Fig. 7. Conceptual hypothetical framework illustrating the factors that influence the preferred walk–run transition speed (PTS). The horizontal axis represents increasing speed and the vertical axis represents the influence of each factor. A critical threshold for the PTS is represented by the broken horizontal lines. The thin lines 1,2,3 arbitrarily represent muscle groups. The thick line in each figure represents a proposed underlying factor that ultimately limits the walk–run transition speed. (A) PTS under normal conditions; (B) PTS at a slower speed as a result of increased demand in muscle group 1; (C) PTS at a faster speed as a result of decreased demand in muscle group 1; (D) PTS at the same speed as in C despite decreased muscle demand in all groups; (E) PTS at a slower speed as a result of another underlying factor that occurs during simulated reduced gravity.