Lecture 14: Electrical Recruitment of Muscle Force

Question 1

TENS
* stands for
* stimulation target
* use

Answer 1

• Transcutaneous Electrical “Nerve” Stimulation
• Stimulation targets are large sensory nerve axons
• Used for relief of Acute or Chronic Pain

Question 2

What’s the big difference between therapeutic and functional electrical stimulation?

Answer 2

• therapeutic: helps to rebuild muscle strength but not necessarily to use in normal patterns of activation
• functional: can help with normal pattern of activation. muscles will be stimulated at specific times and during specific actions

Question 3

TEMS
* stands for
* stimulation target
* use
* what is a requirement of TEMS?

Answer 3

Transcutaneous Electrical “Muscle” Stimulation
* Stimulation targets are motor nerve axons
* Used to Increase Muscle Strength and Endurance
* muscle must be innervated

Question 4

What is TEMs of the paralyzed muscle useful for?

Answer 4

• Prevent disused muscle atrophy
• Increase muscle mass
• Increase muscle force
• Increase resistance to fatigue (endurance)
• Increase blood flow (exercise skeletal muscle pump)
• Reduce edema, remove metabolic waste
• Reduce fibrous tissue growth, adhesions, pain
• Moderate spasticity by facilitating voluntary movement

Question 5

What is endurance index

Patients that received what type of stimulation had higher endurance indexes?

Give an example of how muscle endurance can be measured.

Answer 5

Endurance index is the fraction of force that is still being generated after 3.5 min of intermittent stimulation. The endurance index provides a quantitative measure of a muscle’s ability to resist fatigue during sustained or repetitive contractions. A higher endurance index indicates greater resistance to fatigue

patients that received more daily intermittent stimulation (0 vs 45min vs 8 hours), had a higher endurance index

EI increases progressively in subjects that received increasing durations of daily intermittent stimulation.

Stimulation Protocol: The common peroneal nerve is stimulated with a train of electrical pulses at 40 Hz for 2 seconds, followed by 2 seconds of rest. This pattern is repeated for 3.5 minutes.

Force Measurement: During the stimulation protocol, the force generated by the ankle dorsiflexor muscles is continuously recorded.

Endurance Index Calculation: fraction of initial force that is still being produced at the end of the 3.5-minute stimulation period.
For example, if the initial force generated is 100 N and the force at the end of 3.5 minutes is 60 N, the endurance index would be 0.6 (60/100).

Question 6

What does the classification of muscle types reflect?

Answer 6

Variation in myosin heavy chain isoforms

Isoforms are MHC1, 2A, 2X and 2B

Fiber contractile speeds show corresponding differences

• Type 1 fibers are slow-contracting and fatigue-resistant = Type S
• Type 2A fibers are fast-contracting and fatigue-resistant = Type FR
• Type 2B and 2X fibers are fast-contracting and susceptible to fatigue = Type FF

Question 7

Give the power equation for muscles

Answer 7

force x shortening velocity = power

Question 8

phasic vs tonic firing

Answer 8

tonic = always firing but at different rates (depends on muscle effort) - generally type s fibres

phasic = not always firing; usually type FF or FR

Question 9

How does stimulation activity pattern affect muscle fibre type?

Answer 9

When you stimulate fast muscle at a pattern resembling those found in nerves of slow muscles (10Hz continuous stimulation for several weeks) results in slow contractile characteristics

Sustained high levels of stimulation would induce slow, fatigue-resistant properties. Low or intermittent levels of use would allow fibers to retain, or revert to, a native fast, fatiguesusceptible state.

Question 10

Discuss thresholds of activity as changes in plasticity of muscle

Answer 10

With increasing stimulation, you cross certain thresholds:
1. anaerobic to aerobic –> FF to FR

2. 2x to 2a:
3. MHC 2A to MHC 1

Question 11

Why are the effects of stimulation greater than those of voluntary endurance exercise?

