Fuel oxidation during human walking

Walking around the preferred speed: examination of metabolic, perceptual, spatiotemporal and stability parameters

Walking is the most accessible and common type of physical activity. Exercising at one's self-selected intensity could provide long-term benefits as compared to following prescribed intensities. The aim of this study was to simultaneously examine metabolic, perceptual, spatiotemporal and stability parameters at an absolute 3 km·h-1 speed range around the individual preferred walking speed (PWS). Thirty-four young sedentary adults (18 women) volunteered to walk at seven speeds relative to their PWS in 3-min trials interspaced with 3-min rest intervals. Results indicated a significant main effect of speed on all studied variables. While metabolic, perceptual and spatiotemporal values were sensitive to the smallest change in speed (i.e., 0.5 km·h-1), a significant increase in the rate of carbohydrate oxidation and decrease in %fat oxidation were only observed at speeds above PWS. Results also revealed significantly higher coefficients of variation for stride characteristics at speeds below PWS only. Moreover, analyses of best fit models showed a quadratic relationship between most variables and speed, with the exceptions of metabolic cost of transport, rating of perceived exertion and stride duration that changed exponentially with speed. PWS coincided with optimized mechanical efficiency, fuel oxidation and gait stability. This indicated that walking below PWS decreased both mechanical efficiency and stability of gait, while walking above PWS increased carbohydrate oxidation. Those factors seem to play an important role as determinants of PWS. We suggest that walking at PWS may provide benefits in terms of fat oxidation while optimizing gait stability.

Impact of balance on the energetic cost of walking and gait speed

BACKGROUND

Examine the relationship between balance test performance and the energetic cost of walking (ECW) and gait speed.

METHODS

Cross-sectional and longitudinal analyses of data from the Baltimore Longitudinal Study of Aging. Men (48%) and women aged 60-96 years enrolled in the BLSA between 2007 and 2020 (n = 1132). Balance was assessed using narrow walk (NW) and progressive standing balance tests (SB). ECW measured during 2.5-min usual paced walk while participants wore a portable indirect calorimeter. Gait speed assessed over 6-m. Each test parameterized using validated methods. Statistical analysis to compare balance measures to ECW, and gait speed used generalized logistic regression models and adjustments for age, sex, race, height, weight, and comorbidities.

RESULTS

Cross-sectionally, mean ECW was higher and gait speed slower in persons who failed the NW than those who passed (0.189 vs. 0.164 mL/kg/m, p < 0.0001 and 0.96 vs. 1.15 m/s, p < 0.0001, respectively). Mean ECW was increasingly higher and gait speed slower over three progressively challenging SB tests (0.207 vs. 0.171 vs. 0.164 mL/kg/m, p < 0.0001 and 0.95 vs. 1.05 vs. 1.15 m/s, p < 0.0001). Over an average 2.4 years, those who declined in SB and NW had a higher ECW and slower gait speed than persons who maintained performance (SB: 0.18 vs. 0.160 mL/kg/m, p = 0.0003, and 1.00 vs. 1.13 m/s, p = <0.001; NW: 0.175 vs. 0.160 mL/kg/m, p = 0.002, and 1.04 vs. 1.14 m/s, p = 0.001). Persons who improved had lower ECW and faster gait speed than those who failed at both visits (SB: 0.169 vs. 0.240 mL/kg/m, p = 0.0002, and 0.99 vs. 0.94 m/s, p = 0.67, NW: 0.163 vs. 0.195 mL/kg/m, p = 0.0005, and 1.10 vs. 0.92 m/s, p < 0.001).

CONCLUSION

Instability contributes to higher ECW and slower gait speed which suggests that rehabilitation efforts to improve balance may help maintain function further into older adulthood and delay mobility limitation.

