Neural Adaptations with Chronic Activity Patterns in AbleBodied Humans

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Neural Adaptations with Chronic Activity Patterns
in Able-Bodied Humans

• Roger M. Enoka, Ph.D.
• Department of Kinesiology and Applied
  Physiology
• University of Colorado
• Boulder, CO
• Roger.Enoka_at_Colorado.EDU
• Presented at NIDRR Rehabilitation Research and
  Training Center in Neuromuscular Diseases
  Roundtable 2001 Role of Physical Activity and
  Exercise Training in Progressive Neuromuscular
  Diseases. Sept. 30, Oct. 1-3, 2001, San Diego, CA.

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Outline

• Evidence of neural adaptations
• Locus of neural adaptations
• Types of adaptations
• Cortical maps
• Motor command
• Descending drive
• Muscle activation
• Motor unit activity
• Sensory feedback
• Summary and future directions

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Evidence of Neural Adaptations

• Young subjects trained for 12 weeks with a
  bilateral knee-extension task that involved
  lifting loads of 80 of maximum.
• The increase in the training load was much
  greater than the increase in MVC force due to
  changes in coordination.
• MVC force increased by 15, whereas the training
  load increased by 220. (Rutherford Jones,
  1986)

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Evidence of Neural Adaptations

• Six weeks of immobilization for a forearm
  fracture impairs the voluntary activation of a
  hand muscle (Duchateau Hainaut, 1987).
• The force that could be achieved during an MVC
  (A) was reduced more than the force that could be
  evoked by electrical stimulation (B).
• MVC force (A) was reduced by 55 and Po (B) by
  33.

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Locus of Neural Adaptations

• A typical scheme used to identify the neural
  mechanisms that contribute to observed changes in
  performance.
• The sites include
• 1. Cortical maps
• 2. Motor command
• 3. Descending drive
• 4. Muscle activation
• 5. Motor units
• 6. Sensory feedback
• The sites at which the adaptations occur vary
  with the intervention.

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Adaptations Cortical Maps

• Somatotopic map in the motor cortex.

• Technological advances in imaging and activation
  protocols have provided new information on the
  changes that can occur in the human CNS in
  response to various interventions.
• Alterations in chronic patterns of physical
  activity can change the number of cortical
  neurons involved in a motor task.

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Adaptations Cortical Maps

• Subjects practiced a 5-finger piano exercise for
  2 hours on each of 5 consecutive days
  (Pascual-Leone et al 1995).
• The cortical map for some of the muscles used in
  the exercise program was mapped with transcranial
  magnetic stimulation.
• Calculated contour maps based on the probability
  of evoking a response in the muscles.
• The piano exercise increased the number of
  cortical neurons that responded to the TMS.

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Adaptations Motor Command

• Several weeks of strength training with one limb
  will evoke a strength gain in the contralateral
  limb, even though it was not active during
  training.
• On average, the strength gain in the untrained
  limb is 60 of that achieved in the trained
  limb.
• This phenomenon is known as cross education.

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Adaptations Motor Command

• Five findings suggest that cross education
  involves an alteration in the motor command for
  the task.
• 1. The increase in strength is localized to the
  homologous muscles (Zhou, 2000).
• 2. The improvement in performance is specific to
  the type of contraction performed during training
  (Hortobágyi et al 1997, 1999).
• 3. Cross education has been observed after
  brief-duration training programs with imagined
  contractions with hand muscles (Yue Cole, 1992
  Pascual-Leone et al 1995).

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Adaptations Motor Command

• Five findings suggest that cross education
  involves an alteration in the motor command for
  the task (continued).
• 4. Unilateral endurance training of the calf
  muscles caused a reduction in the central command
  (diastolic blood pressure) when the task was
  performed by the untrained homologous muscles
  (Fisher White, 1999).
• 5. Unilateral high-force contractions often
  involve irradiation of activation to the
  contralateral limb (Zijdewind Kernell, 2001).

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Adaptations Motor Command

• Three weeks of immobilization, however, had no
  effect on the strength of the contralateral knee
  extensor muscles (Hortobágyi et al 2000).

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Adaptations Descending Drive

• Aside from the command signals generated by the
  primary motor cortex, other supraspinal centers
  provide postural support for the movement.
• This includes
• Providing a base of support.
• Orienting the body and its limbs.
• Controlling the actions at adjacent joints due to
  inertial effects and two-joint muscles.

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Adaptations Descending Drive

• These needs are met by an interaction between the
  descending command and sensory feedback at the
  spinal level.
• The adaptations are manifested as changes in the
  timing and intensity of activity among the
  muscles involved in the task and among the
  muscles that provide support for the task.
• There are, however, few descriptions in the
  literature of how coordination changes with
  chronic patterns of activity.

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Adaptations Descending Drive

• Four weeks of immobilization altered the
  distribution of activity among the elbow flexor
  muscles of 7/12 subjects during a fatiguing
  contraction at 15 MVC force (Semmler et al
  2000).

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Adaptations Muscle Activation

• The maximality of the activation provided by the
  nervous system to muscle is often assessed by
  comparing voluntary force to that evoked with
  electrical stimulation (Merton, 1954).

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Adaptations Muscle Activation

• Observations include
• Superimposition of a few stimuli during isometric
  and concentric contractions indicate that
  subjects can achieve maximum activation (Allen et
  al 1995, 1998 Gandevia et al 1998).
• Brief trains of shocks indicate that many
  individuals exhibit a deficit in contraction
  maximality (Kent-Braun Le Blanc, 1996 Miller
  et al 1999).

