Sprint biomechanics of female National Collegiate Athletic Association division track and field athlete

Abstract

Sprinting success is achieved by a fast start such that maximal horizontal velocity can be achieved and maintained (Johnson and Buckley, 2000; Mann and Herman, 1985). Sprint velocity can be defined as the product of stride rate and stride length. Consequently, velocity can be increased by increasing stride rate or stride length or both; however, both factors are interdependent and individual morphologic and physiologic characteristics may influence the individual's motor abilities and utilization of the energy system (Coh et. al., 2001). It has been reported that world class sprinters demonstrate increases in stride length and stride rate, landing angle, thigh acceleration, trunk inclination, and decreases in components such as thigh angle and ground contact time (Kunz and Kaufmann, 1981). However, it is the ratio between the contact time and the flight time that is the most crucial factor in the kinematic structure of the sprinting stride. Successful sprinters demonstrated shorter contact phases and longer flight phases than less successful sprinters (Coh et. al., 2001). Reaction time, technique, electromyographic (EMG) activity, force production, neural factors, and musculoskeletal structures are other biomechanical factors that can influence sprint performance (Mero et. al., 1992). Hypothetically, successful sprinters must have the ability to exert large ground-reaction forces (GRF) in shorter time periods than a less successful sprinters (Alexander, 1989; Kunz and Kaufmann, 1981; Weyand et. al., 2000). Ground reaction forces are only achievable when the body is instantaneously in contact with the surface of the ground (contact phase). The contact phase can be divided into braking and propulsion phases according to the vertical movement of whole body center of gravity (CG) or the negative and positive horizontal reaction forces during foot contact (Luhtanen & Komi, 1978). The braking phase starts from initial foot contact to the lowest position of CG, during which the extensor muscles of the stance leg work eccentrically (Luhtanen & Komi, 1978; Miller, 1983). The velocity of the CG decreases following initial foot impact then the velocity increases during the subsequent propulsion phase (Cavanagh, 1980). It has been reported that the angle between the horizontal running surface and the line from CG to initial foot contact point (landing angle) can affect contact times and CG velocity during the braking phase. In order to minimize decreases in velocity during the braking phase it is crucial to keep the CG close to the point of initial foot contact at touchdown resulting in large landing angles. (Deshon & Nelson, 1964; Kunz & Kaufman, 1981) Payne (1983) reported that the type of foot contact influenced the braking force. He found that running with no heel contact demonstrated the absence of the first vertical force peak and smoother force/time patterns. This type of foot contact was mainly seen in 400 to 800 m specialists and should be considered mechanically more efficient than foot contacts causing high impact forces where the CG was behind the contact foot (Payne, 1983). Nett (1964) studied type of foot contact during sprinting and reported that running speed influenced ground contact. He reported that initial foot contact occurred on the lateral aspect of the 5th metatarsophalangeal joint, high on the ball of the foot. As the running speed decreased, the contact point shifted to a more posterior position, or toward the heel. This can be seen in the 400 m run, where the initial foot contact point shifts back toward the heel and foot plant is somewhat flatter. In distances greater than 1500 m, the initial foot contact occurs on the lateral edge of the longitudinal arch between the heel and the head of 5th metatarsal. Nett further noted that during the load-phase of the contact foot, the heel strikes the ground, even in the case of sprinters; especially when the sprinters are fatigued (1964). Conversely, Mann (1980) and Novacheck (1998) reported that the heel of sprinters did not or "may not" touch the ground throughout the sprint, and that initial ground contact was dependent on gait speed. Consequently, as speed increased initial contact changed from the hind-foot to the forefoot. This issue remains unclear because only five studies have involved examination of type of foot contact during sprinting and its effect on the biomechanics of sprint performance (Nett, 1964; Mann. 1980; Payne, 1983; Novacheck, 1995; Novacheck, 1998) Differences between levels of elite sprinters have been observed biomechanically (Alexander, 1989; Kunz & Kaufman, 1981; Luhtanen & Komi, 1978; Mann, 1981; Mann and Herman, 1985). However, present kinematic research generally does not extend to the influence of type of foot contact during the ground contact phase of the sprint gait cycle. Although two main biomechanical factors - stride length and stride rate - have been widely accepted by researchers as key factors in sprint performance, foot contact type during the contact phase is unclear and controversial. Therefore, the purpose of this research study was to investigate type of foot contact during the contact phase and its effect on the biomechanics of the 200 m sprint.