Knee injuries are very common, especially if you take part in sport. Your knee is a complex joint. Sometimes the bones of your knee can be damaged but it's more likely that you will injure soft tissues such as ligaments Spend five to 10 minutes warming up before exercising and cooling down afterwards. Stretch out your muscles once they are warmed-up - don't do this while they are cold as you could damage them - and then again after you have cooled down. This reduces the risk of injuring yourself and helps to prevent stiffness later.

Hello Panayotis,
Reckoning your concern, i would advise your husband to do static exercises for his knees and ankles, thereby strengthening the muscles around them. Strong muscles would prevent undue strain on the ligaments. Check for quadriceps, hamstrings and ankle strengthening exercises with your physiotherapist or on the internet.