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Adaptations Muscle Activation

• Observations (continued)
• Voluntary activation and MVC force increase after
  1 min of transcranial magnetic stimulation
  (Urbach Awiszus, 2000).
• Concurrent electrical stimulation of muscles in
  one limb increases MVC force of the homologous
  muscles in the other limb (Howard Enoka, 1991).

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Adaptations Muscle Activation

• These findings suggest that the activation of
  muscle is often not maximal and can, therefore,
  be altered with chronic changes in physical
  activity.

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Adaptations Muscle Activation

• Strength training of the adductor pollicis
  muscle for 6 weeks increased MVC force by 22 and
  electrically evoked tetanic force (Po) by 15
  (Duchateau and Hainaut,1988). This difference
  indicates an increase in voluntary activation.

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Adaptations Muscle Activation

• Some investigators use the average EMG to
  estimate changes in muscle activation after
  chronic interventions.
• Although the average EMG during eccentric
  contractions appears to increase after strength
  training, the findings are equivocal for
  isometric and concentric contractions.

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Adaptations Muscle Activation

• There are at least three factors that limit the
  utility of the average EMG as an index of muscle
  activation
• The detection range of bipolar recordings is
  about 2 cm.
• Cancellation of positive and negative phases in
  overlapping action potentials reduces the
  absolute quantity of EMG.
• Changes in the timing of action potentials
  modulates the quantity of EMG without any changes
  in the number of action potentials.

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Adaptations Muscle Activation

• The preferred method to assess the maximality of
  muscle activation, therefore, is to compare
  voluntary and evoked forces.

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Adaptations Motor Units

• More is known about the adaptive properties of
  motor units than any other element in the motor
  system. Our understanding, however, depends on
  an adequate sample size and a set of valid
  measurements.

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Adaptations Motor Units

• Training for 12 weeks with rapid contractions
  while lifting moderate loads (40 MVC) increased
  MVC force, motor unit force, and the initial
  discharge rate during rapid contractions (Van
  Cutsem et al 1998).

Initial discharge rate of motor units in the TA
muscle increased from 77 to 146 Hz.
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Adaptations Motor Units

• Immobilization of the forearm-hand in a plaster
  cast for 6-8 weeks reduced motor unit force in
  hand muscles (Duchateau Hainaut, 1990).

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Adaptations Motor Units

• Immobilization of the forearm-hand in a plaster
  cast for 6-8 weeks reduced maximum discharge rate
  of motor units of hand muscles (Duchateau
  Hainaut, 1990).

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Adaptations Motor Units

• Simulation of the effects of the motor unit
  changes on MVC force.

• MVC force decreased by 44 after immobiliz-ation,
  compared with a 42 reduction in the simulated
  MVC force (b).
• The decline in maximal discharge rate accounted
  for 18 of the decrease in MVC force (a).
• The reduction in motor unit force contributed to
  23 of the decrease in MVC force (b-a).

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Adaptations Sensory Feedback

• Studies on humans, monkeys, and rats have shown
  that it is possible to increase or decrease the
  amplitude of the H reflex with training (Chen et
  al 2001 Feng-Chen Wolpaw, 1996 Wolf Segal,
  1996).
• The adaptation appears to occur in two phases
• Alteration in the corticospinal control over the
  reflex arc, such as presynaptic inhibition.
• Plastic changes in the synaptic terminals on the
  motor neurons.

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Adaptations Sensory Feedback

• These changes probably explain the reduced
  H-reflex amplitude found in ballet dancers and
  after bed rest, and the increase that has been
  observed after strength training (Duchateau,
  1995 Mynark Koceja, 1997 Nielsen et al 1993
  Sale et al 1983).
• These findings indicate that chronic patterns of
  activity can change the efficacy of the
  connections between afferent fibers and motor
  neurons.

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Adaptations Sensory Feedback

• Subjects trained the knee extensor muscles of one
  leg for 6 weeks by performing either voluntary
  contractions (Vol), electrically evoked
  contractions (EMS), or a combination of the two
  (rEMS) (Hortobágyi et al 1999).
• All 3 groups experienced significant strength
  gains during voluntary contractions in both the
  trained and the untrained legs.

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Summary

• Alterations in chronic patterns of physical
  activity are accompanied by adaptations within
  the nervous system.
• The types of adaptations include
• Modulation of cortical maps
• Shifts in the maximality of voluntary activation
• Alterations in the timing and intensity of muscle
  activation
• Changes in the maximum discharge rate of motor
  neurons

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Summary

• Types of Adaptations (continued)
• Variation in motor unit force.
• Modification of reflex amplitude
• Enhancement of voluntary activity by sensory
  feedback
• The adaptations are widely distributed throughout
  the nervous system.
• The changes observed with strength training are
  often not the converse of those found with limb
  immobilization and limb unloading.

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Future Directions

• The state of our knowledge on neural adaptations
  is such that we can provide evidence of
  alterations at specific sites with changes in
  chronic patterns of physical activity.

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Future Directions

• However, we have limited information on
• The effects of various interventions on daily
  levels of muscle activity.
• Adaptations in the activation of muscles that
  provide postural support for a task.
• The relative contribution of the various
  adaptations to the changes in performance.
• The specific adaptations that are unique to each
  intervention.

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Future Directions

• Our knowledge on this topic would likely
  benefit from a strategy that combines
  experimental investigation with modeling and
  computer simulation.