IntroductionStatic stability of the human body can be characterized by the relationship between the total body center of mass(COM) and the support base(SB) provided by the feet. Evaluation of dynamic stability while stepping over obstacles is, however, a more challenging problem from the point of view of biomechanical modelling. The objective of the research project described below was to develop a model for evaluating dynamic stability. The main components of the model were position of COM, SB, and the available response time (ART). ART is the time from the swing toe reaching the obstacle to when the COM would contact the boundary of SB in the event that the swing toe was arrested by contact with the obstacle.Six healthy young male subjects (18-35 years) volunteered for the study. Two OPTOTRAK (Northern Digital Inc.) movement analysis systems positioned either side of the walkway were used to record the displacement/time characteristics of 18 infrared LEDs placed on the joint centers of rotation of the lower limbs and torso. The obstacle to step over was a 100cm wooden rod 10mm in diameter adjusted to heights of 18cm and 38cm. Six trials per subject were conducted in random order at slow, normal and fast walking speeds for each obstacle height. The position of COM and the velocities of foot and COM were calculated using standard equations (Winter 1991). The support base (Figure 1) was defined by the four-sided figure created by connecting markers of the toe and heel of the support limb and the swing limb in the transverse plane. The relative position of the COM in the horizontal plane is also shown in Figure 1. A stability index was calculated based on the time required for the COM to reach the forward boundary of the support base if the swing foot were suddenly arrested by contact with an immovable object.ResultsART showed a significant increase with obstacle height across all three walking speeds, as follows; Slow (S), 250 ms vs 310 ms (p <.001); Normal (N) 180 ms vs 220 ms (p <.001); Fast (F), 120 ms vs 140 ms (p<.001) for the low obstacle (L) and high (H) obstacle respectively, as shown below in Figure 2. In addition, ART decreased significantly with an increase in walking speed independent of obstacle height (p<.001).DiscussionWalking speed and obstacle height were shown to influence ART of CM when stepping over obstacles. The ART decrease with walking speed and obstacle height is consistent with basic mechanical principles. The velocity of COM is positively associated with walking speed and an increase velocity of COM is expected to result in a decrease in the ART of COM. It is reasonable to suggest that the risk of falls will increase as ART decreases due to the reduction in recovery time if the subject contacted the obstacle with their lead foot. It is also interesting to observe that obstacle height also influences the ART of COM. It is possible that this is due to the COM being held back , as it were, when crossing a high obstacle as a means of maintaining stability. This action might be interpreted as a protective reflex specific to this activity.There are few studies with data comparable to those of the present experiment. Cheng et al, (1994) measured ART not for the COM but for the swing (lead) foot in an obstacle avoidance task in which a virtual obstacle (a light band projected onto the walkway) was stepped across. They found that success in avoiding the obstacle was only 11.8% when lead limb ART was 200 ms, increasing to 97% for a 450ms ART. The data presented above indicate COM ART times of 120 ms to 220 ms for normal and fast walking speeds, i.e., the COM would take approximately 200 ms to reach the boundary of SB. These findings imply that in the event of contact with an obstacle swing foot ART of approximately 200 ms would not be sufficiently long to arrest the forward motion of the COM and a fall would occur. Thus, a critical observation arising from these data is that progression of the COM may be related to swing limb ART Subjects may control the velocity of the COM such that, should they contact the obstacle, they would be capable of recovery by putting down the swing foot. In conclusion, the ART of COM may prove to be a useful index in the assessment of dynamic stability in human gait. The conditions under which falls are likely to occur are now becoming better understood via an examination of the relationships between obstacle characteristics (such as height and width), walking speed and the available response times of both the whole body center of mass (COM) and the swing limb.