This week on Neuromechanics Weekly:
The necessary factors for muscle contraction. Stay tuned for what controls muscle tone next week.
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Shawn and Ivo
Ah yes, the Ia and type II afferents.
One of our favorites! Acting as a sentinel from the muscle spindle, concentrated in the antigravity and extensor musculature, Ia and type II afferents live in the belly of the muscle and send information regarding length and rate of change of length to the CNS via the spino cerebellar and inferior olivary pathways. In more simpler terms, think of muscle spindles as small computer chips embedded in the muscle and using la and type II afferents the team act as volume controls helping to set the tone of the muscle and it responsiveness to stretch. If they are active, they make a muscle more sensitive to stretch.
So what does that mean? Muscle spindles turn up the volume or sensitivity of the muscles response to stretch. Remember when we stretch a muscle, it’s response is to contract. Think about when a doctor tests your reflexes. What makes them more or less reactive? You guessed it, the muscle spindle; which is a reflection of what is going on in the higher centers of the brain. The muscle spindles level of excitation is based on the sum total of all information acting on the gamma motor neuron (ie the neuron going to the muscle spindle) in the spinal cord. That includes all the afferent (ie. sensory) information coming in (things like pain can make it more or less active) as well as information descending from higher centers (like the brain, brainstem and cerebellum) which will again influence it at the spinal cord level.
So we found this cool study that looks at spindles and supports their actions:
J Physiol. 2009 Jul 1;587(Pt 13):3375-82. Epub 2009 May 18.
Mechanical and neural stretch responses of the human soleus muscle at different walking speeds.
At increased speeds of walking, the muscles themselves (particularly the soleus in this study) become stiffer due to changes in spindle responsiveness. The decline in amplitude and velocity of stretch of the soleus muscle fasicles with increasing walking speeds was NOT accompanied by a change in muscle spindle amplitude, as was hypothesized.
Clinically, this means that the spindles were STILL RESPONSIVE to stretch, even though the characteristics of the muscle changed with greater speeds of action. This may be one of the reasons you may injure yourself when moving or running quickly; the muscle becomes stiffer and the spindle action remains constant (the volume is UP).
Thankfully, we have another system that can intervene (sometimes) when the system is overloaded, and take the stress of the muscle. This is due to the golgi tendon organ; but that is a post for another day…
Geeking out and exploring the subtleties of the neurology as it relates to the system, we remain…The Gait Guys
A brief gait review from a youtube clip we found:
at :03 notice the shrugged shoulders and trapezius activation, forcing respirations to the upper lung fields. This also facilitates the scalene muscles in the neck (which is probably one of the reasons they flex their neck). Breathing from here is shallow and inefficient. This action (shrugging the shoulders) activates the upper trap and deactivates the lats (which are the functional link between the upper and lower extremities)
at 05: they begin to flex the lumbar spine
at :06 they flex at the waist as well as the neck. This rounds the spine and puts the glutes at a mechanical disadvantage for extending the hips and limiting some of the driving power. They then become hamstring dependent, which isn’t as efficient. Dropping the head defacilitates the extensor muscles neurologically, so they will have some power loss (as well as stiffness loss) as well. They keep their neck flexed till :07, where they really begin to pick up more speed. The torso remains flexed at the waist through most of the footage.
it appears at :07 that the left foot strikes the ground in eversion bottom of foot pointing away from camera) indicating some degree of forefoot pronation. A shot from behind would be helpful to confirm this
The arm swing appears asymmetrical from left to right, right being greater both forward and especially backward. I would wonder what they are hiding (biomechanically) there (so are they increased on the right or less on the left?. Here is where foot age from behind would be instructional).
Ok folks. Hope you enjoyed the ride!
we still are….The Gait Guys…..
Time for a quick pedograph case:
This person presented with arch pain and occasional forefoot pain.
Note the increased size (length) of the heel print with blunting at the anterior most aspect. The midfoot impression is increased, revealing collapsing medial longitudinal arches. The forefoot print has increased pressures over the 2nd metatarsal heads bilaterally, and the 1st on the left. She claws with toes 2-4 bilaterally.
