The Tendon Reflex (T-Reflex) is a feedback mechanism that controls increasing muscle tension by causing muscle relaxation before tension force becomes so great it may damage the muscle. The investigators hypothesized that the amplitude of the T-Reflex will decrease throughout the bed rest period while the latency of the reflex will increase. It is believed that these alterations will be due to two primary changes. First, the compliance of the primary postural muscles is reduced, and second, there is a reduction or atrophy of motor synapse to muscle. Compensation for these changes results in a potential and inappropriate hyper-motor response following a return to the upright position that returns to normal within a few days.
A second hypothesis dealt with a change in otolith bias from the six-degree head down bed rest position, and the influence of descending otolith modulation of spinal interneuron's following a return to the upright position. The specific aims of this experiment were to compare the effects of weightlessness on the amplitude and latency of the torque reflex, with those subjects participating in head down bed rest studies.
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The subjects participated in Motor Stretch Reflex (MSR) T-reflex trial with a varying stimulus in a randomized order with the subject keeping the leg relaxed while stretch reflex is elicited as the motor dorsiflexes the foot. Subjects participated in three pre-bed rest test sessions to determine individual baseline responses. Test sessions were conducted during bed rest to determine whether a decrease in T-Reflex amplitude and increase in latency occurs. Another test was conducted immediately following bed rest to determine whether an increase in response is observed. Additional post-bed rest tests were conducted to track response recovery.
The subjects remained in the prone position on a subject positioning device with the ankle firmly attached to the motor using a foot plate. Electromyography (EMG) electrodes with a high impedance probe were placed on the triceps surae and the anterior tibialis muscle groups. A dorsiflexion torque was applied by the motor, moving the toe forward. For the remainder of these trials, subjects were instructed to provide no resistance to the torque, enabling collection of T-Reflex data. Rectified and filtered EMG data was analyzed for both latencies and amplitudes
T-Reflex Apparatus Setup:Device was configured for ankle dorsiflexion in the supine position.
Amplitudes of 1-10º of dorsiflexion was used.
Subject’s left leg was tested.
EMG Setup: Subjects had electrodes placed in the following locations on the left leg: tibialis (anterior), soleus (medial and lateral), and gastrocnemeius (medial and lateral belly, and at solues interface).
Subjects participated in each of the following trials with varying stimulus in a randomized order.
T-reflex: Subjects relaxed leg while stretch reflex was elicited as the motor dorsiflexes the foot (1-10º, 20 repetitions).
Subject Positioning Device:Subjects laid prone on the mat. Before and after bed rest they were at a 0 degree head down angle. During bed rest they will were at a six-degree head down angle.
Subject's left leg was tested.
The motor accelerated quickly, causing the foot to dorsiflex during the trial type.
This experiment has concluded. Summarized results for this test aren't available.
J. M. Cerisano, M. F. Reschke, I. S. Kofman, E. A. Fisher, and D. L. Harm. Stretch Reflex as a simple Measure to Evaluate the Efficacy of Potential Flight Countermeasures Using the Bed Rest Environment. Presented at the 18th IAA Humans in Space Symposium, Houston, TX, April 11-12, 2011.
Amplitude, fifty percent
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Horizontal ankle measurement
L5 to C1 measurement
L5 to tibialis measurement
Latency, fifty percent
Motor torque, latency
Motor torque, peak
Motor velocity, latency
Motor velocity, peak
Peak amplitude, average
Peak latency, average
Propagation latency tendon reflex repetitions
S1 to C1 measurement
S1 to gastroc soleus measurement
S1 to gastrocnemeous measurement
S1 to soleus measurement
Two peaks monosynaptic reflex
Upside down monosynaptic reflex