Henrik Crantz, Registered Physical Therapist (RPT), BSc PT, MSc PT, Certified Specialist Orthopaedic Manipulative Physical Therapy
Originally published in Manualen Nr 4, 2016.
It is well known that during different phases of muscle action (concentric, isometric and eccentric) the force output differ. It is a common sensation to feel stronger lowering than lifting a given amount of weight in a controlled manner. The concentric muscle action is considered to be the weakest and the eccentric the strongest of phases in muscle action. Factors that affect force output are several, such as energy (metabolic, elastic recoil), neural fatigue and others. Rarely emphasized in the debate, regardless of contraction phase, is how friction as a result of movement affects muscular strength which in itself is very interesting. Intramuscularly, where movement most certainly occurs, the muscle action in its fibers creates movement and therefore friction. If there is movement, there is friction and the contraction phases will differ, as mentioned, and to a great extent. There is an established opinion considering that the phases do not to differ largely in output force (1). In fact, researchers have met the conclusion that isometric and eccentric phases are more equal or, differ very little. This, however, isn’t an uncontroversial claim. On the contrary, much seems to indicate that the phases indeed differ largely.
The basic mechanics of eccentric contractions are still a source of debate. The cross-bridge theory (2, 3, 4) that describes concentric contractions is not as successful in describing eccentric contractions. This theory, which holds the two-filament sarcomere model (actin-myosin), is perhaps the most widely known model for the explanation of contractions. Other models have been suggested (5) mainly focused on the three-filament sarcomere model (actin, myosin and titin). This model suggest that force from stiffening of the protein titin increase when actin-myosin forces decrease (as in the eccentric phase of muscle action). However, the concentric phase of muscular contraction must inevitably overcome the friction that is being produced by the moving actin-myosin filaments when stiffening of titin is not a significant issue.
Intramuscular anatomical structures such as muscle fibers, blood vessels and nerves, usually works well together, but one has to understand the lack of empty space between each of them. The structures are of course separated from each other, but still packed together as a closed system. Therefore, friction is without a doubt present in the structure itself when movement is induced, even if the surfaces are smooth or lubricated.
According to the laws of physics the force it takes to lift a weight at a constant speed is the same, no matter in which phase of contraction. That is, if you lift a barbell that holds a total weight of 50 lbs, it will take exactly 50 lbs (≈222,5 N) of muscular force to lift, hold and lower at a constant speed. Even though it would be impossible for a human being to accomplish that, it’s still interesting to ponder as a mechanical phenomenon in muscular force output. Since the force output clearly differs a great deal between its phases, there must be something hindering the force output. Fatigue is, as mentioned, one aspect, but shouldn’t be the only significant one to consider.
“An object at rest remains at rest if there is zero resultant force acting on it. If the resultant force acting on a moving object is zero, the object continues its motion with constant velocity.” – Isaac Newton
An example on how friction is working against force output during the concentric contraction of a muscle can be described using a simple model. Imagine a string that is attached to a weight at one end placed hanging from the edge of a table. The string is in contact with the edge of the table. If a force that was greater than the weight was acting in a positive direction (equivalent to concentric contraction), the part of the string in contact with the table would create friction (figure 1, left picture). In this situation, friction would work against the force output lifting the weight. The opposite would occur (figure 1, right picture) if the forces in each end of the string were to be switched. Now, friction would work in favor of force output since it is lowered in a negative direction (equivalent to eccentric contraction).
Tiina Lahtinen-Suopanki, Finnish Physiotherapist, along with scientists in Italy studied the importance of fascia tissue (figure 2) and its relation to different pain conditions (6, 7). They could confirm that the fascia is deeply innervated and therefore a possible source of pain and discomfort. The fascia tissue is in fact divided into layers all the way down to the muscle, that each works independently from one another and thus causes inertia.
Even though the amount of friction in these layers’ surely increases or decreases depending on the situation, it should generally be quite noticeable in any circumstance. The fascia clearly holds elastic energy but isn’t as elastic as you could imagine. It is a force transmitter where roughly 30-40 percent of the force produced by the muscle is transmitted to the fascia and not to the tendon of the muscle (8, 9). That makes the muscle dependent on the fascia for growing in strength, especially in the eccentric part of movement. If a limb, or any part of the body that can move, is immobilized, the fascia will stiffer, which will result in an increase of friction while lowering a weight.
In earlier writings (10) it is mentioned that friction increases as muscle action continues. This is due to several phenomena linked to physical activity, one being an increase in fluids between the fascia layers. Blood flow will also increase within the muscle, creating tension within the muscle belly and when the blood flow increases the muscle belly holds some of the fluids momentarily. This “acute hypertrophy” makes the eccentric phase of muscle action decrease at a slower rate when compared to the concentric. When concentric phase-force output is zero, the weight can still be lowered in the eccentric phase at a controlled manner.
All that has been described above causes friction – and a lot of it. For sure, the contraction phases differs a great deal in force output as well. Friction is of course one of many factors that can and should be discussed, but ignoring it altogether is in my view unwise since it undermines our understanding and measurement of true strength. It must be stated that movement, of any kind, is very hard to measure and this is equally true when it comes to muscular strength. Many things in our world follows “the path of least resistance”, and the same goes for the musculoskeletal system. In the world of weight training, however, “the path of most resistance” is generally favorable and can be achieved in numerous ways.
So, what about this obsession with friction? How can this way of thinking be used clinically? As I’ve pointed out earlier, friction is not the only factor to take into consideration, even though it certainly will make itself reminded in an exercise machine. But, as an example in weight training, one can focus on the eccentric phase when it is no longer possible to lift the weight. Or, by focusing primarily on the eccentric phase in all of your lifting even from the beginning of an exercise or set. By changing the direction of force in consideration to a muscle’s strength curve is another example how one can manipulate factors that relate to qualitative resistance, which is the inward sense in all the examples. These are easy steps to assure that qualitative resistance from training is being used by making each repetition harder with emphasis on the eccentric part of movement.
6. Stecco L Fascial Manipulation for Musculoskeletal pain, Piccin, 2004.
7. Stecco L & C. Fascial Manipulation Practical Part, Piccin , 2009.
8. Passerieux et al, Physical continuity of the perimysium from myofibers to tendons. J Struct Biol. 2007.
9. Trotter et Purslow PP. Functional morphology of the endomysium in series fibered muscles. J Morphol. 1992.
10. Crantz, H. Går det att mäta ren styrka? [Blogg]. 21 maj 2016. https://wordpress.com/post/swebodybuilding.wordpress.com (hämtad 2016-10-14).