49 Tibiofemoral joint forces during isokinetic knee extension RALPH NISELL,* MD, PhD, MATS O. ERICSON, MB, GUNNAR NÉMETH, MD, PhD, AND JAN EKHOLM, MD, PhD From the Kinesiology Research Group, the Departments of Anatomy and Physical Medicine and Rehabilitation, Karolinska Institute and Karolinska Hospital, Stockholm, Sweden ABSTRACT Using a Cybex II, eight healthy male subjects performed isokinetic knee extensions at two different speeds (30 and 180 deg/sec) and two different positions of the resistance pad (proximal and distal). A sagittal plane, biomechanical model was used for calculating the mag- nitude of the tibiofemoral joint compressive and shear forces. The magnitude of isokinetic knee extending moments was found to be significantly lower with the resistance pad placed proximally on the leg instead of distally. The tibiofemoral compressive force was of the same magnitude as the patellar tendon force, with a maximum of 6300 N or close to 9 times body weight (BW). The tibiofemoral shear force changed direction from being negative (tibia tends to move posteriorly in relation to femur) to a positive magnitude of about 700 N or close to 1 BW, indicating that high forces arise in the ACL when the knee is extended more than 60°. The anteriorly directed shear force was lowered consid- erably by locating the resistance pad to a proximal position on the leg. This model may be used when it is desirable to control stress on the ACL, e.g., in the rehabilitative period after ACL repairs or reconstruc- tions. In recent years, isokinetic exercises have become common- place in muscle strengthening programs, with special em- phasis placed on knee extension and the quadriceps muscle. Isokinetic exercise has been used as a method for analyzing force-velocity or power-velocity relationships12,25>2’ and as a training/testing device for healthy athletes.2° It has also been widely used for many different clinical purposes, such as evaluating rehabilitation or function of joints. 2,5,8.11, ~5, ls, ~s Many investigators have studied in detail the mechanics and function of the ACL. 1,3,4,9,19,11,14 After ACL surgery, it is important that knee rehabilitation not cause excess stress on the ligament. In spite of all the research in this area, Noyes et al.’9 had to conclude in a review: &dquo;...we know little about the forces that exist within ligaments and we are therefore unable to distinguish objectively between safe and dangerous activities during ligament healing.&dquo; Hence, it is of great value to assess the force magnitudes in ligaments during exercises and various activities in order to give advice that optimizes individual rehabilitation but minimizes risks to the patient. In 1973, Smidt26 presented a biomechanical model of the knee and calculated the knee joint forces during isometric knee flexion and extension. However, in that study, the subjects were lying on their sides and the joint forces during isokinetic movements were not calculated. Using biome- chanical calculations, Johnson 13 showed that during isoki- netic knee extension, the magnitude of the tibiofemoral shear force, and consequently the ACL stress, could be considerably diminished by using dual pad resistance instead of single pad. However, the tibiofemoral joint force magni- tudes were not calculated. The same conclusion, using a knee laxity tester, was stated in the 1985 award winning paper by Jurist and Otis,l4 which showed that a proximally positioned pad would minimize the ACL stress during iso- metric knee extension. The object of the present study was to calculate, by using a two-dimensional biomechanical model, the magnitude of the tibiofemoral compressive and shear forces during isoki- netic knee extension performed at two different speeds and with the resistance pad placed either proximally or distally on the leg. By using data on the tibiofemoral shear force magnitudes, the stress on the ACL was estimated and its postsurgical isokinetic rehabilitation discussed. MATERIALS AND METHODS Eight healthy male volunteers acted as subjects for this study. They were university students and were free from * Address correspondence and repnnt requests to’ Ralph Nisell, MD, PhD, Kinesiology Research Group, Department of Occupational Medicine, Karolinska Hospital, S-104 01 Stockholm, Sweden