How muscles rip

I recently updated my book about muscle strains with more scientific detail about the exact mechanism of hamstring strains, particularly explaining the dominant theory, and then looking at who disagrees and why. Here’s a good sneak peak behind the paywall, including 8 footnotes visible only to logged-in PainSci members. LOGIN

Most muscle tears happen during contractions that are not only eccentric, but fast and powerful.12 They are most common in sprinting, for instance, and the harder you run, the higher the risk.

The hamstrings usually tear this way, mostly in the instant before foot-strike, at the end of the “swing phase.”3 At that point in the gait cycle, the hamstrings are doing the eccentric thing: lengthening and contracting at once, putting the brakes on that forward swing, slowing the foot down before it connects with the ground.4 The biceps femoris muscle may be the most likely part of the hamstrings to rip, because it is working harder and lengthening just a little more — a quirk of anatomy, accentuating the eccentric contraction — than the other “strings” (semitendinosus and semimembranosus).5

Settled science? Mature kinesiology? Probably … not! All those citations are getting pretty old, and research has continued. The late-swing-phase hypothesis was questioned by Van Hooren and Bosch in 2018,6 Maybe injury happens a split second later, due to “large joint torques and ground reaction forces during early stance” — although “direct evidence is still lacking” (as of 2019).7

I followed that with a re-write of a short chapter about the role of movement mistakes in muscle strain: “Movement mistakes and micro-clumsiness: zigging when you should have zagged.” It was always tiny and basic, not really worthy of being called a “chapter.” (I used to call them “sections,” back in the day, when were all much shorter on average.) I probably hadn’t even looked at that particular chunk of text in a decade. But there was a good idea at the heart of it, and that idea has now been expressed more clearly and thoroughly than my original attempt: a "chapter" at last! Albeit still a short one. Here’s some of it:

You could certainly make a case that our wise bodies should “know” better than to ever contract too hard while a muscle is lengthening (eccentric contraction) — and there are indeed dazzlingly intricate reflexes that prevent exactly that, of course, and they mostly work extremely well.

Yes, yes, very impressive … but we are also a species that stubs our toes on the regular. 😜

Movement mistakes happen. Sometimes we zig when we should have zagged! Sometimes the intricate reflexes fail because we made a poor split-second “decision.”8 I speak from plenty of experience as a somewhat clumsy “athlete” (mostly retired now). I suspect many coordination mistakes involve conflicting coordination priorities — quite literally trying to do two things at once (at least), to the detriment of both. I speculate that this is one of the major basic reasons why the intricate orchestration of the sprinting muscles sometimes produces a sour note, and occasionally just fails catastrophically.

Not subtle, technical, kinesiological vulnerabilities (like the “pelvic misalignment” of Oleksy et al). I suspect the unsubtle elephant in the room is mostly just … “clumsiness.” A citation to support this idea would be great, of course, but I don’t have one of those.

That’s me playing ultimate about twenty years ago — coordinating a lot of fast, powerful muscle contractions, mostly without injury. But not always! I think I probably had about a half dozen mild to moderate muscle strains in my 25-year ulti career.