Neural Control of Movement Part III: Golgi Tendon Organs

A couple of weeks ago, we talked about a special type of proprioceptive receptor called muscle spindles.  Muscle spindles detect changes in muscle length and velocity of lengthening, and send signals to the CNS.  This helps the nervous system know about changes in joint angles and muscle length, and can help protect the muscle from lengthening too much and too fast.

Another type of proprioceptive receptor found in muscle (actually between the muscle and tendon) is the Golgi Tendon Organ (GTO).  These receptors are sensitive to changes in muscular force.  Whenever a muscle contracts (shortens), tension is developed within the muscle and tendon, which activates the GTO, causing it to send signals to the CNS.  Thus, the GTO provides feedback to the CNS about the amount of force a muscle is producing.  If a muscle is producing too much force, and is at risk of injury, the CNS can send inhibitory signals back down to the muscle so it will stop contracting and relax, thus reducing the amount of force.  Unlike the muscle spindles, which are are sensitive to changes in muscle length and the rate of change, GTOs are only sensitive to changes in muscle force, not the rate of change.

So, the muscle spindles send information to the CNS about  muscle length and the velocity of lengthening, and GTOs send information about muscular force.  This information allows the nervous system to make quick adjustments so we can move more efficiently and safely.   

Neck Injuries

It seems that this football season, especially last weekend, has seen a very high number of neck injuries.  Devon Walker, a defensive back for Tulane, sustained a cervical spine fracture this past Saturday when he attempted to make a tackle and collided helmet to helmet with a teammate.  Walker had surgery but the extent of the damage is not yet known.  Hopefully he will make a full recovery.

The question is, why are we seeing so many head/neck injuries in football?  Obviously football is a contact sport and there are hundreds of violent collisions every game.  These players are very massive and move at high velocities, meaning they generate a large amount of momentum that is transferred between the players during a collision.  Injuries are going to happen.  Now, I have never played or coached football, I've only worked with football teams as an athletic trainer, and I watch a lot of football.  Through my observations, it seems that many football players attempt to make tackles, or attempt to "run into" a tackler with their necks in a flexed position (think about looking down).  This is the worst possible position for the neck to be in during a collision.  Cervical (neck) flexion removes the natural curvature from the cervical spine, and places the vertebrae in direct alignment.  When the head makes contact with another person, the force is transferred from the head straight down the vertebrae, essentially creating a domino effect.  If there is enough force, an injury such as a cervical vertebrae fracture can occur, which can potentially damage the spinal cord.

I do not think there is a simple solution to this problem.  The best way to avoid this injury would be to tackle with the head up, or to teach the defender to be able to see the person they are tackling.  I've seen several examples of defenders "launching" themselves headfirst into the offensive player.  However, I think a lot of these players have been tackling with their heads down for so long, that it is a difficult habit to break, especially in heat of the game when they have to make a play.  Hopefully improvements will continue to be made to equipment and more research will be conducted to help answer these questions.

Remembering 9/11

It is hard to believe that today is the 11th anniversary of the terrorist attacks of 9/11.  I thought I would do something different with the blog today due to the 11 year anniversary of the day the terrorists attacked our country and many brave men and women lost their lives.  I was a sophomore at Southern Miss on 9/11/2001, and had just finished a morning workout at the Payne Center when the news broke.  It seemed surreal at the time, and it still does.  I was working as a student athletic trainer with the football team, and it was very difficult for anyone to focus on football for a few days, and all the games for that weekend were canceled.  Looking back, it was definitely the right thing to do, although later on I do believe sports played an important part in helping our country heal.  Amy and I had a chance to visit ground zero in New York a couple of years ago, and even though the rebuilding process is underway, you could still sense that something terrible had happened there.  Let's just remember everyone that lost their lives that day or later due to the attacks, and all the brave men and women that are fighting for our freedom.

Neural Control of Movement Part II: Muscle Spindles

When we think about muscles, we often think about the contractile components, actin and myosin, that attach and slide past each other, causing a muscular contraction.  But, there is another component of the muscle that is critical for coordinated movement, and that is the muscle spindle.  In the picture above, the extrafusal muscle fibers are the ones that contract and develop force, while the muscle spindle contains the intrafusal muscle fibers, afferent neurons, and gamma motor neurons.

There are three types of intrafusal fibers: dynamic bag fibers, static bag fibers, and chain fibers.  When muscle lengthens (think about when a muscle is stretched), the intrafusal fibers send signals to the spinal cord through the Group Ia and Group II afferent neurons, which relays information about how much the muscle is lengthening and how fast the muscle is lengthening.  The greater the lengthening or speed of lengthening, the more signals will be sent.  The gamma motor neurons send signals to the muscle spindles from the CNS (central nervous system) that can increase or decrease the sensitivity of the muscle spindle.  The gamma motor neurons help the CNS control the gain of the muscle spindles.

Why are muscle spindles important?  There are two big reasons.  1) The muscle spindles send information to the CNS about muscle length, which helps the nervous system know how joint angles are changing and where the different body parts are located in space.  For example, if you extend (straighten) your elbow, this lengthens the biceps brachii muscle.  The muscle spindles in the biceps will send signals to the CNS, indicating that the muscle is lengthening.  If the biceps is lengthening, then the elbow has to be moving into an extended (more straight position).  Also, if you were to flex (bend) your knee, this would lengthen the quadriceps, which activate the muscle spindles, indicating that the muscle is lengthening and the knee if flexing.  2) Muscle spindles also help protect the muscle from injury due to the muscle lengthening too much and too fast.  If a muscle is lengthening too much and too fast, the CNS can send signals to the muscle for it to contract and shorten.

