Muscles Affected by Sitting All Day

Spending a whole lot of time in an inactive posture isn't just a bad habit — your mom already told you that one. On a larger scale, it's such a widespread health concern that the term sitting disease long ago entered the health care lexicon.

Spending a whole lot of time in an inactive posture isn't just a bad habit.
Credit: Maskot/Maskot/GettyImages

If you do a lot of sitting, you're definitely not sitting alone; a November 2018 JAMA survey of nearly 6,000 American adults reports that one in four people sit for more than eight hours every day. As you might expect, that staggering statistic exerts an equally significant influence on the body — an influence that goes well beyond muscles to affect everything from the cardiovascular system to mental health.

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Sitting primarily engages core ab and back muscles while leaving the leg muscles at rest, but the effects of a sedentary lifestyle reach much further than simple muscle engagement.

What Happens When You Sit?

When your body assumes sitting posture, explains Cornell University, much of your weight is transferred to the pelvis, particularly to the ischial tuberosities, rounded bones at the pelvis's bottom also appropriately known as sit bones. In a sitting position with fairly even weight distribution, your legs meet your hips at roughly 135 degrees, with your knees bent at 45 degrees.

When your body sits, muscles do most of the work against gravity, as long as you practice proper posture. In this position, the muscles deep in your abdomen, pelvis and back — collectively known as the core stability muscles — support your upright posture. Muscles such as the rectus abdominis, iliocostalis lumborum and multifidus muscles help maintain spinal stability.

Because sitting leaves your leg muscles at rest, the Department of Health and Human Services at Victoria, Australia, warns that prolonged sitting can lead to weakening of large leg and gluteal muscles. Extended periods of sitting also stress the hip flexors, causing them to shorten over time and leading to potential hip joint complications.

According to Cornell University, sitting puts about 40 to 90 percent more pressure on your back compared to standing. Another of the key physiological problems with sitting is that you simply expend less energy compared to when you stand or move. And less energy expenditure means less calorie burn, which can lead to weight gain and cardiovascular complications.

Read more: 12 Simple Sitting Adjustments to Reduce Back Pain

How You Sit Counts

Your posture exerts a huge effect on the biomechanics of sitting. For instance, when you're in a neutral, relaxed and unsupported position, your center of mass is just above the ischial tuberosities, and the ground supports about a quarter of your body weight.

A small study of 37 adults published in the January 2019 edition of Gait and Posture finds that slouched sitting engages the bilateral obliquus and transversus abdominis muscles less than sitting more upright. A similarly modest field study of 13 office workers in the April 2013 issue of the Journal of Electromyography and Kinesiology notes that in a relaxed posture, the lumbar muscles experience very low activation. This transmits the load to ligaments and intervertebral discs, which may account for long-term lower back pain.

Your chair matters too. Tall chairs put pressure on the popliteal fold of the knees, which can decrease circulation, while a higher backrest encourages better trunk support and weight distribution. A small study of 70 individuals published in the February 2019 edition of Physical Therapy finds that posture does not affect pain levels in the rotator cuff muscles. Exercises, of course, can help relieve pain from sitting all day — but it's even better if that pain isn't there in the first place.

Too Much Sitting: Potential Effects

According to the Mayo Clinic, obesity is among the most prominent potential effects of too much sitting, often resulting in excess fat around the waist. The amount of calories you burn while sitting versus standing plays a role here, as it does in the many cardiovascular concerns of extended sitting, which include increased blood pressure and abnormal cholesterol levels. All told, people who sit for more than eight hours a day with no physical activity have a risk of dying similar to that of those who smoke.

In January of 2015, the Annals of Internal Medicine published an extensive review of 47 studies investigating mortality rates, hospitalization and the incidence of disease in sedentary adults. Their data found that prolonged sedentary time — including excess sitting — is associated with "deleterious health outcomes" such as an increased incidence of Type 2 diabetes, and increased both incidence and mortality rates for cardiovascular disease and certain types of cancer (including lung, uterine and colon cancers).

In September of 2018, the Journal of Applied Physiology revealed another interesting effect of prolonged sitting via a small study of 15 people. Researchers found that long periods of sedentary behavior such as sitting actually reduces cerebral blood flow, which in turn leads to lower cognitive functioning and an increased risk of neurodegenerative disease. On the same note, a more sedentary lifestyle is associated with increased instances of anxiety and depression.

As blood pools in your leg muscles while you sit, it may also lead to the typically harmless appearance of varicose or "spider" veins. In more serious but rarer cases, this can cause blood clots in the legs that have the potential to break off and travel to other parts of the body (such as the lungs), creating a condition known as deep vein thrombosis.

Read more: Is it Better to Sit or Stand While Eating?

Inactive Posture: Solutions

Filling your life with more movement will do a whole lot more than just help you maintain muscle tone. In an analysis of 13 studies spanning over 1 million people, Mayo Clinic concluded that just 60 to 75 minutes of moderately intense physical activity per day can counter the negative effects of too much sitting. At Mayo, Edward R. Laskowski, MD, says, "The impact of movement — even leisurely movement — can be profound. ... This might lead to weight loss and increased energy."

Some easily accessible ways to counteract too much sitting include:

  • Stand and move around every 30 minutes (download an alarm app on your computer to help).
  • Stand while you watch TV.
  • Use a treadmill as a work surface for your laptop.
  • Do housework or take walks while engaging in audio content such as music, audiobooks or podcasts.

Read more: 14 Exercises to Offset Sitting All Day

You can even burn more calories while at your desk by using a standing desk, taking phone calls while standing up or walking around, or intentionally moving your trash bin away from your desk to encourage yourself to take small, frequent walking trips. When it comes to counteracting the sitting slumps, even a little activity can go a long — and worthwhile — way.

"The impact of movement — even leisurely movement — can be profound." — Edward R. Laskowski, MD, Mayo Clinic

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