Most animal movements are actively controlled by muscle contractions. However, passive forces, such as those due to the elastic properties of muscles and tendons, have been shown to be invaluable to locomotion. But what about passive forces originating in the joints themselves? What role do they play in movement, and how do they relate to the active forces being generated at those joints? Although passive forces originating in the joints of insects have been documented, little else is known about their role in locomotion or their specific importance in different modes of locomotion such as walking and jumping. In a recent study in Current Biology, Jan M. Ache and Tom Matheson from the Universities of Leicester, UK and Cologne, Germany, sought to understand whether passive joint forces are adapted to the requirements of insects with legs specialized for different types of locomotion.
To understand the extent of passive joint forces in generating leg movement, Ache and Matheson measured flexion and extension of the tibia of two insects, the locust Schistocerca gregaria and the false stick insect Pseudoproscopia scabra. In the locust, the authors stimulated muscle contractions of the large and powerful extensor tibiae muscle and recorded the velocity and angle of the tibia during the resulting forced extensions and subsequent passive flexions back to a resting position. In the false stick insect, the authors held dissected legs in fully extended or flexed positions and recorded passive movement back to resting position.
The authors found that tibial flexion from an extended angle occurred without contraction of the flexor muscle in both insects. This passive flexion occurred even after all associated tissues (with the exception of the essential cuticular structure of the joint) were removed, showing that passive forces were originating from the joint itself. In addition, Ache and Matheson discovered that these passive joint forces counteracted active extension, likely an adaptation to help stabilize the joint during powerful kicks. In contrast, the authors found almost no passive extension in the hindlegs of either insect. When they examined passive joint forces in the middle legs of the false stick insect, however, they found that passive flexion and extension were evenly matched. In these middle legs, the passive forces help to support the relatively weaker flexor and extensor muscles. The authors then compared their results with published data on the stick insect Carausius morosus and found that in the stick insect, which has a contrastingly weak extensor muscle and stronger flexor muscle, the joint provides passive extension, not flexion.
Data from all three insects suggest that passive joint forces have evolved to individually match the strengths and requirements of the muscles of each type of leg. While the hindlegs of the locust and false stick insect are adapted for jumping, the false stick insect's middle legs and the stick insect's hindlegs are designed for walking. This finding has important implications for robotics, which have long used insect morphological and locomotor designs as inspiration. As robotics focus more on incorporating passive forces, it will be essential to consider the functions of each limb. In animals where a strong muscle is needed for a specialized movement, strong passive forces could provide a balance for, and may allow a more rapid recovery from, that movement, without the need for neural input. These balancing forces are the key to the passive–aggressive dynamics of limb movement.