Myo-Electrical Stimulation of Muscle Flashcards
What is the purpose of Myo-electric stimulation
Achieve muscle contraction and explore its applications
Def: Electricity
The force created by an imbalance in the number of electrons at two points
Electrons
Negatively charged particles
Current
The movement of electrons from higher potential to lower potential
Ampere
The unit of current or rate of electron movement
Voltage
Electromotive force created by the difference in electron population between two points
Resistance
Opposition to electron flow measured in ohms
Watts
Units of electrical power = to volts x amps
Capacitance
The ability of a material to store electricity
Ohm’s Law
Current through a conductor is directly proportional to the potential difference(voltage) and inversely proportional to resistance
1- 10mA shock
Person will feel little or no electrical shock effects or even the sensation of shock
10-20 mA shock
Painful shock will occur like a jolt but muscle control will not be lost
20-75 mA shock
Shock is more serious. Pain jolt and muscle control will be lost resulting in the inability to let go of the current source
75-100mA shock
Ventricular fibrillation of the heart occurs and damage can result
100-200 mA shock
Heart can stop and death can occur if medical attention is not administered quickly
over 200 mA shock
Severe burns, internal organ damage and the heart can stop due to the pressure that the chest muscles place on the heart
- heart does not experience fibrillation and the person can survive if the source is removed quickly
Methods of stimulation
- Percutaneous electrodes - placed on the skin over the target muscle
- implanted electrodes - surgically placed within paralyzed muscle for chronic use
Electrode set up
- Bipolar arrangement
- Activates muscle tissue between electrodes
Simulator control
- very short square-wave pulse
- short duration pulses enhance safety, reducing the risk of tissue damage
- Voltage adjusted to increase the number of recruited muscle fibers
- Frequency adjusted to increase the firing rate
Effect of voltage on single pulse stim
- low voltage: a small voltage excites only a few muscle fibers, resulting in a small twitch force
- Increased voltage: as voltage increases, more muscle fibers are recruited, leading to a larger twitch force
- Maximal Recruitment: At a certain voltage, all recruitable fibers are active and further increase in voltage do not increase twitch force
Muscle force and stimulus frequency
- Higher stimulus frequency = greater twitch summation
- Modulated by the CNS
Recruitment in voluntary contractions compared to myo-electrical stimulation
VC
- follows Henneman size principle (small to large as force increases)
ME
- recruitment based on proximity to electrode and impedance
Simultaneous Firing in voluntary contractions compared to myo-electrical stimulation
VC
- Fibers in a motor unit fire together, but different motor units have different firing rates
- Results in a smooth, graded contraction and fine motor control
ME
- Simultaneously activates all recruited fibers, treating the muscle as a single motor unit
- less fine control
lack of inhibition in voluntary contractions compared to myo-electrical stimulation
VC
- muscle activation is modulated by mechanisms (stretch reflex, reciprocal inhibition, sensory inputs)
- Safeguard mechanism to prevent injury
ME
- Overrides natural inhibitory mechanisms - increased injury risk
- Requires careful application
pain considerations in voluntary contractions compared to myo-electrical stimulation
VC
- natural activation is painless if the muscle is uninjured
ME
- High activation levels can cause pain
- Full activation of large muscles may be limited by pain tolerance
Functional Electrical Stimulation
Uses myo-electric stimulation to activate muscles
FES in spinal cord injuries
- Jerrold petrofsky’s innovation applied computer-controlled electrodes to a paralyzed student enabling her to pedal a tricycle
- Nan davis is perfect canidate due to minimal muscle atrophy and prior athleticism
Benefits of FES
- Enables ambulation on non-wheelchair-friendly terrain
- facilitates standing to reach objects
- reduced muscle wasting, bone density loss, and bed sores
- provides psychological motivation
Challenges of FES
FINE MOTOR CONTROL
- Current FES lacks the ability to recruit motor units asynchronously or by size principle
- Potential improvement with implanted electrodes enabling multi-channel muscle activation
WEIGHT BEARING
- Paralysis and muscle atrophy (issues holding posture)
- Orthotic solutions (hip, knee, ankle orthotics to improve stability and strength)
- offer reciprocating gait by coupling hip joints for reciprocal motion
FATIGUE
- can’t simulate fatigue-resistant fibers first
SENSORY FEEDBACK
- loss of afferent feedback creating difficult with balance and reaction
Reciprocating Gait Orthosis with FES
- Combines functional electrical stimulation with a reciprocating gait mechanism
- Electrodes placed over key muscles control hip, knee, and ankle flexion/extension via a voice-activated computer
- lightweight brace can reduce muscle force required for weight bearing
Closed System
Alters future behaviour or state of a system, aims for a desired outcome or state
Change-the-state system
Simply changes the state without feedback
Control theory
A strategy to select the appropriate inputs
Basic parts of control system
- Plant: the system to be controlled
- Input: acts on the plant to produce the output
- Output: The result of the plant’s response to input
Closed loop control
- Measures system output with a sensor and compares it to a reference signal
- Comparison creates an error signal sent to the controller
- The controller’s output becomes the input to the plant
Central vs peripheral fatigue
Central Fatigue: Decreased neural activation of muscle
Peripheral Fatigue: Decreased Neural Activation of the muscle
Role of Myo-electrical stimulation on central vs peripheral fatigue
Helps identify the source of fatigue
- Subject maintains 50% MVC until fatigue sets in
- If myo-electric stimulation maintains force longer, central fatigue is the cause
- If stimulation force declines similarly with voluntary effort, peripheral fatigue is the cause
Individual muscle isolation
- Research on muscle mechanics often preformed on animals for isolation of tendons and muscles
- Voluntary contractions in humans cannot isolate individual muscles, but myo-electric stimulation can
- Different strength curves
Outcome of strokes
10% recover almost completely
25% recover with minor impairments
40% experience moderate to severe impairments
10% require long-term care
15% do not survive
Myo-Electric stimulation in stroke rehab
- Complements traditional rehab by using EMG-triggered electrical stimulation
- Enhances neuronal plasticity through proprioceptive and somatosensory feedback
- Facilitates reorganization of damaged brain areas and supports physiological learning processes
Motor Learning and MES
- MES is effective for children with cerebral palsy and hemiparesis due to birth defects
- Provides sensory feedback combined with contraction force
- Encourages use of neglected limbs
MES and strength gain
LIMITATIONS:
- Pain and injury risk restrict maximum stimulation of large muscles
- Strength gains primarily observed in severely atrophied muscles
RECENT ADVANCES:
- Emerging evidence shows benefits comparable to voluntary efforts
- Can be used to prevent atrophy but not for weight loss (burns minimal cal)
