the same time a wide base of support as loading the legs lowers the body and it’s center of gravity.
Legs and hips loading
The wide base of the stance enables the player to load the legs major muscle groups. 
As loading is placed on the one (outside) leg, the wide stance is NEEDED for the player to keep the balance at the loading stage and stretching and li adding the muscles in hips, quadriceps and calves. From this position a player is ready to transfer the powerful energy from the legs to the hips, what is called a LINEAR MOMENTUM.
Upper body loading
Keeping a good posture helps a player to coil and uncoil the upper body. The shoulders usually turn (rotate) further then hips stretching the trunk, back and abdomen muscles and load the upper body. For the top tennis players their trunk rotation is the engine that drives the strokes creating the flowing motion, hitting the ball at the ideal contact point while creating high velocity shots, as a speed of trunk rotation is very much correlated with the racquet velocity. This is usually referred as a TORQUE (ANGULAR MOMENTUM). Top players are rotating the shoulders from the backswing to the full follow through by 180 degrees!
At this moment, a player is fully loaded and ready to uncoil (rotate) and transfer the energy through the kinetic chain, from the legs to the hips (linear momentum) and then hips to the shoulders and wrist to the contact point (angular momentum).
Play Smart Tennis - MIND YOUR GAME 