The muscle groups are a great feature, and it would be even better if the could be keyed directly.
Since multiple muscles affect the same bones, the way I'm picturing this working is that keying a muscle group would create 'virtual keys' in the timeline of the affected bones. These keys would not be editable, and their values would be continually calculated by combining the effects of all the muscles driving them.
For example, a bend of the torso accompanied by a back-and-forth twist would comprise:
- two keyframes for the bend (on the 'torso bend' muscle curve)
- four keyframes for the twist (on the 'torso twist' muscle curve)
- six virtual keyframes on the affected bones, the values of each reflecting the cumulative effect of the keyed bones
At any point the user could 'bake' any individual virtual key to create directly editable normal keys (a one-way process).
The advantage to the user is that different muscles in the same group could be keyed and edited at different points on the timeline, making it possible to (as in the example above) superimpose a back-and-forth twist on a bend, and then go back and separately tweak the bending or twisting element of the motion without having to adjust the keyframes of the other.
I fully expect to be told this is actually possible already, but I really, really did look quite hard this time first before asking XD
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