The functions of muscles revealed with a robot testbed
When humans walk or run they use compliant legs, which was discovered with the bipedal spring-mass model. This model with compliant legs shows similar ground reaction forces like that of humans in both gaits. If the whole leg acts like a spring, there could be a possibility to apply real springs at robotic legs. Let us take a look at the structure of human legs. First we can see the large segment: the thigh, the shank, and the foot segment. They are coupled with joints, i.e. the hip joint, the knee joint and the ankle joint. The rotate these joints, a large number of muscles and tendons exist, which are able to generate torque over one or two joints. In some instances it is difficult to understand the muscles functionality during locomotion, because the muscle activity does not necessarily generate a motion. The muscle activity can be measured as electromyogram, however, the measurement cannot directly be converted into the applied muscle force. The scientists at the Lauflabor Locomotion Lab in Jena thought about the possibility to replace muscles by simple strings in order to make their function and force visible.
For scientific reasons, a simple robotic platform was built that is able to reproduce the gait of walking. The proportions of the robot legs are closed to those of human legs except for a larger foot segment. Both hip joints are powered with a motor, which generates a simple sinus motion like human hips do in walking. The difficulty of stabilizing the upper body is excluded since the upper body is fixed at a boom and rotation is prevented. A major question is which muscles are eminently important for locomotion or which springs should be implemented. Based on the knowledge on biomechanics research and first tests with the robot, four springs representing muscles were mounted, i.e. rectus femoris, biceps femoris, gastrocnemius, and tibialis anterior. The rectus femoris is a biarticular muscle that flexes the hip and extends the knee. The biceps femoris is a biarticular muscle that extends the hip and flexes the knee. Hence, the biceps femoris is the antagonist of the rectus femoris. The gastrocnemius is the third biarticular muscle, which flexes the knee and extends the ankle joint. The last one is the tibialis anterior, a single joint muscle flexing the ankle.
Based on robot experiments and a similar simulation model, the functionality of the implemented springs, respectively the according muscles can be revealed. During the stance phase in walking, the thigh is rotated by the hip motor and the hip joint itself is extending. The knee is slightly extending. Both joint rotations lead to a stretching and an activation of the rectus femoris, which generates a torque at the knee joint. At the lower end of the leg, the knee joint extension leads to an activation of the gastrocnemius generating torque at the ankle joint. As long as the foot segment remains fully at the ground, the force of the gastrocnemius does act as part of the ground reaction force leading to a forward motion of the main body. At a certain point, the heel lifts off and the effect of the gastrocnemius changes significantly. The muscle force generates motion at the lower limb, i.e. the shank moves upward and the knee is flexing. At the same time, the thigh is moving forward due to hip motor action. This leads to a slightly deactivation of the rectus femoris. The leg is moving forward and the knee is slightly flexed due to the previous force of the gastrocnemius. What happens now with the foot segment? At the end of the stance phase, the gastrocnemius extended the ankle, which would lead to an overextension of the ankle. This behaviour is prevented by the tibialis anterior. It flexes the ankle during the swing phase and allows for enough ground clearance when the second leg applies its normal stance phase.
What is the functionality of the biceps femoris during walking? The representing spring is inactive during the whole cycle respectively the spring is rarely or never tensed. The simulation study showed that the biceps femoris is still important at the time shortly before touchdown. It can slightly flex the knee joint in order to keep the leg compliant when the touchdown occurs. In other words, it prevents the knee from overextension and serious injuries.
While the springs in walking are mostly relaxed and activity is mainly shown during midstance, in running a much higher activity is necessary. Using the simulation model the properties of the springs were analysed for the running gait. It is revealed, that the antagonistic pair of muscles, the biceps femoris and the rectus femoris who are stretched over hip and knee, are always active. Their forces are required to generate the required leg stiffness for running. The gastrocnemius generates force during stance phase only but with a much higher amplitude compared to walking. This high force is necessary to extend the ankle joint and to apply the high leg force at the ground. The tibialis anterior as an antagonist to the gastrocnemius does also generate higher forces to ensure ground clearance when the leg swings forward.
The success of the reviewed robotic testbed with biarticular springs led to the development of the professionally manufactured JenaWalker robot. Using the JenaWalker further studies on walking with compliant legs were conducted.