1.14 - Muscle Force Production Flashcards
Define: Contraction
Development of force with or without muscle shortening
Define: Twitch
Brief contractile response elicited by a single action potential. A twitch does not produce sufficient force by itself
Define: Force
Force = mass x acceleration (N)
Define: Torque
Torque (Nm) = Force (N) x radius (m).
Torque is a twisting force
Define: Power
Power=the rate of work done = work per unit time
Power=Force x Velocity
How does the CNS control muscle contraction?
Through two mechanisms:
Tetany
Motor-unit recruitment
Describe Tetany
A single twitch by itself does not produce much force
When a subsequent AP follows before Ca2+ has had time to be removed, the twitch force adds to the previous force (temporal summation)
Tetanus refers to when individual twitches are no longer distinguishable from each other (when the response plateaus)
Describe Motor Unit Recruitment
A motor unit refers to all muscles fibres innervated by
1 α-motor neuron. By recruiting more motor units
(motor-unit recruitment) more muscle fibres will be
activated and hence force will increase
The additional recruitment of motor units is also referred
to as spatial summation or multiple fibre summation
Describe Asynchronous recruitment and its purpose
Asynchronous recruitment of motor units within a
motor neuron pool is performed by the spinal cord to
prevent fatigue.
Motor unit recruitment is shifted between different motor-units to conserve energy
The shift between turning of and off the different motor
units is coordinated smoothly
Asynchronous recruitment can only occur for submaximal recruitment
What are the three types of muscle contraction?
Isometric
Isotonic (Concentric and Eccentric)
Auxotonic
Describe Isometric Contraction
No length change
i.e. no movement – but force (no external force observable)
“easy” case – single variable (force at fixed length)
Describe Isotonic Contraction
length change against a constant force/weight; (external) force observable
“complex” case – two variables (force and length for a fixed load).
There are two types of isotonic contractions:
Concentric: contraction with shortening
Eccentric: contraction with lengthening
Describe Auxotonic Contraction
Auxotonic: contraction of muscle as the load changes (combination of isometric and isotonic). In reality, most muscle contraction is auxotonic
Describe the Length-Tension Relationship
Isometric
Maximum tension occurs where there is maximum overlap of actin and myosin filaments –> Optimal length
Describe the Force-Length Relationship
Isotonic
Set up allows the muscle to contract and pull a weight with a level
The weight is changed in increments from very heavy (an immovable object) to no load
The slope of the length curve = velocity of contraction
When the load is too heavy for the muscle, force will develop but the length will = 0, i.e. it will be an isometric contraction
When there is no load (force=0) the muscle will contract maximally (no load opposing the contraction)
The slope of the length curve = velocity of contraction
The velocity of muscle contraction is dependent on the amount of force (load)
The maximum velocity possible is when load = 0 (no load), and is governed by the max rate possible of the cross-bridge cycling (ATP dependent)
A heavy load or immovable object –> all actin and myosin are engaged in the cross bridge cycle:
If the load reduces slight so that 95% of the filaments are in the cross-bridge cycle to maintain the isometric contraction, the remaining 5% are available to provide the movement –> thus relatively slow movement.
If there is no load –> 100% of filaments are available to contribute to movement, therefore max velocity achieved
