Energy cost of balance control during walking decreases with external stabilizer stiffness independent of walking speed

Human walking requires active neuromuscular control to ensure stability in the lateral direction, which inflicts a certain metabolic load. The magnitude of this metabolic load has previously been investigated by means of passive external lateral stabilization via spring-like cords. In the present study, we applied this method to test two hypotheses: (1) the effect of external stabilization on energy cost depends on the stiffness of the stabilizing springs, and (2) the energy cost for balance control, and consequently the effect of external stabilization on energy cost, depends on walking speed. Fourteen healthy young adults walked on a motor driven treadmill without stabilization and with stabilization with four different spring stiffnesses (between 760 and 1820 N m⁻¹) at three walking speeds (70%, 100%, and 130% of preferred speed). Energy cost was calculated from breath-by-breath oxygen consumption. Gait parameters (mean and variability of step width and stride length, and variability of trunk accelerations) were calculated from kinematic data. On average external stabilization led to a decrease in energy cost of 6% (p<0.005) as well as a decrease in step width (24%; p<0.001), step width variability (41%; p<0.001) and variability of medio-lateral trunk acceleration (12.5%; p<0.005). Increasing stabilizer stiffness increased the effects on both energy cost and medio-lateral gait parameters up to a stiffness of 1260 N m⁻¹. Contrary to expectations, the effect of stabilization was independent of walking speed (p=0.111). These results show that active lateral stabilization during walking involves an energetic cost, which is independent of walking speed.     

The effect of citrate loading on exercise performance, acid-base balance and metabolism

Nine subjects (VO2max 65 +/- 2 ml.kg-1.min-1, mean +/- SEM) were studied on two occasions following ingestion of 500 ml solution containing either sodium citrate (C, 0.300 g.kg-1 body mass) or a sodium chloride placebo (P, 0.045 g.kg-1 body mass). Exercise began 60 min later and consisted of cycle ergometer exercise performed continuously for 20 min each at power outputs corresponding to 33% and 66% VO2max, followed by exercise to exhaustion at 95% VO2max. Pre-exercise arterialized-venous [H+] was lower in C (36.2 +/- 0.5 nmol.l-1; pH 7.44) than P (39.4 +/- 0.4 nmol.l-1; pH 7.40); the plasma [H+] remained lower and [HCO3-] remained higher in C than P throughout exercise and recovery. Exercise time to exhaustion at 95% VO2max was similar in C (310 +/- 69 s) and P (313 +/- 74 s). Cardiorespiratory variables (ventilation, VO2, VCO2, heart rate) measured during exercise were similar in the two conditions. The plasma [citrate] was higher in C at rest (C, 195 +/- 19 mumol.l-1; P, 81 +/- 7 mumol.l-1) and throughout exercise and recovery. The plasma [lactate] and [free fatty acid] were not affected by citrate loading but the plasma [glycerol] was lower during exercise in C than P. In conclusion, sodium citrate ingestion had an alkalinizing effect in the plasma but did not improve endurance time during exercise at 95% VO2max. Furthermore, citrate loading may have prevented the stimulation of lipolysis normally observed with exercise and prevented the stimulation of glycolysis in muscle normally observed in bicarbonate-induced alkalosis.     

Energy metabolism during insect flight: biochemical design and physiological performance

