Why Eccentrics Matter
By Michael MacMillan, MD
REHABILITATION
A muscle, alone, can only do one thing—pull. For it to go back out to length, it has to be stretched by some outside force. The outside force is usually the limb that the muscle is attached to, and usually it is pulled by the antagonistic muscle, which moves the limb in the opposite direction. The prime-mover muscle contracts strongly while the antagonistic muscle contracts weakly. To move a limb, the less-stimulated muscle is pulled out to length by the stronger contraction of the primary muscle. Generally, this paired muscle mechanism is how movement and motion is created. One analogy for how muscles lengthen and shorten is the fishing reel. The fisherman turns the handle to reel in the line or to release the line and let it spool out.
There are situations when muscles are not lengthened by simple motion of the joint, but rather by the limb being acted on by some outside force. When limbs bend to absorb the force of a fall or are suddenly extended by a heavy object, the muscles may suddenly and forcefully lengthen. If left unchecked, the joint could be damaged by hyperextension. To protect the joints from these large outside forces, muscles have receptors that de-
tect unusual muscle activity[CITATIONPro81 11033]. They aie activated if the muscle is “overstretched” and reflexively cause muscle contraction to prevent this. The question is: What happens when a large outside load exceeds the force of the reflexively contracting muscle?
What Is an Eccentric Contraction?
For large forces, once again the fishing reel provides a good analogy for this scenario. Clearly, the answer in response to the large external force in muscles is not to just to let the muscle “spool” itself out. The force would then snap the involved joint and injury would occur. In both muscle and fishing reels, a mechanism exists to protect the suddenly lengthening muscle. In the fishing analogy, this would be the same as when a fish is hooked—an external load. The fisherman’s reflex is to turn the handle to try to reel in the fish. If it is a small fish, it can be reeled in by the fisherman. In that raie case when the catch is something such as a great white shark, trying to reel it in forcefully would cause the line to snap. Fortunately, both fishing reels and muscles have mechanisms for resisting large, dangerous forces.
Those who have fished know that the reel can be set to “back turn” when a certain level of tension is created in the fishing line. This is called “drag.” So in that scenario when the angler is cranking the handle trying to reel in the great white, the tension in the fishing line can reach the breaking point. Fishermen would break a lot of lines if it weren’t for drag. Instead of snapping the line, the great white overcomes some predetermined amount of friction and causes the reel to back turn. The level of force needed to get the reel to back turn is high, but obviously less than the breaking strength of the fishing line. The external load (the shark) forces the reel to give out line even as the fisherman is turning the crank as hard as possible in the opposite direction.
Muscle has the same protective mechanism. When an outside force suddenly loads a limb, the muscle goes into protective mode and maximally contracts. In the case of a large force against the muscle, there is a mechanism to prevent the muscle from being pulled apart even as it is maximally contracting. This mechanism involves a large, coiled spring-like molecule called titin[CITATION Her14 y 1033]. Each titin molecule bridges over every muscle contraction unit and absorbs the pulling force as the external load stretches the muscle. Like the drag on the fishing reel, the stretching of titin molecules allows “back turning” of the muscle, which prevents catastrophic muscle rupture. This protected lengthening of the muscle is called an “eccentric contraction.”
Although it appears traumatic to pull a muscle out to length while it’s trying to contract, it is the job of the titin molecules to make this safe. These molecules can support 40 to 100% more load than the muscle can create on its own and not let the muscle be pulled apart. Of course when extreme loads are applied, suddenly muscle damage can occur. Therefore, there is a “safety zone” of loading when muscles can be challenged by loads that exceed their maximum but do suffer structural damage.
What Is the Effect of Eccentric Loading on Muscle, Bone, and Discs?
In the absence of physical activity to signal the muscle, tendons, and bone, the catabolic breakdown of tissue tends to be more active than the anabolic buildup of tissue, and musculoskeletal mass is lost. The normal forces seen in everyday activity in muscles and bones ai e too weak a signal to maintain the tissue that is already present. Exposing bone to prolonged submaximal loading results in gradually diminishing bone and muscle mass.
However, when loads match or exceed the maximum force of the muscle, it can create strong physiologic mechanisms to signal rebuilding of the muscle-bone complex. One response is for the bones and muscles to react to high levels of force through a process called “mechanotransduction.” Just the way nerve fibers propagate electricity to stimulate cells, mechanical force is carried through musculoskeletal tissue to create tissue growth[CITATION Bas(mi 1033]. The skeleton is intimately related to the muscle in the transmission of force. In our analogy, bone is the fishing rod. The tension in the line causes the rod to bend, and eccentric muscle force likewise causes deflection of the bone. The force that bends the bone and stretches the muscle is what creates the mechanotransduction signal. Bone detects this deformation and becomes structurally stronger because of
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Mechanotransduction also has an effect in the elderly, where there is a loss of tissue during aging. The decreased muscle mass seen in the elderly is different from the muscle mass you lose when you are younger. In earlier stages of life, muscle cells merely shrink during illness or when they are neglected. When they are exercised again, they can regain their former size and function. In the aging cells, the lack of stimulation actually causes the cells to turn off and die in a process called “apoptosis” (Dupont-Verteegden, 2005). If seniors allow their muscles to go unstimulated for prolonged periods, the weakness they suffer in some part will be permanent. In the elderly, this loss of muscle mass is called “sarcopenia.” Similarly, when bone is not placed under load regularly during life, the cells within the bone also undergo this apoptosis, and not only is bone mineral lost, but also living bone tissue.
