What are the biomechanical principles that are crucial for increasing the efficiency of running?

Introduction:

Running is a main component in the majority of land sports. Netballers run up and down a court, Volleyball players run up to spike, Tennis players run to get to a wide ball and Long Jumpers use a run up before they leap. However running is sometimes thought of a natural process, that everyone knows innately how to perform, after ‘going through’ stages of crawling and walking as a child. Conversely running is a skill, that just like any other ability – needs to be taught, demonstrated, experienced and learnt. Running that is taught using biomechanical principles, can be performed more effectively and efficiently. Learning to run with correct technique is essential to lessen the likelihood of injury, whilst also ensuring minimal energy is lost and thus increasing the efficency of the running movement, reducing the time taken to perform the skill and increasing speed.

This article will focus on the biomechanical principles and consequent technique that is required when running, with the purpose to increase the efficiency of high-speed running. This discussion will provide an understanding of the key biomechanical principles that relate to running, and how they can be manipulated to provide a more efficient running model. This paper will look at Newton’s Law of Motion and The Impulse/Momentum relationship in regard to foot strike, Angular Kinetics and mass distribution of the legs, and lastly Conservation of Momentum and the role of the arms during the running process.  This paper will also provide how an efficient running model can be related to other sporting fields, and how biomechanical principles used in this paper can also be transferred into other sporting codes.

The Answer:

Newtons Laws of Motion, the impulse momentum-relationship and foot strike.

In a study conducted on elite runners it was shown that forefoot and mid-foot strikers had significantly faster average race speed due to significantly shorter ground contact times, which developed strong correlations between foot strike, ground contact time and subsequent speed. This aspect discusses the biomechanical principles behind foot strike in running.

It can be understood from Newtons First Law ‘An object will remain at rest as long as the net force equals zero’, and his Second Law ‘The acceleration of a force is proportional to the net force acting upon it and inversely proportional to the mass of the object’ that to change an object’s state of motion, a force needs to be applied that overcome the object’s inertia. Newtons third law demonstrates that ‘every action, has an equal and opposite reaction’, and in regard to running, when an athlete strikes the ground the Earth, a vertical downward force is applied, and at this instance the ground provides a reaction force that is equal and opposite. It is during this time of force produced by the athlete, and reacted equally by the ground, that accelerates an athlete forward if the force is sufficient for the mass.

Figure.1. This diagram shows the vertical and horizontal forces generated from both the athlete and ground. This length of the arrow indicates not only the direction, but also the magnitude of the force, demonstrated by the length of the arrow.

From Newton’s Third law two ideas can be inferred. Firstly, to have a large amount of force exerted unto an athlete, a large magnitude of force must first be exerted. And secondly, force can be used directionally, and if needed to be produced in a specific direction, the force exerted must be equally and oppositely applied. From Newtons Law’s of motion we can establish that an athlete can strike the ground with variable foot placement and produce forces of different durations in various directions. To increase the efficiency of running, an athlete needs to produce a horizontal force great enough to overcome their inertia. To increase the momentum or velocity of a runner, a force needs to be applied over time, which is called impulse. Essentially, the greater the impulse the greater the momentum, and this is described as the impulse-momentum relationship. A runner needs to produce the largest force for the longest time possible to optimize their momentum.

Impulse during running needs to be applied backward to create a force that propels the runner forward. However Figure. 2 shows that a typical foot strike incorporates force first applied in a forward direction, also described as a braking impulse. Later the runner applies a backwards force that propels them forward, a propulsive impulse. The total foot strike produces the direction and magnitude of force, and therefore the braking impulses should be limited and the propulsive impulses increased.  Upon contact with the ground the foot has some time spent applying forward force to the ground, with the braking impulse dependant on the time and magnitude. As the foot moves to apply force backwards, it is propelled forward by propulsive impulses. In summary to maximise momentum, braking impulses need to be lessened and propulsive impulses increased.

Figure 2. It is indicated that braking impulses first reduce force before propulsive forces increase force.

