Fatigue Mechanism in a High-Power Exercise

[Sasha Pachev]

What is the fatigue mechanism in a high power exercise like maximum bench press?

If you run 100 meters all out, you can try it again a few minutes later, and you will achieve a comparable if not better performance. However, if you do a maximum bench press, it will take you as long as a couple of days or maybe even more before you can do it again with the same results.  Why such a difference?

[Jon Allen]

I'm not a muscle expert, but I can see a difference.  In bench press, you are pushing your muscles to their maximum strength limit.  In the run, your muscles are actually not even coming close to their strength limit- they are only lifting/pushing your body, which isn't much for those muscles.  Your speed is limited mainly by stride length and turnover, with strength playing a lesser role.  That is why you can run again a few minutes later.  As for the fatigue mechanism in the muscles, I don't know.

[adam]

I don't necessarily agree with the idea that after an all-out 100m you would be able to produce something just as good or better after a brief rest. This may be true for distance runners who won't see a 10 in front of their time, but if this were the case for sprinters, we would see more 100m runners sprinting all out before their races in order to get a better time.

In any case, Jonathan is on the right track. Muscles recruit their fibers according to the intensity of the exercise. Lifting a light weight recruits less fibers than a medium, and a heavier weight more than a medium. An all out bench will recruit all the associated muslce fibers possible to produce the necessary force against the resistance of the weight. A one Rep Max is very dangerous for this very purpose. It is hardly every done anymore because of the risk of muscle injury is very high.

In running, though we may think differently because it hurts so much, we hardly ever need to recruit all muscle fibers to produce to force necessary for the movement.

The fatique effect comes from level of recruitment. The more muscle fibers recruited, the longer lasting the fatigue.

[Adam R Wende]

I have to agree with the other Adam on this one. I would agrue that the two are not comparable they way you point out. However, here is the first description I could find in reviews on the topic. If you are interested I can try and get the full copy of the review article for you...

Ann Readapt Med Phys. 2006 Jul;49(6):257-64, 348-54
Although everyone knows fatigue personally, it is a difficult concept to define. For muscular fatigue, one must know the aspect of performance affected. The most obvious demonstrations are decreased maximal force and slowed muscular answer. Fatigue can have a central origin, by reducing cognitive performance or lowering excitation of motoneurons. Various mediators are in question (serotonin, moduline, dopamine). The fatiguing muscular contractions are accompanied by reduced discharges of motoneurons. The neuromuscular junction does not seem to be in question. Cold reduces muscular power, whereas a hot environment limits exercise by a central mechanism, which starts the normal behavioural response to stop the exercise. Fatigue can also be the consequence of overtraining. In the periphery, the electric activity of the membrane's surface is the first possible sign of failure, which explains high-frequency fatigue: the accumulation of potassium outside the cell blocks the sodic channels to block the potentials of action or slow down their propagation. With fatigue, less calcium is released and limits the number of attached actin-myosin bridges connections of actin-myosin. The slowing down of the muscular answer represents a deterioration of the function of actin-myosin bridges. On the metabolic level, the most-often evoked changes are reduced pH and increased intracellular lactate level. However, these variations cannot all describe fatigue, since patients with Mc Ardle disease do not exhibit these variations but very quickly experience tiredness. In fact, an association of small metabolic intracellular variations could explain tiredness. The fast fibres are larger than slow fibres; their metabolic needs are higher and they are thus more sensitive to tiredness. The half time of recovery is within approximately 1 min: normal values of force and power are recovered after 5 to 10 min. During endurance activities, the limiting factors are glycogen reserves and levels of oxidative enzymes. On the whole, mechanisms of fatigue must be explored to completely understand the governing phenomena.

[Sasha Pachev]

So what exactly happens to you after a one-time max bench press that makes you so incapable afterwards? Is it just the muscle tear? Something in the nervous system? Do we even know?
My understanding of fiber recruitment that for each individual fiber it is either on or off, not such thing as a half-powered individual fiber contraction, correct? If that is the case, then if you recruit every possible fiber for an all out effort, each individual fiber suffers no more than it would from a less than maximum effort lasting the same amount of time were it to be recruited for that effort - the difference is that when you lift a fraction of your max, you recruit a fraction of your fibers, but that fraction works exactly like it would in a max contraction while the remainder of the fibers are resting. Is that a correct assumption, or are things more complex?

[Adam R Wende]

Sasha, I'm trying to get you a more detailed reply. I've sent your questions to two exercise physiologists I know. If they reply I'll let you know. If not I should have some time this weekend to look up additional info...

[adam]

Sasha,

I was refering to the fiber type when I was writing about the fiber recruitment, not about an individual fiber. Sorry for the confusion.

Most sources will say you recruit muscle fibers in this order: Type 1a (slow twitch), then Type 2a (partial fast/partial slow twitch), and then Type 2x (fast twitch) depending on the intensity of the exercise. So before anything else, your body tries to recruit all the slow twitch fibers it has, then adds to it more and more fast twitch.
A heavier weight would recruit a higher percentage of your total muscles fibers in a given muscle group.

This is why I was trying to say a trained distance runner would hardly ever reach the intensity to require a full recruitment as the person lifting a max weight would. It is also a reason why some say that weights are important in training distance runners, as it may be some of the only exercise that can completely target your fast twitch fibers. This idea is probably much different for trained sprinters, as they most likely do get to the intensity needed to recruit a higher percentage of fibers. This may be a reason why we see so many sprinters pull a hamstring 40m into the race and might be related to why a one rep max can also cause injury.

[James Winzenz]

I suspect that with a one rep max effort, you would be dealing with microtears within the muscle fibers.  Since the function of a muscle fiber contraction is similar to that of an 8-man crew (actually a pretty good visualization, I think), although I seem to recall that the cells within the muscle that contract actually stick out in all directions along the axis of the fiber (i.e. looks more like a set of golf clubs), you can visualize all of the members of the crew team sticking their oars into the water and pulling with all their might, but the water is more like molasses.  Are some of the oars going to break at the head where the force is furthest from the fulcrum?  Probably.  Then you end up with two things - one, a decreased capacity of the muscle to contract and two, soreness within the muscle after 1-2 days as the muscle is forced to heal.  Just my opinion from my exercise physiology classes from years bygone . . .

[adam]

You're right about the microtears. The eccentric contraction involved in lowering the weight lengthens filaments beyond their optimal length, causing tears in the muscle fibers. This is one of the main causes of primary muscle soreness, and one of the causes of DOMS. Runners will most likely expierence this effect from running hard downhill.

[adam]

Good article on fatigue in the November-December Running Times

[Sirenesque]

The simplest explanation is that during a max bench effort the fibers develop micro tears during the eccentric phase of the motion, the lowering of the weight.  The muscle tissue is damaged and takes time to repair.  It would be overly simplistic to say that all of the fibers would be damaged, but a percentage certainly is.  In regards to running the 100, no where near maximum contraction of muscle is required, and the eccentric forces are generally limited to absorbing body weight - limited muscle damage.  Most force required to move the body is with concentric force.  The limiting factor becomes the amount of phosphocreatine contained in the muscle tissue.  PC is used for short burst high intensity exercise - generally only the first maybe 7-15 seconds. PC stores are replenished relatively quickly and the body can be ready to go fairly soon.  Provided the muscle fibers are still intact and there is adequate fuel, the feat can be replicated.  If recovery time is too short, the body is forced to rely on anaerobic glycolysis with lactic acid being a by-product.  Once blood lactate levels rise beyond a certain point normal muscle function is not possible until it has be metabolized.  PC stores will generally replace within a few minutes.