The second effect is that the stretching of the titin molecule is accompanied by micro-tears in the contractile proteins of the muscle[CITATIONLie93U 1033]. These areas of micro-injury elicit a powerful series of events that not only greatly strengthen the affected muscles and involved skeletal elements, but also have a positive effect on the entire body. One of the unique benefits of eccentric contractions is to rebuild and repair muscle, and not just to maintain its structure but to make it stronger. The rebuilding process involves recruiting stem cells, which use powerful growth factors to stimulate muscular repair. These growth factors include human growth hormone, insulin-like growth factor, and a host of cytokines that stimulate cellular function|CIIAIIONDuzl21 l()r'i. These important factors are not only produced locally in the muscle, but they also aie released in circulation and enhance tissue growth at sites distant from the exercised muscle. It should be emphasized that this powerful endocrine response only occurs when muscle is forcefully lengthened by an external overload.
Even ligamentous structures such as intervertebral discs may be stimulated by the application of load. Certain parameters of mechanical loading (i.e., high load, low volume, low frequency) can stimulate disc physiology. This loading pattem has been implicated in the healing and regeneration of the intervertebral disc[CITATION ste15 y 1033]
Applying the Load: Amount, Rate, and Duration
Although a forced lengthening of the muscle can occur accidentally, it can also be applied purposefully. The application of an overload must be done under very controlled conditions. Since the muscle can create a certain level of force, there is some known amount of load that can exceed the muscle force. Determining these values is relatively easy. Simply test the maximum force the muscle can generate and apply a load that is at or slightly above that level of force. The key point is that there is a known relationship between the amount of weight applied and the resultant tension in the muscle[CITATIONLin01 u 1033].
This is the reason why the only safe means of causing an eccentric contraction is with isotonic resistance through standard weight machines. These devices give a known quantity of resistance. There are eccentric technologies being promoted that use a motorized arm to force a muscle to lengthen. This form of resistance creates higher and higher tension in the muscle until it fails. The muscle reaches maximal failure and catastrophic injuries occur. It is the equivalent of the muscle being “drawn and quartered.”
Once you determine the amount of weight that slightly exceeds the force capacity of the muscle and can be safely applied, the next question is how fast the load should be lowered. It should always be remembered that in eccentrics the weight is applied to a muscle that is trying to maximally contract. If the amount of
weight applied was chosen correctly, it should be fairly close to the level of maximum muscle force. The muscle will only lengthen very slowly. Therefore, in eccentrics the weight should be lowered as slowly as possible. If the weight is causing the muscle to stretch rapidly, then too much weight has been applied and there is a risk of injury.
In practical application, patients need some guidance beyond saying, “Lower the weight as slowly as possible.” Pausing during the lowering of the weight not only ensures that weight being used is safe and appropriate, but it also lets the weight exert its full effect on the muscle. Thus, a momentary pause increases the safety and effectiveness of an eccentric muscle lengthening.
The ideal set of repetitions to apply a safe and effective eccentric load would be done in conjunction with a concentric weight of about 50 to 60% of the one repetition maximum, performed for about 10 repetitions. This combination stimulates structural transformation of the desired muscle-bone complex and improves the physiologic function of the muscle as well.
How Frequently Should Eccentrics be Applied?
Eccentric overloads create a structural deformation of the muscle-bone complex. Deformation causes stress, which challenges the bone’s structural integrity. This level of force is a signal for cells to produce additional structural proteins.
In muscle, the deformation can cause microfailure in the tissue. These widespread micro-injuries ai e the powerful stimulus for muscle repair and regeneration. The key event in the repair process is the activation of cells near the injury and the migration of stem cells from elsewhere. These cells create a healing environment with a wide range of hormones, cytokines, and growth factors. Through these factors, muscle proteins are formed in larger amounts and muscular hypertrophy occurs. Like any area of healing, it requires at least seven days to go from injury to inflammation to healing[CITATIONMoo°5 ' 1033].
The more extensive the area of structural failure, then the longer the time required for the muscle to recover. The advantage of applying the exact amount of force to create this signal without causing widespread damage is that it can be assured that the muscle is fully recovered in a week’s time.
The Energy Cost of Eccentrics
Although it sounds like having patients perform eccentric movements with maximal levels of resistance would be extremely difficult, another property of eccentrics makes it very tolerable. The energy expended to pull a muscle out to length comes primarily from the external force. The energy consumed during an eccentric lengthening is actually very low[CITATIONMut851 1033]. This is evidenced by the common activity of going up and down stairs. The physical effort to climb 10 flights of stairs can be very taxing, but walking down the same 10 flights rarely causes any significant cardiopulmonary stress. Thus, doing eccentric training once a week gives even elderly or untrained individuals the opportunity to improve their musculoskeletal health without intense effort.
Why Do Eccentrics Matter?
Eccentrics, therefore, transmit an important level of force that signals the growth and development of our musculoskeletal system. Eccentric training need only be performed once a week and can be performed by people with minimal cardiopulmonary functioning who have intact joints. There aie a number of age-related conditions that can be treated by understanding and applying the principles of eccentric contractions. These conditions include osteoporosis, sarcopenia, degenerative disc disease, and arthritis. In addition to preventing disease, increasing the strength of the muscles and bones can help people, and even athletes, of all ages increase their strength and physical abilities to function better and enjoy life more.
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Dr. MacMillan went to North Carolina State University and then graduated from medical school at the University of North Carolina in 1980. He did his orthopaedic surgery residency at the University of Florida and his spinal fellowship at Southern Illinois University. He has been involved in eccentric muscle research for over 25 years.
He can be contacted through email at mikemacmillanfrcox.net.
