Lecture 10 - Resistance Training II Flashcards
Motor unit
Consists of motor neuron and all the much fibres it innervates.
Motor units often contain fibres of a
similar type (i.e., type I vs type II)
-smaller = slow twitch
-fast twitch = bigger
Hennemans size principle
Recruitment based on size and force output
Firing frequency of active motor units increases
* More motor units are recruited
Smaller (slow twitch/type I) motor units are recruited first
* Lower threshold for activation
Physiological underpinning of size principle
Ohms law: Voltage different = (electrical current)(resistance)
Membrane resistance depends on number/density of ion channels
- Smaller neurons will have larger membrane resistance
- Less surface area = less ion channels
- Membrane resistance decreases “leak” of current back out of cell
activation
The amount of calcuim released within a muscle fibre
-activation responses to amount of force produced
At whole muscle level, we control force by increasing firing frequency and/or increasing the number of active motor units (MU recruitment)
Recruitment vs rate coding in the control of muscle force
Force exerted by a muscle depends on
the number of motor units that are
activated (recruitment) and the rates
at which these motor units discharge
action potentials (rate coding)
- Relative contribution of these two
processes varies depending upon the
muscle and its function
What does this have to do with muscle hypertrophy
As we fatigue, more fast twitch/type II motor units will be recruited
It is generally accepted that fast twitch motor units have a higher growth potential from resistance training than slow twitch motor units
Therefore, in order to stimulate the muscle fibres with the highest potential for growth, we must get sufficiently close to muscular failure to ensure that
these motor units are recruited
Hypertrophy vs hyperplasia
Hypertrophy – increase in size of individual muscle fibres
- Hyperplasia – increase in number of muscle fibres (scientifically questioned).
It is generally thought that in humans*,
hypertrophy is the predominant mechanism by which muscles grow
Protein balance and muscle hypertrophy
Muscle hypertrophy must come
from a net increase in the amount of
protein within the muscle
Resistance exercise increases muscle
protein breakdown, but not as much
as the increase in muscle protein
synthesis
Primary signaling pathways in muscle hypertrophy
Downstream targets ultimately shift muscle protein balance to favor
synthesis over degradation
-lots of intercellular pathways involved
KNOW mTOR and Calcium
Mechanistic target of rapamycin (mTOR) : increasing protein synthesis following resistance training
* Important signaling molecule in muscle hypertrophy
* Induced by mechanical loading
Ca2+ (through muscle activation) also acts as a potent signaling molecule
for increased muscle protein synthesis through its activation of a number
of signaling pathways
Mechanotransduction in muscle
Mechanical tension is a key factor in muscle hypertrophy, but the specific mechanisms of its signaling pathway is
poorly understood
Signaling proteins located near structural elements may play a crucial role in mechanotransduction in muscle
In addition to initiating muscle contraction, Ca2+ plays a key role in
numerous signaling pathways in muscle
Gene expression and hypertrophy
When a gene is expressed, its corresponding protein is manufactured
Transcription and translation
From signal generation to functional
improvement
Exercise initiates cellular signals that
activate a number of proteins involved in gene transcription
Repeated exercise bouts over
prolonged periods of time result
in accumulation of protein (increase mRNA expression and protein synthesis)
Long term adaptation to training
is due to cumulative effects of
each exercise bout
Parallel vs serial muscle hypertrophy
Parallel hypertrophy = radial
growth
* Increased force production capacity
Serial hypertrophy = longitudinal
growth
* Increased fibre/fascicle length
* Results from adding more sarcomeres in = series (sarcomerogenesis)
* Increased shortening velocity/power
production capacity
Fascicle length and muscle performance
As each sarcomere shortens independently, more sarcomeres in series will allow a higher muscle
fibre maximum shortening velocity
Additionally, for a given muscle fibre shortening velocity, more sarcomeres in series means that individual sarcomeres will not have to shorten as
fast
This will have positive impact on force/power due to force-velocity relationship
Sarcoplasmic vs myofibrillar hypertophy
Myofibrillar hypertrophy
- Increase in number and/or size of individual myofibrils inside each muscle fiber
Sarcoplasmic hypertrophy
- Expansion of the sarcoplasm (the cytoplasm of the muscle) inside the muscle fiber
- Conceivably enhances muscle bulk without concomitantly increasing strength
General overview of signaling mechanisms of muscle
hypertrophy
Mechanical tension (longitudinal and lateral)
- Forces experienced by muscle fibres
- Most important factor in training induced muscle hypertrophy
Metabolic stress
* Exercise induced accumulation of
metabolites
* Associated with fatigue
Muscle damage
* Can set off series of subsequent reactions
* e.g., inflammation, satellite cell infiltration, release of growth factors
Mechanical tension versus activation
mTOR can be activated by tension per se (activation not required)
- To increase muscle protein synthesis, it seems that both tension AND activation is required
What role does damage play in muscle hypertrophy?
Result of high intensity training
Muscle protein synthesis will be
increased acutely to repair damaged
tissue, but this may not reflect net
protein accretion
What role does metabolic stress play in muscle hypertrophy?
High rates of anaerobic energy input
leads to buildup of metabolites
Bodybuilders often employ training
that results in high amounts of
metabolic stress (i.e., fairly high reps
per set, limited rest between sets)
Plays role in hypertrophy
Several mechanisms have been postulated linking metabolic stress with muscle hypertrophy
* As with activation and muscle damage, it is difficult to tease out effects of metabolic stress per se from other factors
Important
Blood flow restriction training and metabolic stress
BFR entails performing resistance training while restricting blood flow to working limbs via a cuff placed proximally to the limb
- Less blood flow
- Less oxidative metabolism
- More buildup of metabolites
BFR has gained popularity for its ability to promote muscle growth with
comparatively low loads
* E.g., rehab purposes
Blood flow restriction training and muscle hypertrophy
Significant increases in leg size have been reported following walking with BFR
Yet, applying BFR cuffs to biceps immediately following resistance training has been shown
to impair hypertrophy
