Q. What are some explanations of fatigue regarding muscular contractions?
A. Fatigue
is usually perceived as any reduction in physical or mental
performance. For the exercise physiologist, the word has a more
restricted meaning. In the case of isometric contractions, force is a suitable outcome measure. However, when one considers dynamic contractions, "contractile response"can
replace "force". This includes shortening, power, or velocity of
shortening. The rate of fatigue depends on the muscles employed, the
relative intensity of the exercise, and whether or not the contractions
are continuous or intermittent. Although there is no singular cause of
fatigue, it is task specific and its causes are multifaceted and vary
from occasion to occasion. However, when discussing various aspects of
training, fatigue can be described as failure to maintain the expected
force, or the inability to maintain a given exercise intensity or power
output level (Meeesen
2006).
There are two types of fatigue: peripheral and central.
Fatigue during exercise is often due to impairment within the active
muscle themselves, in which case fatigue is peripheral to the central
nervous system. Specifically, the muscle contractile proteins are not
responding to their neural stimulation. Depletion of muscle glycogen
(for fuel) is thought to be an important factor in peripheral fatigue,
especially during prolonged exercise (Jentjens, 2003).Peripheral
fatigue also encompasses events that occur independently of the CNS,
including disturbances to axon terminals, neuromuscular junction,
excitation contraction coupling, and contraction itself. Interestingly,
this is where the majority of fatigue originates. Recently, strong
evidence suggests the central fatigue hypothesis as an exercise-induced
increase in extracellular serotonin concentrations in several brain
regions contributed to the development of fatigue during prolonged
exercise. (Meeesen 2006).
Central
fatigue is concerned with the descending motor pathways from the brain
and spinal cord. Bishop and colleagues (2008) explain that brain
messages may signal reductions or complete cessation of exercise
performance. The central fatigue hypothesis suggests that the
brain is
acting as a protective mechanism to prevent excessive damage to the
muscles.During contraction in athletic competitions, muscular fatigue
usually appears to be a peripheral phenomenon and due to fatigue of the
muscles. The central nervous system and the associative motor neurons
and neuromuscular junction appear to be far superior to skeletal muscle
in maintaining function. However, as evidence suggests, the CNS can
become a major contributor of fatigue function during exercise, as the
CNS may operate to limit performance of muscles, thereby protecting the
heart, brain, and other vital organs from damage.
Through
the wonders of contemporary science and technology, a whole series of
devices and techniques are becoming available to researchers. Many of
these devices will be of assistance in understanding muscle fatigue,
such as nuclear magnetic resonance (NMR), positron emission tomography (PET), and near infared spectroscopy (NIRS).
However, further research is warranted on these techniques with working
muscles, brain, and other tissues during exercise.Denervation Apoptosis
of muscle fibers, re-innervation of surviving fibers
References
- Bishop PA, Jones E, Woods AK. Recovery from training: a brief review: brief review.J Strength Cond Res. 2008 May;22(3):1015-24.
- Jentjens,
R, & Jeukendrup, A. (2003). Determinants of post-exercise glycogen
synthesis during short-term recovery. Sports Medicine. 33(2):117-144.
Meeusen
R, Watson P, Hasegawa H, Roelands B, Piacentini MF. Central fatigue:
the serotonin hypothesis and beyond.Sports Med. 2006;36(10):881-909
~Jonathan Mike, CSCS, USAW, NSCA-CPT
Doctoral Student, Assistant Editor |