You're lifting weights. The first time feels easy, but each lift takes more and more effort
until you can’t continue. Inside your arms, the muscles responsible for the lifting
have become unable to contract. Why do our muscles get fatigued? We often blame lactic acid
or running out of energy, but these factors alone don’t account
for muscle fatigue. There’s another major contributor: the muscle’s ability to respond
to signals from the brain. To understand the roots of muscle fatigue, it helps to know how a muscle contracts
in response to a signal from a nerve. These signals travel from the brain to the
muscles in a fraction of a second via long, thin cells called motor neurons. The motor neuron and the muscle cell
are separated by a tiny gap, and the exchange of particles
across this gap enables the contraction. On one side of the gap, the motor neuron contains a
neurotransmitter called acetylcholine. On the other side, charged particles, or ions, line the muscle cell’s membrane: potassium on the inside,
and sodium on the outside. In response to a signal from the brain, the motor neuron releases acetylcholine, which triggers pores on the muscle
cell membrane to open. Sodium flows in, and potassium flows out. The flux of these charged particles
is a crucial step for muscle contraction: the change in charge creates an electrical
signal called an action potential that spreads through the muscle cell, stimulating the release of calcium
that’s stored inside it. This flood of calcium causes
the muscle to contract by enabling proteins buried in the muscle
fibers to lock together and ratchet towards each other, pulling the muscle tight. The energy used to power the contraction
comes from a molecule called ATP. ATP also helps pump the ions back
across the membrane afterward, resetting the balance of sodium
and potassium on either side. This whole process repeats
every time a muscle contracts. With each contraction, energy in the form of ATP gets used up, waste products like lactic
acid are generated, and some ions drift away from the muscle’s
cell membrane, leaving a smaller and
smaller group behind. Though muscle cells use up ATP as they
contract repeatedly, they are always making more, so most of the time even heavily fatigued muscles still
have not depleted this energy source. And though many waste products are acidic, fatigued muscles still maintain pH
within normal limits, indicating that the tissue is effectively
clearing these wastes. But eventually, over the course of
repeated contractions there may not be sufficient concentrations
of potassium, sodium or calcium ions immediately available near
the muscle cell membrane to reset the system properly. So even if the brain sends a signal, the muscle cell can’t generate the action
potential necessary to contract. Even when ions like sodium,
potassium or calcium are depleted in or around the muscle cell, these ions are plentiful
elsewhere in the body. With a little time, they will flow back to the areas
where they’re needed, sometimes with the help of active sodium
and potassium pumps. So if you pause and rest, muscle fatigue will subside as these ions
replenish throughout the muscle. The more regularly you exercise, the longer it takes for muscle fatigue
to set in each time. That’s because the stronger you are, the fewer times this cycle of nerve signal
from the brain to contraction in the muscle
has to be repeated to lift a certain amount of weight. Fewer cycles means slower ion depletion, so as your physical fitness improves, you can exercise for longer
at the same intensity. Many muscles grow with exercise, and larger muscles also
have bigger stores of ATP and a higher capacity to clear waste, pushing fatigue even
farther into the future.
