1. Electromyography: Physiology
D. Gordon E. Robertson, PhD, FCSB
Biomechanics Laboratory,
School of Human Kinetics,
University of Ottawa, Ottawa, Canada

2. Biomechanics Laboratory, University of Ottawa
Nervous System
Central Nervous System (cerebellum, cerebrum, brain stem, spinal cord) – conscious control, motor programs
Peripheral Nervous System (afferent and efferent motor nerves, various sensory organs) – reflex control, sensory feedback
Somatic nervous system – connect with skeletal muscles
Muscles can be excited (contracted) by either system
E.g., messages can travel from motor cortex directly to a-motoneurons via pyramidal nerve cells in spine,
Or a stretched muscle (tendon tap) can send a message via Ia-afferent nerves attached to muscle spindles directly to a-motoneurons to cause a reflex contraction
Biomechanics Laboratory, University of Ottawa

3. Central Nervous System
Pyramidal nerves (or corticospinal nerve tract)
carry messages from motor cortex to grey matter (anterior or ventral horns) in spinal cord
fastest conduction speeds
most cross from one side of brain to opposite side of body at the medulla oblongata
synapse directly with alpha-motor nerves but most synapse through interneurons
Biomechanics Laboratory, University of Ottawa

4. Peripheral Nervous System
Efferent nerves
excite muscles to contract directly (alpha motoneurons) or indirectly (gamma motoneurons)
Afferent nerves
carry sensory messages to brain or to motoneurons
Muscle Spindles
sense stretch and velocity
Golgi Tendon Organs
sense force in the tendons
Interneurons
carry messages from one nerve to another
usually inhibitory
Biomechanics Laboratory, University of Ottawa

5. Biomechanics Laboratory, University of Ottawa
Muscle Spindles
Muscle spindles (g-efferent nerves)
have excitable muscle fibres called intrafusal fibres that allow spindle excitability to be modulated
in parallel with muscle fibres
act primarily to excite muscles to contract via direct connection with alpha motoneurons and indirectly relax muscles to inhibit contractions of opposing muscles
Biomechanics Laboratory, University of Ottawa

6. Biomechanics Laboratory, University of Ottawa
Muscle Spindles
Muscle Spindles (Ia- and II-afferents)
when a muscle is stretched, spindles send messages to the spinal cord via Ia-afferent nerves that can cause the same muscle to respond with a contraction, called the stretch or myotatic reflex
Ia-afferents sense both muscle length and velocity changes
secondary II-afferents sense degree of stretch but not velocity
Biomechanics Laboratory, University of Ottawa

7. Biomechanics Laboratory, University of Ottawa
Golgi Tendon Organs
Golgi Tendon Organs (Ib-afferent nerves)
located in tendons and thus are able to sense tension changes
in series with muscle fibres
transmit signals to brain via Ib-afferent nerves
act primarily via interneurons to prevent tearing of muscle by inhibiting contractions
Biomechanics Laboratory, University of Ottawa

8. Biomechanics Laboratory, University of Ottawa
Reflexes (Examples)
Stretch Reflex (or monosynaptic or myotatic) are fastest skeletal reflexes since there are no interneurons
cause a stretched muscle to contract with least delay
Biomechanics Laboratory, University of Ottawa

9. Biomechanics Laboratory, University of Ottawa
Reflexes (Examples)
Flexion Reflex (or nociceptive withdrawal reflex) use interneurons and act to cause flexor muscles to contract after a painful or hot stimulus
Biomechanics Laboratory, University of Ottawa

10. Biomechanics Laboratory, University of Ottawa
Reflexes (Examples)
Reciprocal Innervation – when agonists flexors are excited, antagonist ipsilateral extensors are relaxed
Crossed Extensor Reflex – excitation of contralateral extensors after ipsilateral flexors have contracted due to a withdrawal reflex
Tonic Neck Reflex – flexion of neck facilitates flexor muscles of extremities; neck extension acts vice versa
Long-loop Reflex (or functional stretch reflex, transcortical reflex) – acts like the stretch reflex but takes longer and is trainable. Is part of the reason that prestretching a muscle (via a counter-movement) creates a stronger contraction.
Biomechanics Laboratory, University of Ottawa

11. Biomechanics Laboratory, University of Ottawa
Motor Unit
One a-motoneuron plus all the muscle fibres it enervates
Innervation ratio varies with number of fibres per motor unit (large leg muscles have many fibres per motoneuron for stronger responses, facial and eye muscles have few fibres and therefore permit finer movements but weaker contractions)
Biomechanics Laboratory, University of Ottawa