The position of the foot in relation to the body is crucial for determining the time spent using braking and propulsive impulses. When running, the braking impulse is usually greater when the foot lands further in front of the body, and is reduced if the foot contacts the ground below the body. Elite sprinters are able to extend their hips, as shown in Figure. 3, so that their body travel past their foot during the propulsion phase, and are therefore able to produce force for a longer time. It should be noted here that sprinters are advised to have a short contact time with the ground, and this occurs because of the high velocity of speed they are travelling at, and occurs as a result of their speed, rather than causing their speed. 

Figure. 3. This runner large hip extension allows a larger distance from the foot past the body, resulting in a greater period of time to apply force.

Angular Kinetics, Moment of Inertia and Mass Distribution.

Research has found that the location of greater muscle proximally rather than distally, and therefore using the leg as a pendulum was an efficient running model. Biomechanical principles such as radius of gyration and mass momentum can be used to explain this occurrence.

As discussed earlier, we are able to run forwards due to the force we apply to the ground in the opposite direction. Therefore our running speed is a result of the amount of force we can produce (the impulse) and our bodies ability to frequently repeat this cycle. Therefore the body’s ability to cycle through the running stage with more frequency is essential to increasing the efficiency and speed of running.  This following part of the paper follows the distribution of mass and its relationship with inertia, and how it is most effective for runners to overcome this inertia by manipulation of mass distribution and running technique. This aspect talks about two phases of running – the swing phase (the motion of moving the leg from in front to behind) and the recovery phase (the motion of moving the leg forward again).

During the Swing phase, the leg swings with the hip as its pivot point, and consequently the leg needs to overcome inertia. The moment of inertia describes how mass overcomes inertia in regard to the distance of mass from a pivot point. It has been found that the further in distance mass is located from a pivot point, the greater the inertia that the body needs to overcome. Torque, the moment of force that produces the rotation of the leg, is directly proportional to the acceleration of the hip joint. From these biomechanical principles we can establish that leg rotation in running can be increased if the Torque is increased or the Moment of Inertia decreased.

Keeping mass located closer to the pivot point can decrease the moment of inertia, during the swing phase. As seen in figure 4 faster animals and humans, have greater muscle mass in areas around their hip pivot points. Therefore mass developed at the hip, directly affects the speed and ability of a runner. The recovery phase also needs to be optimised, and completed as quickly and efficiently as possible. This phase centres on overcoming inertia, and to do this, runners need to bend or ‘tuck’ their legs as close to their centre of mass as possible, as seen in Figure. 4. The moment of inertia has been found to be significantly reduced by ‘tucking’ of the leg during the recovery phase. 

Figure. 4. This runner reduces the moment of inertia by flexing or tucking his leg during the recovery phase. 

Law of conservation of momentum and arm swing:

Using the arms whilst running has often been emphasised. However research varies on the how much contribution arm swing has on propulsion during running, with discrepancies ranging from just 1% to 10%. However the majority of research has found that arm swing is able to balance the angular momentum of the legs.  Even though the arms have a much smaller mass than the legs, they are able to balance the legs due to their location and position from the bodies’ centre of mass. This aspect of the paper will explain the concept of momentum and will focus on how runners can deliberately control rotations of body segments to optimise force production, and in particular centring on the role of the arms during running.

The Law of Conservation of Motion states ‘that the total angular momentum of a system remains constant unless external forces influence the system’. During running, the legs are characterised as a cycling movement, and once again due to Newton’s third law every action has an equal and opposite reaction, an opposite ‘reaction’ needs to also occur. During running, when the left leg is in front of the body, and the right directly behind it, the body now has zero momentum and velocity. Therefore, a runner accelerates their left eg both backwards and down towards the ground. However it is in this movement, that the left leg rotates around the bodies’ centre of mass, in an anti-clockwise direction. In the opposing direction the upper body is rotated away towards the right side. And this cycle repeats oppositely when the right leg is cycling in front of the body. This Torque would result in the body moving from right to left during running, an efficient way to move forwards at speed. Therefore the arms can be seen as a counter-balance of these forces. Swinging the right arm from front to back, essential rotates the arms clockwise around the body, causing an anti-clockwise motion of the upper body. However, at the same time, the left arm is swung from back to front which cause the body to rotate clockwise. Therefore arm swing (and it’s affect on upper body rotation) is responsible for conserving angular momentum in a runner by counteracting nearly all the rotations produced by the legs. The contribution of the arms and their angular momentum increases proportionally to the speed of the runner.