Nonequilibrium thermodynamics and mitochondrial protein content predict insulin sensitivity and fuel selection during exercise in human skeletal muscle

Introduction: Many investigators have attempted to define the molecular nature of changes responsible for insulin resistance in muscle, but a molecular approach may not consider the overall physiological context of muscle. Because the energetic state of ATP (ΔGATP) could affect the rate of insulin-stimulated, energy-consuming processes, the present study was undertaken to determine whether the thermodynamic state of skeletal muscle can partially explain insulin sensitivity and fuel selection independently of molecular changes. Methods: 31P-MRS was used with glucose clamps, exercise studies, muscle biopsies and proteomics to measure insulin sensitivity, thermodynamic variables, mitochondrial protein content, and aerobic capacity in 16 volunteers. Results: After showing calibrated 31P-MRS measurements conformed to a linear electrical circuit model of muscle nonequilibrium thermodynamics, we used these measurements in multiple stepwise regression against rates of insulin-stimulated glucose disposal and fuel oxidation. Multiple linear regression analyses showed 53% of the variance in insulin sensitivity was explained by 1) VO2max (p = 0.001) and the 2) slope of the relationship of ΔGATP with the rate of oxidative phosphorylation (p = 0.007). This slope represents conductance in the linear model (functional content of mitochondria). Mitochondrial protein content from proteomics was an independent predictor of fractional fat oxidation during mild exercise (R2 = 0.55, p = 0.001). Conclusion: Higher mitochondrial functional content is related to the ability of skeletal muscle to maintain a greater ΔGATP, which may lead to faster rates of insulin-stimulated processes. Mitochondrial protein content per se can explain fractional fat oxidation during mild exercise.

Optimization of energy and time predicts dynamic speeds for human walking

Humans make a number of choices when they walk, such as how fast and for how long. The preferred steady walking speed seems chosen to minimize energy expenditure per distance traveled. But the speed of actual walking bouts is not only steady, but rather a time-varying trajectory, which can also be modulated by task urgency or an individual's movement vigor. Here we show that speed trajectories and durations of human walking bouts are explained better by an objective to minimize Energy and Time, meaning the total work or energy to reach destination, plus a cost proportional to bout duration. Applied to a computational model of walking dynamics, this objective predicts dynamic speed vs. time trajectories with inverted U shapes. Model and human experiment (N=10) show that shorter bouts are unsteady and dominated by the time and effort of accelerating, and longer ones are steadier and faster and dominated by steady-state time and effort. Individual-dependent vigor may be characterized by the energy one is willing to spend to save a unit of time, which explains why some may walk faster than others, but everyone may have similar-shaped trajectories due to similar walking dynamics. Tradeoffs between energy and time costs can predict transient, steady, and vigor-related aspects of walking.

Fuel Selection in Skeletal Muscle Exercising at Low Intensity; Reliance on Carbohydrate in Very Sedentary Individuals

Background: Resting skeletal muscle in insulin resistance prefers to oxidize carbohydrate rather than lipid, exhibiting metabolic inflexibility. Although this is established in resting muscle, complexities involved in directly measuring fuel oxidation using indirect calorimetry across a muscle bed have limited studies of this phenomenon in working skeletal muscle. During mild exercise and at rest, whole-body indirect calorimetry imperfectly estimates muscle fuel oxidation. We provide evidence that a method termed "ΔRER" can reasonably estimate fuel oxidation in skeletal muscle activated by exercise. Methods: Completely sedentary volunteers (n = 20, age 31 ± 2 years, V̇O2peak 24.4 ± 1.5 mL O2 per min/kg) underwent glucose clamps to determine insulin sensitivity and graded exercise consisting of three periods of mild steady-state cycle ergometry (15, 30, 45 watts, or 10%, 20%, and 30% of maximum power) with measurements of whole-body gas exchange. ΔRER, the RER in working muscle, was calculated as (V̇CO2exercise -V̇CO2rest)/(V̇O2exercise - V̇O2rest), from which the fraction of fuel accounted for by lipid was estimated. Results: Lactate levels were low and stable during steady-state exercise. Muscle biopsies were used to estimate mitochondrial content. The rise of V̇O2 at onset of exercise followed a monoexponential function, with a time constant of 51 ± 7 sec, typical of skeletal muscle; the average O2 cost of work was about 12 mL O2/watt/min, representing a mechanical efficiency of about 24%. At work rates of 30 or 45 watts, active muscle relied predominantly on carbohydrate, independent of insulin sensitivity within this group of very sedentary volunteers. Conclusions: The fraction of muscle fuel oxidation from fat was predicted by power output (P < 0.001) and citrate synthase activity (P < 0.05), indicating that low mitochondrial content may be the main driver of fuel choice in sedentary people, independent of insulin sensitivity.

