What is leg drive?
The first link in the kinetic chain motion of the serve, using the ground reaction forces as the player push off the ground and its efficiency depends on the coordinated and synchronized extension of three joints (starting from the ankles to the knee joints and then to the hip joints). Successful synchronized extension has crucial role in creating the linear momentum upwards enabling player to increase the impact height, enabling the transfer of angular momentum from the legs to hips and shoulders and increasing the length of the forward swing through the ball, making it longer and therefore more efficient. And it has been proven that the efficient leg drive place less load on the shoulder joints in the external and internal rotation phase.
Foot stance and the leg drive
The foot stance and the force production (leg drive) can be different depending on the feet positioning before the serve motion, but the basics are similar. The most important aspect of the efficient leg drive is that the player’s foot positioning should provide the player stable base of support. In simple words, this means that during the preparation of the serve and then while landing and during the hitting phase player should be coordinated and balanced.
When tennis players are pushing against the ground with their toes to create the reaction force, their feet can be in two different positions: feet apart (foot back) and feet together (foot up) position.
1. Feet apart technique, in which back foot stays in the initial serve position, the player has a wider base of support and this allows the player that with the greater squat depth, create greater ground reaction force, so this technique requires greater knee joint extension. Front foot acts as the strong post, following the rear foot upwards in the acceleration phase but blocking the left side of the body in the hitting phase which allows the rotational momentum of the hitting hips and shoulder forward. Back foot is pushing the body upwards (off the ground) enabling the hitting hip and shoulders to be above the non hitting ones in the hitting phase. The very important consequence of the rear leg extension is that allows the hitting arm (while holding the racquet) to be drawn more downwards. Body going upwards while the hitting arm dropping downwards at the same time, creates the situation (opposite reaction force) where the arm is getting further away from the players back. In this way the player can create much higher racquet acceleration, protecting the shoulder joints.
2. Foot-up technique, in which the back foot advances to join the front foot for a push-off, deploys less ground reaction forces, but generates higher vertical ground reaction forces. Front foot acts as the strong post, while the back foot coming up and close is leaning forward bending the knees, with enhanced ground reaction towards the higher impact point.
Consequences/signs of the inadequate leg drive
1. Ankle joint
As the lower body center of balance is in the ankles and the leg drive begins with the healthy ankle joint extension, it is very important that the player has a balanced stance. Pressing against the ground in the unbalanced position can result in the complete unbalanced and uncoordinated serve motion. Player pushing against the ground with the toes, rather then the whole foot will always be creating much efficient ground reaction force, which is a necessary condition for the efficient leg drive movement.
2. Knee joint
Pressing the toes against the ground will automatically lead to having knees bent forward, which should be a condition to perform the efficient squatting movement. Knees bent away (or inside) from each other due to the foot movement in the loading stage can seriously disturb the kinetic chain and efficient transfer of energy from the ground (ankle joints) up to the hip joints.
3. Hip joint
Starting from the back leg, around 80% of the body weight should be at the back and then slowly transition to the front. If the hips rotate while legs going down or if there is not efficient hip rotation and the body weight is equally divided between the front and back foot, the transition of energy is very low and inefficient from the leg drive. Pushing the hip front can lead to injuries as the weight is on the front. Pro players do bring hip up but it’s a consequence of the back hip turn and placing the weight on the back leg. Therefore back hip drops so the front hip ends up moving forward and up as the opposite reaction what is happening at the back.
Next blog: Serve preparation phase (storing the energy)