Excessive sitting will shorten your hip flexors. I have read this sentence many times. What I have never read is a citation to supporting evidence at the end of the sentence. It’s one of those claims that no one ever questions, like you should drink eight glasses of water a day. Let’s see how it stands up to some critical inquiry.
How might we test the hypothesis that sitting will shorten the hip flexors? I can think of three ways. If the hypothesis were true, we would predict that: (1) there would be a correlation between time spent sitting and hip flexor length; (2) there is a plausible physiological process for how sitting would shorten the hip flexors; and (3) healthy animals engaged in natural functions would avoid placing their joints in shortened positions for long periods of time, like we see in sitting. Let’s examine these predictions in turn.
1. Studies on sitting and hip flexor length
Are there any studies that find a correlation between length of time sitting in a chair and hip flexor tightness? Searches on google scholar and pubmed turn up nothing. Strike one, but absence of evidence is not evidence of absence. So let’s move on.
2. What is the Mechanism?
Is there any plausible physiological process that could explain how excessive sitting could shorten the hip flexors? It is well known that completely immobilizing a joint for an extended period of time can lead to loss of muscle sarcomeres and contracture and cross linking of connective tissue. However, it appears that one can fully prevent any negative effects of extended immobilization on tissue length with only short and infrequent bouts of movement. In one study, just half an hour of stretch a day preserved range of motion and muscle length in a muscle that was immobilized in a shortened position for the the rest of the day. This suggests that sitting in a chair almost all day every day would not cause loss of tissue length, provided you get up to go the refrigerator every once in a while.
3. Is it "natural" to keep the hips flexed for long periods of time?
If sitting in a chair many hours a day leads to shortened hip flexors, we might predict that it is generally unhealthy to hold the joints in one place for a long period of time, and that healthy animals engaged in natural behaviors would avoid this. But this does not appear to be the case. It is easy to think of examples of animals who hold static limb positions for long periods of time without loss of function. Dogs and cats sleep twenty hours a day with a minimum of movement or shifting around. When they rise, they stretch half a second and then go sprint. They don’t experience shortening of the muscles that were contracted while sleeping.
Many people sleep on their sides with their hips flexed at ninety degrees for eight hour stretches. Hunter gatherers surely spend many hours a day sitting on the ground with flexed hips, and in a deep squat position, which involves far more hip flexion than sitting in a chair. Why would sitting in a chair tend to shorten the hip flexors any more than these completely natural uses of the limbs? Surely human joints evolved so they do not start to knit themselves together after a few hours in the same position.
And consider the shoulder. How much time do you spend every day with your hand at your side, as opposed to fully stretched over your head or far out to the side? Probably not much, but range of motion in these directions are preserved with just the occasional reach. It's fine to spend most of the day with your hand by your side, and its probably also fine to spend long hours with the hips in flexion.
Of course it is true that sitting in a chair at a computer all day isn't what nature intended for the human body. But the unnaturalness of this activity does not consist in having the hip joint flexed to ninety degrees for hours at a time. The worst part of sitting in a chair is probably that it displaces many other healthy movements we could be making instead. In fact, excess computer time probably messes with human movement in many significant ways, but I doubt very much that any of them involve making the hip flexors structurally shorter.