How does fixing the knee at a flexed angle affect motor unit recruitment and lower leg muscle activation?

When we look at the data, we found that 26.0 studies support, 0 studies refute, this pattern. What we have found so far suggests that keeping your knee bent alters motor unit recruitment, which is simply how your nervous system selects and activates specific muscle fibers. In practical terms, holding your knee in a bent position appears to slow down the calf muscle that points your foot downward. At the same time, it seems to speed up the muscle group that helps you push up onto your toes. The evidence we have reviewed leans toward the conclusion that the exact angle of your knee directly influences how your brain coordinates these lower leg muscles ¹.

We want to be clear that this is a partial view based on the research we have analyzed to date. Our current analysis shows a consistent pattern, but we continue to track new findings as they emerge. The evidence we have reviewed leans toward a clear shift in muscle firing patterns when the knee is fixed in flexion, though we acknowledge that individual responses may vary.

For everyday movement, this means that simply adjusting your knee angle can change which lower leg muscles do the most work. If you are doing exercises or daily activities that require you to keep your knee bent, you can expect your toe-lifting muscles to take on more of the load while your downward-pointing calf muscle rests a bit more. We will keep monitoring the research to refine this picture as more data becomes available.