Our expert says:
For all intents and purposes, yes it is. There are some very subtle biomechanical differences, but these are insignifcant during walking, and I would not consider them in this argument. So then the exercise is the same. The only tiny difference is that there's no wind resistance on a treadmill and that's why normally a gradient of 1% is considered to simulate outdoor exercise. That's probably less during walking though - maybe 0.5%.
As for the knee, what does "bad knee" mean exactly? If it's a simple injury, then maybe strengthening the muscles in your quads would help. If it's a serious chronic problem, then you need to seek out assistance, because only a one on one consult would help identify the cause, and hence the treatment.
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