Humans trade off whole-body energy cost to avoid overburdening muscles while walking

Metabolic cost minimization is thought to underscore the neural control of locomotion. Yet, avoiding high muscle activation, a cause of fatigue, often outperforms energy minimization in computational predictions of human gait. Discerning the relative importance of these criteria in human walking has proved elusive, in part, because they have not been empirically decoupled. Here, we explicitly decouple whole-body metabolic cost and 'fatigue-like' muscle activation costs (estimated from electromyography) by pitting them against one another using two distinct gait tasks. When experiencing these competing costs, participants (n = 10) chose the task that avoided overburdening muscles (fatigue avoidance) at the expense of higher metabolic power (p < 0.05). Muscle volume-normalized activation more closely models energy use and was also minimized by the participants' decision (p < 0.05), demonstrating that muscle activation was, at best, an inaccurate signal for metabolic energy. Energy minimization was only observed when there was no adverse effect on muscle activation costs. By decoupling whole-body metabolic and muscle activation costs, we provide among the first empirical evidence of humans embracing non-energetic optimality in favour of a clearly defined neuromuscular objective. This finding indicates that local muscle fatigue and effort may well be key factors dictating human walking behaviour and its evolution.

Spinal electrical stimulation to improve sympathetic autonomic functions needed for movement and exercise after spinal cord injury: a scoping clinical review

Spinal cord injury (SCI) results in sensory, motor and autonomic dysfunction. Obesity, cardiovascular and metabolic diseases are highly prevalent after SCI. Although inadequate voluntary activation of skeletal muscle contributes, it is absent or inadequate activation of thoracic spinal sympathetic neural circuitry and sub-optimal activation of homeostatic (cardiovascular, temperature) and metabolic support systems that truly limits exercise capacity, particularly for those with cervical SCI. Thus, when electrical spinal cord stimulation (SCS) studies aimed at improving motor functions began mentioning effects on exercise-related autonomic functions, a potential new area of clinical application appeared. To survey this new area of potential benefit, we performed a systematic scoping review of clinical SCS studies involving these spinally mediated autonomic functions. Nineteen studies were included, 8 used transcutaneous and 11 used epidural SCS. Improvements in BP at rest or in response to orthostatic challenge were investigated most systematically, whereas reports of improved temperature regulation, whole body metabolism and peak exercise performance were mainly anecdotal. Effective stimulation locations and parameters varied between studies, suggesting multiple stimulation parameters and rostrocaudal spinal locations may influence the same sympathetic function. Brainstem and spinal neural mechanisms providing excitatory drive to sympathetic neurons that activate homeostatic and metabolic tissues that provide support for movement and exercise and their integration with locomotor neural circuitry are discussed. A unifying conceptual framework for the integrated neural control of locomotor and sympathetic function is presented which may inform future research needed to take full advantage of SCS for improving these spinally mediated autonomic functions.

Multi-objective control in human walking: insight gained through simultaneous degradation of energetic and motor regulation systems

Minimization of metabolic energy is considered a fundamental principle of human locomotion, as demonstrated by an alignment between the preferred walking speed (PWS) and the speed incurring the lowest metabolic cost of transport. We aimed to (i) simultaneously disrupt metabolic cost and an alternate acute task requirement, namely speed error regulation, and (ii) assess whether the PWS could be explained on the basis of either optimality criterion in this new performance and energetic landscape. Healthy adults (N = 21) walked on an instrumented treadmill under normal conditions and, while negotiating a continuous gait perturbation, imposed leg-length asymmetry. Oxygen consumption, motion capture data and ground reaction forces were continuously recorded for each condition at speeds ranging from 0.6 to 1.8 m s^-1, including the PWS. Both metabolic and speed regulation measures were disrupted by the perturbation (p < 0.05). Perturbed PWS selection did not exhibit energetic prioritization (although we find some indication of energy minimization after motor adaptation). Similarly, PWS selection did not support prioritization of speed error regulation, which was found to be independent of speed in both conditions. It appears that, during acute exposure to a mechanical gait perturbation of imposed leg-length asymmetry, humans minimize neither energetic cost nor speed regulation errors. Despite the abundance of evidence pointing to energy minimization during normal, steady-state gait, this may not extend acutely to perturbed gait. Understanding how the nervous system acutely controls gait perturbations requires further research that embraces multi-objective control paradigms.

