The answer is yes, there is a point at which muscles will stop working, but this is rare. Most commonly there is a point that muscles will work sub-optimally which is often manifested as fatigue. The causes are many.

The body has three types of muscle. Smooth muscle exists in the walls of several types of organs such as the GI tract, bladder and blood vesicles. It is under involuntary control and responds to various stimuli, both neuro and chemical, as released by the body. Smooth muscle is what acts to propel food from one end to the other and what helps us maintain our blood pressure. It does not often fatigue, but can go awry causing plenty of problems. It can contract just right, too much or too little. If you think about it, we’ve all experienced this to a degree.

Cardiac muscle is pretty much isolated to the heart. It is also an involuntary muscle and obviously has fantastic endurance qualities. It can be conditioned and de-conditioned and even fatigued, but usually only in pathologic conditions or extreme conditions (prolonged episodes (days) of sustained rapid heart rates for example). An amazing muscle since it starts beating before we are born and in most of us, keeps going day in and day out while responding to the various demands placed on it automatically.

Voluntary muscle, or skeletal muscle is probably the muscle most identified with the sense of fatigue. Looking at how skeletal muscles work can help explain some causes of fatigue. Nerves deliver impulses to muscles that allow the muscles to contract. The nerve impulses are delivered to the muscles by small vesicles containing neurotransmitters that are released from the ends of nerves. The chemicals cross the small space between the nerve and the muscle, bind to the muscle and result in a biochemical cascade that causes muscle contraction. Most often there are plenty of neurotransmitters at the end of the nerves just waiting to be called on and cross the neuromuscular junction. In extreme cases, it is possible to temporarily run out of neurotransmitter, or reduced supply of neurotransmitter, resulting in weakness. It would be incredibly rare to run out of transmitter in non-pathological states. Various pathologic conditions and even some poisons (think nerve gas or pesticides) effect neurotransmitter release at the nerve, or binding at the muscle, and cause weakness or prolonged spasm (overstimulation) or tetany (muscle contraction that cannot be broken, rendering the muscle useless). In normal people, neurotransmitter regeneration takes place pretty quickly.

Within the muscle cell contraction involves many different things. Once the neurotransmitter binds to the muscle walls, a cascade of events takes place involving energy stores (ATP), calcium release and binding and sliding of the myofibril elements actin and myosin. Actin and myosin are the actual contractile elements within the muscle cell. They lie next to each other and overlap each other, and with the right impulse and metabolic environment, they act like a ratchet mechanism causing shortening of the muscle that we experience as muscle contraction. There is even such a thing as optimal actin / myosin overlap to generate an optimal contraction.

Of course all of this metabolic muscle contraction activity generates metabolic waste and there is some thought that some of the metabolic waste, if accumulated quickly, can result in fatigue. At one time it was thought that lactic acid accumulation played a major role in this, but it is less clear now.  Running out of an energy source (glucose, glycogen, ATP) can absolutely give a sense of fatigue. And then there is the mental aspect. Mind over matter if you will. What is fatiguing to some, is not fatiguing to others.

Bottom line is that there is a very complex combination of physiologic factors and psychologic factors at play. Conditioning, diet and appropriate resting of the muscles is the key- which can be extremely difficult on this type of adventure, especially in storm conditions. The good news is that Rich’s muscles are unlikely to just stop working- but it may feel like they’ll stop!

Question submitted by Rich, aboard Great American IV