The great thing about running is that you can do it almost anywhere and with very little equipment. Although one of the most attractive places to go for a run is the beach, the softer surface of the sand presents your body with different kinds of stresses that can show up in your joints. And as soft sand running takes more energy and offers more resistance than running on a road or on wet sand, start with shorter runs.
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Foot and Ankle Problems
When you are running at the beach, the first contact between you and the sand takes place at the heel of your foot. That landing sets the stage for the rest of your foot to land properly. If your heel rolls in, the arch of your foot will collapse earlier than it should, and you might develop tarsal tunnel syndrome or plantar fasciitis due to the pronation. If your heel rolls outward, you might supinate, so the arch of your foot might stay high, and you will lose some of your natural shock absorption. That sets the stage for problems higher up, in your knees and back.
The most common running injury at the beach is knee strain. If your foot doesn't track straight, but rolls in or out, it places abnormal stress on your knee, which is a hinge joint and is only designed to bend in a straight line. If you add lateral stresses to it, like you do with pronation or supination of your foot, you can strain the ligaments or stress the internal ligaments in the knee.
Hip and Lower Back Problems
Following the stress from running up your leg, the hip is the next joint to bear the stress. Iliotibial band syndrome occurs when your hip is twisted with each step, irritating the ligament that runs from just over your hip down to the outside of your knee. Your lower back works like a shock absorber for the rest of your spine, and whenever the leg doesn't absorb shock the way it is designed to, more demands are placed on the joints of your lower back. This can stress the discs of the spine, causing more wear.
Prevention and Management
You will do better if you stretch before you run. Pay particular attention to calf stretches. Also, watch the surface in front of you. Wet sand is usually harder, but might get soft when the wave runs in. Try to run on the flattest part of the beach because the slope might be stressful to your feet and knees. Remember that your feet hit the ground about 1,000 times per mile, so a little stress can add up. Try to run only on the sand; not switch from road running to sand running, as the stress on your Achilles tendon may be too much.