Abstract
The impact that occurs when the heel strikes the ground causes some problems in athletes and patients. Strong impact forces may damage or degenerate joints. The cushioning effects of soft tissues of joints or muscles around a joint have been studied experimentally. The configuration of the leg landing on the ground has also been found to be important in absorbing the impact force. The impact force at heel contact with the ground was evaluated using accelerometers (Nihon Koden, 1.7g). The accelerometers were pretested on human cadavers to determine the relationship between the skin-mounted accelerometer output and the bone surface acceleration. The acceleration of the bone was measured by the accelerometer mounted on a screw (1.7g, 2.5cm long). The screw was inserted into the bone. The impact force transmission through the bone up through the human body was measured as the acceleration by both accelerometers. They made little difference. So the reliable acceleration of the impact force through the bone could be obtained noninvasively by the skin-mounted accelerometer. The experimental results for normal walking (100 step/min) were obtained in two cases: with subjects' ankle joints free or were fixed using an ankle brace. The results showed that the transfer of acceleration of impact force to upper joints was attenuated greatly at the ankle and knee joints with normal and fixed-ankle walking, respectively. Passive impact forces in the vertical reaction were also measured to examine relation to the acceleration at leg joints. The leg configuration just before it strikes the ground was discussed using a planar model. The model has five massive links, in which the feet were assumed to make a point contact with the ground. Then the model having the feet was introduced to examine the function of a foot. A convenient method was proposed to represent the relationship between the velocity of the foot just before striking the ground and the impact force by a contact with unit velocity of 1.0m/s. The model with the feet reduced the impact remarkably, in agreement with the experiment. And the magnitude of the impact simulated depended on the leg configuration at the heel contact: an extended knee made the impact great; some flexed knee reduced the impact and minimized the force. Physical parameters of the model were chosen based on normal persons.
