Running
In rugby all players must run to move from phase to phase, to carry the ball, to catch an opponent or to reach a ball that has been kicked. Many of those actions of running are done at relatively high speed if not flat out. Generally the player is accelerating as the distance run between phases is less than the distance needed for an athlete to reach maximum speed. Accordingly the action can be compared to sprinting even though it is seldom identical to track sprinting.
In general any running action can be broken into three phases:
A take-off phase where the centre of mass is ahead of the point of contact of the foot and the ground
A flight phase where both feet are off the ground
A landing phase where the foot is ahead of the centre of mass
These can be related to the action of the leg in driving the body forward and itself moving behind the body, recovery where the leg is in the air and returning in preparation for the next stride and support where the foot lands and arrests the downward motion of the body.
Whenever the foot is in contact with the ground it experiences reaction forces against its movement and against the weight of the runner.
The forces are split into the normal reaction force upwards and a frictional force which may be horizontally forward or backward depending on the direction of movement or tendency to move of the foot at that instant. Generally once the foot is in contact with the ground it is stationary relative to the ground, but is subject to forces from the leg tending to move it forward or backward depending on the phase. The friction with the ground reacts against these producing propulsive forces on the body.
In the take-off or driving phase (as shown) the horizontal reaction force is greater than the air resistance and the body accelerates forwards. The centre of pressure of the air resistance, however, acts above the ground so that it exerts a backward torque about the point of contact. A sprinter who tried to run while perfectly upright would fall over backward. To balance this torque, the runner leans forward so that his weight is ahead of the point of contact supplying an equal and opposite torque about the point of contact.
In the flight phase the air resistance is unbalanced by any driving force from the track so that the body slows slightly.
When the foot again makes contact with the ground it is moving forward relative to the ground and is generally ahead of the centre of mass. The horizontal reaction force is thus backward tending to decelerate the body. The greater the forward reach of this leg the greater the decelerating component will be. The backward lean that results from this will give a backward torque about the point of contact reducing the angular momentum of the centre of mass about the point of contact. That angular momentum is, however, so large that it rotates the body rapidly into a forward lean where the driving phase may be repeated. A ”pawing action” may be used in an attempt to create a forward reaction force at the instant of contact but this requires the foot to be travelling backward at the same speed as the runner is moving forward and reduces the contact time. The contact phase is of course required to arrest the vertical motion and requires time to do this without excess force on the joints. When running at a constant speed, the horizontal reaction force is just larger than the air resistance so that it can accelerate the body sufficiently to recover the energy lost in the support phase.
In track sprinting only speed is relevant so that the sprinter will move to the most efficient position for the recovery and driving phases. In this situation the body is substantially upright, the knee is fully flexed to reduce the moment of inertia during recovery and the knee and hip reach near full extension at the end of the driving phase. In rugby on the other hand the runner must be prepared for body contact which tends to require some lean to the body, ground contact for as long as possible and a reasonably wide base. This can be achieved if the runner maintains relatively large knee flexion, shortens the stride and keeps the centre of gravity relatively low.
In both the above situations the direction of the horizontal reaction is in the direction of running to maintain straight line motion. The bipedal gait does mean that the force is not directly through the line of the centre of mass and body rotation is induced to counter this with the aid of the arm swing and movement of the trunk. The rugby player carrying a ball has less scope for arm action and may have greater upper body reaction. It is also necessary for the rugby player to be aware of surrounding action so that the upper body is rarely maintained in a fixed attitude relative to the vertical axis.
