Now obviously we don’t have a bicycle wheel connecting the femur to the pelvis. But there are some attributes of a bicycle wheel, particularly the older kinds with wire spokes, that lend themselves to a better understanding of the hip joint and how it can function as a tensegrity structure.
(And it lends itself to understanding failures that can break the hip joint.)
Understand here that in some cases it can help to work with an actual bicycle wheel. At other times we can work with a theoretical “modified” bicycle wheel. At other times it can help to think of the process of building or fixing a bicycle wheel. And at other times it can help to think of how we could build a bicycle wheel to actually act like a hip joint.
Note that we’ll ignore the knee and sacroiliac joint for now. The rim of a wheel could be thought of as the pelvis and the hub as the femur (though it could be helpful to switch these points of view around from time to time).
So lets begin.
Bicycle wheels, are tensegrities, at least they are when the spokes are tightened (but not too tight.)
If you relax the tension in the spokes to the point of flaccidity, you have a hub that can flop around relative to the rim. The wheel is then not really useful for riding on. But even with little or no tension, the spokes are still long enough that they keep the hub and rim from impinging or butting into each other.
1. So there’s one way in which a bicycle wheel is similiar to the hip joint. You can relax tension so that the hub can flop around freely relative to the rim, and you can tighten tension. You can do the same with the hip, tighten muscles to make the hip stable, loosen them so that the thigh can flop around relative to the pelvis.
Now legs’ say that you had a bicycle wheel with a spoke layout such that you could pull (or push) the hub into contact with the rim. You could tighten one set of spokes (and perhaps loosen their opposite members) to pull the hub tight to the rim in one direction. Or you could slacken one set of spokes to the point that if weight where added to the hub you could allow the hub to sink towards the rim.
2. This too is something that can happen with the hip. Muscles (the equivalent of “active” or self-adjusting spokes) could be over active to the point that they pull the ball of the hip into contact with a portion of the hip socket. Or muscles could be flaccid so that in specific weight bearing situations the roof of the hip socket (or whatever part of the hip socket is uppermost) sinks down onto the ball of the femur.
If you’ve ever tried to true a bicycle wheel using a spoke wrench you may have noticed that the more tension you added to a spoke the harder it became to tighten further. The opposite happens when you loosen a spoke. It becomes easier to loosen the more you loosen it. Thus you could “feel” how whether a spoke was tight or loose. You would have to actively try to tighten or loosen the spoke but in this action you directly experience how much tension a spoke had. You could also try “plucking” a spoke to get a feel for how much tension was in it.
3. Working against, say, the weight of the leg you can feel tension in the various muscles of the hip. You can directly experience this tension by say, picking up a leg and lowering it and noticing the on/off changes in muscle tension. Or you could use a hand to feel the changes in muscle tension as you lift and lower a leg. You could even watch yourself in a mirror to see these changes.
Sticking with truing a wheel, if you are using a truing jig, or say a brake pad to true your wheel, you’ll notice that as you add tension to a spoke you can actually pull that section of rim to which the spoke attaches to one side. If the spoke you are tightening attaches to the right side flange of the hub then increased tension will pull that section of the rim to the right. This is a way of truing a bent wheel (though for a bend that is too big, it’s time to get a new rim). You either add tension to pull a section of rim towards the spoke or decrease tension to see if the section of rim retracts away from the spoke.
4. This experience is useful in understanding that when the hip joint is stable it is possible to use hip muscles in such a way to change the shape of the pelvis. As an example, pulling outwards or pushing inwards on the ischial tuberosities or ASICs.
Older type spokes wheels generally have spokes that cross one or two other spokes. (There are “radial” spoke patterns for wheels but we’ll ignore those as not relevant in modeling and understanding the hip joint). Lets focus on wheels where only two spokes cross. Generally the crossing point is close to the flange of the hub. The crossing helps to make the wheel more stable.
5. If we look at muscles of the hip joint they often cross close to the their point of attachment to the femur. You could think of this as overlap. Working around the hip, some muscle sets that overlap or intertwine include: gluteus medius and minimus, psoas, and iliacus and pectineus, obturator externus and the previous three, glute max (fibers that attach to femur) and quadratus femoris, adductor brevis and longus, piriformis and obturator internus, obturator externus and quadratus femoris.
Wheels are round and spokes are equally spaced around the whole circumference. As the wheel turns, no matter what position it is in (but assuming it is on a bike that is being ridden and the rubber side stays down) the spokes always help to maintain the distance between hub and rim. The hub (bearing the weight of the bike and body) is prevent from sinking relative to the rim.
6. Standing upright the obturators and gemelli can be used to provide a slight amount of lift of the pelvis relative to the thigh. In a standing forward bend, the pectineus, obturator externus and gluteus minimus may serve to keep the pelvis lifted. In a back bend, quadratus femoris (maybe) the gemelli and obturator internus may help.
If you had spokes which not only had adjustable tension but adjustable length, you could use these spokes to rotate the rim relative to the hub or vice versa. You could also use them to tilt the hub relative to the pelvis and vice versa. Even without these adjustable length spokes (actual spokes are lengthened or shorted by loosening or tightening them, but the change is slight) it is still possible to adjust spokes to vary the relationship between the hub and rim.
7. And that’s more or less the way muscles work. Increase tension in the muscles that rotate the thigh with respect to the pelvis in one direction (and reduce tension in the muscles that resist that movement) and you can move the thigh relative to the pelvis.
Bounce a wheel with loose spokes on the ground enough times (while it is bearing weight) and you’ll wreck it. The rim will bend and spokes will break.
8. Put all or even some of your weight on a hip that isn’t engaged, or jump or kick and land or strike with a leg in which the hip isn’t engaged and you’ll hurt the hip.
If you break a spoke or two, you could possible adjust tension in the remaining spokes. You’d also have to be careful when riding, perhaps on putting weight on the wheel where there are enough spokes in place. Hardly ideal but what can you do?
9. If you have inactive or injured hip muscles, pain may be used to signal when you are approaching a position where the hip won’t have the necessary support. And other muscles can be used to share the load, putting up with extra tension as a result.
Bicycle wheels have tires to absorb the tiny little shocks and make riding a more comfortable experience. The greater the tire pressure the less likely the rim is going to bump into the road.
10. That’s a little bit like the joint capsule of the hip. The joint capsule is the equivalent to the tire but instead of air it contains fluid. The joint capsule is tensioned to maintain fluid pressure so that whenever there is an impact the bones of the hip joint are prevented from crashing into each other. Now imagine, no matter how much weight the joint is bearing, joint capsule tension is varied so that fluid pressure is always sufficient to prevent bones from impinging. This could be the equivalent of varying tie pressure to suit the weight of the individual that is riding.