Energy expenditure associated with walking speed and angle of turn in children

Purpose

Recent studies have suggested that turning is power intensive. Given the sporadic and irregular movement patterns of children, such findings have important implications for the assessment of true energy expenditure associated with habitual physical activity. The purpose of this study was to investigate the influence of walking speed and angle, and their interaction, on the energy expenditure of healthy children.

Methods

20 children (10.1 ± 0.5 years; 10 boys) participated in the study. On two separate days, participants completed a turning protocol involving 3-min bouts of walking at one of the 16 speed (2.5, 3.5, 4.5, and 5.5 km h^− 1) and angle (0°, 45°, 90°, and 180°) combinations, interspersed by 3 min seated rest. The movement involved 5 m straight walking interspaced with prescribed turns with speed dictated by a digital, auditory metronome. Breath-by-breath gas exchange was measured, in addition to tri-axial acceleration and magnetic field intensity recorded at 100 Hz.

Results

Mixed models revealed a significant main effect for speed (p < 0.006) and angle (p < 0.006), with no significant interaction between speed and angle (p > 0.006). Significant differences to straight-line walking energy expenditure within speed were established for 3.5 and 5.5 km h^− 1 for 180° turns (~ 13% and ~ 30% increase, respectively).

Conclusion

These findings highlight the importance of accounting for the magnitude and frequency of turns completed when estimating children’s habitual physical activity and have significant implications for the assessment of daily energy expenditure.

Carbohydrate and fat oxidation in persons with lower limb amputation during walking with different speeds

BACKGROUND

Studies suggest that the energy expenditure of healthy persons (control) during walking with the preferred walking speed in steady-state conditions is dominated by fat oxidation. Conversely, carbohydrate and fat oxidation during walking is little investigated in transfemoral amputees.

OBJECTIVES

To investigate carbohydrate and fat oxidation, energy cost of walking, and percent utilization of maximal aerobic capacity [Formula: see text]during walking.

STUDY DESIGN

Eight transfemoral amputees and controls walked with their preferred walking speed and speeds 12.5% and 25% slower and faster than their preferred walking speed.

METHODS

Energy expenditure and fuel utilization were measured using a portable metabolic analyzer. Metabolic values are means ± standard deviation.

RESULTS

For transfemoral amputees (37.0 ± 10.9 years) and controls (39.0 ± 12.3 years), fat utilization at the preferred walking speed was 44.8% ± 7.2% and 45.0% ± 7.2% of the total energy expenditure, respectively. The preferred walking speed of the transfemoral amputees and controls was close to a metabolic cross-over speed, which is the speed where carbohydrate utilization increases steeply and fat utilization decreases. When walking fast, at 90 m min^-1 (preferred walking speed plus 25%), transfemoral amputees utilized 70.7% ± 5.6% of their [Formula: see text], while the controls utilized 30.9% ± 4.5% ( p < 0.001) at the matching speed (control preferred walking speed). At 90 m min^-1, carbohydrate utilization was 78% ± 4.7% and 55.2% ± 7.2% of the total energy expenditure for the transfemoral amputees and controls, respectively ( p < 0.01). Compared to the control, energy cost of walking was higher for the transfemoral amputees at all speeds (all comparisons; p < 0.001).

CONCLUSION

At the preferred walking speed, carbohydrate, not fat, dominates energy expenditure of both transfemoral amputees and controls. For the transfemoral amputees, consequences of fast walking are very high [Formula: see text] utilization and rate of carbohydrate oxidation. Clinical relevance Research on the relationships between physical effort and fuel partitioning during ambulation could provide important insights for exercise-rehabilitation programs for lower limb amputees (LLA). Regular endurance exercise will improve maximal aerobic capacity and enable LLA to walk faster and at the same time expend less energy and improve fat utilization.

