Friday, September 18, 2015

The Physics of Aqua Jogging Compared to Running

A large problem with running is the stress that is placed on the runner’s body due to the constant motion and pounding of hitting the ground—it is estimated that up to 85% of runners have been injured at some point. A way that many runners are combatting that are by integrating aqua jogging into their routine as a cross training exercise. The question is how this compares to the physics of running, and how effective it can be as a training technique. The technique is growing in popularity with many collegiate runners training in this way, and even some NCAA champions have used this method. So is it really effective or is it just a fad?

The biggest differences between normal running and aqua jogging are the forces of gravity and resistance. In regular running, the full effect of gravity is felt. The runner is feeling the acceleration of -9.8 m/s2 in the y direction when coming down from each step. This brings in the importance of the runner’s general center of mass, which must be well balanced in order to prevent compensation in other areas of the body and to create minimal impact. When running on land, it is important to have good posture in order to prevent over striding (which offsets the balance of the enter of mass). Injury occurs when the runner is in the support phase of a stride, the time when the runner is on the ground (as opposed to the flight phase when the runner is in the air). So if a runner has certain weaknesses that are made worse by poor posture and a stride that keeps them on the ground for long periods of time, there is a high likelihood for injury. The goal is to run with legs underneath the general center of mass in order to make running as impact free as possible (accelerating without too much effect of gravity). The air resistance in the x direction in regular running is about 1N on a wind-free day.

Aqua jogging provides an option that can reduce body weight up to 90%, thus reducing the normal force that is placed on the runner (because they are not on a surface). Water also provides a more resistive force because it is 800 times denser than air. Thus, aqua jogging replaces the force of gravity with the force of resistance in the x direction, which the body (in lower leg injuries) is better able to cope with. Studies have shown that baseline heart rates are similar when comparing aqua jogging and regular running, it is just hard to get the maximum heart rate when running equivalent to that when swimming. Thus it can act as a good way to maintain fitness, but aqua jogging is not a good method to improve on pacing and speed training for specific races. Aqua jogging reduces the need to balance the general center of mass (since the mass is greatly reduced by the reduction of the normal force). Thus, there is less stress in the y direction of acceleration, which helps the runner to stay injury free. 

With these ideas in mind, aqua jogging is presented as a useful technique for combatting injury. It provides a way to continue training without losing fitness during an injury, and for those who are not injured, it provides a way to maintain fitness without adding an additional day of impact.

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