This study will test a spinal cord reflex called reciprocal inhibition before, during, and
after learning a motor skill to see if the reflex becomes stronger by learning the movement.
People learn new motor skills throughout their lives. At first, performing a new skilled
movement such as riding a bicycle takes effort and is clumsy, but with practice, it becomes
relatively automatic. The motor cortex (a part of the brain) is very active when a new
skilled movement is learned, but becomes less active when the movement is over-learned.
This study will determine whether the spinal cord helps coordinate the pattern of activity
between groups of muscles once a motor skill is learned.
Healthy volunteers between 21 and 65 years of age may be eligible for this study.
Candidates are screened with a medical history and neurological examination.
Participants are divided into two groups. Group 1 has movement training sessions only and
Group 2 has both movement training sessions and physiology sessions, as follows:
Movement training sessions
For 15 minutes every weekday, participants practice moving their wrist back and forth to
make a cursor on the computer screen follow a target. Activity of the arm muscles is
monitored with surface electrodes taped to the skin. The sessions continue until the
participant can perform the movement well. Group 1 participants return to the clinic a week
after the last session to perform the movement again to see if their skill level has
changed. Each session lasts about 20 to 30 minutes.
In three separate sessions, the reflex for reciprocal inhibition is measured before and at
several times during the movement task. This is done with nerve conduction studies. A
probe placed on the skin delivers a low-intensity electrical stimulus. Wires taped to the
skin record the nerve impulses. To measure reciprocal inhibition, several dozen stimuli are
given to two nerves in combinations. Each session lasts 2 to 3 hours.
Once a skilled movement is thoroughly learned, it can be performed relatively automatically.
The motor cortex is active when learning a new motor skill, but becomes less activated once
the skill has become over-learned. We hypothesize that learning a skilled movement is
associated with more efficient use of subcortical motor circuits. Subcortical motor
circuits can coordinate features of the intended movement such as the timing and patterns of
activation of different muscles. The goal of this study is to determine whether learning a
motor skill strengthens spinal interneuron circuits that facilitate the movement. Subjects
will learn to perform a movement consisting of alternating wrist movements. The strength of
reciprocal inhibition between antagonist muscles will be tested.
22 healthy adult volunteers
The study has two experiments. Both experiments use a within-subjects design. The purpose
of the first experiment is to determine whether learning to accurately perform an
alternating wrist flexion and extension movement task is associated with reduced
co-contraction of wrist flexor and extensor muscles. Subjects will practice making accurate
wrist flexions and extensions to keep a cursor on a target for fifteen-minute sessions every
weekday until they are able to obtain 95% accuracy. Activity of the arm muscles will be
monitored during the sessions with surface electrodes on the arm muscles. The first
experiment will also determine the best training parameters for learning the task. In the
second experiment, a second group of subjects will learn the task using the best training
parameters. The strength of the reflex for reciprocal inhibition will be measured before,
during and after training. Reciprocal inhibition will also be measured after the movement
has become highly learned.
In the first experiment, the outcome will be the percent of movement time in which
co-contraction occurs, as recorded from surface electrodes, while the subject learns to
perform the movement to 95% accuracy. In the second experiment, the outcome measure is the
strength of spinally mediated reciprocal inhibition, as measured using reflexes.
Healthy individuals between the ages of 21-65 years who are willing to participate in
daily training sessions and physiological studies.
Any history of peripheral nerve injury, cervical radiculopathy, arthritis, tendonitis, or
surgery on the wrist.