More on weak muscles. Just WHY are they weak? Know before you activate!

Dr Allen’s post last week on chronic ankle instability (click here for post) served as an inspiration for many of us. It brings to mind the many reasons muscles can become “weak”.

So why does a muscle become weak? We like to categorize the causes as follows:

  • local
  • segmental
  • long loop/cortical

Local causes include muscle injury and muscle pathologies, like muscular dystrophy and neuromuscular endplate disorders like myasthenia gravis. Segmental causes are largely due to reflexes which occur at the spinal cord level. Long loop and cortical causes ae due to an increased inhibition or lack of drive from higher centers, such as the motor cortex and cerebellum.

Lets examine local causes in more detail. To understand causes we must understand what makes a muscle contract.

Muscles are composed of many proteins, 2 of which are actin and myosin (see above). Actin has 2 forms, F (filamental) and G (globular) actin. Imagine 2 grapefruits side by side (G actin) held together in the middle by small filaments (F actin). Now imagine these another set immediately below, in a repeating pattern. These groups of 2 are held together at the sides by an additional protein called tropomyosin. This whole complex looks a little like train tracks. Along the strands of tropomyosin, at regular intervals is yet another protein called troponin. We like to think of troponin as a triangular shaped protein and each part of the triangle has a particular binding site: one for tropomyosin, one for actin and another for calcium ions.

Myosin is another component of muscle, that looks similar to a bunch of golf clubs. The head of the club will, under the right circumstances, interact with actin, the body (tail) of the club interacts with other myosin bodies.

Globular actin and myosin heads are like 2 teenagers and like to interact with one another. Normally, in a resting state, troponin protein covers the active site of myosin binding on G actin. In the presence of calcium, there is a change in shape of the troponin molecule, moving it off of the active site of actin, allowing myosin to bind there. When this happens, the head ratchets and muscle contraction occurs. In the presence of adequate fuel (ie ATP) the myosin head detaches from actin and “recocks”, ready for another contraction cycle (see 2nd picture above).

So where does the calcium come from? It is stored in areas of the muscle called the terminal cisterns. It is released when an action potential fires the peripheral nerve to the neuromuscular endplate of a muscle.

Can calcium be released any other way? Sure it can. How about if the terminal cisterns are damaged, from an injury to the muscle? How about if they are damaged from a disease process?

So, when calcium is released, no matter how it is released, muscles contract. If calcium is not released, then muscles do not contract.

From a local cause, If a muscle is weak, one of the following are usually causing the weakness:

  • there is physical damage to the muscle causing fewer of the working units of the muscle (called sarcomeres) contract

this is by far the most common, due to overuse or trauma

  • there is a problem with the connection of the nerve to the muscle

Disruption of nerve to muscle connections can be also be due to trauma or disease. Weakness that is becoming progressive and worsening, needs to be evaluated further and may be the signal for a progressive muscular or neurological disorder (muscular dystrophy, myasthenia gravis, Gullian Barre, etc)

  • there is insufficient neurotransmitter at the neuromuscular end plate to fire the muscle

this is usually due to a disease process

  • Insufficient calcium could theoretically hamper a muscles contraction, but since calcium is involved with nerve transmission as well, tetany (ie sustained contraction and spasm) would most likely occur due to other reasons that we will not explore at this juncture.

OK, so that sums up local causes. Look for a follow up post about segmental causes next…

We are: The Gait Guys

One simple hip screen that gives you lots of information.

This is the one leg standing test. We use it as a hip function (abduction) screen(as well as an exercise), to see if a person’s gluteus medius is working in a functional situation (as opposed to manual muscle testing).

As you may remember (don’t remember? Click here), the gluteus medius fires throughout stance phase (ie; when the foot is on the ground). It keeps the pelvis level while the foot is on the ground and works in conjunction with the opposite quadratus lumborum muscle (if you have not read up on this, please see our groundbreaking work on the problematic cross over gait, found here, here and here).

The test is simple; try it on yourself while watching yourself in a mirror. Stand on one leg on your foot tripod (the heel, base of big toe and base of little toe). Raise the opposite foot off the ground by flexing the thigh. Observe.

You should see the pelvis remaining level with no shift of the torso or hips. 

Watch for:

  • ·      Pelvic drift to the side you are standing on
  • ·      Pelvis drop on the side opposite you are standing on
  • ·      Body lean to the side you are standing on
  • ·      Excessive hiking of the opposite, non weight bearing hip
  • ·      Any combination of the above

 

Seeing any (or all) of these means the gluteus medius is probably having some trouble.  The reason we say probably is that a person with a hip problem (like arthritis) or an anatomically short leg may do some of these things in compensation.

The question you are hopefully asking is why do they drift, lean, hike, etc? Not everything you see is muscle weakness per se.

  • ·      Maybe they have a balance issue
  • ·      Maybe they have a disc injury
  • ·      Maybe they have injury to the nerve going to the gluteus medius
  • ·      Maybe they have a knee/ankle/foot issue
  • ·      And the list goes on…

So, if it were a muscle weakness, how could you fix it? Determine the cause. Begin at the bottom with foot exercises: tripod standing, lift/spead/reach with the toes etc. Then have them repeat the exercise IN A MIRROR, maintaining a level pelvis. Yes, it is that simple. Now see if they can translate that to their gait cycle. If so, great. If not, start again and repeat till they can.

The Gait Guys. Making it real, each and every day.

all material copyright 2013 The Gait Guys/ The Homunculus Group. All rights reserved. Please ask before using!

You have heard us talk about weakness (short weak and tight weak) muscles all the time on Youtube: The Gait Guys; now here is the neurology behind it. Think about this concept the next time you see a gait compensation and think of the muscles (agonists/antagonists) involved.