An appendage that can generate considerable force may be capable of damaging itself. Many injuries occur when something disrupts a planned motion. Failure to gain adequate traction (slippage) during a movement can damage joints, muscle or bone. In a recent paper in JEB, Bayley, Sutton and Burrows, inform us that, “During running, rowing, baseball, cricket and ballet dancing, humans generate high forces that can fracture arm, leg or rib bones.” They then proceed to calculate the force exerted on the leg of a grasshopper when it jumps and conclude that the leg is at most about 20 percent stronger than the force it can generate. This suggests that the grasshopper pushes the limits of the force its legs can accommodate. What happens when movement goes wrong, such as slipping during the execution of a jump? If the leg slips, the force that should have been transferred to lifting the body from the ground (jumping) is instead transferred to the end of the leg.
Grasshopper leg anatomy has a built in feature to absorb the shock of unintended slipping. Just below the joint of the femur and tibia of the hind leg (the grasshopper knee) is an area of the tibia that can buckle and allow the tibia to bend. Bayley and colleagues have demonstrated that the area that buckles contains the elastic protein resilin, the same elastic protein present in grasshopper joints. When a grasshopper leg slips, the tibia buckles and allows the end of the leg to extend. The leg will resonate in a swinging motion until all the force of the failed jump is dissipated. The fail-safe buckling of the tibia prevents damage to the muscles and tendon of the leg and preserves the integrity of the leg itself. The leg can bend so it will not break.
Tibia of a grasshopper hind leg has a shock absorbing area (arrow) that allows the tibia to bend.
Upper: Leg in relaxed position.
Lower: Same leg with tibia bent in the buckle region.
Note: Bayley, Sutton and Burrows have some nice images in their JEB paper.