Answer 11

With electrical stimulation, you recruit the largest motoneurons first. These are physiologically the hardest to recruit.

electrical stimulation overrides the physiological recruitment order
and can activate all the fibers in the muscle simultaneously.

takes effort to recruit them all

Question 12

What were Salmon’s findings when he studied the response of rabbit fast muscles to 10, 5, and 2.5
HZ delivered for 10 months

Answer 12

Muscle mass, maximum shortening velocity, maximum isometric tension, and power declined most at 10Hz, then followed by 5 and 2.5Hz.

Continuous stimulation at 10 HZ transformed the myosin isoform composition completely to the slow pattern. 10/s x 60s/min x 60min/hr x 24 hr/day = 892,800 stimuli/day

The 2.5-HZ pattern had brought about only a partial transformation, with a disappearance of MHC2X, predominance of MHC2A, and slight induction of MHC1.

At 5 HZ the results varied between one animal and another, resembling the 2.5-HZ response in some cases, and the 10-HZ response in others.

This suggests that the aggregate amount of activity is more important
than the frequency. The variable response at 5 HZ is exactly what would be expected if this stimulation pattern
placed the muscle close to the threshold for fast-to-slow transformation. Muscles stimulated at 2.5 HZ received least aggregate activity but showed the highest levels of
both oxidative and glycolytic
activity.

Thus, you want to stimulate enough to reach second threshold, don’t overstimulate, or you’ll convert all to type S.

Question 13

What is FES of a paralyzed muscle useful for?

Answer 13

restore voluntary control of paralyzed muscles
 patterned stimulation of motor nerves
 partially restore sensory-motor integration
 sensory nerve stimulation can support force production
 provide sensory cues
 reestablish auditory, vision, touch perceptions
 alleviate spasticity
 stimulation of afferents can modify reflex gain
 alleviate chronic pain
 cutaneous or epidural afferent stimulation (neuromodulation)

Question 14

Describe vastus intermedius

Answer 14

vastus intermedius: always engaged; non fatiguable. soleus of the quad

motor point is inaccessible from the skin surface

Question 15

List 10 body locations
where FES may be
applied to restore
some use of
paralyzed muscles

Answer 15

Skin surface
intramuscular
Epimysial: muscle surface
Peripheral nerve: ex: tibia
Ventral roots: may result in other muscles moving too
Dorsal roots: to generate a reflex. may not be specific to one muscle
Motor nuclei: grey matter of ventral cord
Spinal pattern generators: in spinal cord. neurons that supply synergists
Deep brain:
Cortex

Question 16

Besides having a means to stimulate individual muscles (or muscle nerves), what else is generally needed when you put together a FES system?

Answer 16

To compensate for unpredictable muscle output, movement control mechanisms include sensory feedback

• sensors report back to control circuitry which modulates signals sent to stimulator and electrodes

Question 17

Why are closely spaced electrode pairs generally preferable to distantly spaced electrode pairs for FES?

Answer 17

Current is passed through a smaller volume of tissue. Results in more selectivity of recruited muscle

Question 18

In muscles that remain innervated but are paralyzed six months after a CNS lesion, roughly how long can
it take to increase muscle strength and muscle endurance using electrical stimulation? What stimulation
patterns would you use for efficient exercise?

Answer 18

It can take about 6 weeks to observe noticeable increases in muscle strength and endurance using TEMS.

The duration of daily intermittent stimulation directly influences the improvement in muscle fatigue resistance (endurance index). Subjects who received 8 hours of daily stimulation showed greater increases in endurance index compared to those who received 45 minutes or no stimulation

The long-term influence on muscle phenotype depends primarily on the aggregate number of impulses delivered (not on the frequency). For power and endurance gains, aiming for Type 2A muscle fiber characteristics is ideal. This can be achieved with a stimulation pattern that doesn’t exceed 40,000 impulses per day (however delivered is irrelevant)

However, higher-frequency bursts can enhance muscle protein synthesis, leading to increased muscle mass and force.