Winiarski S, Siemie ski A (2006) Peak ground reaction force and loading rate from kinematics in the ACL deficient patients. Zeszyty Naukowe Katedry Mechaniki Stosowanej; Z.26, s.377-382. Slawomir WINIARSKI, Adam SIEMIE S KI , Department of Biomechanics, University School of Physical Education in Wroclaw PEAK GRO UND REAC TIO N FO RC E AND LOADING RATE FRO M KIN EMA TIC S IN TH E ACL D EFIC IEN T PA TIEN TS Summary. In clinical gait analysis ground reaction force measurement is the gait parameter which can validate the state of disorder of the patient’s movement. 3D kinematic gait analysis was conducted on normal and ACL-deficient subjects to test the usefulness of ground reaction force (GRF) measurement obtained from the kinematic data of the body center of gravity (COG) in clinical condition. The 3D displacement of the COG was calculated using the Clauser model and acceleration was calculated using double differential operation. Peak force and loading rate in gait was estimated from kinematics and directly from force plate measurement. 1. INTRODUCTION Direct measurement of loads in the structures of the human locomotor system is difficult and often not feasible at all. That is why investigators are usually constrained to indirect methods that use externally measured forces and torques acting on the human body to infer about internal loads. For example, when the ground reaction force during walking or running is available, the joint torques at hip, knee and ankle can be estimated. Interestingly, precision of these estimates can be significantly enhanced if kinematics of the moving human body is also taken into account (Cappozzo et al., 1975). Moreover, for any mechanical system the acceleration of its centre of mass is proportional to the resultant force vector applied to it so the ground reaction force could be, in principle, removed altogether from such an analysis. In fact, theoretically, when the human body is modeled as a system of linked rigid bodies, all the forces and torques acting between its segments could be determined based on kinematics alone. This has led a number of authors to speculate on the possibility of using kinematics data to assess the ground reaction force and joint torques. For example, Bobbert et al. (1991) calculated segmental contributions to the vertical ground reaction force from positional data for the landing phase in running, Sato and Andrew (2002) proposed a model for estimating ground reaction forces during human walking with the view of using it to supplement qualitative gait analysis in situations in which force plates are not available, and Hattori (1998) estimated the vertical component of the ground reaction force using a simplified procedure consisting in replacing the centre of mass by a marker attached on the lower end of the sternum; he found a regular cyclic pattern of the ground reaction force in normal subjects but in hip patients there was a significant decrease of downward acceleration at the mid- stance of the affected side. In this paper we describe an experimental procedure leading to the determination of peak ground reaction forces and loading rates based solely on displacement data. We also bring results of applying this procedure in a group of patients under rehabilitation following an ACL operation and in a control group of healthy volunteers.