The racquet swing phase is the consequence of the efficient previous loading and cocking phase of the service kinetic chain motion. In the cocking phase, the elastic energy is stored in the major slow moving muscle groups, and through the acceleration phase, this energy is released through the faster moving muscle groups to the ball.
Leg drive: As the acceleration begins from the ground up, the leg drive is the crucial element of the efficient movement (leg drive explained in details in the past blogs). Efficient leg drive enables hips to elevate, so the back hip achieves the higher vertical velocity then the front hip, which will enable the shoulder over shoulder rotation. This linear momentum from the leg drive in combination with the angular momentum of the hip and trunk rotation creates the most efficient serve motion.
Feet and ankles: The player is creating the energy from the ground up, pushing against the ground with the toes. The front foot, front leg and hip are providing body the stability needed for the efficient trunk rotation in the angular phase of the acceleration. The energy for the powerful vertical velocity is stored mostly on the slow back leg muscles, supported with the back foot and ankles. So in simple words, front side is the stability side and the back side is the power side.
How does the energy transfers through the ankles? The strongest movement of the ankle or foot is a plantar flexion, as the larger muscle mass is contributing to the movement. Plantar flexion is used to propel the body forward and upwards, and that’s exactly needed for the powerful but stable acceleration.
Knees: More powerful knee extension produces much faster acceleration phase. The producer of the extension at the knee is the quadriceps femoris muscle group, which is one of the strongest muscle groups in the body. Increase in the knee flexion and extension is being associated with creating higher racquet velocity forces and with placing less load on the hitting shoulder. Peak extension is achieved at 50-70 degrees of knee flexion.
Hips: From the leg drive the energy is transferred into the hips and it’s powerful muscles (gluteus). The back leg drive drives the back hip upwards and then forward into the angular movement to the impact zone. As the back hip was in the lateral tilted position, this hip contributes, together with the back leg drive, to the rapid transfer of the energy to the trunk and the back, hitting shoulder. With the trunk uncoiling, the back hip aligns with the front hip just before the contact point (if this happens long before the contact, the kinetic chain energy transfer is inefficient).
Trunk: The acceleration phase is the phase in which the trunk muscles are getting used the most. Trunk rotation contributes with around 10-20 percent of racquet speed at the impact.
In order to generate the maximal amount of angular momentum possible for an effective serve, there is a necessity for a fast, rapid movement of the trunk, which is in fact uncoiling. The efficiency of the angular rotation of the trunk is directly dependent on the efficiency of the hips and shoulders rotation. This angular momentum is created in combination with the linear momentum, maximizing the usage of the leg drive.
Shoulders: The upper arm is involved in explosive inward rotation prior to ball impact, which places great deal of stress on the shoulder region. The most efficient consequence of the leg drive is that it forces the racquet in a downwards direction. Direction away from the back of the athlete. As the shoulder is in an external rotation in the cocking phase, the leg drive gives an additional degree of the external shoulder rotation as racquet and arm are pulled downwards by gravity rather then muscle force. This (healthy) additional external rotation has for a consequence more efficient and explosive internal rotation of the shoulder towards the impact zone. At the same time, the player creates an extra distance from the tip of the racquet to the contact point before the forward rotation of the trunk. Longer distance = higher torque.
As the hips muscles begin the trunk rotation, the shoulders are following the motion and hitting shoulder from the external rotation is rotating internally through out the acceleration phase(from away of the ball to the contact with the ball).
Very important aspect of the shoulder rotation is the cartwheel rotation, shoulder OVER shoulder. In the acceleration phase the hitting shoulder moves above the front (stability) shoulder, creating the rotation up and forward rather then just forward. This movement is a crucial element of the efficient wrist flexion at the contact point (whip motion). Left shoulder is very stable and enables the efficient rotation of the hitting shoulder forwards to the impact zone.
Elbows: During the acceleration phase there is high triceps activity as the hitting elbow is extending. Hitting elbow joint contributes around 15% of the force produced during the tennis serve. Left elbow has a very important role in the acceleration phase as it initiates the forward trunk rotation (quite similar to the forehand stroke).
Forearm and wrist: In the cocking phase forearm should be in the supination as adds additional rotation to the ball. But that’s not the main reason for the importance of the supination. The powerful pronation is the result of the supination. Forearm supination is closely connected with the efficient wrist flexion (whip action). Research confirms that maximum torque for the wrist flexion is highest in the supination position. As whip action contributes at around 40% of the power of the serve and fully controls the direction of the serve, we come to the conclusion that the supination of the forearm represents a very important aspect of the serve. Throughout the acceleration phase, the forearm rotates towards the pronation while shoulder is rotating internal and inwards, wrist is moving towards the impact zone from the position of the full extension to the powerful flexion.
Next blog: Follow through and landing phase