Gait appears most robust to weakness of hip and knee extensors, which can tolerate weakness well and without a substantial increase in muscle stress. In contrast, gait is most sensitive to weakness of plantarflexors, hip abductors, and hip flexors. – van der Krogt
In the past few weeks I have shared my thoughts on some articles regarding low back paraspinal musculature fatigue and the subsequent effects on motorneuron pools, specifically excitability of the soleus and quadriceps. These shared thoughts are from recent papers in the literature (search the blog over the last week). These effects are suggested to indicate a postural response to preserve lower limb function. In other words, as paraspinal fatigue set in, lower extremity muscle compensation ramped up to sustain postural locomotion demands. Obviously, one should think this a step further and translate it all into questions of assessment of ankle dorsiflexion (ankle rocker) and control of progressing knee and hip flexion when pertaining to these muscles. The issues of stability and mobility should heighten. The one big problem in these studies, and you have even likely had these thoughts during your clinical examinations, is that one cannot truly fatigue one muscle group alone especially during activity, nor can one assess a single muscle group during manual testing. Luckily we have EMG testing capabilities in this day and age and we can more easily look into the function and reaction of a muscle and its’ direct response reactions.
Today I have an article by van der Krogt that we read long ago, but that which one of our readers brought back into our wheelhouse. This is pretty amazing stuff.
“This study examines the extent to which lower limb muscles can be weakened before normal walking is affected. We developed muscle-driven simulations of normal walking and then progressively weakened all major muscle groups, one at the time and simultaneously, to evaluate how much weakness could be tolerated before execution of normal gait became impossible. We further examined the compensations that arose as a result of weakening muscles. Our simulations revealed that normal walking is remarkably robust to weakness of some muscles but sensitive to weakness of others. Gait appears most robust to weakness of hip and knee extensors, which can tolerate weakness well and without a substantial increase in muscle stress. In contrast, gait is most sensitive to weakness of plantarflexors, hip abductors, and hip flexors. Weakness of individual muscles results in increased activation of the weak muscle, and in compensatory activation of other muscles. These compensations are generally inefficient, and generate unbalanced joint moments that require compensatory activation in yet other muscles. As a result, total muscle activation increases with weakness as does the cost of walking.“-van der Krogt
So, if your client comes in with knee, hip or ankle pain and a history of low back pain, you might want to pull out these articles. You may want to consider which muscles are, according to this article, most robust and sensitive to weakness. Remember what I mentioned when i reviewed the soleus article ? I mentioned that the reduced ankle dorsiflexion range may be from a soleus muscle postural compensation reaction to low back pain. Today’s article seemed to confirm that this muscle group is sensitive to weakness. In today’s discussion, not only is the impairment of the hip ranges of motion or control of the knee (from quadriceps adaptive compensation) possibly related to low back pain, in this case, paraspinal fatigue but it may be a muscle group robust to weakness which is a darn good thing when the paraspinals go to nap.
Sometimes the problem is from the bottom up, sometimes it is from the top down. It is what makes this game so challenging and mind numbing at times. If this is all too much for you, in teasing out this quagmire of a system, just throw corrective exercises at your client and hope for the best. What is the worst that can happen if you get it wrong ? Stronger compensations on already present compensations … . . why not, it is good for return business (insert sarcasm emoticon). But, lets be honest, if it was easy everyone would be doing it the right way. But the truth is that it is a long journey, and we are on the same bus of discovery with you all.
Dr. Shawn Allen, one of the gait guys.