Core as an important part of kinetic chain
The hip musculature generates the majority of power for the successful stroke. In simple words, if the player uses the wrong sources of power, most likely that she/he will lose the stability and balance during the acceleration phase. As mentioned in previous blogs, the source of the power comes from the ground, and its forces. These forces are transferred from the ground to the hip muscles and core, and at this point is where rotational and linear forces work together. The transfer of energy in linear and rotational mode wouldn’t be possible without a“stiffened” core and serape musculature.
Stiffness of the core and serape
Stiffness is the essential precursor to stability and the efficient transfer of forces, together with being one of the key factors of the injury prevention.
How does core stiffness enhance the speed and strength of the torso rotation and how does that contribute to acceleration of the hitting arm in tennis?
At the loading phase, there is a twist of the core due to the winding up of the hip. In simple terms, the hitting hip is rotating back and away from the stabile non hitting shoulder. The elastic muscle forces are created in this way, via the transverse rotation of pelvis and shoulders creating a diagonal stretch. This stretch works on the bases of the stretch shortening cycle – “rubber band effect”. In simple words these forces act as an elastic band. The more the band is stretched, the more potential energy can create.
From the moment of the cocking phase, or the phase of complete loading, to the moment of acceleration, the core is a translator of the power from loaded hips to the shoulder and arm, all the way to the follow through. For core to do this action efficiently, the core has to be stiff. The stiffness of the core enables winded and loaded hitting hip to unload into controlled and balanced follow through. When stiffened, the core can provide enough stability of the non hitting side, for the hitting side to accelerate through the ball.
Core muscles working together
Without the contribution of the diaphragm, the spine can not fully stabilized and in that case a player has to rely on other (peripheral trunk) muscles to support the motion. This kind of muscle functional redistribution is limiting the spinal stability and efficient transfer of energy. In this case, the peripheral muscles, which are most suited for the force production and energy transfer, need to help out in providing stability to the moving body, so the player can not utilize those muscles in full their potential.
Serape as the main source of creating the acceleration forces
Posterior and anterior serape
The player is using the posterior serape for the loading and anterior serape for the unloading. A player loads activating the muscles of the posterior serape (posterior diagonal musculature from the power leg (power hip) to the stability shoulder).
Using the posterior serape, a player can generate the backward (away from ball) momentum, which is considered as loading momentum. Loading momentum is achieved using the muscles of the loading leg and hip (gastrocnemius, soleus, hamstrings and gluteus). At this point as the shoulders rotate as well, the muscle used is the non hitting side latissimus (back) muscles.
Anterior serape presents the diagonal musculature from right (hitting) shoulder to the stability hip (and foot) (left foot in forehand, right foot in backhand in right handed players). At the acceleration phase, both hips accelerate using the serape musculature (serratus anterior, internal and external oblique) with the support from the hip flexors and adductors of the non hitting side.
This support helps stabilizing the acceleration and creating the rotational momentum. The non dominant foot and hip have to stabilize to enable complete transfer of forces (forward momentum) by the hitting hip forwards. Stabilization is created by planting the (non hitting side) foot against the ground, which gives solid stability to lower limbs and hip. At this point stabilization is achieved using the leg and hip muscles of the non hitting side (gastrocnemius, soleus, hamstrings and gluteus).
Complete hip stabilization is achieved with the help of non hitting arm which begins tucking in towards the hip when the acceleration begins and it’s completely tucked in during the contact point and forward follow through motion.
Serape muscles
The muscles associated with the serape effect are the rhomboids, serratus anterior, external obliques, and internal obliques (picture shows original Logan&McKinney serape concept). 
The Serape diagonal musculature is crucial in translating the ground forces from the lower extremities to the upper extremities. Serape muscles accept the ground reaction forces to create efficient rotational movement of the torso. The translation of the forces from the loading phase begin from the ground up, when the serape musculature due to the kinetic chain transfer of energy, accept the energy from the gastrocnemius, soleus, hamstrings and gluteus and transfer it in a rotational momentum during the acceleration phase forward.
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