Swimming is a low-impact activity that can help prevent injuries, but weakness in your left leg can interfere with your workout. Contact your doctor if you have significant weakness in one leg while swimming because you might have an underlying medical condition that requires treatment.
Background
Strong, consistent kicking helps you tread water and perform lap-swimming strokes such as the freestyle, breaststroke and butterfly. You might not be able to swim as fast or straight with a weaker left leg compared to your right, or you might have trouble staying afloat. If the weak feeling develops as you start to become tired, you might be able to prevent the weakness by warming up slowly and pacing yourself during your workout. If you cannot figure out the cause of recurring weakness in your left leg while swimming, your doctor might be able to help.
Muscle Cramp
Cramps are prolonged contractions of your muscles, and they can occur during repetitive exercise. Lower leg cramps in your calf or other muscles in your legs can cause pain and weakness, according to MayoClinic.com. You might have trouble controlling your leg or completing your activity. You are at higher risk for cramps while you are swimming or doing other exercise if you have not gotten enough potassium in your diet. Many fruits, vegetables and whole grains are good sources of potassium, according to the Linus Pauling Institute Micronutrient Information Center.
Muscle Imbalances
You might not notice muscle imbalances when you are not exercising, but a weak left leg will become apparent while you are swimming. Movements in swimming require coordination among the muscles in your hips, quadriceps, hamstrings and calves. Exercises targeting these muscles can make your left leg as strong as your right leg so that neither side gets too fatigued while swimming.
Considerations
A pinched sciatic nerve can cause sciatica and symptoms of weakness or pain down one leg, according to MedlinePlus. Reducing your exercise temporarily might improve your symptoms. You might need medical attention if you have numbness or tingling while you are swimming. Possible symptoms of a stroke include weakness or numbness on one side of your body, according to MedlinePlus. Other possible reasons for a weak left leg while swimming include a vitamin B-12 deficiency and use of certain medications.
