Measurement of knee muscle strength and instability is important for diagnosing and estimating the surgical results for knee cruciate ligament injury. When knee muscle strength and instability is estimated, the side difference is usually ignored. The muscle power of the weak side is about 90% compared with that of strong side. This difference in knee muscle strength between the weak and strong side is statistically meaningful. Regarding knee instability, 10% of normal knees show more than a 2mm side difference. When knee muscle strength and instability is estimated, the side difference must also be taken into consideration.
