Fig. 1.

A: the electromyography (EMG) level with respect to both joints is approximated by a plane. A multiple regression analysis was used ($M=⌊a{\text{T}}_{\text{k}}+b{\text{T}}_{\text{h}}⌋$), where M is the EMG level, T_k and T_h are the knee and hip joint torques, respectively, and a and b are fitting parameters. Additionally, $⌊x⌋=\max \left(x,0\right)$ where EMG was restricted to be 0 or greater, and only EMG above 0 was used for analysis. B: contour plot indicating the activity level of a muscle obeying cosine tuning on the torque plane. Darker shades are larger muscle activity. The preferred direction (PD) is the direction in which the muscle activity level increases the greatest. C: mean value of PDs of each (thigh) muscle from experimental data (n = 11). BFL, biceps femoris long head; BFS, biceps femoris short head; GAS, medial gastrocnemius; GM, gluteus maximus; RF, rectus femoris; ST, semitendinosus; VL, vastus lateralis; VM, vastus medialis. A and C are reproduced with modification from Nozaki et al. (2005b), their Figures 4A and 1C, respectively. B is reproduced with modification from Nozaki et al. (2005a), their Figure 3A.