BPK 142 Study Guide - Midterm Guide: Skeletal Muscle, Angular Velocity, Motor Unit
27 Apr 2018
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Skeletal Muscles:
- How muscles control (force)?
C. Force Control of a Motor Unit.
There are two methods by which a muscle can control force production:
1. Multiple Motor Unit Summation. A large muscle may contain up to 2,000 motor units.
A skeletal muscle can increase force production by activating more motor units. A
low force requires the activation of a small number of motor units while a higher force
requirement progressively enlists more motor units.
2. Frequency or Wave Summation. If a single action potential travels down a motor
neuron axon, the motor unit response is a twitch. If many action potentials travel down
the axon at a rate faster than the twitch response time of the motor unit, then the
mechanical force response will summate. At high motor neuron firing rates (i.e., many
action potentials in a short period of time) this effect can generate on average about 5x
the force of a single twitch.
When all motor units are activated, and they are all activated at a high firing rate, then the
muscle is maximally activated.
Size principle of motor unit recruitment – there is a recruitment order of motor units as
exercise intensity increases. As the muscle force requirement increases, motor units with
progressively larger axons are recruited. Slow twitch motor units with the lowest activation
threshold are selectively recruited during light to moderate effort. More rapid, powerful
movements progressively activate FOG motor units and then FG motor units, until all the motor
units of all types are activated.
D. Mechanical Factors Which Influence Muscle Force Production.
1. Muscle Length-Tension Relationship
An isolated muscle can exert its maximal force or tension while in a resting stretched position.
As the muscle shortens, less tension can be exerted.
When a muscle is stretched or shortened, the length of each individual sarcomere also
increases or decreases, respectively. Within the range of sarcomere lengths there is an optimal
length at which provides for the greatest possible number of cross-bridge formations. Longer or
shorter than this optimal length, the number of cross-bridge formations is reduced and thus
active force production is reduced.
Length – Tension Curve For An Isolated Muscle
- Muscle stretched/shortened. Length of each individual sarcomere increases/decreases.
Document Summary
How muscles control (force): force control of a motor unit. There are two methods by which a muscle can control force production : multiple motor unit summation . A large muscle may contain up to 2,000 motor units. A skeletal muscle can increase force production by activating more motor units . A low force requires the activation of a small number of motor units while a higher force requirement progressively enlists more motor units: frequency or wave summation . If a single action potential travels down a motor neuron axon, the motor unit response is a twitch. If many action potentials travel down the axon at a rate faster than the twitch response time of the motor unit, then the mechanical force response will summate. At high motor neuron firing rates (i. e. , many action potentials in a short period of time) this effect can generate on average about 5x the force of a single twitch.
