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The purpose of this study was to investigate the possible existence of systematic differences between moment-length properties of the rectus femoris muscle of cyclists/speed skaters and runners. In cycling/speed skating the rectus femoris is used at a shorter length than in running because of the pronounced flexion at the hip joint. It was speculated that using the rectus femoris chronically at different lengths would result in different moment-length relations for the two groups of athletes. Moment-length relations of rectus femoris muscles were determined using an adaptation of procedures outlined in the literature. Four subjects in each group performed 13 isometric knee extensions on a Cybex II dynamometer in each of three testing sessions. Knee and hip angles were varied in a systematic way to allow the determination of moment-length relations over a wide range of normal rectus femoris lengths. It was found that cyclists tended to be stronger at short compared with long rectus femoris lengths, whereas the opposite was true for runners. This finding may be associated with an adaptation of the rectus femoris muscle to the requirements of cycling and running or may show an inherited difference in the muscles of the athletes that existed before they became involved in their respective sports. The data of this study do not allow us to distinguish between these two possible factors.