When using bi-articular muscle (like the hamstrings) how does the body choose which joint in moves?
In: 3
By tensioning other muscles to “freeze” the joint you don’t want to move.
For the hamstring example, contracting your hamstring can bend your knee or bend your entire leg back (or both).
To just bend your knee, you use other muscles around your hip to lock your hip joint, so only the knee can flex.
To just bend your hip joint, you use your lower leg muscles to lock your knee joint, so only the hip can flex.
This isn’t a conscious decision by your body, it’s just part of how your coordination works.
Edit:typo
From what I remember in anatomy the mechanics of these muscles isn’t understood very well. However for the hamstrings, they work with several other muscles to produce a range of movements. Therefore when the brain sends a signal for a specific movement a specific group will be triggered. Quadriceps femoris is the only extensor of the knee (I think) and so that would guide the overall movement of extension. Extension is most useful in walking, if the knee is extended this then dictates that the next movement should come from the hip.
It should also be noted that within muscle groups, muscles are recruited for movement in a specific order to facilitate a smooth movement which will pose the lowest risk of injury. The nervous system is responsible for telling the muscles when to contract. So for muscles across more than one joint, it would make sense that the joint they affect when they contract will be determined by the other muscles which are stimulated at the same time. Hamstrings affect a certain amount of torque on the knee that means it would be in constant flexion, however the extensors of the knee can overpower this.
Overall, the bi-articular muscles are thought to be “designed” as such so they operate within their optimal range (of lengthening and shortening)
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