THE ROLE OF THE NERVOUS SYSTEM
This "epidemic" of recurring hamstring injuries is, in part, due to the under-valued role of the nervous system. There is often too much focus on the structural "damage" and isolated limitations vs the overall dysfunction. I'm not saying that treating the damaged tissue is not important. But, in order to minimize recurrence, we must look beyond the isolated injury and seek to understand the integrated, "big picture" of the problem.
IS IT ACTUAL DAMAGE OR SOMETHING ELSE?
One interesting concept to understand is that the tension of our muscles
are completely under the control of our nervous system. Your brain is in
constant awareness of the tension of all your muscles. Unless you change
your nervous system, you will continue to sense tightness. As mentioned in Part 1, stretching isn't necessarily the answer to resolving tension. The relief felt by isolated muscle stretching only lasts for about 20 minutes. This is
why you can stretch routinely, yet feel as if you are not making any gains.Along the same lines, a contraction of muscle fibers also gives a signal of "tension" to our brains. If the tension you are feeling is actually due to a localized muscle contraction, the plan of attack should be to get those muscle fibers to stop contracting. Something referred to as "inhibition". You would be surprised how many hamstring "strains" are actually a localized spasm! The nature of each injury must be understood in order to know what the nervous system needs. The sensation of a "pull" during and after injury does not always indicate that something has been torn.
It is important to note that most hamstring injuries are diagnosed and rated without an actual MRI. Grading (rating of severity) is most commonly dependent on the severity of pain, how much of the muscle is tender, strength and range of motion. Presence of bruising and deformity are also used to determine hamstring strain severity. When MRI or Ultrasound is used in the diagnosis, a positive finding is when there is an "increased signal" along the muscle. Increased signal indicates inflammation and healing, but there is not necessarily a visible disruption (tear) of the fibers. Sure, inflammation indicates trauma, but the question is, what exactly is the cause?
To further discuss the concept that recurring strains are highly correlated to the nervous system, let's look at the location that most injuries occur. Multiple studies have found that the most common part of the hamstring to become damaged is at the Musculotendinous junction. Or, where the muscle fibers join with the tendon fibers. This is a key point to understanding the mechanism of muscle injuries. It is very rare to actually damage the contractile fibers in the middle of a muscle, which is often caused by an actual blow to the muscle vs a muscle contraction.
Musculotendinous (M-T) junctions are quite unique and do not simply function as a mere extension of the muscle. The M-T junction contains specific nerve endings called Golgi Tendon Organs (GTO's). Those nerve endings detect stretch from a tensing muscle, and, when a certain threshold is met, they trigger an inhibitory (relaxation) response to the muscle attached. That being understood, think about our mal-positioned hamstring that is starting with an increased "pull" at the tendon. The elevated attachment at the pelvis is also taking up slack from above the M-T junction. The threshold to elicit the GTO response is much smaller. This reflex also affects the opposite limb, in an opposing response. The activation of the inhibition response from a GTO on the left hamstring will cause the left hamstring to relax and the right, to contract. So, a forward left pelvis can cause over-activity of a right hamstring. We must look at both sides of the body when exploring the causes of hamstring injuries.
We have another type of nerves in our muscles, called Muscle Spindles. These are located within the muscle fibers, themselves, and their job is to detect the stretch of the muscle belly. When a certain threshold of stretch is met, they will elicit a rapid contraction of the muscle to prevent over-stretching. A muscle that is on a constant "stretch" will have muscle spindles that have developed a larger threshold for response. This means the shortening response is dulled and it requires much more sensory input from the muscles before activating.
The combination of an earlier-than-normal inhibition of a sprinting muscle and the slower response of the muscle spindles causes decreased sensitivity and confusion in the nervous system...right at the moment that the muscle is most mechanically vulnerable! This confusing stimulus could either lead to an isolated spasm (via isolated Muscle Spindle activation as an attempted response to the rapid stretch during a sprint), or an actual tear to the M-T junction.
