Lecture 7, Control of Muscle Tendon and Training Adaptations Flashcards
Neural Control - Review (motor units)
motor unit: a motor neuron and all of the fibers it innervates
- one motor neuron can innervate several fibers, which may be distributed between up to 100 different fascicles
during a muscle contraction, multiple motor units are “recruited”
- increase in motor unit recruitment -> increase in muscle fibers contracting -> increase in muscle tension (increase in cross bridge-cycling (contract))
motor units are recruited on a rotating basis
- function: delays fatigue of muscle fibers, while allowing for maintained muscle tension
Order of Recruitment of Motor Units
sequence of motor unit recruitment goes from smallest to largest:
-> smallest motor units contain the fewest and slowest muscle fibers
-> larger motor units contain faster, more powerful muscle fibers
maximum muscle tension = all motor units are recruited and in a state of fused tetanus (firing and generating the most tension possible)
Motor Unit Recruitment
motor units contains one type of muscle fibe
- each motor neuron innervates the same muscle fiber type
->slow, oxidative motor units contain only type l fibers
fast, oxidative-glycolytic motor units contain only type lla fibers
-> fast, glycolytic motor units contain only type llx fibers
remember: motor unit recruitment goes from smallest to largest
* type l fibers have the smallest diameter
* slow, oxidative motor unit are recruited first
* fast, glycolytic units are recruited last
no muscle contains only one muscle fiber type
- the proportion of type l, type lla and type llx fibers in a muscle is called the muscle’s fiber type proportion
◦ aka fiber type distribution
Fiber Type Distribution
type 1 fibers have high endurance, low power
- muscles that need to maintain their activity for long periods of time are more likely to have a higher proportion of type l fibers
◦ postural muscles (ex. intrinsic back muscles,
abdominals, etc.)
◦ respiratory muscles (ex. diaphragm, intercostals,
etc.)
type lla/llx have lower endurance and higher power
- muscle that are often used to generate lots of muscle tension in short bursts are more likely to have a higher proportion of type lla/11x fibers
◦ ex. leg muscles in a sprinter or long-jump athlete
comparisons between sport:
- average proportion of type l fibers in the gastrocnemius of an elite distance runner: 75%
- average proportion of type ll fibers in the gastrocnemius of a sprinter: 75%
Motor Unit Size
motor units have different sizes - largely depends on the function of muscle
- muscles for fine movements have small motor units (less fibers/MU) (greater control)
◦ eyes, playing the guitar and writing
- muscles for powerful movements have large motor units (more fibers/MU) (thousands of muscle fibers present within one motor unit/greater output at the expense of control)
◦ power lifting, sprinting
Henneman Size Principle
the orderly recruitment of motor units from smallest to largest
- smallest MUs are always recruited first
- increasing the required force -> progressive recruitment of larger MUs (more overall active fibers)
light load contractions recruit predominately slow, oxidative MUs (predominately type l fibers)
- the heavier the load, the more MUs are recruited
Skeletal Muscle Adaptations to Resistance Training
consistent resistance training results in an increase in muscle strength
- the type of adaptations at the muscle level depends on the type of exercise performed
initially, the increase in muscle strength occurs due to neural adaptations
- improved mind-muscle connection
- changes in nervous system
later, subsequent increases in muscle length occurs due to changes at the muscle level
- hypertrophy: an increase in muscle mass due to an increase in muscle size
-> increases in strength can occur without increases in size
Skeletal Muscle Adaptations to Resistance Training
adaptations to training depend on FITT:
- frequency: how often you exercise
- intensity: the load placed on the muscle (% of maximum)
- type: type of exercise performed
- time: duration of each exercise sessions
resistance training: using a muscle or muscle length to lift a heavy load for a low number of repetitions
- progressive overload: the gradual increase in weight, frequency, and/or # of repetitions in a resistance training routine
a proper resistance training program elicits several changes at the muscle level:
