Muscle Response in Isometric Exercises

An isometric exercise is a static contraction in which you hold a contracted muscle in a fixed position against resistance without changing the length of the muscle. A standing calf stretch for your legs or pushing against a wall to work your upper body are examples of isometric exercises. Your muscles respond the same whether you are doing isometric exercises manually, using your body or the ground for resistance, or with the assistance of a prop or partner.

  1. Muscle Fibers

    • As you start an isometric stretch or exercise, only a subset of the muscles fibers in the group you're focusing on will lengthen; the other fibers will remain at rest. When the contraction is held, the contracting muscle fibers pull on the resting muscle fibers -- this causes your resting muscle fibers to stretch. Your muscles respond to isometric stretching over time and help to increase flexibility.

    Blood Flow

    • During an isometric exercise or stretch, blood is forced out of the muscle you’re working and into your bloodstream. As the muscle becomes tense and blood moves out, your blood pressure increases. According to the Mayo Clinic, a large increase in muscle tension all at once during an isometric exercise can lead to significant changes in blood pressure. When the blood flow in your muscle is reduced, metabolic by-products start to build up. As these accumulate, they stimulate chemical and mechanical receptors within the muscle that send information through your nervous system and back to your brain. It's at this point when your brain starts to send signals that you’re feeling tension or some discomfort in the muscle.

    Fatigue

    • Isometric exercises that are sustained or repeated over a period of time result in muscle fatigue. As blood leaves the muscle, lactic acid begins to build up, and the balance of available electrolytes -- such as calcium and potassium -- is disrupted. Due to this process, the muscle will become fatigued and lose the ability to sustain a contraction.

    Considerations

    • Consult with your doctor or health-care provider to make sure isometric exercises are right for you. According to Dr. Edward Laskowski, a physical medicine and rehabilitation specialist at the Mayo Clinic, isometric exercises are usually not recommended for people with heart problems or high blood pressure. If you're medically cleared for isometric exercises or stretches, do them after your body is warmed up or as part of your cool-down. Doing isometric or static exercises as part of a warm-up can decrease your performance and endurance in the activity or sport you're getting ready for.