Feature

Swimming for Success

Physics can tell us who will swim faster.

Toshimasa Yanai

When you perform front-crawl swimming, your torso rolls from one side to the other, synchronised with your arm movements. The torso always rolls in the direction so that the shoulder of the recovery side moves upward as the recovery arm is lifted out of the water. As the recovery arm enters the water, the torso starts rolling toward the other side. This rolling action of the torso is called bodyroll.

How do you bodyroll? You may say that it is a natural phenomenon or that it is the result of certain learned skills. Mechanically, such a periodic rotation of a torso can occur only when the torso is pushed by an external source(s) to generate the roll. The mechanical quantity that describes the magnitude and direction of such a push is called torque.

There are two distinct methods to generate such a torque. First, by pushing the water with the arms and legs. As you push the water in one direction, the water, in turn, pushes you back in the other direction in accordance with Newton's third law. When this torque is transmitted through the arms and legs and reaches the torso, the torso rolls. In this case, the mechanical cause of the bodyroll can be stated as "push by the water".

Second, the torque can be generated by the muscular effort made at shoulder and hip joints. As a skeletal muscle runs across one or more joints and attaches to two or more bones, a contraction of a muscle pulls the attached bones toward the centre of the muscle. For example, a muscle called the hip flexor, which runs across the front part of the hip joint connecting from the torso to the thigh, can be activated to move the thigh forward (such as when raising the legs). The same muscle can also be activated to move the torso forward (as when performing a sit-up).

If these exercises are performed in a weightless situation, both the torso and the thigh move toward each other by rotating about the hip joint. This phenomenon can be described as the angular equivalent of Newton's third law. It means that when one part of the body is rotated in one direction about a joint, the rest of the body rotates in the other direction about the same joint.

You can feel this by quickly moving your outstretched arms toward the right while sitting on a freely swivel chair. The movement of the arms to the right causes the rest of the body to move toward the left. This mechanism can be used to bodyroll by moving the arms and/or the legs in the direction opposite to the intended direction of the bodyroll. In this case, the mechanical cause of the bodyroll can be stated as "muscular effort."

A study was conducted to determine which mechanism was primarily used by elite swimmers to generate bodyroll. The performances of the swimmers were recorded using two periscopes to provide information on their three-dimensional movements which could be computed later using mathematical formula. The torque due to the "push by the water" and the torque due to the "muscular effort" were calculated on the basis of the Newtonian mechanics involved.

The results clearly indicated that the "push by the water" was the primary source for the bodyroll. The contribution of the muscular effort was found to be small. This suggests that the swimmers consciously or unconsciously push the water to generate the torque to bodyroll.

How do the swimmers generate the torque? Further mathematical analysis was conducted to identify specific source(s) for the torque. The torque was divided into many components of signals at various frequencies and those signals that match the movement pattern of each body segment were determined. The results suggest that two major sources of the torque were the buoyant force and the kicks.

The buoyant force, which pushes you upward in water to help you float, acts through the geometric centre of the immersed part of the body (called the centre of buoyancy). When the body is streamlined and floats horizontally on the water surface, the centre of buoyancy is aligned vertically with the centre of gravity through which the weight acts, and thus has no effect on bodyroll. However, when an arm is lifted out of the water and not subject to the buoyant force, the centre of buoyancy shifts in the direction away from the arm. Because the centre of buoyancy no longer aligns vertically with the centre of gravity, a torque is generated. This can be observed during the recovery of each arm. The push by the water in this mechanism is virtually effortless and available whenever you are floating on the water surface. It is certainly an efficient method to generate bodyroll.

Swimmers put all their efforts into swimming fast. The forces that propel the body must be directed forward; in contrast, the forces that generate bodyroll must be directed up-and-down or side-to-side. Generating bodyroll through the buoyant force and kicking might improve the efficiency of the performance by sparing the arms from the effort of generating the force in non-propulsive directions. As the major propulsion in front-crawl swimming comes from the arms, the arms should be used entirely to generate the propulsion. An effective use of the buoyant force and the kicks for bodyroll might be a key for a success.

Dr Toshimasa Yanai lectures in the School of Physical Education at Otago University.