Instantaneous Metabolic Cost of Walking: Joint-Space Dynamic Model with Subject-Specific Heat Rate

A subject-specific model of instantaneous cost of transport (ICOT) is introduced from the joint-space formulation of metabolic energy expenditure using the laws of thermodynamics and the principles of multibody system dynamics. Work and heat are formulated in generalized coordinates as functions of joint kinematic and dynamic variables. Generalized heat rates mapped from muscle energetics are estimated from experimental walking metabolic data for the whole body, including upper-body and bilateral data synchronization. Identified subject-specific energetic parameters-mass, height, (estimated) maximum oxygen uptake, and (estimated) maximum joint torques-are incorporated into the heat rate, as opposed to the traditional in vitro and subject-invariant muscle parameters. The total model metabolic energy expenditure values are within 5.7 ± 4.6% error of the measured values with strong (R2 > 0.90) inter- and intra-subject correlations. The model reliably predicts the characteristic convexity and magnitudes (0.326-0.348) of the experimental total COT (0.311-0.358) across different subjects and speeds. The ICOT as a function of time provides insights into gait energetic causes and effects (e.g., normalized comparison and sensitivity with respect to walking speed) and phase-specific COT, which are unavailable from conventional metabolic measurements or muscle models. Using the joint-space variables from commonly measured or simulated data, the models enable real-time and phase-specific evaluations of transient or non-periodic general tasks that use a range of (aerobic) energy pathway similar to that of steady-state walking.

The Pittsburgh Fatigability scale for older adults: development and validation

OBJECTIVES

To describe the development of the Pittsburgh Fatigability Scale (PFS) and establish its reliability and concurrent and convergent validity against performance measures.

DESIGN

Cross-sectional.

SETTING

University of Pittsburgh, Pittsburgh, Pennsylvania.

PARTICIPANTS

Scale development sample: 1,013 individuals aged 60 and older from two registries; validation sample: 483 adults aged 60 and older from the Baltimore Longitudinal Study of Aging (BLSA).

MEASUREMENTS

The scale development sample and BLSA participants self-administered an initial 26-item perceived fatigability scale. BLSA participants also completed measures of performance fatigability (perceived exertion from a standard treadmill task and performance deterioration from a fast-paced long-distance corridor walk), a 6-m usual-paced corridor walk, and five timed chair stands.

RESULTS

Principal components analysis with varimax rotation reduced the 26-item scale to the 10-item PFS. The PFS showed strong internal consistency (Cronbach's alpha 0.88) and excellent test-retest reliability (intraclass correlation 0.86). In the validation sample, PFS scores, adjusted for age, sex, and race, were greater for those with high performance fatigability, slow gait speed, worse physical function, and lower fitness, with differences between high and low fatigability ranging from 3.2 to 5.1 points (P < .001).

CONCLUSION

The 10-item PFS physical fatigability score is a valid and reliable measure of perceived fatigability in older adults and can serve as an adjunct to performance-based fatigability measures for identifying older adults at risk of mobility limitation in clinical and research settings.

Walking energetics, fatigability, and fatigue in older adults: the study of energy and aging pilot

BACKGROUND

Slow gait speed increases morbidity and mortality in older adults. We examined how preferred gait speed is associated with energetic requirements of walking, fatigability, and fatigue.

METHODS

Older adults (n = 36, 70-89 years) were categorized as slow or fast walkers based on median 400-m gait speed. We measured VO2peak by graded treadmill exercise test and VO2 during 5-minute treadmill walking tests at standard (0.72 m/s) and preferred gait speeds. Fatigability was assessed with the Situational Fatigue Scale and the Borg rating of perceived exertion at the end of walking tests. Fatigue was assessed by questionnaire.

