How does muscle produce force?
Signal from AMN tells muscle to contract by releasing ACh at neuromuscular junction
AP propagates across surface and down t-tubule in center of muscle to the voltage gated receptors (DHPRs)
Once exposed to DHPRs, they change shape and calcium is released to bind to tropin which opens binding on actin/myosin
Cross bridges from sliding produces contraction
What is the relationship of nerves to muscle?
1 nerve to 1 muscle fiber relationship
Voluntary muscle contraction recruits (Henneman's Size Principle)
small slow fatigue resistant to large fast fatiguable fibers
Voluntary recruitment is
activating more motor units
Voluntary rate coding is
asynchronous rate coding (random over time)
With estim, recruitment is analogous to
With estim, rate coding is analogous to
Can increase muscle force with estim by
increasing intensity or frequency
How are contractions activated voluntarily verses stimulated by estim?
Starts in CNS with voluntary and is bypassed using the periphery when using estim
Estim stimulation recruits by
increasing intensity (electrical field)
Estim stimulation rate coding by
increasing frequency synchronously at a fixed rate
Where is estim placed to stimulate muscle contraction?
over superficial motor points
Rate coding is
frequency of action potentials produced by estim that drive the activated motor units up to a higher rate of firing to summate (does not recruit more motor units like recruitment)
Increasing intensity and recruitment varies in
activation pattern from person to person even with same intensity level and electrode placement
Toe region of the force frequency curve (rate coding) shows
Muscle twitches (low-rate TENS)
Why does an increase in frequency from 5 to 10 hertz show no increase in force?
The frequency is set so low that time lapses to allow the action potentials to return to baseline
How can you get a stronger muscle contraction when thinking of the force frequency curve?
Low intensity with high frequency (rate coding) to summate the action potentials
What does the linear region of the force frequency curve (rate coding) show?
Muscle twitches close enough together to summate and generate a higher force
Do slow fibers summate faster or slower compared to fast fibers?
Faster because they relax more slowly to return to baseline (better chance to summate)
Slow fibers frequency level for smooth muscle contraction (fusion)
30 to 40 hertz
Fast fibers frequency level for smooth muscle contraction (fusion)
50 to 80 hertz
If estim continues to increase after force plateaus then the force will
How can you correct electrical failure or fatigue?
Allow rest times
Electrical failure or fatigue
At a high frequency, there is an ion build-up to prevent membrane potentials within the narrow t-tubule, throwing off the sodium potassium gradient (small amount) and no longer depolarizes for muscle contraction
How do you recover from electrical failure or fatigue?
Can recover instantaneously
With direct stimulation of the nerve, normal recruitment order is
Estim frequency causes muscles to fire at a
synchronous rate (arm contraction beats rather than smooth contraction)
Estim recruits fibers
randomly (whatever is close by)
Which will fatigue faster when stimulating, 10% verses 80% MVC?
Will fatigue at the same rate, because e-stim recruits randomly (both large fatiguable and small fatigue-resistant at the same time)
Submax NMES tends to produce more rapid what compared to submax voluntary contraction? Why?
Rapid fatigue due to random recruitment of fibers
What is the goal of muscle contraction?
Assist with movement (not overload, stabilize joints to move)
Improve motor control (coordination of muscle for correct timing of fire rates)
Motor level pain control
Strength involves more than muscle size, what else does it depend on?
Neural drive/central activation
Muscle contraction stimulation can be generated by
Pulsed currents (biphasic)
Interrupted or burst AC (Russian)
Standard protocol for muscle contraction stimulation
Russian or burst (interrupted) AC
Russian or burst (interrupted) AC can be used for what level of stim aside from motor level
sensory level (analgesia)
Can high volt produce muscle contractions?
Yes, but not a good choice for strengthening (overloading) a person due to it's small phase charge
Typical parameters for NMES
Duration--at least 200 microseconds (varies with muscle)
Frequency--depends on the task to get smooth muscle contraction
Electrode placement for NMES
Over motor points (superficial nerves), can change when skin moves
Can you strengthen with NMES?