12. All-or-none and Tetanus
All-or-none Rule – once a motoneuron fires all its muscle fibres must fire
Graded muscle force occurs by increasing the rate of muscle firing until tetanus occurs, i.e., fusing of twitches that achieves higher tension in muscles.
Biomechanics Laboratory, University of Ottawa

13. Biomechanics Laboratory, University of Ottawa
Orderly Recruitment
Further graded muscle responses occur because of orderly recruitment of motor units, i.e., lowest threshold motoneurons (type I) fire first followed by the next lowest threshold fibres (type IIa). Highest threshold and strongest motor units fire last (type IIb).
Biomechanics Laboratory, University of Ottawa

14. Motor Unit Action Potential
When an action potential reaches the muscle at localized motor points (innervation points) the sarcoplasmic reticulum and t-tubule system carry the message to all parts of the muscle fibre
Biomechanics Laboratory, University of Ottawa

15. Motor Unit Action Potential
A rapid electrochemical wave of depolarization travels from the motor point causing the muscle to contract
Followed by a slower wave of repolarization and a brief refractory period when it cannot contract
The wave of depolarization can be sensed by an electrode and is called the electromyogram (EMG). The repolarization wave is too small to detect.
Biomechanics Laboratory, University of Ottawa

16. Biomechanics Laboratory, University of Ottawa
Electrodes
A surface electrode detects the wave of depolarization as it passes below.
As the wave approaches, the voltage increases; as it passes underneath the voltage goes to zero; finally as it departs the voltage reverses polarity and gradually declines.
This yields a biphasic signal.
The biphasic signals are so small that other electrical signals from the environment (called interference) mask them.
Solution is to use a differential
amplifier.
Biomechanics Laboratory, University of Ottawa

17. Differential Amplifier
Subtracts one signal from another.
By placing two electrodes in series over the muscle, the wave of depolarization passes under each electrode one after the other but with a slight delay.
Subtraction makes any common signal disappear and identical biphasic signals arriving at different times become a triphasic signal, called the electromyogram (EMG).
Biomechanics Laboratory, University of Ottawa

18. Biomechanics Laboratory, University of Ottawa
Electromyogram
EMG1 under electrode 1
EMG2 under electrode 2 has a slight delay
EMG1–EMG2 is the triphasic EMG signal
Biomechanics Laboratory, University of Ottawa

19. Biomechanics Laboratory, University of Ottawa
Electromyogram
two motoneurons exciting
five muscle fibres with their
associated EMG patterns
Each motor unit’s wave of depolarization may be detected by the electrode pair and will have approximately the same shape if the electrode stays at the same place with respect to the muscle. Thus, it is possible to detect the recruitment of single motor units.
In most contractions, however, there are many motor units some large, some small, some close and some far from the site so it is usual impossible to tell how many are firing and which fibres are firing (large vs. small).
But, EMGs may be used to roughly estimate the level of recruitment and the timing of muscle contractions.
Biomechanics Laboratory, University of Ottawa

20. Electromyogram cont’d
Except in very special situations it is NOT possible to use EMGs to estimate the level of force in a muscle.
It is also NOT suitable to compare the magnitude of one muscle’s EMG compared to a different muscle’s even in the same person.
The magnitude of the EMG depends of many factors unrelated to the force and therefore is a relative measure for each muscle.
Thus, EMGs are often normalized to specific values such as the muscle’s maximal voluntary contraction (MVC) or to some standard load.
Biomechanics Laboratory, University of Ottawa

21. EMGs and Vertical GRFs of Gait Initiation
R. erector spinae
L. erector spinae
R. tensor fasciae latae
L. tensor fasciae latae
R. adductors
L. adductors
R. tibialis anterior
L. tibialis anterior
R. (lead) vertical force
L. (trail) vertical force
Biomechanics Laboratory, University of Ottawa

22. EMGs and Vertical GRFs of Gait Initiation
yellow line shows start of gait, right leg’s vertical force increases while left’s decreases
right TA then left TA fire to assist forward lean, not shown is the relaxation of the plantar flexors
also firing is the right TFL, an abductor
Biomechanics Laboratory, University of Ottawa

23. EMGs and Vertical GRFs of Gait Initiation
left and to lesser extent right ES turn on just before right toe-off (green line)
left TFL fires strongly before single support start and continues to toe-off of left leg (red line)
lastly adductors are not fully recruited until full speed gait starts
Biomechanics Laboratory, University of Ottawa