The investigation was designed to determine the relationship between the electromyographic (EMG) activity of the calf and elbow muscles and the level to which their functional integrity can be maintained in prolonged space flight. The selected muscles to be tested were located in the calf of the right leg and the upper right arm: the ankle dorsiflexor (m. tibialis anterior, TA) and plantar flexor (m. gastrocnemius medialis, GM, and m. soleus, SO) of the calf, the elbow flexor (m. biceps brachii, BB) and elbow extensor (m. triceps brachii, TB) in the arm.
The specific objectives of the experiment were:
1. To determine daily activity of the musculus soleus (SO), m. gastrocnemius medialis (GM), m. tibialis anterior (TA), m. biceps brachii (BB), and m. triceps brachii (TB) before, during and after space flight.
2. To determine the EMG-torque relationship for the SO, GM, TA, BB and TB muscles during maximal and submaximal efforts before, during and after flight.
3. To determine the changes in motor control of the ankle, elbow and hand-grip.
4. To determine the fatigability of the ankle plantar flexors (SO and GM) before, during and after flight.
5. To determine the blood plasma levels of immunoreactive growth hormone (IRGH), insulin-like growth factor (IGF1), growth hormone carrier protein, thyroxine (T4), triiodothyronine (T3), testosterone, insulin, and cortisol before, during, and after flight.
6. To determine the bioassayable growth hormone (GH) response to exercise before, during and after space flight.
7. To determine the relationship between plasma hormone levels and the decrease in plantar flexor torque associated with muscle fatigue.
The functional integrity of the calf muscles, and the elbow and wrist flexors, was assessed during maximal and submaximal efforts, using the Torque-Velocity Dynamometer (TVD) and Hand Grip Dynamometer (HGD). The calf muscles of the right leg were tested in a protocol using the TVD, followed by the 8913020 protocol of Dr. Fitts and a fatigue test. The elbow flexors of the right arm were also tested in a similar protocol on the TVD, followed by the E407 protocol of Dr. diPrampero. The stand-alone Hand Grip test of the wrist flexors was composed of a reference test and a force curve. The protocols on the TVD were composed of six different tests: 1) reference test, 2) estimate position, 3) torque curve, 4) maintain constant torque, 5) dynamic stiffness and 6) position curve. These six tests monitored different aspects of the subject's motor control capabilities, and allowed determination of the torque-velocity/EMG relationship. Motor control capability of the ankle and arm were tested through a variety of tasks requiring precision in force production and displacement. Specific temporal and amplitude characteristics of the EMG signal were correlated with the maximal torque, velocity, and power. EMG of predominately slow and predominately fast ankle plantar flexors and elbow flexor and extensor muscles during the maximal and submaximal efforts were studied to compare the magnitude of the presumed decrease in total EMG throughout repeated 24-hour periods before, during, and after space flight with torque, velocity, and power potentials of these muscle groups. Similarly, the changes in torque, velocity, and power potentials associated with prolonged flight were related to plasma hormonal changes during space flight.
During both the early and late mission right leg TVD-test protocol, small amounts of blood were collected prior to and immediately after the fatigue test, to measure the level of growth hormone (GH), which increases in the bloodstream during exercise. It was hypothesized that the stay in microgravity would affect GH production and result in a lower value. In addition to the described measurements, plasma levels of other hormones that play a major role in regulating muscle protein metabolism were analyzed. Additional samples were taken pre- and postflight.
These data will help define the relative importance of muscle function to muscle maintenance in prolonged space flight. Furthermore, these data will provide insight into the relative impact of neural and muscular components on changes in movement control and maximal performance capacity as might be needed in an emergency egress procedure.
The 24-hour electromyography (EMG) showed an increase in muscle activation on FD 1 for the soleus (SO), tibialis anterior (TA), biceps brachii (BB) and triceps brachii (TB) muscles. The SO and TA activation was increased during the entire space flight, whereas a slight decrease in the gastrocnemius medialis (MG) daily level of muscular activity was observed. The TB values were increased for the entire space flight, while for BB the daily values measured on FD 7 and FD 13 were similar to the baseline and recovery values.
The measured torque values (for Hand Grip Dynamometer force values) showed no obvious effects caused by space flight. A small but significant (p<0.05) increase in elbow flexor torque was observed across subjects during flight. In addition, a decrease in elbow flexor torque (p<0.01) was seen in 3 of 4 subjects at FD 2-3, although one crewmember showed larger elbow flexor torque during this measurement session. The steep decrease in torque seen in the plantarflexor muscle group on FD 2-3 with a sudden recovery at the next session (FD 6-7) was caused by problems with seating and securing the subject to the TVD during the FD 2-3 session.
For changes in muscle output estimation (subjects were asked to apply a certain force level, expressed in percent of MVC) there was no specific trend observed. However, in a number of instances output estimation was altered inflight relative to preflight (elbow extensors for all subjects, plantarflexors for 3 subjects, dorsiflexors and hand grip for 2 subjects). In all such instances the direction of the change was toward an overestimation of output at lower force levels.
No significant difference between pre- and inflight for the ability to maintain a constant torque output at a faster (1 Hertz) or slower (0.5 Hertz) passive isokinetic movement was observed. Similarly, there was no significant difference in the ability to maintain torque with or without continuous feedback, with the exception of the 50% MVC ankle plantarflexion. All subjects were significantly (p<0.05) more accurate at maintaining a constant torque for ankle plantarflexion at 50% MVC when given continuous feedback, and the torque was underestimated by 10% without feedback. For elbow testing, subjects consistently overestimated 10% MVC and underestimated 50% MVC for elbow extension. In opposition, for elbow flexion, 10% MVC was consistently underestimated while 50% MVC was overestimation.
Torque output for the ankle flexor and elbow flexor muscles were the most affected during space flight. On the second inflight test (FD 4-5) there was a significant decrease in torque levels for ankle dorsiflexion at 50% MVC with feedback. For elbow flexors, on the first day of space flight, the mean difference from requested torque increase significantly for elbow flexion at 10% MVC, such that subjects underestimated to a greater degree. This effect continued throughout the flight and for the first eight days postflight.
The EMG measurements during the experiment sessions revealed no significant changes for agonist or antagonist muscles whether or not subjects were given continuous feedback. However, significant changes were observed for the agonist EMG activity recorded during low level ankle extension or elbow flexion. The EMG signals for both the MG and BB muscles decreased significantly during the 10% MVC early into the flight. SO and TB demonstrated no significant alterations in EMG activity.
The hormone response (BGH response) to exercise was clearly present before space flight (p<0.05), blunted at FD 2-3, and was essentially absent by FD 13-14. Following space flight, the BGH response remained absent at R+2 but had returned by R+4. In conclusion, the exercise induced release of BGH was inhibited during space flight and early recovery, but has returned to normal four days after recovery.
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