Muscle recruitment in response to resistance

[Sasha Pachev]

How does this really work? We all know that the muscles work a lot harder when we are pushing against some kind of resistance. But what exactly is the process? Let's consider the example of running. You land and feel the ground. You try to kick the Earth but find that it is too heavy for you to kick. That resistance somehow helps you generate the force from your muscles to which you get a ground reaction force that pushes you up and forward. Questions:

How does the brain know to start recruiting the muscles?

What controls the amount of lag between the time you feel the ground and the time you apply the force?

The force increases gradually, I assume, correct?

What controls the rate at which that force increases?


 

[Jon Allen]

I don't dare answer all these questions.  But I do have 2 comments.

I imagine the rate of force increase is very rapid- the amount of time it takes for the measured force to travel to the brain (very quick), for the brain to analyze (not quite as quick, but pretty good), and then for the instructions from the brain to travel to the muscle (very quick).  I am guessing it is a control feedback loop- i.e. if the brain decides the action is not happening fast enough, it signals for more muscle force.

As for rate of increase- I think it is initially controlled by how much the brain thinks is required.  For example, if you are going to run a 100 m sprint, the brain knows to tell your legs to go all out right from the start.  Likewise, if you are going to lift a heavy box, you start with a strong muscle force.  But we have all had the experience where we pick up something that we expect to be heavy but is actually light and almost throw it into the ceiling.  Hence, our expectations of how much force is required play a part in how much force is initially exerted.

[adam]

How does the brain know to start recruiting the muscles?

-look up golgi tendon organ (GTO), muscle spindle, action potential, and motor unit.

What controls the amount of lag between the time you feel the ground and the time you apply the force?

-time from sensory nerve to CNS to motor nerve to motor unit

The force increases gradually, I assume, correct?

-strength can increase gradually (ascending), decrease gradually (descending), or be bell curved, depending on the type of action and range of motion. Actions that allow you to move more weight only at the last half or last quarter of the motion are ascending (ie, squat, bench press). Actions that allow you to move more weight only at the beginning or first half of the motion are descending in strength (ie, upright rowing motion). Actions where you can move more weight during the middle portion are bell-curved in nature (ie, single joint exercises like curling your arm, knee extension). Force changes according to muscle action and velocity of movement- which is different for concentric, eccentric, isometric, etc.

What controls the rate at which that force increases?

-all three answers above, rate and frequency of the action potential, plus the level of training of the individual. For the majority of people (especially in trained marathon runners), at any intensity of exercise, you will recruit first through all the ranges of Type 1 slow-twitch fibers available, then add on more and more of availbe Type 2 fibers only if the stimulus or the intensity is truly high enough (which in most cases will not occur, except through true sprint/power/resistance training). In other words, marathoners and others who are not anaerobically sprint/power trained will first use those fibers that require less activation (type 1) and last the longest. Even through it may be what they may consider an intense running exercise, the level of recruitment of type 2 fibers is still minimal when compared to what would be recruited during an intense power or resistance exercise.

Those who are sprint/power trained can and will practice selective recruitment, meaning they can bypass type 1 fibers and use type 2 fibers initially to perform actions that require high amounts of force in less time. This is why a trained marathoner will not be able to compete with a power trained person in vertical jump tests, sprints, or lifts. The marathoner has to recruit Type 1 through Type 2 to increase the force at a moments notice. The power trained person (when trained well) can jump right into Type 2 to increase the force right then for a that quick motion.