- hypertrophy: the increase in cross-sectional area of a muscle fiber, leading to an increase in the cross-sectional area of the whole muscle
- an increase in the FT: ST fiber cross sectional-area
◦ selective hypertrophy of fast twitch fibers
- increase in [glycolytic enzymes]
Skeletal Muscle Adaptation to Endurance Training
- frequency: how often you exercise
- intensity: to load placed on the muscle (% of maximum)
- type: type of exercise performed
- time: duration of each exercise sessions
endurance training: using a muscle or muscle group to generate a low relative tension, for a high number of repetitions - also referred to as aerobic training or cardio
consistent endurance elicits several changes at the muscle level: - increased mitochondrial biogenesis (more mitochondria and increased mitochondrial density)
- increased capillary density
consistent endurance training also results in adaptations in the cardiovascular and respiratory systems, in improving blood flow to the musle - improved oxygen delivery and metabolic removal
Muscle Fiber Type Distribution
muscle fiber distribution is largely influenced by genetics, and is a good predictor of exercise performance
- greater proportion of type l fibers = more likely to succeed in slower, long-distance events
- greater proportion of type llb/llx fibers = more likely to succeed in high velocity, short-distance events
there is some degree of trainability for muscle fiber type distribution
fiber type plasticity: the ability for a muscle fiber to transition from one type to another
- individuals are genetically predisposed to their fiber type distribution, but there is some degree or trainability
- resistance training may induce the transition of slow twitch fibers to fast twitch fibers
◦ increased glycolytic capacity
- endurance training may induce the transition of fast twitch fibers to slow twitch fibers
◦ increased oxidative capacity, capillarization,
myoglobin…
the transition is not instantaneous
- fibers that are in the middle of a transition are sometimes referred to as “hybrids”
◦ ex. type l/lla hybrid, or type lla/x hybrid
fiber type switching occurs at a lower extent, the
more well-training an individual is
Muscle Cramps
muscle cramps are voluntary tetanic contractions of a muscle
- action potential firing rates are higher than that of an MVC (maximum voluntary contraction)
cause is not fully elucidated, but is likely related to electrolytes imbalances
- electrolytes: a substance that can be broken down into ions when dissolved in a solvent/fluid
electrolyte imbalances at the tissue-level may impair the ability of the muscle or neuron to maintain the concentration gradient
- at rest, the resting membrane potential of a neuron is -70 mV
- at rest, the resting membrane potential of the muscle cell is -70 to -90 mV
with an imbalance in electrolytes, this might impair the ability of the neuron/muscle to depolarize and/or repolarize
Muscle Cramps (2)
another suggested cause of muscle cramp is:
- stimulation of sensory receptors in the muscle, which activate a reflex arc with the motor neuron being the effector
- ingestion of spicy foods may reduce the incidence of muscle cramps
◦ supposedly by activating the reflex arc that
decreases motor neuron output
◦ chemical stimuli at the mouth/GI level is the
stimulus
electrolyte imbalances may result from overexercising or dehydration
- electrolyte drinks are often recommended to rebalance the body’s electrolyte concentration and alleviate cramping
- heat therapy, massage, hydration and rest also often help
Hypocalcemic Tetany
hypocalcemic tetany: the involuntary tetanic contraction of skeletal muscles that occur when the extracellular Ca2+ concentration decreased to about 40% off its normal value (need to have high normal value, to maintain normal polarization)
- intramuscular [Ca2+] is low because the majority of the Ca2+ resides in the terminal cisternae of the SR, rather than in the sarcoplasm
- extracellular and plasma [Ca2+] is relatively high
- concentration gradient of of Ca2+ across the membrane
◦ allows for depolarization
◦ a change in the [Ca2+] would impair the ability of
the neuron or muscle to depolarize and repolarize
- low extracellular [Ca2+] results in an opening of Na+ ion channels, leading to depolarization and spontaneous action potential firing
◦ importance of [Ca2+] regulation/homeostasis
◦ maintain it close to its homeostatic state
hypo = less or under
calcemic = relates to calcium
hypocalcemic = low [Ca2+]
