concentric Archives - Cor Kinetic

A term I tend to hear quite a lot in the gym is 'muscle firing'. A classic example of this is "I have been told my glute's don't fire." I must admit I am a little confused by this term.

As far as I can tell this term means muscle activation and this is where I start to get even more confused. This activation usually refers to a muscle shortening (concentric) and producing force. What we have to ask ourselves is, is this how muscles work?

In these situations I always like to refer back to my principles of muscle function. In this case specifically the principle of muscles lengthening before they shorten. This eccentric lengthening is the activation or trigger for the concentric force production.

This is a pretty solid principle. Go to throw a ball and first you rotate the opposite direction. Go to jump and first we go down. Even standing from a seat we flex forward before we extend. So if my muscles aren't firing is that because we have not given them the input (eccentric) to activate? Especially during function related and non-contrived movement away from the gym!

I have seen various muscle firing tests, many lying down while being palpated, usually with the diagnosis that the muscles are not firing properly or in sequence. Well what do you expect?? If we ask a muscle to shorten without first eccentrically lengthening as we see in functional movement how can we expect it to 'fire' properly?

Also what would the 'firing' sequence be when lying down. Different movement patterns will have different muscle activation patterns based on joint angle. As the angles change so will muscle activation. If my glutes are not firing when I lie on my front then that makes sense to me. No eccentric activation has occurred, nor is it likely to as my ability to flex is blocked by the floor (according to classical anatomy). We also have to ask ourselves what movements activate (lengthen) the glutes. If we look at the oblique angle of the muscle it would lead me to believe that internal rotation and adduction would also play a huge role. If we look down the body at the sub talar joint (STJ) the similar direction/angle of the joint (42°) to the glutes action line means that the joint moves predominantly in the frontal and transverse planes (Eversion and abduction). Why then do we see the largest glute, the glute max, as a predominantly sagittal plane muscle? In fact the relationship between the two parts to me is obvious. The axis of the STJ reacts to ground reaction forces going up and in, inferior lateral to superior medial, the glute action line will run perpendicular to this running down and out (pointed out by my good friend Ola!). A perfect relationship to help eccentrically decelerate STJ motion before producing force. In fact as the great Gary Gray calls the STJ "The switch that turns on the engine (glute max)." So for glute activation do we need to be upright and moving? I would say yes!

Here we can see the perpendicular relationship of the action line/striation of the glute from origin to insertion and the axis of the STJ.

How can someone tell the split second difference in activation between different muscles, especially through palpation, without the aid of expensive EMG equipment? What sequence are muscles meant to fire in the situation you are testing them? We need to know that first before we inform someone it is wrong or is not happening!

So lets put this a little bit into some context. A classical way to reduce knee valgus or abduction during gait is to fire the glute 'complex' sometimes specifically the glute med. This will create concentric force to abduct the hip to reduce the knee 'falling in'. This occurs in the front foot (pronation) phase of gait.

So if we want to create abduction then should we not first create adduction? During an energy sensitive, in terms of efficiency, activity such as walking creating an eccentric movement first would help generate elastic energy from both the passive and active muscular components.This means less metabolic energy expenditure. This eccentric activation also creates a subconscious mechanical trigger to produce concentric force to move our centre of mass. This means we can walk and talk and freeing our conscious mind to worry about what we are going to have for dinner.

Well this is what happens! But in reverse. When our foot hits the ground we go through an adduction moment at the hip. Don't just believe me, lets refer to a scientific study -
"The hip adducts in the absorption (front foot) phase because the ground reaction force falls medial to the hip and the hip abductor moment is less than the external adduction
moment due to gravitational and acceleration loads" ("Biomechanics of running, T Novacheck 1998). So the external forces outweigh the internal muscular forces.

Novacheck T, Gait and Posture 7 (1998) 77 – 95

Many times an increase in 'gravitational and acceleration loads' can come from a foot that increases pronatory forces. The hip will never have a hope of producing the torque to overcome increased force if it cannot overcome the force in a normal functioning individual with normal pronatory forces occurring.

So we have to ask ourselves if non functional concentric activation or 'firing' into abduction without external forces present (when we are lying on our side or back) is going to help us produce stiffer muscles (resistance to lengthening) that can resist a function related eccentric adduction moment or even an increased adduction moment? Is this even the rationale used? Or are we trying to 'fire' individual muscles such as the glute med (even less chance of overcoming force) to overcome a force we cannot overcome and then blaming a muscle not working?

