Muscle activities during walking and running at energetically optimal transition speed under normobaric hypoxia on gradient slopes

Abstract

Energy cost of transport per unit distance (CoT; J·kg-1·km-1) displays a U-shaped fashion in walking and a linear fashion in running as a function of gait speed (v; km·h-1). There exists an intersection between U-shaped and linear CoT-v relationships, being termed energetically optimal transition speed (EOTS; km·h-1). Combined effects of gradient and moderate normobaric hypoxia (15.0% O2) were investigated when walking and running at the EOTS in fifteen young males. The CoT values were determined at eight walking speeds (2.4-7.3 km·h-1) and four running speeds (7.3-9.4 km·h-1) on level and gradient slopes (±5%) at normoxia and hypoxia. Since an alteration of tibialis anterior (TA) activity has been known as a trigger for gait transition, electromyogram was recorded from TA and its antagonists (gastrocnemius medialis (GM) and gastrocnemius lateralis (GL)) for about 30 steps during walking and running corresponding to the individual EOTS in each experimental condition. Mean power frequency (MPF; Hz) of each muscle was quantified to evaluate alterations of muscle fiber recruitment pattern. The EOTS was not significantly different between normoxia and hypoxia on any slopes (ranging from 7.412 to 7.679 km·h-1 at normoxia and 7.516 to 7.678 km·h-1 at hypoxia) due to upward shifts (enhanced metabolic rate) of both U-shaped and linear CoT-v relationships at hypoxia. GM, but not GL, activated more when switching from walking to running on level and gentle downhill slopes. Significant decreases in the muscular activity and/or MPF were observed only in the TA when switching the gait pattern. Taken together, the EOTS was not slowed by moderate hypoxia in the population of this study. Muscular activities of lower leg extremities and those muscle fiber recruitment patterns are dependent on the gradient when walking and running at the EOTS.

Fig 1. Schematic illustration of Cost of Transport (CoT) and gait speed (v) at normoxia and hypoxia.

(a) Upward and leftward shifts of the U-shaped CoT-v relationship in walking and an upward shift of the linear CoT-v relationship in running result in a slower energetically optimal transition speed (EOTS, circles) and economical speed (ES, triangles) at hypoxia. (b) Upward shifts of both CoT-v relationships result in unchanged ES and EOTS. Solid and dotted lines mean normoxia and hypoxia, respectively. Both red arrows indicate a possible ‘shifting model’ for explaining slower ES and EOTS at hypoxia.

Fig 2. U-shaped (walking) and linear (running) CoT-v relationships on different gradient slopes at normoxia and hypoxia.

The energetically optimal transition speed (EOTS; km·h^-1) was 7.447 ± 0.311 km·h^-1 at normoxia and 7.678 ± 0.324 km·h^-1 at hypoxia, and the economical speed (ES; km·h^-1) was 4.993 ± 0.185 km·h^-1 at normoxia and 5.056 ± 0.210 km·h^-1 at hypoxia on the level slope (Black panel). The EOTS was 7.679 ± 0.342 km·h^-1 at normoxia and 7.653 ± 0.295 km·h^-1 at hypoxia, and 5.198 ± 0.192 km·h^-1 at normoxia and 5.133 km·h^-1 ± 0.243 km·h^-1 at hypoxia on the downhill slope (Blue panel). The EOTS was 7.412 ± 0.480 km·h^-1 at normoxia and 7.516 ± 0.415 km·h^-1 at hypoxia, and the ES was 4.984 ± 0.238 km·h^-1 at normoxia and 5.082 ± 0.223 km·h^-1 at hypoxia on the uphill slope, respectively. ^# downhill > uphill within normoxia or hypoxia. * downhill > level = uphill within normoxia and hypoxia. † (p < 0.05), ‡ (p < 0.01), and § (p < 0.001) indicated significant differences of the energy cost of transport per unit distance (CoT; J·kg^-1·km^-1) between normoxia and hypoxia on each slope. Data were shown as mean ± standard deviation (S.D.).

Fig 3. Comparisons of muscle activities during walking or running at the EOTS.

Deep colors (black, blue, and red) and thin colors (grey, light blue, and pink) are normoxia and hypoxia, respectively. Solid and dotted bars are walking and running, respectively. WN, RN, WH, and RH mean walking at normoxia, running at normoxia, walking at hypoxia, and running at hypoxia, respectively. * walking > running within normoxia or hypoxia, § walking < running within normoxia or hypoxia, and † normoxia > hypoxia within walking or running, respectively. Data are mean ± S.D.

Fig 4. Comparisons of mean power frequency during walking or running at the EOTS.

Deep colors (black, blue, and red) and thin colors (grey, light blue, and pink) are normoxia and hypoxia, respectively. Solid and dotted bars are walking and running, respectively. WN, RN, WH, and RH mean walking at normoxia, running at normoxia, walking at hypoxia, and running at hypoxia, respectively. * walk > running within normoxia or hypoxia and † normoxia < hypoxia within walking or running, respectively. Data are mean ± S.D.