Flying insects achieve the highest known mass-specific rates of O(2) consumption in the animal kingdom. Because the flight muscles account for >90% of the organismal O(2) uptake, accurate estimates of metabolic flux rates (J) in the muscles can be made. In steady state, these are equal to the net forward flux rates (v) at individual steps and can be compared with flux capacities (V(max)) measured in vitro. In flying honeybees, hexokinase and phosphofructokinase, both nonequilibrium reactions in glycolysis, operate at large fractions of their maximum capacities (i.e., they operate at high v/V(max)). Phosphoglucoisomerase is a reversible reaction that operates near equilibrium. Despite V(max) values more than 20-fold greater than the net forward flux rates during flight, a close match is found between the V(max) required in vivo (estimated using the Haldane relationship) to maintain near equilibrium and this net forward flux rate and the V(max) measured in vitro under simulated physiological conditions. Rates of organismal O(2) consumption and difference spectroscopy were used to estimate electron transfer rates per molecule of respiratory chain enzyme during flight. These are much higher than those estimated in mammalian muscles. Current evidence indicates that metabolic enzymes in honeybees do not display higher catalytic efficiencies than the homologous enzymes in mammals, and the high electron transfer rates do not appear to be the result of higher enzyme densities per unit cristae surface area. A number of possible mechanistic explanations for the higher rates of electron transfer are proposed.     

Energy cost and muscular activity required for propulsion during walking.

We reasoned that with an optimal aiding horizontal force, the reduction in metabolic rate would reflect the cost of generating propulsive forces during normal walking. Furthermore, the reductions in ankle extensor electromyographic (EMG) activity would indicate the propulsive muscle actions. We applied horizontal forces at the waist, ranging from 15% body weight aiding to 15% body weight impeding, while subjects walked at 1.25 m/s. With an aiding horizontal force of 10% body weight, 1) the net metabolic cost of walking decreased to a minimum of 53% of normal walking, 2) the mean EMG of the medial gastrocnemius (MG) during the propulsive phase decreased to 59% of the normal walking magnitude, and yet 3) the mean EMG of the soleus (Sol) did not decrease significantly. Our data indicate that generating horizontal propulsive forces constitutes nearly half of the metabolic cost of normal walking. Additionally, it appears that the MG plays an important role in forward propulsion, whereas the Sol does not.     

Energy cost and muscular activity required for leg swing during walking.

To investigate the metabolic cost and muscular actions required for the initiation and propagation of leg swing, we applied a novel combination of external forces to subjects walking on a treadmill. We applied a forward pulling force at each foot to assist leg swing, a constant forward pulling force at the waist to provide center of mass propulsion, and a combination of these foot and waist forces to evaluate leg swing. When the metabolic cost and muscle actions were at a minimum, the condition was considered optimal. We reasoned that the difference in energy consumption between the optimal combined waist and foot force trial and the optimal waist force-only trial would reflect the metabolic cost of initiating and propagating leg swing during normal walking. We also reasoned that a lower muscle activity with these assisting forces would indicate which muscles are normally responsible for initiating and propagating leg swing. With a propulsive force at the waist of 10% body weight (BW), the net metabolic cost of walking decreased to 58% of normal walking. With the optimal combination, a propulsive force at the waist of 10% BW plus a pulling force at the feet of 3% BW the net metabolic cost of walking further decreased to 48% of normal walking. With the same combination, the muscle activity of the iliopsoas and rectus femoris muscles during the swing phase was 27 and 60% lower, respectively, but the activity of the medial gastrocnemius and soleus before swing did not change. Thus our data indicate that approximately 10% of the net metabolic cost of walking is required to initiate and propagate leg swing. Additionally, the hip flexor muscles contribute to the initiation and propagation leg swing.     

Serotonin metabolism in the rat brain during water deprivation and hydration

Water deprivation (WD) decreased the serotonin (5-HT) level and significantly increased the 5-hydroxyindoleacetic acid (5-HIAA) in the rat midbrain and hypothalamus, the catabolic 5-HIAA/5-HT ratio increasing three-fold. Hydration (H) produced a moderate increase in the 5-HT and 5-HIAA levels in the hypothalamus with no changes in the 5-HIAA/5-HT ratio. Hydration exerted no significant effect upon the 5-HT level and metabolism in the midbrain. A two-fold increase of corticosterone concentration in water deprivation and its decrease in hydration were shown to occur in peripheral blood plasma.     

