Bumps on your outer thighs after a workout could be caused by a number of triggers but are likely due to physical activity. Exercise leads to an increase in body temperature and sweat, which is known to cause a few conditions that result in bumps and rash. The outer thighs are not the most common area to be affected. However, the thighs could be the only place you get bumps or one of several areas.
Bumps on your outer thighs after working out may be due to exercise-induced urticaria. Exercise-induced urticaria causes allergy-like symptoms, such as hives or welts. Hives are typically flat, raised bumps ringed with red. The hives may appear as blisters, blotches or red spots anywhere on your body, including the thighs. This condition may also cause itchy skin, headaches, trouble breathing, stomach cramps, and swelling of the hands or face. Exercise-induced urticaria may be fatal in very rare situations, so see your doctor for medication if your symptoms do not disappear within 10 minutes of ceasing exercise.
Miliaria is a condition that leads to bumps on the skin in areas affected by clogged sweat ducts. It is also called sweat rash. Being in hot climates, exercising, wearing too many clothes or sitting too near a source of heat are often contributing factors. Miliaria can occur on anywhere on the body though the usual sports are in skin folds or areas affected by clothing friction. Skin under prolonged exposure to sweat may cause a blockage of sweat ducts for about three weeks. The exact cause is unknown but could be linked to an increase in skin bacteria.
Exercising hard enough that you sweat extensively can cause heat rash. This condition causes bumps and itchy skin. It can also lead to symptoms of heat exhaustion, such as nausea, dizziness or a rapid pulse. Heat rash develops when clogged sweat ducts trap sweat beneath the skin, causing inflammation and rash. Exercise can contribute to the clogged sweat ducts, though the reason this happens is unknown. Heat rash often affects the groin, neck, chest and shoulders. Therefore, if you have bumps from heat rash on your outer thighs, you likely have bumps on your inner thighs and upper body, too.
These conditions do not necessarily mean you have to stop exercising, although your doctor may recommend you avoid certain types of exercise to decrease sweating. Your doctor may also prescribe medications or topical lotions. Antihistamine medications may help people with exercise-induced urticaria. Antibiotics may help miliaria. Topical steroids may also help people with miliaria and extreme cases of heat rash. Workout only in cool, air-conditioned places and wear light, breathable fabrics to prevent triggering these conditions. Breathable fabrics help keep you cool and are less irritating to the skin.