There are essentially 3 ways to hit harder as a volleyball player: improve lower body power, improve upper body power, and improve core stability, which is what we are going to look at today. In a nutshelll, a stable core will allow for an efficient transfer of power from your lower body to your upper pody. Do not miss this part - core stability is much more then just doing thousands of crunches, bicycles and v-ups. It's an entirely different concept that is a little tricky to understand. Let's start at the feet and work our way up so you can understand this. The ankles are meant to be mobile, meaning they should be able to move in all different directions. If you have heard people say they need to improve ankle mobility, this is what they are talking about. The big concern about ankle injuries is scar tissue building up and preventing some of this movement. Moving up to the next joint, the knees are meant to be stabile - they shouldn't move in too many directions. If they have excessive movement, then we're going to have some problems. Then we get to the hips - they are supposed to be mobile. When you see track athletes going over and under hurdles and doing all sorts of crazy leg swings and stretches, this is what they are working on. The hips need to be able to move - they are what generates most of our power in most of our movements. This is key in understanding core stability because the core is the next link. The core should be stable because it is what supports the spine and what the upper body relies on to produce power. Finally, the thoracic spine and shoulder should be mobile. We need a large range of motion in the upper back and shoulder in many sports we play (including volleyball).
Alright, that might sound a little confusing to some of you volleyball players/coaches. What's important to know with regards to hitting harder, is that the hips (and pelvis) are going to generate power, the core (being very stable) will transfer it to the upper body, which results in a hard hit ball. The hips are generating the rotational movement, the core is simply staying stable and allowing the force to travel up. Remember, the muscles of the hips attach to the pelvis and spine; the abdominal muscles attach to the pevlis and the spine. This is how force is transferred. This is called a kinetic chain - different parts of the body make up the chain. Energy generated by one part is transferred to the next. For example, hitting a ball hard requires coordination of a lot of different systems. Jumping off the ground explosively is just a piece of the link - the arms swing to initate the movement, hips rotate to generate force, arms cocked to start arms swing and so on. When there's a weakness in a certain area, the transfer of energy is impeded and some of it is lost. A weakness could be a physical weakness - such as the core not being stable, or it could be mechanical - as in the arms aren't swinging properly, or even mobility issues - the hips aren't mobile enough to create force. Just glance at the pictures below and see how everything is connected. Really focus on how the muscles of the lower body (glutes, ilopsoas, hamstrings, quads...) attach onto the spine or pelvis and so do the abdominal muscles.
The hips and core are inseparable. Let's look at an athlete hitting the ball. Newton's Third Law of Angular Motion sates that in an airborne body, the sum of the torques must remain constant. This means that the hips and the upper body have to balance eachother out. When the upper body is rotated backwards, the hips must come forward to create balance. When an athlete is mid air, the shoulders are rotated backwards so they are sideways to the net, and the hips rotate forward to face the net. This keeps the torque balanced. Core stability is key here because it's role in the kinetic chain is to prevent buckling of the spine and to return it back to equilibrium following pertrubation. That's a fancy way of saying that the core is protecting the spine from doing too much work and breaking.
Check out the picture below. During a hit or jump serve, the back is hyperextended during the peak height of a jump. The hips are also hyperextended and the knees are maximally flexed to maintain balance. The hyperextension of the trunk causes the rectus abdominis (your 6 pack) muscles to stretch. Remember, core stability means returning the body to equilibrium. So this massive stretch on the abs causes it to contract and pull the upper body forward. The same is true with the medial rotators of the shoulder and elbow extensors - they experience a massive stretch before they contract and make contact with the ball.
What does all this mean? The hips are going to generate massive amounts of force when jumping off the ground. As a volleyball athlete is airborne, body positioning is vital to be able to transfer power from the lower body, through the core, to the upper body. The rotation of the hips just prior to contact and at contact allows the force to cross through the abdominal muscles, through the upper body, and into the ball. Having a technical volleyball coach who can teach proper body positioning is vital to hitting the ball well. Having a strength coach who can teach proper power production, core stability, and shoulder health is also vital to hitting hard. Having the combination of coaches, plus hardwork.... that's a deadly combination. Next in the series, we will look at how increasing upper body power will help you hit harder.