SPORTS MEDICINE FOUNDATION Volume 6 Fall/Winter 1999
Editors Note: former Steadman-Hawkins Fellow Dr. John Wright conducted an analysis of CADS at the Steadman¨ Hawkins Sports Medicine Foundation to provide a biomechanical explanation as to why skiers who use CADS experience less quadriceps fatigue and less knee pain. After completing his Fellowship with Drs. Steadman and Hawkins in July 1999, Dr. Wright joined the Steadman-Hawkins Clinic in Vail and Breckenridge to specialize in hip and knee replacement.
If you or a friend has given up skiing, or youre thinking about giving up skiing because of knee pain and fatigue, you should know about CADS. "Constant-force Articulated Dynamic Struts" (CADS) were conceived by engineer/inventor Walter Dandy to alleviate quadriceps muscle fatigue and knee pain in skiers. The paired devices consist of fiberglass rods and cords which run between a pelvic harness and thick elastic bands which are anchored to the ski boots. This suspension system transfers weight from the legs to the rods.
For several years, Steadman-Hawkins patients who have used CADS have claimed the system reduces pain and fatigue. Many patients suffering from arthritis and/or patella femoral pain report that CADS allow them to ski with significantly less knee discomfort.
A static free body analysis was performed to quantify knee joint forces during flexed-knee stance. The unloading effect of the CADS at various degrees of knee flexion was calculated by having subjects sit upon a CADS simulation apparatus.
Ordinarily, when a skier bends his or her knees, the quadriceps muscles must contract to prevent the skier from falling over. This is why wall-sits cause a burning sensation in the quadriceps. The heavier the skier, the greater the quadriceps must work to maintain a flexed-knee stance. Furthermore, the deeper a skier flexes his or her knees, the more the quadriceps must contract. Stronger quadriceps contraction in turn produces increased compressive forces across the knee joint.
When a skier wearing CADS bends the knees to assume a skiing position, the elastic bands of the CADS are stretched. The stretch in the elastic bands is transmitted via the cords over the pulleys to the pelvic harness. This reduces the skiers effective boy weight and, hence, diminishes the required quad-contraction strength. Decreased quadriceps contraction, in turn, reduces compressive forces across the surfaces of the knee joint. The study documented that, at 50 degrees of knee flexion, CADS reduced effective body weight, quad contraction force, and joint contact force by 22 percent. CADS-induced reductions in these forces are even greater at higher degrees of knee flexion.
The results of this biomechanical analysis of CADS provide a scientific rationale for the clinical observation that skiers who wear CADS experience decreased quadriceps muscle fatigue and reduced knee pain.