Is "Efficient" Movement Unsafe?


I have made the claim several times on this blog that quality of movement is primarily determined by its efficiency. In fact, I have stated that "efficiency is the "essence of coordination" and that the best movement is always the most efficient movement.

I defined efficiency as "the ratio of useful work performed to the energy expended to do the work." The logic of this idea is discussed here, but to briefly summarize, great movers such as athletes and dancers appear to move in an effortless fashion. For example, an elite marathon runner seems to float over the ground, and in fact their gaits are more energy efficient than less accomplished runners. Further, our bodies must have evolved to move in a way that is both energetically efficient and mechanically unstressful, so it would be surprising if one quality came at the expense of the other.

Well I recently read a post from Bret Contreras, a blogger I respect, and he basically took the exact opposite position! In this post (which in fairness is really addressing a separate issue) he discusses his idea that most "crappy" movement patterns such as valgus knees actually result from being too energy efficient, and that our natural inclination to efficiency is kind of like a pathological laziness that causes bad form and will compromise joint safety in the long run! I am not convinced this is true, but he does have an interesting point that I want to address.

Are Rag Dolls Efficient?

The problem with defining optimal movement purely in terms of energy efficiency is illustrated by a simple example. The most energy efficient way to land a jump is to just collapse on the floor in a heap like a rag doll. That would minimize energy expenditure, but I think we can all agree this is not the optimal way to land a jump.

That is because even though the "collapsing in a heap" method imposes a very small cost in terms of energy, it imposes a large cost in terms of mechanical stress on the body. So, in the context of jump landing, there is a conflict between energy expenditure and joint safety - you can either minimize the damage to the joints or the energy expended, but not both at the same time.

Is this just a weird exception to my general rule that good movement patterns are energy efficient? Or are there other examples of a conflict between energy efficiency and joint stress?

Bret's post refers to many of the movement flaws he sees as a trainer, such as overpronation, valgus knees, excessive low back flexion, or other forms of postural collapse. These "floppy" movements reduce energy demands on local muscles because they transfer the work of stabilization to ligaments and other connective tissues. This saves energy but creates mechanical stress because body weight is just "hanging" passively off the ligaments and connective tissues.

I agree that hanging off your ligaments can reduce the work of local stabilizing muscles. But I think it is probably the case that in most contexts, any local gains in energy efficiency from "floppy joints" are more than offset by a general loss of energy efficiency that comes from poor alignment of the bones and inadequate stabilization of the joints.

In other words, valgus knees, rounded backs, and overpronated feet are not actually energy efficient at all, because they sacrifice the stabilization and proper bony alignment which is the key to efficient movement and posture. In fact, they create energy leaks that destroy any ability to efficiently transfer force to its intended target.

The bones transfer compressive forces through the body at no energetic cost, and this benefit is lost if the bones are not well aligned. For example, a valgus knee or flat foot might save some muscular work in the lower leg or glutes, but it will compromise the stability and bony alignment that is necessary to efficiently transfer ground forces to the rest of the body. If you want to create effortless power in a throw, jump or punch, you need good bony alignment. This will make the movement safer by reducing shearing forces, and more powerful by accurately channeling energy to a target.

Thus, the floppy joint strategy usually fails in terms of energy efficiency, just as it does in terms of joint safety. That is why the most efficient athletes don't look floppy. They are great examples of alignment that would please any trainer concerned with minimizing mechanical stress. So if we think in these terms, there is not so much of a tradeoff between energy efficiency and joint safety.

But the jump example shows that there is at least some tradeoff, and a study I came across recently (via Alex) also provides some interesting food for thought on this issue.

Running from Paris to Beijing

In this study, researchers gathered data on an experienced ultra marathoner three weeks before, three weeks after, and five months after running from Paris to Beijing. Yes, he ran from Paris to Beijing.

Three weeks after the race, his gait had changed in several ways - his stride was shorter, less forceful, involved less aerial time, and was less energy efficient than his stride before the race and 5 months after.

The researchers concluded that as the joint stress of running increased, he changed his movement strategy in a way that imposed less cost on the joints and more cost on energetic demand to move. In other words, it appears that maximum energy efficiency was not consistent with maximum joint protection, at least for this (completely insane) runner.

So here is evidence that we sometimes need to choose between movements that are safe and those that are energy efficient.

But as I said before, I think this is the exception to the rule. If you watch the athletes who have the highest level of skill in being energetically efficient, you will generally see movements and postural alignment that also maximizes joint safety for the particular job being done. So for the most part, energy efficiency and joint safety travel together. (But maybe not all the away from Paris to Beijing.)

What do you think? Let me know in the comments.