EXTENSOR KNEE MOMENT ARMS

Abstract

INTRODUCTION The quadriceps is the main muscle acting across the knee, and numerous models of the extensor apparatus have been suggested to understand movement co- ordination in Sportsmen . These models are also used to formulate rehabilitation programs for Sportsmen following knee extensor injuries. In the past, this center of rotation for the knee has been variously understood to be the cruciate crossing points, the instant centers or evolute centroids. Brand calculated the moment arm of the musculo-tendinous unit being studied from the derivative of relative excursion of - - that unit with respect to joint angulation. Recent work has shown that the knee has a fixed flexion-extension (FE) axis and the quads and patella track perpendicular to this axis. I used Brand's method to measure the mechanical advantage of the quadriceps tendon in fourteen fresh frozen cadaver knees. RESULT The moment arm for the patellar tendon was longest (3.44cm s.d.+ 0.36 ) at 30' of flexion, decreasing to a minimum (2.57cm s.d.+ 0.34) at 90' of flexion (fig 2). The difference between the two curves of each knee is the mechanical advantage of the patella (fig 3). The moment arm fell rapidly after as the patella entered the inter-condylar groove and moved towards the FE axis. CONCLUSIONS The quads has a substantial moment for knee FE. The maximum moment arm is in the terminal 30' of extension. Extensor moments and moment arms . .- calculated referring to ICRs or crucite crossing point location follow a set trend. The moment arms are smaller in the first 30' of flexion due to the close proximity of the lCRs to the extensor apparatus; they then increase for the 45- 60' range as the lCRs migrate posteriorly, and the extensor apparatus enters the inter-condylar groove (fig 2). Further joint flexion approximates the quads tendon in the inter-condylar groove and the ICRs, whose posterior migration has slowed. The amount of tension in the quads required to supply the same torque is increased by at least one-third. This results in an increase in the patello-femoral joint reaction force and may account for some of the difficulties in rehabilitation following sportsrelated injuries in Athletes. A fundamental knowledge of the kinematics of gait and running is indispensible for evaluating and improving athletic performance. These implications are profound for force and moment calculations about the knee. The design of protective braces etc will also be profoundly affected. REFERENCES 1 Brand PW: Clinical Mechanics of the Hand. CV Mosby Co., St.Louis, 1985. 2 Fick AE, et al: Anatomisch-mechanische Studie uber die Schultermuskeln Verh phus-med Ges Wurbz 11 :123-152, 1877. 3 Singh AK: A Century of Kinematics of the Knee. Univ of Strathclyde Glasgow 1993.