The answer has long been attributed to motor functions: the arms flex as movements are applied – and when you’re working on a task that causes your head to rotate. This is how people can move their arms to pick up an object. That’s not the whole story, however. It turns out that as the arm makes the transition from being an extension – an “up” from the shoulder – to being something that moves independently, the muscles that control movement also change. This creates two different muscles in the lower half of the arm: a longer and more flexible forearm, which also flexes, and a longer and less flexible forearm, which also tends to be a bit stronger and more efficient at lifting objects. In other words, humans are able to use their arms more flexibly when they move more easily.
The study is the first to use data from motion-capture video to measure these properties in full, rather than on just a single participant. Although this means that the effects observed aren’t as strong at individual joints – the researchers know, for example, that some muscles in the lower arm are harder to capture using motion capture cameras than others – and because it’s an observational study, it provides a much richer set of data than would be possible simply from participants doing simple tasks.
The researchers used an accelerometer to measure the movement of the wrist as the participant walked. This was done by measuring how rapidly the hand moved over the wrist during some of the movements that each participant had to make – and how accurately the hand was tracking that movement (e.g. how slow or fast the hand was moving over the wrist). They also measured the force that each of the three movements produced, and the torque that each action produced from the wrist joint (or arm joint, as some experts would have it). In summary, they were able to capture over 150 individual wrist motions of about 2.5 degrees in length, each in roughly the same speed as the movement.
The researchers analysed these data to see how they differed between the different people, both when the wrist motion was recorded and from motion captured without motion-capture. They found that both the wrist and arm motion were different in people with various levels of mobility. People with weaker mobility, such as a lack of hip mobility or a large amount of wrist movement, had a different motion-capture effect compared to those with stronger mobility. The movement in the lower arm – more flexible as a result – was also different if the person was standing or sitting; this
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