RESULTS

Preferred gait speed over 400 m (range: 0.75-1.58 m/s) averaged 1.34 m/s for fast walkers versus 1.05 m/s for slow walkers (p < .001). VO2peak was 26% lower (18.5 vs 25.1ml/kg/min, p = .001) in slow walkers than fast walkers. To walk at 0.72 m/s, slow walkers used a larger percentage of VO2peak (59% vs 42%, p < .001). To walk at preferred gait speed, slow walkers used more energy per unit distance (0.211 vs 0.186ml/kg/m, p = .047). Slow walkers reported higher rating of perceived exertion during walking and greater overall fatigability on the Situational Fatigue Scale, but no differences in fatigue.

CONCLUSIONS

Slow walking was associated with reduced aerobic capacity, greater energetic cost of walking, and greater fatigability. Interventions to improve aerobic capacity or decrease energetic cost of walking may prevent slowing of gait speed and promote mobility in older adults.

Vigor of movements and the cost of time in decision making

If we assume that the purpose of a movement is to acquire a rewarding state, the duration of the movement carries a cost because it delays acquisition of reward. For some people, passage of time carries a greater cost, as evidenced by how long they are willing to wait for a rewarding outcome. These steep discounters are considered impulsive. Is there a relationship between cost of time in decision making and cost of time in control of movements? Our theory predicts that people who are more impulsive should in general move faster than subjects who are less impulsive. To test our idea, we considered elementary voluntary movements: saccades of the eye. We found that in humans, saccadic vigor, assessed using velocity as a function of amplitude, was as much as 50% greater in one subject than another; that is, some people consistently moved their eyes with high vigor. We measured the cost of time in a decision-making task in which the same subjects were given a choice between smaller odds of success immediately and better odds if they waited. We measured how long they were willing to wait to obtain the better odds and how much they increased their wait period after they failed. We found that people that exhibited greater vigor in their movements tended to have a steep temporal discount function, as evidenced by their waiting patterns in the decision-making task. The cost of time may be shared between decision making and motor control.

Skeletal Muscle Fuel Selection Occurs at the Mitochondrial Level

Mammals exponentially increase the rate of carbohydrate oxidation as exercise intensity rises, while birds combust lipid almost exclusively while flying at high percentages of aerobic capacity. The fuel oxidized by contracting muscle depends on many factors: whole body fuel storage masses, mobilization, blood transport, cellular uptake, and substrate selection at the level of the mitochondrion. We examined the fuel preferences of mitochondria isolated from mammalian and avian locomotory muscles using two approaches. First, the influence of substrates on the kinetics of respiration (KMADP and Vmax) was evaluated. For all substrates and combinations, KMADP was generally two-fold higher in avian mitochondria. Second, fuel competition between pyruvate, glutamate, and/or palmitoyl-l-carnitine at three levels of ATP free energy was determined using the principle of mass balance and the measured rates of O2 consumption and metabolite accumulation/utilization. Avian mitochondria strongly spared pyruvate from oxidation when another substrate was available and fatty acid was the dominant substrate, regardless of energy state. Mammalian mitochondria exhibited some preference for fatty acid over pyruvate at lower flux (higher energy state), but exhibited much greater tendency to select pyruvate and glutamate when available. Studies in sonicated mitochondria revealed two-fold higher electron transport chain electron conductance in avian mitochondria. We conclude that substantial fuel selection occurs at the level of the mitochondrial matrix and that avian flight muscle mitochondria are particularly biased toward the selection of fatty acid, possibly by facilitating high β-oxidation flux by maintaining a more oxidized matrix.

Differences in contractile behaviour between the soleus and medial gastrocnemius muscles during human walking

The functional roles of individual lower limb muscles during human walking may differ depending on walking speed or duration. In this study, 11 volunteers walked on a treadmill for 60 min at speeds corresponding to both optimal and 20% above optimal energetic cost of transport whilst oxygen consumption and medial gastrocnemius (MG) and soleus fascicle lengths were measured. Although energetic cost of transport was ∼12% higher at the faster speed, it remained constant over 60 min at both speeds, suggesting that humans can walk for prolonged periods at a range of speeds without compromising energetic efficiency. The fascicles of both muscles exhibited rather 'isometric' behaviour during the early to mid stance phase of walking, which appears to be independent of walking speed or movement efficiency. However, several functional differences were observed between muscles. MG exhibited time- and speed-dependent decreases in operating length, and shortened faster during the pushoff phase at the faster walking speed. Conversely, soleus exhibited consistent contractile behaviour regardless of walking speed or duration, and always shortened slower than MG during pushoff. Soleus appears to play a more important functional role than MG during walking. This may be especially true when walking for prolonged periods or at speeds above the most energetically efficient, where the force potential and thus the functional importance of MG appears to decline.

