lecture 55

• Myo-electric stimulation is the electrical stimulation of muscle
• functional electrical stimulation (FES) which elicits contraction of paralyzed muscle
• 1. Percutaneous Electrodes - pairs of electrodes are placed on the skin over the target muscle or over the nerve.
• Implanted electrodes - chronically implanted electrodes are surgical placed within paralyzed muscles.
• Stimulator control: Increasing the voltage recruits more fibers and increasing the frequency increases the firing rate of the recruited fibers.
• V = I x R
• As the frequency of the stimulus train of pulses increases, there is increased twitch summation in a similar way that the central nervous system activates muscle by increased motor unit firing rates.
• an increased voltage but no further increase in the twitch force. This means that all of the fibers have been recruited with the last three stimuli.
• Comparison with voluntary contractions:

  1. recruitment
  2. simultaneous firing
  3. lack of inhibition
  4. pain consideration
• my-electric stimulation does not recruit motor units
• The stimulus recruits fibers based on the proximity to the electrodes and the input impedance of the fiber.
• all fibers within a motor unit fire together in an all-or-none fashion but different motor units have different firing rates and they are not synchronized. This allows for smoothly graded contractions and fine motor control.
• Myo-electric stimulation simultaneously activates all of the fibers that have been recruited. This is analogous to treating the muscle as one large motor unit. This contributes to less fine control of the force.
• lack of inhibition: The stretch reflex, reciprocal inhibition, and other modulations of muscle activation from afferent or sensory inputs is over-ridden by myo-electric stimulation. Since these aspects of muscle control safeguard against injury, care must be taken with myo-electric stimulation.
• pain considerations:  myo-electric stimulation can be very painful at high levels of activation. Many large muscles cannot be full activated with myo-electric stimulation because of pain tolerance. 
• he was able to apply electrodes with computer controlled stimulators to a student at Wright State University to cause her lower extremity muscles to ride a tricycle.
• Benefits: of fES  - ambulation on terrain not friendly to wheelchairs

                   - standing to reach objects

                   - reduced muscle wasting, loss of bone density, and bed sores

                   - psychological "I will walk again!"
• Challenges of FES

  • Fine motor control
  • Weight bearing
  • Fatigue
  • Sensory feedback
• Firing of motor units in FES

  In voluntary activation, the firing of different motor units is asynchronous, while in electrical stimulation, they are synchronous. This requires higher firing rates in electrical stimulation, which are known to cause fatigue
• Weight bearing in FES

  Can be offset by a hip-knee-ankle-foot orthotic or reciprocating gait orthosis
• Aspects of fatigue in FES

  • Voluntary activation
  • Electrical stimulation
• Fatigue in FES

  During voluntary activation, the most fatigue resistant motor units are recruited first while in electrical stimulation, muscle fibers that have poor fatigue resistance are often recruited at low levels of stimulation
• peripheral fatigue= loss of contractility in the muscle
• central fatigue= decreased activation of the muscle
•  If myo-electric stimulation is used to produce the same level of force (50% MVC), and if the force is maintained for a longer period of time, then the fatigue was caused by central factors. If the stimulated force declines at the same time as the voluntary contraction, then the source of fatigue was in the muscle.
• Voluntary contractions by living human subjects cannot isolate an individual muscle but myo-electric stimulation can.
• The EMG triggered electrical stimulation is a positive influence on the neuronal plasticity of the brain due to proprioceptive and somatosensory feedback.  This leads to a reorganization of cerebrally damaged areas as well as physiological learning processes. 
• Myo-electric stimulation has helped children with cerebral palsy and hemiparesis due to birth defects.  The stimulation gives sensory feedback like vibration but also adds contraction force which helps the children use the neglected limb.
• Due to pain limitations and risk of injury when stimulating large muscles at maximum levels, strength gains have only really been achieved with severely atrophied muscle. However, recent advances are showing myo-electric stimulation to have benefits similar to voluntary efforts. 
• Prevention of Atrophy

  prolonged space flight

  • long term immobilization
• Weight Loss

  False advertising on infomercials include the following statements"Now you can get rock hard abs with no sweat" . Mild stimulation of small muscles contracting isometrically does not burn many calories.