Some runners may experience injuries when running on concrete surfaces. The problem with running on concrete is that the body adjusts to this flat surface running, and some of the muscles will become weaker, along with the added impact of running on a harder surface. Therefore, it is advised to change terrain occasionally – such as trail, beach, or grass running. This is more unstable ground and allows the legs to strengthen different muscles. Runners should be wary of twisting their ankles on such terrain. Running downhill also increases knee stress and should, therefore, be avoided. Reducing the frequency and duration can also prevent injury.