Effect of age and speed on the step-to-step transition phase during walking

Gait is a powerful measurement tool to evaluate the functional decline throughout ageing. Falls in elderly adults happen mainly during the redirection of the center of mass of the body (CoM) in the transition between steps. In young adults, this step–to–step transition begins before the double contact phase (DC) with a simultaneous forward and upward acceleration of the CoM. We hypothesize that, compared to young adults, elderly adults would exhibit unbalanced contribution of the back leg and the front leg during the transition. We calculated the mean vertical push-off done by the back leg (F_BACK) and the mean impact force on the front leg (F_FRONT) during the transition. Eight young (mean ± SD; age: 24 ± 2 y) and 19 elderly (age: 74 ± 6 y) healthy adults walked on a force-measuring treadmill at five selected speeds ranging from 0.56 to 1.67 m·s⁻¹. Results show that, at mid and high speeds, elderly adults exhibit a smaller F_BACK compared to young adults, possibly linked to the decreased plantar flexion of the back foot. As a consequence, F_FRONT is significantly increased and the transition begins lately in the step, at the beginning of DC. Also, elderly adults show an inability to accelerate the CoM upward and forward simultaneously. Our findings show a different adaptation of the step–to–step transition with speed in elderly adults and identify two potential indicators of gait impairment with age: the F_FRONT/F_BACK contribution and the synchronization between the upward and forward acceleration of the CoM during the transition.     

The effect of strenuous exercise and beta-adrenergic blockade on the visual performance of juvenile rainbow trout,Oncorhynchus mykiss

It is hypothesised that the visual performance of rainbow trout, Oncorhynchus mykiss, will be impaired by strenuous exercise as a result of metabolic stress (blood lactacidosis) that activates the Root effect and limits the oxygen-carrying capacity of blood flowing to the eye. The ability to resolve high contrast objects on a moving background, as a measure of visual performance, was quantified pre- and post-exercise using the optomotor response. Strenuous exercise induced a metabolic acidosis (8.0 mmol l(-1) blood lactate) and a significant red cell swelling response but no change in the optomotor response threshold (120 min of arc) was observed. Beta-adrenergic blockade (propranolol) abolished post-exercise red cell swelling but optomotor response thresholds were still maintained at 120 min of arc despite a significant blood lactate load (7.8 mmol l(-1)). The choroid rete mirabile of the trout is extremely well developed (rete area:eye area = 0.39) and may maintain visual performance by ensuring a relatively direct supply of oxygen to the central regions of the avascular retina. Exercised fish under beta-adrenergic blockade exhibited an enhanced optomotor response at 240-300 min of arc. Assuming that these responses reflect "tunnel vision", adrenergic regulation of red cell function may preserve a high ocular PO(2) gradient that satisfies the oxygen demand of peripheral retinal cells.     

Energy metabolism reactions in ruminant muscle: responses to age, nutrition and hormonal status

Energy expenditure in muscle comprises reactions related to intermediary metabolism and those of posture and activity. The metabolic reactions respond to a wide range of nutritional and hormonal stimuli and are often apparently co-ordinated; in magnitude, however, their contribution to energy requirements can be minor compared with locomotion and posture. Metabolic reactions include protein turnover, ion transport and substrate cycles. In young ruminants muscle protein synthesis responds to intake but effects on energy expenditure are less pronounced; the situation with the adult is unclear. The involvement of insulin in ruminants may differ from that in monogastrics but effects are observed with thyroid hormones. Ruminant muscle may have a higher energy requirement for Na+, K+ transport which responds in proportion to total oxygen uptake to alterations in intake. Thyroid hormone treatment and, probably, the catecholamines enhance both Na+, K+ and Ca2+ transport. Muscle has fewer substrate cycles than liver and each may contribute only 1-3% toward oxygen consumption. Several are sensitive to insulin, but larger responses are observed with thyroxine and epinephrine and under stress conditions, therefore, may account for significant proportions of heat increment. Energy costs of standing may be considerable and posture movements may change with diet quality and quantity. Locomotory activity may mask changes in the contribution of metabolic reactions in response to different stimuli. Approximately 80% of energy costs for muscle in vivo are accounted for by protein turnover (20-25%), ion transport (25-30%), substrate cycling (5-8%) and standing (30%). Better integration of experiments in vivo and in vitro is required to improve the quantification and resolve data anomalies.     