So, muscle spindles play a crucial role in providing feedback to the CNS about muscle length and speed of lengthening.  This information helps the body know how joint angles are changing, and it can serve to help protect the muscle against injury.

Neural Control of Movement Part I: Please Do Not Say Muscle Memory

This semester, I am teaching a class called "Neural Control of Human Movement."  This is a very challenging course for both myself and the students, because the nervous system is very complex.  To me, it is the most complex and difficult system in the body to understand.  I am going to do a series of blog posts discussing how the nervous system works with the muscular system to produce coordinated movement.

When we think of voluntary movement, such as walking, running, hitting or catching a baseball, etc., we often focus primarily on the muscles and the bones involved in the movement.  What we fail to consider is that none of this motion would be possible without the nervous system.  This simplified view of movement has given rise to a very commonly misused term called "muscle memory."  I hear sportscasters, coaches, and even so called scientists use this term often, and every time I hear it I cringe.  The ESPN segment called
"Sports Science" was airing the other day and the host used the term "muscle memory" to describe how a baseball player caught a ball. 

Why is "muscle memory" not correct?  The biggest reason is that there is no memory structure in the muscle.  A skeletal muscle cannot contract unless it is stimulated by the nervous system.  Now, it is true that through practice and experience, movements become more coordinated, efficient, and require less attentional demands, and many people want to label this as "muscle memory."  The next few blog posts will discuss the interaction between the nervous system and the skeletal system, and the actual processes that occur that lead to an improvement in performance that involves both the nervous and muscular systems.

Thoracic Outlet Syndrome

I was talking with my Dad the other evening about St. Louis Cardinals pitcher Chris Carpenter and his return from surgery to relieve his symptoms due to thoracic outlet syndrome (TOS).  This is not a very common injury, but it can lead to a lot of pain and weakness, which is obviously a major problem for a baseball pitcher.  I will outline the basic principles of TOS below.

Before talking about the syndrome, we first need to define the thoracic outlet.  If you look at the top picture, you will see a group of nerves coming out of the spinal cord, and running down between the scalene muscles, behind the clavicle, and in front of the first rib, and then down into the arm.  This group of nerves is known as the brachial plexus.  The opening between the scalene muscles and the ribcage is the thoracic outlet.  There are also blood vessels not pictured above that pass through this space.  Any time there is a nerve or group of nerves passing through a tight space, there is a chance that some of the structures can "press" on the nerves, which can lead to pain, tingling, numbness, and weakness in the affected area.  In TOS, it could be the scalene muscles, the clavicle, or the first rib pressing on the nerves.

What causes TOS?  There are many potential causes, including a fractured clavicle, tightness or scar tissue in the scalene muscles, the presence of an extra first rib (yes, some people actually have 13 instead of 12 pairs, but the presence of an extra rib does not necessarily lead to TOS), pressure from the normal 1st rib, repetitive stress, such as repeating the overhead throwing motion, or poor posture.  The signs and symptoms include pain along the side of the neck, the upper arm, and possibly the lower arm.  Numbness, tingling, and weakness in the shoulder and arm may also be present.  These symptoms are similar to other conditions, such as a herniated disc, which can make diagnosing TOS difficult.

In the case of Chris Carpenter, his TOS was likely caused due to the repetitive stress of the overhead throwing motion.  He had similar symptoms back in 2008, but they did not resurface again for several years.  His symptoms first resurfaced during spring training, when he was diagnosed with a herniated disc.  He was prescribed rest followed by strengthening exercises for the neck, shoulder, and arm.  After a couple of months of rehab, he attempted to pitch again, but could not because the pain returned.  He was then referred to a specialist who made the diagnosis of TOS and recommend surgery to remove his first rib and release some of the scar tissue around the scalene muscles (I am not sure if he had an extra first rib or not).  He has responded well since the surgery and is attempting to pitch again this year, but at the least he should be healthy to start next season (or relatively healthy for a 37 year old pitcher).

In many cases, therapy and postural changes can relieve most of the symptoms of TOS.  Surgery is often seen as a last option, because it does carry some risks since the surgeon has to operate close to many nerves.  

Gruesome injury

Last Thursday night during a preseason NFL game, Tennessee Titans wide receiver Marc Mariani suffered a compound fracture of his tibia and fibula while returning a punt.  I'm going to place the picture of the injury and video at the bottom of the post in case you don't want to see it.  This was a very bad injury.  It likely resulted in an open fracture, where the broken bone(s) punctures the skin.  This can lead to complications from infection because of the open wound.  What type of loading caused this injury?

The type of loading that caused this injury was bending.  Bending occurs when there is tension (think about making an object longer and skinnier) on one side of the bone, and there is compression  (think about making an object shorter and wider) on the other side of the bone.  To fully understand the injury, you must watch this video.  As Mariani is moving forward, his left foot is stepped on by one of his teammates.  This stops the momentum of his foot.  However, his lower leg and the rest of his body continues to move forward due to their inertia.  His lower leg makes contact with the other players leg a few inches above his ankle.  This stops the momentum of his lower leg, but the rest of his body continues to move forward.  The point of contact between Mariani's lower leg and his teammates lower leg basically acts as a fulcrum, and the bending forces placed on the tibia and fibula exceed the strength of the bones, and they fail (break).  Adult bone is weaker in tension than compression, so the fracture likely started on the side opposite of where his leg made contact with his teammate's leg.

Human bone is very strong, and it can withstand a large amount of force before breaking.  In these cases of extreme fractures on the football field, it is often times an example of all the forces being lined up in just the right positions to cause a fracture like this.  If Mariani's teammate had not stepped on his foot, there is a good chance he would not have sustained an injury at all.  The Titans are reporting that surgery went well, and hopefully he will have a chance of returning to the field next year.