Biarticular Actuation of Robotic Systems

In biology, biarticular muscles are muscles that are passing two joints. One of the biarticular muscles that is passing the knee and the ankle joints is the Gastrocnemius muscle (see Fig. 1). Functions of the biarticular muscles during human movement have been studied extensively by many researchers but these functions still remain unclear. One of such functions is the transportation of mechanical energy from proximal to distal points (Ingen Schenau, 1989). It is believed that this transportation causes an effective transformation of rotational motion of body segments into translation of the body centre of gravity. The mechanism by which this is achieved is the timely activation of biarticular muscles before the end of the explosive phase of the movement. The activation of the biarticular muscle prior to the end of the explosive phase of the motion enables the transportation of the power generated by the proximal joint extensors from the proximal joint to the distal joint. This transfer of mechanical energy can be explained using the following example of the gastrocnemius muscle activation. During the push-off phase of the human jump, the knee joint is rapidly extended as a result of the positive work done by the knee extensor muscles. If the biarticular gastrocnemius muscle contracts isometrically (its length does not change), the additional mechanical work is done at the ankle joint because of the gastrocnemius muscle, which contributes no mechanical work by itself. A part of the energy generated by the knee extensors appears as mechanical work at the ankle joint and the height of the jump is significantly increased. This is because, as the jump proceeds and the knee straightens, the angular position changes of the knee have progressively less effect on vertical velocity of the jumper centre of gravity. By gastrocnemius muscle activation, a rapid extension of the foot is produced. This extension has a greater effect on the vertical velocity than the extension of the almost straightened knee. The energy is more effectively translated into vertical velocity and a greater height of the jump is achieved. However, the timing of the gastrocnemius muscle activation is critical to obtain a maximum effect. This was demonstrated by an articulated physical model of the vertical jump by Bobbert et al (1986). Besides biarticularity, the gastrocnemius muscle has one more interesting feature. It is connected to the foot by an elastic tendon (see Fig. 1). The elasticity in the muscle fibres and tendons plays an important role in enhancing the effectiveness and the efficiency of human performance. An enhanced performance of human motion has been most effectively demonstrated for jumping and running (Cavagna, 1970; Bobbert et al., 1996; Shorten, 1985; Hobara, 2008). An important feature of elastic tissues is the ability to store elastic energy when stretched and to recoil this energy afterwards as a mechanical work (Asmussen and