• Joseph Hamill


INTRODUCTION - The measurement of ground reaction forces (GRF) has been used for many years in biomechanics to quantify external forces during locomotion. The use of force platforms to measure GRF's dates back to Marey in the late 1890's. Since the 19701s, GRF data have been used in the evaluation of sport shoes. The GRF components measured consist of three force components (vertical, antero-posterior (Alp) and medio-latera1 (MIL)) and three moments about the corresponding axes. These values can be used to calculate the center of pressure, the free moment and the resultant force. Furthermore, a number of GRF parameters have been derived to evaluate shoe. FORCES IN LOCOMOTION The vertical force for heel-toe running usually exhibits two peaks; an initial peak often referred to as the passive or impact peak and a second peak referred to as the active peak. The impact peak generally occurs about 5 to 30 ms after ground contact. The active peak generally occurs in the middle of the support phase between 100 and 250 ms after ground contact. Impact forces are the result of the collision of the foot and the ground. The magnitude and the time at which the peak occurs depends on a number of factors including running speed, running style and shoe construction. The primary portion of the shoe that influences the impact peak is the midsole. Midsoles are constructed of many types of materials and have various geometrical constructions. Generally, peak impact forces occur earlier in the support phase during barefoot running compared to shod running and with firm midsole shoes than with soft midsole shoes. The second or passive peak is generated by movements that are controlled by muscular activity. The magnitude and time of occurrence for this peak is not generally affected by footwear construction. The mediolateral GRF component is often linked to the pronation and supination actions of the foot. .Attempts to make this link have not proven particularly fruitful. Bates et al. (1 983) reported significant differences in the MIL impulse between running shoes and suggested these differences were related to pronation. The free moment is often used as a friction coefficient to evaluate the resistance of a shoe to rotation. However, it has also been used to measure the pronation action of the foot. Holden and Cavanagh (1991) used the free moment to evaluate shoes that were specifically constructed to place the foot in a pronated, supinated or neutral position. They reported that the footwear could be differentiated using this technique in that greater the maximum free moment the greater the degree of pronation. Attempts to relate the center of pressure (COP) to differentiate footwear has been unsuccessful. Vililliams (1985) suggested that this was because the COP is a global measurement and does not account for subtle changes that might have occurred during the support period. GRF INTERPRETATION - Cavanagh (1 987) suggested that footwear can affect the GRF patterns recorded although not as drastically as one might have imagined. Bobbert et al. (1991) calculated an estimation of the vertical GRF component from the positional data of the center of mass of each body segment. They calculated the vertical GRF component as: Fz = ~ i =nml i (azi - g) where mi is the mass of the ith segment, azi is the vertical acceleration of the ith segment and g is the acceleration due to gravity. Thus, GRF-data reflect the motion of the center of mass of the runner and not necessarily the motion of the foot at the foot-ground interface. It is evident, therefore, that attempts to differentiate between footwear types is extremely difficult. Far example, the differences between hard and soft midsole shoes are not clear when evaluated by GRF data. In some studies, the GRF data indicate that there are no differences in impact characteristics between hard and soft midsoles although impact tests on the midsole materials show differences. Theoretically, a hard midsole will increase the impact peak and decrease the time to impact. Then the impact peak is summed with the active peak, the result is no change in the peak value. Also, subjects may adjust their kinematics to place the body in a position to better attenuate the impact. In some studies, the difficulty in differentiating footwear is a result of an inappropriate number of trials (Bates et al., 1983). However, most of the problem concerns the fact that GRF data are not a direct measure of the forces at the foot. That is, GRF data represents the force acting on the centre of mass, although it is applied at the foot/ground interface. Conclusions—GRF data have often been used to evaluate athletic footwear. However, given appropriate methods and statistical procedures, the data still must be viewed with caution because GRF data represent the accelerations of the center of mass. Thus the difficulty in interpretation of shoe differences is that GRF data are a “remote” measure of lower extremity action.