Fig. 1.

Experimental protocol. a Outline of the system for the analysis of knee rehabilitation exercises. b Muscles related to the knee joint and their recording positions with surface electrodes. The four prime knee movers whose activities were recorded: rectus femoris (RFM), vastus lateralis (VL), vastus medialis (VM), and biceps femoris (BFLH). c Procedure of the straight leg raise (SLR) as an experimental task. To initiate the exercise, the subject was first asked to lie on a mat. The subject then rested for 1 s. After 1 s, the subject was required to lift his right leg within 1 s (raise up). Then, the subject was asked to hold his right leg at that position for 6 s (maintain). After that, the subject was required to lower his right leg on to the mat (put down) and rest for the next trial (rest). In particular, to extract the characteristics of muscle fatigue more accurately, we limited the analysis of each parameter to the stable range (range of analysis) within the maintain range