On an anecdotal note many people who display increased pronation and knee abduction/valgus who I functionally assess seem to lack adduction of the hip. It maybe that the hip muscles/capsule in response to increased force closes down motion. Capsular restrictions many times affect all motions/planes at the hip. Another strategy then maybe to increase the ability of the hip to adduct to help decelerate movement of the femur into adduction. Just because the real bone motion of the femur is towards the mid-line this does not mean the relative hip joint motion is adduction.

Another rationale would be to address the increased force occurring at the foot. A rationale rarely used. That's why we see the single leg squat over the 2nd toe, hopefully to teach the body to resist a force greater than it can by 'firing a muscle' that would be in fact out of the functional sequence. From a functional perspective if we do not decrease force through the decelerative lengthening of adduction that the glute/hip will provide, the force coming up from the floor will just be transferred to another structure, possibly the lumbar spine.
This is my humble opinion on the complex subject of muscle 'firing' and its application. As usual more questions are posed than answers given. For more info check out www.facebook.com/corkinetic

www.cor-kinetic.com

Thanks for reading!!!

Ben Cormack

This blog comes courtesy of a conversation I had with my good friend Mike. Its all about optimum range of a muscle. It kind of followed on from this piece of info…

The eccentrically-loaded muscle will start its contraction weak and then get stronger; the concentrically-loaded muscle will initiate strong, but get weaker as the contraction continues.

I had never given this much thought before but this makes a lot of sense when we think of length tension relationships. A muscle will struggle to produce force when both too long and too short. Being weak in both positions. Cross bridge attachment has an optimal range. This will be true of both force production and conservation of energy. Too much cross bridge detachment will also cause a more thermodynamically expensive scenario as we split ATP and dissipate energy as heat.

Elastic energy will also I believe have an optimal range. Studies have shown that spring stiffness (ability to return energy) comes from optimal joint angles or ranges. Going beyond this range means that we dampen or absorb energy, again dissipating as heat through visceoelasticity of tissue. Different tissues have varying levels of stiffness and compliance, different ranges will bring into play these different characteristics as will our neurological intention (land or jump again) to move control stiffness through efferent spindle stiffness regulation.

If we look at the way we jump when we want to jump again, we can see that we use a shorter joint range than when we land for the final time. When we finish we have a large bend of the knee to absorb ground reaction forces rather than reuse them. This has implications for our understanding of height, range and repetition programming for our training me thinks!!

Ecconcentric (both eccentric and concentric muscle contractions occurring in different planes) muscle action may also play a role in optimal cross bridge attachment. If a muscle was to lengthen in all three planes this may cause a scenario where we are going beyond the optimal range for the muscle in terms of force production and elastic energy recoil. By mitigating elongation of the tissue in a plane of motion through concentric shortening we may also keep an optimal range. It maybe this would happen in a more sub maximal scenario where energy return and energy conservation are more important than maximal force production. I feel that gait is a great example of this. Although maximal force production may also be mitigated by creating too much loading through joint range that is hard to transform.

This then got me thinking about how we train. Many times we are looking for maximal ranges in our training. Maybe we should be looking more at optimal ranges. This may have more implications for sports where we can control the range through skill however. Running is a great example. Controlling stride length will keep us within optimal joint ranges. We must also remember that optimal will be governed by the individual. This will be affected by tissue ability, limb length, speed ability and event. I expect it will be that different events within running e.g. 400 metres will need different joint ranges from a marathon as we balance need for all out power, power-endurance and endurance. Going beyond optimal means our ability to start the next phase of movement, either eccentric to concentric or vice versa, will be compromised. I think that deceleration and acceleration are part of running (unlike the pose method ideology!) However we can mitigate excessive amounts of both having to occur, increasing energy conservation.

If we look at a game of tennis it is much easier to hit a powerful shot when we can manoeuvre our bodies into position. When we are out of position our range of movement may have to be extended to reach the ball. The transformation from eccentric to concentric is sub optimal and affects the power of the shot. The tennis player many times at end range will hit a defensive shot back, aiming to get it in the court rather than a winner! Increases in amortisation from eccentric to concentric reduces cross bridge attachment also decreasing energy return. The closer we get to and stay at end range stretching for the ball the longer we have the amortisation phase reducing the energy gained in the loading motion.

This is a very theoretical piece and mainly my own thoughts (so blame me!!) but it may give us food for thought when we programme ranges/heights for our clients to move through when training.

More may not be better in all circumstances!!!