Oh, My Aching Back

Oh, my aching back. I was unloading a dresser from a van, and I thought I could handle it myself. What a mistake. I think I strained a muscle in my lower back; probably the erector spinae. Then I aggravated the injury when mowing the lawn. Ouch.

Back pain is interesting. Mine is intense for a few specific movements, and otherwise hardly bothers me. For instance, if I lean over to tie my shoes, it hurts. I feel fine when walking my dog Harvest, except when I bend over to pick up her poop. I was able to paint the bathroom two days after the injury—which involves a lot of reaching up—with no discomfort. Rising from a chair, however, is painful. Driving is no problem; my car seat feels particularly comfortable. Here’s one that surprised me: my back hurts when I sneeze.

I know better than to lift a heavy load like that, using my back instead of my legs. Every time I teach Biological Physics (PHY 3250), the students and I solve Problem 10 in Chapter 1 of Intermediate Physics for Medicine and Biology, which begins “consider the forces on the spine when lifting…” The problem is intermediate in difficulty, and the figure associated with it is shown below.

My back pain feels lower than where the erector spinae muscle attaches to the spine (the insertion point). I suspect the injury was to the other end of the muscle (near its origin), where it attaches to the pelvis (or more correctly, the sacrum).

The homework problem is a simplification of the true geometry of the spine. It is a toy model, which is useful for gaining insight but should not be taken literally. For instance, the erector spinae is actually a muscle group consisting of the iliocostalis, the longissimus, and the spinalis, which all have slightly different origins and insertion points. The spine is neither stiff nor straight.

An interesting feature of this homework problem is that you can solve it using one of two natural coordinate systems: horizontal (x) and vertical (y), or along (x’) and transverse to (y‘) the spine. In the solution manual for IPMB, Russ Hobbie and I use x‘ and y‘. Students might benefit, however, from solving the problem both ways, so they can see that the choice of coordinate systems doesn’t matter.  

The solution manual has a short preamble for each problem, explaining its goal. The preamble for Problem 10 says

This problem helps students develop physical intuition about forces and torques, and is our first example of a mathematical model in which the students can examine limiting cases to build physical intuition.

The main message of this problem is that you should lift with your legs while keeping your back upright. If you lean over to lift—like I did—the forces must be huge to balance the torques acting on the spine. The solution manual says

The force on the spine by the pelvis is over seven times larger if the spine is horizontal than if it is vertical. You really should “

lift with your legs (θ = 90º), not with your back (θ = 0º)”!

I guess you could say I’ve developed a new laboratory experiment for IPMB: first lift with your legs and then with your back, and see which one hurts the most!

How am I treating my injury? Mostly with ibuprofen (for the pain and inflammation). Once the worst of the pain is gone (it’s healing rapidly), I’ll begin gently exercising my back, slowly building up strength. I’m not prone to these types of injuries, so I hope this is a one-time problem that will soon be resolved.

From now on, I’m going to take the lessons learned from Intermediate Physics for Medicine and Biology more seriously.