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.

Peer Review #3 of "Why do we transition from walking to running? Energy cost and lower leg muscle activity before and after gait transition under body weight support (v0.3)"

“…These results were equivalent to those of our recent studies that measured TA activity during walking and running at the EOTS (Abe, Fukuoka & Horiuchi, 2017;Abe et al, 2018), which suggested that the motor unit recruitment pattern of the TA shifted more toward Type I (slow twitch) fibers rather than Type II (fast twitch) fibers after the walk-run transition. Several studies have also reported that an abrupt increase in TA activity associated with the walk-run transition (Hreljac, 1995;Hreljac et al, 2008;Bartlett & Kram, 2008;Malcolm et al, 2009;Shih et al, 2016).…”

“…An abrupt increase in muscle activity of the dorsiflexor (tibialis anterior; TA) has been observed when participants walked at a speed close to the PTS (Hreljac, 1995;Hreljac et al, 2008;Prilutsky & Gregor, 2001; Bartlett & Kram, 2008;Malcolm et al, 2009;Shih et al, 2016), and our recent studies showed muscle activity of the TA decreased when participants switched from walking to running at the EOTS (Abe, Fukuoka & Horiuchi, 2017;Abe et al, 2018). In association with a decrease in TA activity when switching the gait pattern, mean power frequency (MPF; Hz) of the TA became lower, suggesting that more Type 1 muscle fibers were recruited in the TA during running than walking at the EOTS.…”

“…An alteration of the MPF values between walking and running allows us to evaluate muscle fiber recruitment pattern in each gait. The sum of the rectified EMG (µV•sec) was normalized by measured time (sec) and number of steps to cancel the effects of step frequency (Abe, Fukuoka & Horiuchi, 2017;Abe et al, 2018). The EMG values obtained were further normalized to those obtained at 1.11 m•s −1 under each condition.…”

Energy cost and lower leg muscle activities during erect bipedal locomotion under hyperoxia

“…Indeed, hyperoxia depresses peripheral chemoreceptor, resulting in a decrease in ventilatory response [ 16 , 17 ]. As far as we know, the ES and/or EOTS have not been measured under different O 2 conditions except for our recent hypoxic studies [ 10 , 18 ]. Saving the whole-body energy expenditure at a given gait speed can provide an effective use of endurance capacity, that is, a possible combination of the rightward (faster) and downward (lesser metabolic) shifts of the U-shaped and linear CoT- v relationships will occur under hyperoxia relative to normoxia (Additional file 1 : Figure S1).…”

Peer Review #2 of "Why do we transition from walking to running? Energy cost and lower leg muscle activity before and after gait transition under body weight support (v0.2)"

“…An abrupt increase in muscle activity of the dorsiflexor (tibialis anterior; TA) has been 54 observed when participants walked at a speed close to the PTS (56 2016), and our recent studies showed muscle activity of the TA decreased when participants 57 switched from walking to running at the EOTS (Abe, Fukuoka & Horiuchi, 2017; Abe et al, 58 2018). In association with a decrease in TA activity when switching the gait pattern, mean power 59 frequency (MPF; Hz) of the TA became lower, suggesting that more Type 1 muscle fibers were 60 recruited in the TA during running than walking at the EOTS.…”