Immobility can lead to joint stiffness, weakness and an undernourished cartilage, so you need small movement to increase blood flow and promote healing. This is called biomechanical stimulation. The tradeoff for this activity or stimulation is that the little bit of pain you may feel in the short run will give you optimal flexibility and strength in the long run. That's why many walking boots and braces these days allow you to have some range of motion (casts not so much). This, of course, is an example of a biological trade-off, like breaking an addiction or dieting: The short-term pain is small compared to the long-term gain, which can be huge.
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