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Gradation of Muscle
Force
Two neural mechanisms
responsible for force
gradations:
1. Recruitment
 Spacial summation
2. Rate coding
 Temporal summation
Recruitment
Varying the number of motor units activated.
Larger motor units
Largest motor units
Low stimulus threshold
Higher stimulus
threshold
Highest stimulus
threshold
The Size Principle
Amount of Force Required During Movement
↑
Number & Size of Motor Units Recruited
↓
Small motor units
Rate Coding
Rate coding refers to the motor unit firing rate.
–
Active motor units can discharge at higher frequencies to
generate greater tensions.
Recruitment vs. rate coding
–
–
Smaller muscles (ex: first dorsal interosseous) rely more on
rate coding
Larger muscles of mixed fiber types (ex: deltiod) rely more on
recruitment
The firing of individual motor units occurs as a stochastic process
Firing rate is a better term to describe the global changes in firing
frequency (i.e., rate coding)
Rate Coding
Rate
coding
100
Larger
muscles
Smaller
muscles
% Maximal
Voluntary
Motor Unit
Recruitment
50
Motor unit
firing
frequency
0
0
50
% Maximal Voluntary Force Production
100
Rate Coding
Rate coding occurs in two stages
– Treppe (the treppe effect)
A phenomenon in cardiac muscle first observed by H.P. Bowditch;
If a number of stimuli of the same intensity are sent into the muscle
after a quiescent period, the first few contractions of the series show
a successive increase in amplitude (strength)
– Tetanus
A state of sustained muscular contraction without periods of
relaxation
Caused by repetitive stimulations of the α-motor neuron trunk (axon)
at frequencies so high that individual muscle twitches are fused and
cannot be distinguished from one another, also called tonic spasm
and tetany
Two forms of tetanus
– Incomplete tetanus – occurs when there are relaxation phases allowed
between twitches
– Complete tetanus – occurs when the relaxation phases are completely
eliminated between twitches
Important!
Smaller muscles (ex: first dorsal
interosseous) rely more on rate coding
Larger muscles of mixed fiber types (ex:
deltiod) rely more on recruitment
How do we measure motor unit
recruitment and rate coding?
Surface electromyogram and
mechanomyogram
Electromyography (EMG)
Muscle force controlled by CNS
Neural “signals” sent to muscle by CNS
“Signals” are called action potentials
Action potential that originates from the cell body
of the primary motor neuron
– Motor unit action potential (MUAP)
Action potential that propagates along the
sarcolemma (from the neuromuscular junction to
ends of the fibers)
– Muscle action potential (MAP)
Electromyography (EMG)
EMG is the recording of the MAP.
EMG is defined as the algebraic sum of
MAPs that pass within the recording area
of the electrodes
Electromyography (EMG)
Used by clinicians
Used by ergonomists
Used by physiologists
Used by biomechanists
Recording EMG Signals
Electrodes
– Surface of the skin
– Subcutaneous
– Intramuscular
Needle or fine-wire
The size and location of the electrodes
determine the composition of the EMG
signal
Recording EMG Signals
Surface EMG provides a global measure
of MAPs
Fine-wire EMG records single action
potentials in a few adjacent muscle fibers
Surface EMG
Bipolar electrode arrangement
– Electrode pair over the muscle
– A reference (ground) electrode over boney
area
– Signal = (E1 + ground) – (E2 + ground)
Common mode rejection
Placing Surface EMG Electrodes
Identify the innervation zone
Place bipolar arrangement distal to the
innervation zone
Use interelectrode distance (IED) of
approximately 2 cm
Position to avoid “cross-talk”
Contaminating Factors
Electrode and lead movements
Volume conduction = cross talk
Electromagnetic radiation of power
sources
Two common methods to reduce these:
– Bipolar recording
Common mode rejection
– Signal filtering
Isometric
muscle
action at
30% MVC
Displacement
Sensor
Accelerometer
Laser
Beam
Bipolar EMG
Electrodes
Force
Transducer
Orizio, C., Gobbo, M., Diemont, B., Esposito,
F., Veicsteinas, A. Eur J Appl Physiol. 2003.