Physiology of Strength and Power Flashcards
How is a muscle action produced?
A result of the activation signal produced by the nervous system and the force produced by the muscle-tendon unit. This leads to joint action
Innovation Ratio
The ratio between the number of fibres that are innovated by a single neuron.
What is a motor unit comprised of?
Alpha motoneurone
Spinal cord
Axons
Fibres
Why is the size of the motor neuron and motor unit important?
The activation signal from the motor unit needs to be transformed into contractile activity. Done in an orderly fashion (size principle)
How do neurotransmitters depolarise cells?
Through the post-synaptic membrane
Muscle spindles
receptors in the muscle that are sensitive to changes in the muscle length
Muscle contraction
Muscles contract to shorten the muscle and reduce the stimulus. This happens quickly and allows us to regulate where we are in space
Golgi-tendon organs
Receptor that controls tension in the muscle
They send an inhibitory signal into the alpha motoneurone - protective mechanism against excessive tension of the muscle.
The size principle
Smaller motor units are recruited first. As a greater force is produced, bigger motor units are recruited.
As motor units are recruited, a twitch occurs (basic contractile response to an action potential).
Lot of twitches are produced at the same time
Producing very high forces
We have to discharge motor units at very high frequency and this leads to a summation of the twitches (tetinus).
Max contraction
Max contraction for a few seconds (jumping before a sprint) the twitch amplitude of a motor unit will increase and force production will increase acutely.
What does a warm up do?
Primes the nervous system to drive the muscles at a higher capacity than it would do with no warm up.
How to increase the force producing
You need to recruit more motor units to increase twitch size or discharge motor units at a higher frequency (rate coding)
What happens when you recruit new motor units?
the twitches summate and the force production is bigger
Fine motor control muscle (muscles of the finger)
Muscles will recruit units at 50% of max force so any further increase will come from discharging at a higher frequency. This is to maintain the resolution of the action, son it remains accurate with low errors.
High force muscles
Recruit more motor units to increase the output
Rate coding
The first recruited units (smallest units) will reach higher discharge rates that those units recruited later - has more time to develop the discharge rate.
What dictates how fast you can produce force?
How high the rate coding is at the beginning and how quickly you are able to recruit all motor units.
Fatigue and max force production
When we fatigue our max production capacity decreases. New units are recruited and the rate coding is adjusted. The discharge frequency is increased in the motor units.
The change point of fatigue and force production
The later the direction of change point the longer the task (using this you can predict how long a person can perform that action). The later that point the longer the task.
Contractile element of the muscle fibre
Active portion of the muscle
Characterised by force length
Sarioelastic element of the muscle fibre
can be divided into active (elasticity of the myofibril and the sliding filament/cross bridges) and passive components (tendons and connective tissues)
Paraelastic element of the muscle fibre
modulator that transmits the force from the fibres onto the skeleton
Tendons
Passive component of force production through recall of the tendon.
At longer length muscles the recall has a greater contribution
Can train the force-length relationship
Force - Velocity relationship
Muscles can adapt for lots of different loads, by modulating the velocity at which the muscle contracts.
High loads = slower contraction to reach the force needed
Can modulate by increasing the temperature of a muscle, all values will increase
When is max power produced?
About 1/3 velocity is when you produced the max power
How are fibres are arranged in the muscle?
Series
Parallel
Series
range of motion of muscle is maximised because length at which muscle can operate increases, maximises velocity in the force production
Parallel
maximises force, increases strength
Maximising muscle mechanical properties
To maximise force, operate at about 1/2 length of the muscle. Sacrifice speed
To max power, sacrifice force production
Stretch-shortening cycle
Series of actions makes the movement more economical.
Allows tendon to store elastic energy, can invoke some reflex into that muscle - produce higher forces.
How to increase muscle performance?
Improve neural activation and muscle-tendon force.
There is a skill element as well - training in a specific way helps you to be strong in the way you need for sport
Training adaptations in first few weeks of resistance training
An untrained person starting resistance training will increase strength rapidly.
First few week = no increase in muscle mass, increase is due to skill adaptations and neural adaptations
Training adaptations after training for a few year
Increases in strength are largely a result of an increase in muscle size and increase in contractile proteins that are in muscle tissue.
Other adaptations are relevant but they are smaller contributors to strength increase.
Neural adaptations
Adaptations in the cortical centre (motor cortex) due to strength training result in an decreasing inhibitory input in the motor cortex.
Alpha motoneurone is the main transducer of activation signals to muscle contractions
Where do inputs from the nervous system converge?
