Originally Posted by Johnstown
i would think running on sand would be good for the joints???
another thing i have heard is varying the surface you run on is extremely bad. Your body can not adapt and hence it can make you very pron to injuries (happen to soccer players who also run on the street).
Indirectly running on sand will be good for joints since you're using more muscles (greater muscular conditioning effect, reduced fatigue after adaptations) and getting more sensory input to mechanoreceptors on your feet, however if you did a lot of volume in a session it would be every bit as risky as running on pavement.
Varying the surface might be harmful if you spend a lot of time on one or the other. When a soccer player runs on grass it is soft so their system is stiff, they rely more on recoil of tendons which results in a lower metabolic cost due to less muscle activation.When they are running on a hard surface their system is softer or more compliant so they are using their muscles more to power them. When they switch to a hard surface their muscles may not be conditioned enough so they fatigue quicker and increase their injury risk. The forces to the knees etc. are the same, your body regulates force very well.