The literature contains some hypotheses regarding the most favorable ground reaction force (GRF) for sprint running and how it might be achieved. This study tested the relevance of these hypotheses to the acceleration phase of a sprint, using GRF impulse as the GRF variable of interest. Thirty-six athletes performed maximal-effort sprints from which video and GRF data were collected at the 16-m mark. Associations between GRF impulse (expressed relative to body mass) and various kinematic measures were explored with simple and multiple linear regressions and pairedt-tests. The regression results showed that relative propulsive impulse accounted for 57% of variance in sprint velocity. Relative braking impulse accounted for only 7% of variance in sprint velocity. In addition, the faster athletes tended to produce only moderate magnitudes of relative vertical impulse. Pairedt-tests revealed that lower magnitudes of relative braking impulse were associated with a smaller touchdown distance (p< 0.01) and a more active touchdown (p< 0.001). Also, greater magnitudes of relative propulsive impulse were associated with a high mean hip extension velocity of the stance limb (p< 0.05). In conclusion, it is likely that high magnitudes of propulsion are required to achieve high acceleration. Although there was a weak trend for faster athletes to produce lower magnitudes of braking, the possibility of braking having some advantages could not be ruled out. Further research is required to see if braking, propulsive, and vertical impulses can be modified with specific training. This will also provide insight into how a change in one GRF component might affect the others.

These results suggest that runners who have developed stride patterns that incorporate relatively low levels of impact forces, and a moderately rapid rate of pronation are at a reduced risk of incurring overuse running injuries.

Leg length, height of takeoff, and vertical velocity of takeoff are all possible sources of a negative interaction between step length and step rate. The very high step lengths and step rates achieved by elite sprinters may be possible only by a technique that involves a high horizontal and low vertical velocity of takeoff. However, a greater vertical velocity of takeoff might be of advantage when an athlete is fatigued and struggling to maintain a high step rate.

No abstract

In the high-velocity tennis serve, the contributions that the upper limb segments' anatomical rotations make to racket head speed at impact depend on both their angular velocity and the instantaneous position of the racket with respect to the segments' axes of rotation. Eleven high-performance tennis players were filmed at a nominal rate of 200 Hz by three Photosonics cameras while hitting a high-velocity serve. The three-dimensional (3-D) displacement histories of 1 1 selected landmarks were then calculated using the direct linear transformation approach, and 3-D individual segment rotations for the upper limb were calculated using vector equations (Sprigings, Marshall, Elliott, & Jemings, 1994). The major contributors to the mean linear velocity of the center of the racket head of 31.0 m . s-' at impact were internal rotation of the upper arm (54.2%), flexion of the hand (31.0%), horizontal flexion and abduction of the upper arm (12.9%), and racket shoulder linear velocity (9.7%). Forearm extension at the elbow joint played a negative role (-14.4%) and reduced the forward velocity of the center of the racket at impact.As every rally in tennis is started with a serve, it is logical to assume that this stroke plays an important role in determining the final outcome of a match. Players of all levels certainly seek to develop a high-speed serve as an integral part of their game. Sport scientists and coaches agree that the speed of the racket and therefore the postimpact speed of the ball, the height of impact, and the amount of forward rotation of the ball are the key factors determining the angle the ball should be directed for a successful serve (e.g., Elliott, 1983). The speed of the racket head at impact is therefore a critical feature of a successful serve that can be varied considerably by the player, particularly through the individual movements of the upper limb segments. As such, the role of the angular velocity vectors of the upper arm, forearm, and hand in generating maximum racket impact point speed