Last Updated on November 23, 2008 by Jimson Lee
Sprinting: A Biomechanical Approach By Tom Tellez.
Scientific Overview and Common Errors
Part 1 was presented yesterday
At the start, mass is at rest. The property of inertia inherent in the mass resists changes in velocity (a vector with magnitude and direction) and according to Newton’s first law will maintain this velocity unless acted on by an external force. The amount of inertia within the mass is proportional to the amount of mass; objects with more mass require more force to move initially. When the sprinter applies force to the blocks, inertia is overcome and linear movement begins.
Stride length and frequency are low because of inertia. This is why the first stride is the slowest and shortest. Force is initially applied backward and downward where the shin angles somewhat close to the ground and behind the knee. This motion is piston-like as it pushes the mass forward from its rest state. Horizontal impulse is greatest at this point (Weyand et al. 1991) but diminishes as mass increases speed. It is the net force that causes the mass to accelerate: the difference in mass forward force and the pushing force. At the start of a run when resistance is at its highest, it is easier to increase stride length because force needs something to work against.
Shortly after starting, stride length and frequency increase. As momentum builds, stride length tends to stabilize as momentum builds, resistance diminishes and net force approaches zero. Concurrently, frequency increases because resistance constantly decreases.
In short sprints, it is important to accelerate over the longest possible distance in the shortest amount of time and try to maintain and decelerate gracefully. After the athlete has been successful in setting the body in motion, and the center of mass begins to gain momentum, a toppling would occur if the feet continued to go backward and downward. The acceleration process then goes through a mechanical transition where the legs go through the form of cycling motion. The lever lengthens to give a mechanical advantage and shortens on the recovery phase to give a speed advantage. The position of the body dictates how reactive forces affect the center of mass.
The center of mass accelerates at a decreasing rate until maximum velocity (top speed) is attained. At max speed, the net horizontal force applied is zero. The ground tends to recede faster when the body picks up speed, resulting in reduction of contact time as well as effective force because of reduction in resistance. At this point in the race, the athlete tries to keep the effective force at zero until the net horizontal force becomes negative. When body momentum becomes greater than that of the legs, the body starts toppling over, causing a braking action each time the foot hits the ground. At this point, the athlete tries to relax and lets momentum of the mass take the body down the track.
The human body is engineered to utilize a series of lever systems (third class) where force acts between the axis and the resistance. For any movement to occur, the lever must rotate around its axis. Based on this premise, it is safe to say that the origin of movement is rotary.
Rotary motion simply refers to bodily motion. It is the interaction of physical laws and body levers. For an object to rotate, it must have two different types of forces acting on it at right angles: a centripetal or centrifugal force and tangential force. Centripetal and centrifugal forces cause objects to move toward the point of rotation, known as the axis. The axial or tangential force causes object to move perpendicularly in relation to the radius.
In the human machine, force is closer than resistance to the axis. To illustrate, a sprinter resists the ground while producing movement and force with the thigh. This relationship makes the lever very inefficient because more torque is required for movement. For the sprinter, it is essential that the lever move through the full range of movement fast, requiring a torque (turning force) both forward and backward.
Torque (force X movement arm) contributes to the angular velocity of the lever. If the length of the lever is constant, the lever would oscillate like a pendulum. But because lever length changes so does angular velocity, for any given force. Thus the angular velocity ties the length of the lever produces the linear velocity at the point farthest away from the axis.
Both the arm and leg movement efficiency benefit from the ability to lengthen or shorten levers. By lengthening and shortening the lever, the athlete can change the moment of inertia (resistance to movement). The farther the moment of inertia (mass X radius) is away from the point of rotation, the harder it is to change motion.
With a given amount of force, a shorter lever with the same mass as a longer lever would rotate faster than the longer but would have a mechanical disadvantage. It will therefore take more torque (moment of inertia X angular acceleration) to turn the longer lever than it would to turn the shorter. If no additional turning force is applied to the lever, angular momentum (moment of inertia X angular velocity) is conserved. An increase or decline in the moment of inertia will cause decline or increase in angular velocity respectively. This inverse relationship between the moment of inertia and angular velocity in the conservation of angular velocity in the conservation of angular momentum is one of the keys to great sprinting.
Stressing stride length or stride frequency
Debate on the best way of improving sprint speed focuses on two issues:
increasing stride length versus increasing frequency. We are limited physiologically to the amount of strides than can be performed in a second. It is true that most good sprint athletes have just about the same frequency. So, an athlete with adequate conditioning who takes the longest strides usually wins.
Stressing stride frequency alone results in inefficient sprint technique.
Common knowledge portrays stride frequency as a speed advantage. However, stride frequency differs from stride speed. Stride speed is angular velocity of a stride, while frequency is the number of strides, or impulses, per second. Even if a sprinter has the fastest turnover, without proper force application, stride length will be small. This is because frequency alone does cause linear motion; applying force to the ground does. Proper force application results in stride length and frequency increases.
Stride speed is involuntarily increased by the conservation of angular momentum. Shortening the radius, thus reducing the moment of inertia, results in an increase of angular velocity. The shorting of the lever occurs after the foot breaks contact with the ground. This movement is the response of the forces being applied correctly to the ground and is non-volitional.
Illusion of Speed
There is an illusion of speed when then lever does not go through the full range of motion; each movement looks and feels faster. For example if stride frequency is stressed, an athlete may not allow the hip extend through full range of motion to reap whatever benefits he created from applying force to the ground. Lack of hip extension detracts from momentum and ultimately decreases speed because of inefficiency. But the objective is not to move the lever fast but rather transport the mass down the track in the least time possible. Avoiding full range of motion also sacrifices proper force application.
Leg preactivation: pawing action
Pawing action is actually an illusion resulting from rapid hip extension. Too much voluntary action at the knees and ankles causes a reduction in angular velocity of the hip, which is the prime generator of force. Force causes motion while speed is a measurement of motion.
Cyclic force is applied from the hip (radial force) which results in tangential motion of the foot. At foot placement, the shin should be approximately 90 degrees to the ground (Fig. 5). As the center of mass passes over the point of support, the heel briefly touches the ground and the ankle angle closes. This motion puts the Achilles tendon and calf in a stretch position while the knee is bent, allowing a greater push off force from the ball of the foot. Hips extend in one continuous motion from the knee lift position through the end of the push off with no pauses in hip extension at foot contact.
High knee lift
Please see “Knee action” in Part 1.
Reach and Pull
Running action such as reaching and pulling with the hamstrings has been scientifically proven not to produce the most efficient movement (Weyand et. al. 1998). This running style is inefficient because it does not utilize the stretch reflex, but instead requires more muscle forces and volumes per unit of force applied to the ground (Weyand et al. 1998).
Scientific research across the world has yet to penetrate the track and field world. Athletes can be better sprinters with scientifically proven sprint technique. Better sprinters require coaches who are willing to learn scientific principles as well as a method of communicating their knowledge to the athlete.
Novacheck, Tom F. The biomechanics of running. Motion Analysis Laboratory, Gillette Children’s Specialty Healthcare, University of Minnesota. 1998; 77-95.
Vaughan C.I. Biomechanics of running gait. Crit Rev Eng 12: 1-48
Weyand, Peter G., Deborah B. Sternlight, Matthew J. Bellizzi, and Seth Wright. Faster top running speeds are achieved with greater ground forces not more rapid leg movements. Journal of Applied Physiology 89: 1999-2000.