Figure. 5. This figure shows how angular momentum of the arms are opposite and equal to the momentum produced in the lower body.

The length and position of the arm is essential throughout each running cycle. During one leg rotation the velocity of the leg changes. The leg is not moving at its highest velocity as it starts its downward and backward movement, instead the angular momentum of the leg increases and peaks at contact with the ground. Therefore the arms must also counter this momentum by also peaking velocity just before the foot contacts the ground. Figure. 6 shows how the arm moving backwards is first shortened at the front of the body, with arm mass located close to the shoulder, resulting in low velocity. B shows how the arm is extended so that the mass of the arm is located further from the body, producing higher velocity. It is at this time the arms and the leg are simultaneously travelling at high velocities. As the leg passes under the body, the velocity is again quite low, and the arms shorten again to reduce arm length and consequent velocity. To conclude this aspect, arm swing rotates oppositely to the legs. The angular momentum of the legs vary throughout the running cycle, and the arms shorten and lengthen to compensate for this change.

Figure. 6. The length and position of the runners arm vary throughout the running cycle, is shortened during moments of lower leg velocity, and is extended at higher leg velocity, to counteract rotation.

How Else Can We Use This Information?

Running is a main component of many sports, and therefore we can use this information of running biomechanics to increase performance in other areas. Optimal running efficency enhances performances such as the distance a long jumper can jump, the height a footballer can jump when performing a mark or the ability of a Netball defender to intercept a ball. However, the biomechanical principles that were used to develop a running model or guide can also be used in areas of skill that don’t contain running.

The concept of Newton’s Laws and the knowledge that the direction of acceleration is produced equally and oppositely to the force applied can be transferred to a range of sports. Examples of this occur in swimming where the swimmer applies some downward force which in turn lifts the body slightly and in ball sports where the force applied produces spin, such as projecting force down unto the ball to produce topspin.

The impulse-momentum relationship and length of time for force application can be transferred to skills such as ball-bat games or activities with strokes such as swimming or rowing. The ability to increase the time force is applied develops greater levels of acceleration, which is why rowers use longer strokes and shot-putters develop rotation techniques that allow them to develop force for longer. The ability to produce the most amount of force in the shortest time is the goal for many sports such as during a tennis serve or baseball hit. 

This paper found that the length of the arm directly affects its velocity, with extended arms increasing the speed. This information can be transferred to bat-ball sports where it is apparent if arms are outstretched (without inhibiting movement patterns) the velocity of the bat is also increased and the adaptation of techniques that resemble this principle can result in an increased ball speed. This information can also be used by pitchers and servers to throw a ball close to an opponent’s body, to reduce their ability to extend their arms.

References:

Blazevich, A. (2013). Sports Biomechanics : The basics: Optimising Human Performance. Retrieved from http://www.eblib.com

Hamner, S., Seth, A., & Delp, S. (2010). Muscle contributions to propulsion and support during running. Journal of Biomechanics. 43(14), 270.

Hayes, P. (2012). Foot strike patterns and ground- conact times during high-calibre middle-distance races. Journal of Sports Sciences. 30(12).

Magness, S (2014). Science of Running. Retrieved 20 June 2014, from http://www.sceinceofrunning.com/2010/080how-to-run-running-with-proper.html.

Myers, M., & Steudel, K. (1985). Effect of limb mass and it's distrubition on the energetic cost of running. The Journal of Experimental Biology, 1(1), 363-373.