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| Location of GTO's and Muscle Spindles in a single muscle. Note the relationship. Opposite responses. Both negatively affected by a lengthened position of the hamstrings. |
IS IT POSSIBLE FOR THE OPPOSITE PHENOMENON TO OCCUR?
Can we have a hamstring muscle that is actually in a shorter-than-ideal resting position? The answer is, yes. But, it is not as common and usually occurs on the right side. I will explain this in more detail in a later post, but let's take a minute to discuss how a muscle that is in a constant shortened position is also vulnerable. I venture to say these are most likely to be the isolated spasm injury. A muscle in this state will have a hyper-sensitive Muscle Spindle reaction and an under-sensitive GTO response. This means it will be more likely to contract earlier or out of sequence than a muscle in an ideal starting length. Not an ideal situation, either.
ECCENTRIC FORCES and A DYSFUNCTIONAL NERVOUS SYSTEM = BREAKDOWN
We also know that most hamstring injuries occur during the late swing phase (right before the swing leg touches the ground). Peak hamstring M-T stretch occurs at this phase of gait, whether we are walking or sprinting. At this phase, the hamstrings are stretched across both the knee and the hip joints and must also contract to slow the leg down. This combination of contracting a muscle as it is also being lengthened is called an "eccentric" contraction. This is a very powerful contraction and happens mostly near the tendon to more quickly control the "reigns" of the bone it's attached to. It is this powerful eccentric contraction that is blamed for most hamstring injuries. But there is another key thing that occurs at late swing phase - the brain must prepare for landing. Especially as the baserunner approaches a base, or a running back stops and starts again during a route, or a distance runner approaches an object in the road and must quickly adjust. The more primed your brain is to make those adjustments, the less likely damage will occur. As discussed above, a M-T junction that is already starting in a stretched position (or a shortened state) will be less capable of quick adaptations. The nerve input and output is slowed.
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Stretch and Power forces of hamstrings through gait cycle. Note the peak stretch and peak power output occur at nearly the same time. This is also the phase most injuries occur.
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| EMG (muscle activation) activity through multiple muscles during walking and running gait. Notice the hamstrings are nearly the only muscles working at Terminal Swing (approx. 90% through gait cycle). The other muscle activating is the front of the shin in order to lift the toes. |
Lastly, treatment of hamstring injuries can not just be isolated to the knee and hip. It is crucial to understand that the body works as an integrated, complex system. Our muscles work in "families", meaning certain muscles are influenced by the actions of another. This is a concept unique the Postural Restoration Institute (PRI) www.posturalrestoration.com. For instance, the hamstrings are affected by the activation of our Triceps and abdominals (specifically, the Internal Obliques and Transverse Abdominis). We cannot correct the neuro-motor (brain and muscle communication) function of the hamstrings without also looking at the Triceps and Abdominals.
Another undervalued influence of the nervous system on hamstring injuries is the influence of foot contact and a body's center-of-mass. Even the vision system can affect hamstring function. Where your brain senses contact on the foot or where it perceives visual "center" to be will determine which muscles it decides to activate. Pressure on the heel, balls of the feet and arch each elicit a different response. An imbalanced visual system (uncorrected near or far-sightedness or more acuity in one eye vs another) or imbalanced visual habits (focal vs peripheral and looking at the ground vs the horizon) will affect which muscles we use to maintain upright balance.
TO SUM IT UP
The nervous system is highly involved in hamstring function.
Not all hamstring "injuries" are actual tears. Some are actually localized spasms. Know the difference!
Uncoordinated, eccentric loads are most common cause of hamstring injury. Eccentric loads in various positions, activities, and stimuli should be trained to minimize risk of injury.
A thorough hamstring injury prevention and rehabilitation programs should also address:
- Pelvis position - Center of mass during static and dynamic activity
- Foot Contact - Influence of muscle function & joint position in other regions
- The Visual System
Stay tuned for Part 3 of this segment - discussing the rotational component of the hamstrings!
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