Energetics, Locomotion, and Female Reproduction: Implications for Human Evolution

In our reconstructions of human evolution, a few key questions consistently rise to the surface. These questions tend to revolve around how the morphology of previous hominin species would have allowed them to gain access to resources during key life-history events, particularly gestation and lactation. Here the data surrounding the interactions between these key issues are assessed, making particular notes of recent advances in the fields of energetics and biomechanics as they relate to locomotion during reproduction. Reconstructions of body mass, lower limb length, and pelvic breadth suggest diverse mobility strategies for different hominin species and may offer some clues about the demographic shifts occurring in the late Pleistocene.

Mitochondrial function in sparrow pectoralis muscle

Flying birds couple a high daily energy turnover with double-digit millimolar blood glucose concentrations and insulin resistance. Unlike mammalian muscle, flight muscle predominantly relies on lipid oxidation during locomotion at high fractions of aerobic capacity, and birds outlive mammals of similar body mass by a factor of three or more. Despite these intriguing functional differences, few data are available comparing fuel oxidation and free radical production in avian and mammalian skeletal muscle mitochondria. Thus we isolated mitochondria from English sparrow pectoralis and rat mixed hindlimb muscles. Maximal O2 consumption and net H2O2 release were measured in the presence of several oxidative substrate combinations. Additionally, NAD- and FAD-linked electron transport chain (ETC) capacity was examined in sonicated mitochondria. Sparrow mitochondria oxidized palmitoyl-l-carnitine 1.9-fold faster than rat mitochondria and could not oxidize glycerol-3-phosphate, while both species oxidized pyruvate, glutamate and malate–aspartate shuttle substrates at similar rates. Net H2O2 release was not significantly different between species and was highest when glycolytic substrates were oxidized. Sonicated sparrow mitochondria oxidized NADH and succinate over 1.8 times faster than rat mitochondria. The high ETC catalytic potential relative to matrix substrate dehydrogenases in sparrow mitochondria suggests a lower matrix redox potential is necessary to drive a given O2 consumption rate. This may contribute to preferential reliance on lipid oxidation, which may result in lower in vivo reactive oxygen species production in birds compared with mammals.

Comparison of substrate oxidation during walking and running in normal-weight and overweight/obese men

OBJECTIVE

The aim of the present study is to examine the differences in fat and carbohydrate (CHO) oxidation during walking and running between normal-weight and overweight/obese young adult men.

METHODS

19 healthy, normal-weight (age = 21.9 ± 0.7 years, BMI = 22.6 ± 0.4 kg, n = 10) and overweight (age = 21.4 ± 0.6 years, BMI = 31.6 ± 1.1, n = 9) young men volunteered to participate in this study. Body composition was assessed by bioelectrical impedance. Maximal oxygen uptake and maximal fat oxidation rate were determined with indirect calorimetry by using an incremental exercise test on a motor-driven treadmill. The participants' individual preferred transition speeds between walking and running were determined. Indirect calorimetry was used to calculate fat and CHO oxidation during the resting, walking and running tests.

RESULTS

Maximal fat oxidation rates during the graded exercise test were not significantly different between the groups. Changes in CHO and fat oxidation in the resting, walking and running tests were similar in the normal-weight and overweight groups.

CONCLUSION

The study results suggest that with regard to changes in CHO and fat oxidation, normal-weight and overweight/obese individuals have similar responses to walking and running at preferred speeds, despite significant differences in oxygen uptake during activity and body composition.

