Lecture 10 - Resistance Training II

Question 1

Motor unit

Answer 1

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

Question 2

Hennemans size principle

Answer 2

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

Question 3

Physiological underpinning of size principle

Answer 3

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

Question 4

activation

Answer 4

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)

Question 5

Recruitment vs rate coding in the control of muscle force

Answer 5

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

Question 6

What does this have to do with muscle hypertrophy

Answer 6

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

Question 7

Hypertrophy vs hyperplasia

Answer 7

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

Question 8

Protein balance and muscle hypertrophy

Answer 8

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

Question 9

Primary signaling pathways in muscle hypertrophy

Answer 9

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

Question 10

Mechanotransduction in muscle

Answer 10

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

Question 11

Gene expression and hypertrophy

Answer 11

When a gene is expressed, its corresponding protein is manufactured

Transcription and translation

Question 12

From signal generation to functional
improvement

Answer 12

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

Question 13

Parallel vs serial muscle hypertrophy

Answer 13

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

Question 14

Fascicle length and muscle performance

Answer 14

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

Question 15

Sarcoplasmic vs myofibrillar hypertophy

Answer 15

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

Question 16

General overview of signaling mechanisms of muscle
hypertrophy

Answer 16

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

Question 17

Mechanical tension versus activation

Answer 17

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

Question 18

What role does damage play in muscle hypertrophy?

Answer 18

Result of high intensity training

Muscle protein synthesis will be
increased acutely to repair damaged
tissue, but this may not reflect net
protein accretion

Question 19

What role does metabolic stress play in muscle hypertrophy?

Answer 19

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

Question 20

Blood flow restriction training and metabolic stress

Answer 20

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

Question 21

Blood flow restriction training and muscle hypertrophy

Answer 21

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