Whether stumbling over a rocky river bed or clambering up hill, every creature continually adapts their movements as they travel. Each joint is operated by several muscles, some of which work together to flex and extend,while others work in opposition. Tim Higham explains that it had commonly been assumed that muscles that work together contribute equally to joint movements,and that each muscle contributes uniformly. But Higham and Andy Biewener decided to test this. Could muscles that work together contribute differently to joint movements? And do muscles contribute uniformly along their length?Higham and Biewener put their favourite biped, the guinea fowl, through its paces to see how much work and force the synergistic medial and lateral gastrocnemius muscles contribute as the bird walks and runs on the flat and uphill (p. 2303).
`Guinea fowl are great to work with,' says Higham, `they run forever, are happy on a treadmill and have large limbs'. However, despite the birds'sizeable legs Higham admits that placing force transducers on the birds'tendons, and EMG electrodes and sonomicrometry crystals in the birds' medial and lateral gastrocnemius muscles was a rather daunting process. After hours of surgery, Biewener and Higham eventually had four birds ready to set running and walking on flat and inclined treadmills to see how the muscles performed.
When the birds were walking on the flat, the medial and lateral gastrocnemius muscles contributed similar amounts of work. But it was a different matter when the birds started running. The work done by the lateral gastrocnemius rocketed, while that done by the medial gastrocnemius increased much less. The muscles were not contributing equally as the birds ran. However, when the birds walked and ran up hill, their muscles only worked a little harder than if they were moving on the flat; `speed influences the distal muscles of guinea fowl to a much greater extent than incline,' says Higham.
The biggest surprise came when the team turned their attention to the medial gastrocnemius. With length measuring sonomicrometry crystals embedded in the ankle and knee portions of the muscle, the team could see how much mechanical work different regions of the muscle were doing. Amazingly, the region of the muscle closest to the ankle did virtually no work; `the distal part of the muscle didn't change length at all,' says Higham. At first the team were surprised, but after watching each bird run they realised that the differences were real and even greater than the differences between the two muscles.
Why do the two ends of the medial gastrocnemius contribute so differently to the mechanical work that moves the bird's leg? Higham explains that there are regional differences in fibre types along the length of the medial gastrocnemius, which result in different performances in the two regions. Also, the ankle portion of the muscle is encased by a relatively stiff aponeurosis membrane, which restricts the amount of mechanical work that the ankle portion of the muscle can do.
Having found that different muscles, and even different regions of the same muscle, contribute disproportionately to the mechanical work required to move guinea fowl leg joints, Higham and Biewener are keen to find out more about the mechanisms that fine tune limb movements.