Effects of spaceflight on Rhesus quadrupedal locomotion after return to 1G

Locomotor performance, activation patterns of the soleus (Sol), medial gast rocnemius (MG), vastus lateralis (VL), and tibialis anterior (TA) and MG te ndon force during quadrupedal stepping were studied in adult Rhesus before and after 14 days of either spaceflight (n = 2) or flight simulation at 1G (n = 3). Plight simulation involved duplication of the spaceflight conditio ns and experimental protocol in a 1G environment. Postflight, but not posts imulation, electromyographic (EMG) recordings revealed clonus-like activity in all muscles. Compared with preflight, the cycle period and burst durati ons of the primary extensors (Sol, MG, and VL) tended to decrease postfligh t. These decreases were associated with shorter steps. The flexor (TA) EMG burst duration postflight was similar to preflight, whereas the burst ampli tude was elevated. Consequently, the Sol:TA and MG:TA EMG amplitude ratios were lower following flight, reflecting a ''flexor bias.'' Together, these alterations in mean EMG amplitudes reflect differential adaptations in moto r-unit recruitment patterns of flexors and extensors as well as fast and sl ow motor pools. Shorter cycle period and burst durations persisted througho ut the 20-day postflight testing period, whereas mean EMG returned to prefl ight levels by 17 days postflight. Compared with presimulation, the simulat ion group showed slight increases in the cycle period and burst durations o f all muscles. Mean EMG amplitude decreased in the Sol, increased in the MG and VL, and was unchanged in the TA. Thus adaptations observed postsimulat ion were different from those observed postflight, indicating that there wa s a response unique to the microgravity environment, i.e., the modulations in the nervous system controlling locomotion cannot merely be attributed to restriction of movement but appear to be the result of changes in the inte rpretation of load-related proprioceptive feedback to the nervous system. P eak MG tendon force amplitudes were approximately two times greater post- c ompared with preflight or presimulation. Adaptations in tendon force and EM G amplitude ratios indicate that the nervous system undergoes a reorganizat ion of the recruitment patterns biased toward an increased recruitment of f ast versus slow motor units and flexor versus extensor muscles. Combined, t hese data indicate that some details of the control of motor pools during l ocomotion are dependent on the persistence of Earth's gravitational environ ment.