Alpha motoneurone
Performing resistance training on an untrained person will cause:
Recruitment of more motor units
Discharging motor units at a higher frequency
The effect of strength training on the threshold of recruitment
The threshold at which recruitment will occur will decrease with strength training - more effective at producing force.
Derecruitment
time at which the muscle relaxes (motor unit is not active). Unaffected by training.
Happens in certain situations (stroke)
What happens when we are performing an action?
We don’t just use the agonist, there are also lots of synergists involved as well as the antagonist. These protect the joints and the bones, but are counterproductive to force production
H reflex
The electrophysiological equivalent to a tendon-tap reflex
Muscle spindles
Muscle receptor that detects length and rate of change of length in the muscle
Project via the interneuron (transmits information from one side of the body to the other)
Have an inhibitory effect - when they are excited they inhibit the antagonist muscle, so we can move freely not stiffly.
Electromyography (EMG)
Not the best way to measure muscle activation - has limitations
It is a very simplistic view of the role of the antagonist
Antagonist coactivation
After 8 weeks of training, strong evidence that by using complex simulation procedures on nerves you can reduce the activation of the antagonist.
Increase the activation of the agonist and decrease the activation of the antagonist - a combo of these increase force
Why is muscle size important?
there is a close linear relationship (high correlation) between cross sectional area of the muscle and the force they can produce
What is strength also influenced by
Genetic components
Muscle mechanical properties (how tendons attach)
Environmental factors - can’t control these
Estimated force =
cross sectional area x tension a fibre can produce
How to increase muscle size
By increasing the amount of contractile proteins you have in the muscle tissue and connective tissue (hypertrophy)
Specific Tension
how much tension a specific fibre can theoretically produce. Bigger fibres have greater tension
Myonuclei
Fills up with contractile proteins when you put stimulus on the muscle.
Limits you very quickly on building up muscle.
Can only take on so many proteins
Satellite cells
Single nuclei cells
Activated by protein synthesis
Signalling pathways to trigger protein synthesis involve gene expression
Progress into myonuclei so can be filled with contractile proteins as well
Ratio of protein synthesis and breakdown
Each training session you do causes protein breakdown
If ratio between breakdown and synthesis is positive = growing muscle
If ratio is negative no muscle growth = overtraining
What happens when there is a muscle stimulus?
You will trigger the signalling pathway which activates mTOR (enzyme)
Signalling protein goal
activate mTOR which leads to protein synthesis and hypertrophy.
Proteins are stacked in parallel (maximises force production) - increases force producing capacity of the muscle
Proteins in series would maximise length of the muscle and the velocity with which it would contract
Why is mechanical tension needed?
to maintain the muscle mass you have for normal daily function
Exercising individuals at different intensities
protein synthesis will be maximised by using higher loads - high loads not necessary for hypertrophy
As long as you reach a given threshold of a muscle you will increase the muscle mass. Need to go into failure and need to do a lot of sets
H-zone
where the thick and thin filaments overlap during contractions
Why aren’t sarcomeres aligned perfectly?
They are at a slight angle.
When there’s enough tension and the thin filaments are being pulled it causes a rupture in the Z-disk and that causes the repair of the muscle (protein synthesis)
Mechanical tension has to be high or the Z-disk won’t rupture
Muscle damage
Occurs when you perform certain actions (something new)
Causes a disruption in the extracellular matrix of the muscle which leads to a cascade of processes that are linked to information that excites receptors in the muscle and you feel soreness
Soreness typically delayed - after 2 days, slowly disappears
Why is muscle damage more prevalent in eccentric contractions
forces are high and doing contractions at longer muscle lengths
You are putting your muscle in a position where it will produce higher forces and it will cause a greater disruption in the sarcomere
Muscle damage and mechanical tension
Difficult to separate
Difficult to introduce muscle damage without mechanical tension and it’s difficult to induce hypertrophy without mechanical tension
Blood flow restriction training
Reducing blood flow causes the accumulation of metabolites during exercise in the muscle. Blood can’t get in to remove the metabolites
Done at fairly light loads
At a given percentage of max it induces hypertrophy
Hypertrophy
accumulation of proteins within a given muscle fibre
Specificity of strength
The way we express and how we express the neural activation and power is important.
More typical resistance training causes increase of strength in concentric contractions, not as much eccentric
Hyperplasia
the splitting of a muscle into two cells. The daughter eventually becomes an adult muscle cell.
Increases cell count in the body and size in a more effective way
