Applying Science to Your Coaching
Stabilization of the Spine and the Role of the Trunk in Swimming
During the past few years, the function of the trunk and spine in swimming has gained attention. First, we outline spinal stability at it applies to human motion. In any activity that involves using our extremities to move in space, such as running or swimming, the trunk must remain stable while the arms and legs are used for propulsion.
To accomplish this motion, a transfer of kinetic energy takes place. The energy originates at the stable end of the body and moves up the kinetic chain to amplify the end result. Briefly explained, the body can be viewed as a series of links, or segments, connected by the joints. When functioning effectively, this link system can help transmit the forces generated by the muscles, which are attached to these links. By invoking what is called the segmental interaction principle, the forces acting between the segments of a body transfer the potential and contractile energy generated by the muscles during the motion through the segments, and the final result is an increased application of force. This is what takes place when, for example, a swimmer’s hand is pulled through the water.
How does this apply to swimming stroke mechanics? First, we need to remind ourselves that when floating in the water, ground reaction force is not present. That is, we do not have the luxury of starting out, with the exception of when pushing off the wall or launching off the blocks with our feet planted against an immovable surface. Without this stable base, our ability to impart force is significantly decreased.
A pilot study conducted at our laboratory demonstrated what takes place when ground reaction force is progressively decreased (Prins, 2007). Using high-speed video cameras, a subject was filmed while performing an overhand throwing action using a water polo ball. When measuring the velocity of the ball at the moment it left the thrower’s hand, the results were very predictable. As shown in figure 6.5, the ball velocity was highest when the subject stood on land with her feet in a power stance (i.e., one foot in front of the other). The lowest velocity was recorded when the subject floated in deep water using a flotation vest.
This study confirms what our intuition tells us. In order to exert the highest throwing velocity, we must start with the most stable base we can muster—in this case, standing firmly on the ground. When we attempt to throw while floating unsupported in deep water, the primary base of support is located in the region of the spine and hips, which provides the sole platform from which to initiate muscular forces. This does not imply that the hips should remain immovable. They must, however, provide the necessary base of support to allow for maximum force to be exerted at the other end of the link.
The Biomechanical Implications of Conscious Body Roll
Body roll plays an integral part when swimming freestyle and backstroke because, when swimming these two strokes, the torso rotates around the longitudinal axis of the body. The underlying question is whether hip action is accomplished
1. by conscious, voluntary hip rotation performed either immediately before or during the movements of the arms, or
2. as a consequence of what the arms are doing (i.e., during the hand entry and arm extension, the torso begins and continues to roll without additional conscious motion).
And—this is important—the trunk ceases to roll at the conclusion of the extension phase, immediately before the initiation of the catch phase of the pull.
As discussed previously, when floating on the surface, no ground reaction force is available. In the absence of firm footing, the trunk has to provide the stable platform for the muscles that control both the upper and lower extremities to generate propulsive forces.
We are not suggesting that the hips be held in a rigid, fixed position during swimming because holding any part of the body in a relatively immovable position is clearly unwarranted. Furthermore, voluntarily contracting the anterior abdominal or posterior trunk muscles with the intent of preventing the rotation of the trunk is unnecessarily fatiguing and will interfere with rhythmic, bilateral movements of the arms and legs. Observing the longitudinal motion of the hips in elite freestylers from an underwater lateral view demonstrates the extent to which the torso rotates around a longitudinal axis.
When observing elite competitive swimmers—competing in events ranging from 50 meters to 1,500 meters—it becomes evident that the shorter the event, the flatter and more stable the body position. This reduction in body roll is not performed consciously but rather stems from the need to apply more powerful muscular pulling forces when pulling in an attempt to swim faster. Based on this evidence, we can state that it should not be a question of how much a swimmer’s hips should roll but of how much emphasis on active hip rotation should be recommended. Our contention is that recommending conscious rolling of the hips and torso during the course of a stroke cycle is not conducive to the optimal application of force.