Asphalt Roads/SidewalksRun Freely: If you're not particularly injury prone and not rehabbing an injury—although Achilles tendinitis becomes less aggravated on asphalt's stiff surface, which keeps the Achilles in a shorter, less-tensed position. Tread Lightly: If you've recently had shin or knee pain, or a fracture, muscle pull, or iliotibial-band syndrome (ITBS). Asphalt can rattle bones, joints, muscles. Stretch and strengthen your knees with these exercises
Dirt TrailRun Freely: If you've struggled with runner's knee, ITBS, shinsplints, or any injury aggravated by impact. Just be wary of technical trails that cause your feet to land at an angle. Tread Lightly: If you've had an ankle sprain. The ligaments of that joint never recover 100 percent. That increases your risk of a repeat sprain, which is more likely to occur on soft, uneven surfaces.
SandRun Freely: Rarely. Sand is unstable and puts a ton of torque on the knee, ankle, and hip. That said, this surface requires the strength of muscles that are often neglected so it can be beneficial for strength building. Just keep the runs short.
Tread Lightly: If you have a history of ankle sprains or Achilles tendinitis. A 2008 study found that running on sand increases the risk of Achilles pain tenfold.
Synthetic TrackRun Freely: If you're prone to ankle sprains or just recovered from a fracture. The surface is predictable with no roots or curbs to trip over. It's also more cushiony than asphalt, but not so soft as to cause instability. Tread Lightly: If you've had ITBS or calf strains. Your outside calf is shortened as you circle your way around. Tight corners can also stress your inside leg's ITB.
Prevent and heal most of your injuries with this guide to the runner's body.
GrassRun Freely: If you have knee pain or are returning to running after a fracture. On grass, the bulk of the energy from your footfall goes into the ground instead of reverberating back up your leg.
Tread Lightly: If you're prone to plantar fasciitis. You're more likely to overpronate on this soft, uneven surface, which puts extra torque on the ligament that runs along the bottom of your foot.
Run Freely: If you're recovering from injury or are increasing mileage and want a break from asphalt. The belt's cushioned surface reduces stress to your back, hips, knees, and feet. And it's a clear path free of obstacles.
Tread Lightly: If you're training for a road race. Running exclusively on a treadmill won't prepare you to navigate uneven terrain or cope with the impact of asphalt, and that could lead to injury.
Athletes who do plyometric drills on sand improve their sprinting ability compared to those who train on grass.