The gait laboratory force plate at the Cleveland VA Medical Center.

A key element in a gait laboratory is a force plate which may be used to measure the forces and moments applied by the foot onto the walking surface during a gait cycle. While many laboratories have relied solely upon motion data as obtained from using various camera techniques for analysis of gait, it is only with the inclusion of a force plate that the dynamic aspects of gait are appropriately considered. A number of researchers have used a force plate to obtain a clinical evaluation of gait . Jacobs et al . (8) used the vertical component of the force exerted in walking for the diagnosis of hip joints : by analyzing this force as the sum of its harmonic components and by describing it by other mathematical parameters, different types of gait were classified . Yamashita and Katoh (13) used a specially designed force plate to analyze the moving pattern of the point of application of the vertical component of resultant force during level walking. Suggested by Boccardi et al . (1) is a new technique of displaying force plate data, namely a "Butterfly Diagram" Force vectors are displayed on a storage screen where each vector represents the projection, onto the sagittal plane, of the ground reaction with its magnitude, inclination, and point of application clearly shown. Other investigators such as Bresler and Frankel (2), Cunningham (5), Endo and Kimura (6), and Iwai and Kozumi (7) have studied different characteristics of the vertical force obtained through the use of a force plate. In much of the previous work as just outlined, the reduction of the data obtained from the force plate was accomplished manually, and was often considered separately from other data acquired . In order to circumvent some of the inherent difficulties associated with these approaches, the force plate at the Cleveland Veterans Administration Medical Center has been interfaced to a PDP 1 1 /45 minicomputer so that the data-reduction step may be automated, and so that force data may be combined with motion and EMG information to provide a more complete evaluation of gait. In this paper, the characteristics of force plates in general will first be discussed . Next, the construction of our force plate will be outlined-its physical construction, transducers used, and force components directly measured. After that the hardware interface to the computer will be discussed followed by a description of the software programs that have been written for calibration, data acquisition, and display . Finally, the mathematics of the force plate is detailed in the Appendix.