Most of us take our bodies for granted; that is until something goes wrong. Pull the wrong muscle and you can be hobbling for months. Frank Nelson explains that most joints are flexed and extended by several muscles, which poses the question how much each muscle contributes to a joint's movement. According to Nelson, some thought that all muscles associated with a joint contribute equally to its movement, while others suggested that a muscle's contribution was proportional to its cross sectional area. However no one had directly measured individual muscles' contributions to a joint's swinging movement, and none of the few measurements that had been made on limbs in contact with the ground came down hard for any of the hypotheses. Curious to find out how muscles work together, Nelson and his team leader, Tom Roberts,decided to test muscles around the turkey ankle joint to find out how they share the load ().
According to Nelson, the turkey's ankle joint is ideal for such a study. Firstly the bird is a biped, like us, and the ankle joint has three major muscles to extend the joint, one of which doesn't contribute while the leg is being swung forward. Only the remaining lateral and medial gastrocnemius muscles extend the ankle during swinging. What is more, the muscles' tendons are calcified; ideal for fitting strain gauges. Even then, Nelson admits that successfully fitting four strain gauges to the tendons in each bird's ankle was challenging, but eventually he had three birds ready to set running to see how the muscles divided their force.
Filming the birds as they ran on a treadmill at speeds ranging from 1 to 3.5 m s–1, Nelson also increased the force on the bird's ankles by fitting 30 g and 60 g weights to see how the muscles responded. Analysing the movies, Nelson was able to calculate the forces exerted on each bird's foot as it swung forward and to see how they matched up with the strains generated in the muscle tendons. If the muscles were sharing the load equally, then the strains on each tendon would be 50% of the forces acting on the foot.
Focusing on the stance phase of the stride, when the bird's foot was in contact with the treadmill, the strains that the team measured in both muscle tendons were equal. It seems that the force is shared equally when the foot is in contact with the ground.
But Nelson and Roberts were in for a shock when they looked at the forces generated during each stride's swing phase. Far from contributing equally,only the lateral gastrocnemius muscle supplied the force during the second half of the swing. `This was quite spectacular' says Nelson, and adds that no work has been done on the forces involved in limb swinging, so no one had thought that force sharing would break down.
Nelson suspects that that the failure of force sharing during the swing phase could be a general phenomenon applying to all non-weight bearing limb movements. He adds that it could help us to understand how we swing our arms,as well as explaining why a single strain can be such a pain.