This demonstrates poor intrinsic stability of the foot (as evidenced by the increased heel impression and midfoot collapse) and well as decreased ankle rocker (as evidenced by the increased forefoot pressures).
We also see increased ink under the distal second digit (esp on the right). This suggests some possible incompetence of the first ray complex and big toe, which is represented by the medial ink presentation under the great toe (suggesting a pinch callus, which is seen when there is spin of the foot and insufficient great toe anchoring and push off). When the great toe function is compromised, we tend to see increased activity of the 2nd digit long flexors, represented well here by increased ink under the 2nd toe.
The pedograph truly does provide a window to the gait cycle!
We remain: Gait Geeks
And now for something totally random….
How Much Water should I drink? The basics of hydration
We see many athletes and weekend warriors and are often posed this question; so here you go…
Water is the elixir of life. Too much (hyper hydration) or not enough (dehydration) can both be detrimental to your performance, but how much is enough?
Our bodies consist of about 60-70% water at any point in time. Most men have a higher percentage because they have a higher percentage of muscle mass (unfortunately, adipose tissue contains little water!). We generally lose between 1.5 and 2.5 liters of fluid a day through breathing, urinating and general metabolism. The body must keep a balance of water both inside (intracellular) and outside (extra cellular) your cells. Because the body is in equilibrium, if you sweat, breathe, or urinate too much, you will lose water from your extra cellular compartment. The body will then take water from inside your cells and shift it to outside. Likewise, your brain (hopefully) will stimulate you to drink more and urinate less, helping you to fill up the extra cellular compartment again. The water will then diffuse back into the cells and equilibrium is established again. A lot of this movement of water has to do with electrolytes (charged particles in your blood and body fluids) and their movement across cell membranes. The electrolytes that are most important for us are Sodium (Na+) and Potassium (K+).
When you lose too much water, your blood becomes more viscous (remember, you are losing water, not cells. Less water plus same number of cells equals more viscous liquid). This makes your heart have to work harder to push the blood around. This takes more energy and resultantly your heart rate increases, causing a phenomenon called cardiac drift. An example is when you are exercising for a while at the same intensity and your heart rate increases over time. A loss of 2-3% of your body’s water will decrease your performance by 3-7%! The amount you lose will depend on your exercise intensity and duration as well as temperature. A 20 degree increase in temperature can increase your heart rate as much as 10 beats per minute!!
So, it seems if we drink a lot of water, all will be well. Well, yes… and no. Your body can only absorb about 24-28 ounces of water per hour, any more and it just makes you pee more. You can sweat up to 3 liters (that’s more than 96 ounces!) per hour. Hmm. looks like we will probably be in a deficit. How much we absorb depends on whether we can get the water out of our stomach and into our intestines where it can be absorbed. As you can imagine, there are some things which speed gastric emptying and some which slow it down.
Protein and fat are the 2 main things which slow the trip through the stomach down. If too much of these are in your drink or already in your stomach the water will end up sloshing around and probably leave you not feeling too good. Guess that means lots of protein before or during an endurance workout is probably a bad idea, especially if you are trying to stay hydrated! Small amounts of protein, when combined with carbohydrate can be beneficial, but that’s another subject for another day. So much for all that marketing hype!
Small amounts of carbohydrate (up to 60 grams per hour) can enhance water absorption from the intestines and speed emptying of the stomach. The body can’t process any more than this, and it will actually start to slow stuff down if you do too much.
Sodium (50-70 mg, about a pinch) also helps with water absorption. It has the added bonus of stimulating your hypothalamus to tell you to drink more. If you wait until you are thirsty, it will be too late. You need to drink before you are thirsty!
So, what’s a person to do? Here are some tips:
- Drink small amounts often, especially in hot weather. 6 ounces every 15 minutes is a good pace
- Consider adding some sugar to your water sucrose (table sugar) or maltodextrin are a good start. Remember, no more than 60 grams per hour
- If you don’t like sweet drinks while working out, consider using a gel or goo
- A little salt is a good thing. It improves the taste of the water, helps with its absorption, and stimulates the thirst mechanism.
- Research your workout drink. Ask questions. Many claims are marketing hype and not based on science or physiology.
- Consult with your chiropractor, physical therapist, doctor or trainer with questions
The Gait Guys…telling it like it is