The single joint torque exertion task has been widely used as a very efficient method to control muscle activity. In the task, only the relevant joint torque is specified, and generally the other joint torques are not taken into consideration. However, our previous study has shown that the muscle activity level is indefinite even for mono-articular muscles unless the neighboring joint torque is specified. Therefore, the neglect of the neighboring joint could make the activity level of the relevant joint muscles ambiguous. Here we investigated how much hip joint torque was generated while subjects tried to exert knee joint torque and how it affected the activity of the knee joint muscles. Twelve healthy subjects were requested to exert various levels of isometric knee joint torque (extension and flexion). The knee and hip joint torques were obtained using a custommade force measurement device. Since no information about hip joint torque was provided to the subjects, the hip joint torque measured here was a secondary one associated with the knee joint torque exertion. The subjects could be divided into two groups according to the profile of the hip joint torque: for example, knee extension torque accompanied either large hip flexion torque or almost no hip joint torque. Such a difference in the hip joint torque was not negligible, because the knee joint muscle activity level with respect to the knee joint torque level, as quantified by surface electromyography (EMG), changed significantly when the subjects were requested to use the other strategy. This change occurred in a very systematic manner: in the case of the knee extension, as the hip flexion torque was larger, the activity of mono- and bi-articular knee extensors decreased and increased, respectively. These results indicate that the conventional single knee joint torque exertion has the drawback that the inter-subject and/or inter-trial variability is inevitable in the relative contribution among mono- and bi-articular muscles due to the uncertainty of the hip joint torque. We propose that this drawback can be overcome by controlling both joint torques simultaneously. It is hoped that this viewpoint will bring insights into various controversial problems such as the shape of the EMG-force relationship, neural factors that help determine the effect of muscle strength training, and so on.