Fuel oxidation at the walk-to-run-transition in humans

Multiple factors (including anthropometric, kinetic, mechanical, kinematic, perceptual, and energetic factors) are likely to play a role in the walk-to-run transition in humans. The primary purpose of the present study was to consider an additional factor, the metabolic fuel source. Indirect calorimetry was used to measure fuel oxidation, and perception of effort was recorded as 10 overnight-fasted adults locomoted on a level treadmill at speeds progressing from 1.56 to 2.46 m s(-1) in increments of 0.11 m s(-1) and 10.0 minutes under 3 conditions: (1) unconstrained choice of gait, (2) walking at all speeds, and (3) running at all speeds. The preferred transition speed was 2.08 ± 0.03 m s(-1). Gait transition from walking to running increased oxygen consumption rate, decreased the perception of effort, and decreased the rate of carbohydrate oxidation. We propose that, in an evolutionary context, gait transition, guided by the perception of effort, can be viewed as a carbohydrate-sparing strategy.

Walking and running economy: inverse association with peak oxygen uptake

PURPOSE

The purpose of this study was to test the hypothesis that V˙O2peak is positively correlated with the regression coefficients of the curve-linear relationship between V˙O2 and speed during a protocol consisting of submaximal walking and running.

METHODS

Nineteen healthy men (mean ± SD: age = 26.4 ± 6.4 yr, height = 179.9 ± 7.2 cm, weight = 77.7 ± 8.7 kg, % fat = 16.3 ± 7.3) and 21 healthy women (age = 25.6 ± 4.9 yr, height = 167.2 ± 5.4 cm, weight = 61.6 ± 7.7 kg, % fat = 24.0 ± 6.8) underwent an incremental treadmill test to determine VO2peak and on two separate days performed an exercise protocol consisting of treadmill walking on a level grade at 2.0 mph (54 m·min−¹), 3.0 mph (80 m·min−¹), and 4.0 mph (107 m·min−¹) and running at 6.0 mph (161 m·min−¹). Subjects exercised for 5 min at each velocity, with 3 min of rest in between each exercise bout. Pulmonary ventilation (VE) and gas exchange were measured breath-by-breath each minute. The average of VO2 values obtained during the last 2 min of exercise for both exercise sessions was used in polynomial random coefficient regression analysis.

RESULTS

In the polynomial random coefficient regression analysis for walking speeds only, both linear (r = 0.31, P = 0.053) and quadratic (r = 0.35, P = 0.029) coefficients were modestly correlated with VO2peak. Steady-state VO2 during walking at 3.0 and 4.0 mph and running at 6.0 mph was also modestly correlated with VO2peak (r = 0.30-0.48).

CONCLUSIONS

The results confirm our hypothesis and suggest that, as walking speed increases, the increase in VO2 is positively correlated with the VO2peak. Our findings are consistent with the notion that cardiorespiratory fitness and exercise economy are inversely related.

Reliability of the VmaxST portable metabolic measurement system

The purpose of this study was to evaluate the reliability of the VmaxST portable metabolic measurement system. Forty-five healthy adults (age = 25.7 +/- 5.9 yr; height = 171.8 +/- 9.1 cm; weight = 69.6 +/- 12.8 kg; VO2peak) = 40.7 ml/kg/min; percent fat = 21.7 +/- 11.0) performed two separate and identical exercise routines on different days consisting of treadmill walking at 2.0 mph (53.6 m/min), 3.0 mph (80.5 m/min), and 4.0 mph (107.3 m/min) and running at 6.0 mph (160.9 m/min). VE and gas exchange were measured continuously breath-to-breath. A random effects model on log-transformed data yielded coefficients of variation (CV) and intraclass correlation coefficients (ICC) for VO2 and VE of 5.2 - 7.6 %, and 0.77 - 0.92, respectively, for all walking and running trials. For VCO2, CVs were higher (10 - 12 %) and ICCs lower (0.70 - 0.81). Ordinary least squares regression between the individual difference scores and the individual mean scores for VE, VO2 and VCO2, respectively, indicated no systematic bias (all p > 0.05). Bland-Altman analysis also illustrated no systematic bias between repeated measurements. The VmaxST provides reliable measurements of VO2 and VE during walking and running eliciting VE and VO2 at least up to approximately 56 and 2.2 l/min, respectively. The system appears to be less reliable for measuring VCO2.