In addition to causing postural disturbances, space flight has been shown to
modify the reflexes of antigravity muscles. In 1971, a form
of stretch reflex, referred to as the functional stretch
reflex (FSR), was located in the gastrocnemius muscle during
application of a dorsiflexion force. This reflex appeared
as a burst of activity in the electromyogram (EMG) roughly
120 ms after the force was applied. This response was termed
"functional" because, unlike the early (20-30 ms) monosynaptic
response (T-reflex) that contributes little to overall muscle
tension; it directly corresponded with a sustained rise in
force generated by the muscle.
Image right: Measurement of sensorimotor function in 6° head down position.
Reflex is evoked with ankle rotation via 100 lb/ft servo
controlled DC torque motor. Weightlessness affects the amplitude of the FSR, making it a good candidate for
proprioceptive assessment testing. The Sensorimotor Laboratory
has been established to evaluate the effect of bed rest on
the FSR (where muscle unloading is the primary driver for
change) and to compare these results with those observed
as a function of flight. Namely that the amplitudes of both
the FSR and T- reflexes will decrease throughout the bed
rest campaign as the reflex serves no functional purpose
during this period. This paradigm is essentially one of exclusion
(an exclusionary analog) where the unloading of bed rest
can be compared to the unloading and sensorimotor changes
driven from vestibular/proprioceptive changes as a function
of space flight. As an analog to flight the Sensorimotor
Laboratory investigates the FSR and related reflex components
as a function of centrifugation. 
Image left: Close-up of functional stretch testing
During testing of the FSR, subjects
are positioned in a 6o head down supine position with the
ankle firmly attached to the dynamometer (or 100 lb/ft torque
motor) head in a position of 0 degrees of plantar and dorsiflexion.
EMG electrodes with a high impedance probe are placed on
the triceps surae and tibialis anterior muscle groups. A
dorsiflexion torque is applied at 250o/sec to randomized
angular amplitudes of 5, 10, and 15 degrees. For half of
these trials EMG data on the FSR is obtained by instructing
subjects to resist the force. For the other half of these
trials subjects are instructed to provide no resistance to
the torque, enabling collection of T-reflex data. EMG data
is then analyzed both for latency times and amplitudes.
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