Yes, must use approximately 50% MVC in order to see results
What is the problem usually with intensity with studies looking to use NMES for strengthening?
Poorly controlled with intensity because:
Stim is usually set at a max tolerable level for the person (varies from person to person)
Subjects often produced weaker (lower MVCs) contractions than compared to their voluntary max
(25% MVC with stim verses 60% MVC with voluntary)
What is the key to strengthening with NMES?
Train based on force, not stim output
What can decrease neural drive or central activation?
Lack of motivation
True inhibition of that muscle--edema in joints, ligament sensory loss or pain
How can we assess a decrease in central activation?
Use estim by using instrumented dynamometry
How does assessing CNS drive to skeletal muscle work?
Estim and voluntary movement superimposed to show force
Dip in force after stimulation artifact with estim testing central activation
Antidromic signal from stim counteracting against orthodromic
Increase in force when stimulating shows
Deficiency at baseline where the person is not recruiting all of their motor units (CNS is not driving all the units)
Central activation testing is done by
MVC of isolated muscles or groups with maximal stim (supramaximal is better)
How do you measure central activation deficit?
Central activation ratio (CAR)
Twitch or doublet interpolation
Central activation ratio (CAR)
Voluntary force divided by voluntary force + stim force
Less than 1 CAR shows
some central activation deficit
1 minus twitch force during MVC divided by post-contraction force twitch force times 100
Uses single pulse rather than burst
Patient population with profound central activation deficits
3 patient populations that typically have central activation deficits
Weakness is dependent on
muscle size and neural/central activation
What is more of a problem with strength loss, atrophy or central activation?
How might central activation failure affect response to exercise?
Affects exercise because you can only overload those fibers that are recruited (may see gains, but not to their fullest potential as if all of their motor unites were recruited)
estim is useful!
How can one never reach their rehab goal with just exercise (example, patient with quad lag with a goal to match uninvolved limb)?
Patient could have a central activation deficit (example 20%) or failure and may never reach goal with volitional exercise (will never use their motor units that are not recruited)
How might NMES be of use?
Can activate fibers that the CNS cannot do by activating the PNS. Sensory input and antidromic stim with NMES causes cortex to light up (lots of feedback) to wake up the system
NMES for ACL post-op, standard protocol
Modified Russian protocol
What is NMES based on rather than using current or voltage parameters
Achieving a force (MVC)
3 reasons for NMES beyond strengthening
FES (functional movements--gait, reach, stand)
Splinting or prothesis (scoliosis, shoulder subluxation)
What changes are typically made with NMES being used for muscle re-education, FES or splinting (anything other than strentgh)?
Ramp time and on/off times
What does the ramp and on/off times need to do with FES?
Mimic functional task (example, heel switch for foot drop)
Muscle re-education also typically combines volitional with electrical contractions, why might this not be the best idea?
Antidromic colliding with volitional movement (stim first with no volitional contraction, then turn off stim and ask patient to make the same volitional contraction)
Describe how the FES foot drop stimulator works
Stim turns on the dorsiflexors when heel rises to clear foot in swing and then stim ramps down and turns off when heel hits the ground in initial contact
Parastep for SCI
Works on systems for more complex movements that are sometimes with implanted electrodes
Where is FES typically placed for shoulder subluxation?
Deltoid and supraspinatus
Results of study on FES-splinting for shoulder subluxation
Decreased subluxation with estim and showed carryover effects due to waking up the CNS and gaining strength
What are the limitations of treating peripheral nerve injury with estim?
Maintenance of mass/strength, ROM and function while nervous system recovers; however requires much greater current due to greater capacitance of the muscle
Why is estim for a denervated muscle controversial?
Could have negative effect by inhibiting the recovery of denervated muscle if muscle is trying to send out neurotrophic factors to attract nerves to it. If contracting by estim then it won't secrete these factors or find it necessary to promote reinnervation
What is different about stimulating a SCI patient compared to other denervated injuries?
SCI patients still have PNS intact above and below lesion (studies with implanted electrodes do show some improvement in functional return with continuous stim)