Physiological, metabolic, and performance implications of a prolonged hill walk: influence of energy intake.

We aimed to examine the effects of different energy intakes on a range of responses that are relevant to the safety of hill walkers. In a balanced design, 16 men completed a strenuous self-paced mountainous hill walk over 21 km, under either a low-energy (2.6 MJ; 616 kcal) intake (LEI) or high-energy (12.7 MJ; 3,019 kcal) intake (HEI) condition. During the hill walk, rectal temperatures were measured continuously, and blood samples for the analysis of metabolites and hormones were drawn before breakfast and immediately after the walk. Subjects also completed a battery of performance tests that included muscular strength, reaction times, flexibility, balance, and kinesthetic differentiation tests. During the LEI, mean blood glucose concentrations leveled off at the low-middle range of normoglycemia, whereas, on the HEI, they were significantly elevated compared with the LEI. The maintained blood glucose concentrations, during the LEI, were probably mediated via the marked fat mobilization, reflected by a two- to fivefold increase in nonesterified fatty acids, 3-hydroxybutyrate, and glycerol concentrations. The LEI group showed significantly slower one- and two-finger reaction time, had an impaired ability to balance, and were compromised in their ability to maintain body temperature, when compared with the HEI group. The modestly impaired performance (particularly with respect to balance) and thermoregulation during the LEI condition may increase susceptibility to both fatigue and injury during the pursuit of recreational activity outdoors.     

Water balance during vitellogenesis by the American cockroach: effect of frontal ganglionectomy.

Frontal ganglionectomy of adult female American cockroaches causes a retardation of crop emptying and decreases water consumption. Haemolymph volume declines markedly and oöthecal production stops. Ganglionectomized animals eat approximately the same quantity of food as sham-operated animals and there is no increase in faecal pellets or diuresis. The tissue water remains relatively constant and does not appear to be in equilibrium with the haemolymph.The loss of haemolymph water and the subsequent drop in oöthecal production is attributed mainly to the impairment of drinking and crop emptying. However, neural pathways involved in diuresis and ventilation movements may also be affected.     

Effects of breed and age on the performance of crossbred hill ewes sourced from Scottish Blackface dams

The aim of this study was to evaluate the effects of age and breed on the reproductive performance and lamb output of crossbred hill ewes relative to purebred Scottish Blackface (BF). BF ewes were compared alongside Swaledale (SW) × BF, North Country Cheviot (CH) × BF, Lleyn (LL) × BF and Texel (T) × BF ewes on six commercial hill farms across Northern Ireland, on which all the ewes were born and reared. Ewes were mated to a range of sire breeds, balanced across breeds, for up to five successive breeding seasons. Mature live weight of adult BF, SW × BF, CH × BF, LL × BF and T × BF ewes was 52.8, 54.9, 60.3, 55.6 and 58.6 kg (P < 0.001), respectively. Compared with the pure BF, the number of lambs born per ewe lambed was higher with LL × BF and SW × BF (P < 0.05), whereas the number of lambs weaned per ewe lambed was greater for LL × BF and T × BF (P < 0.01). Total litter weight at birth of all the crossbred ewes was heavier (P < 0.01) than the pure BF, except in primiparous 2-year-old ewes. Lambs born to CH × BF and T × BF dams were 0.24 to 0.35 kg heavier at birth (P < 0.01) than the other ewe breeds, whereas lambs born to CH × BF, LL × BF and T × BF dams were, on average, 1.7, 1.3 and 1.5 kg, respectively, heavier (P < 0.01) at weaning than those from BF dams due to their higher (P < 0.05) average daily gain. Compared with the pure BF, total weaned lamb output per ewe lambed was 3.7, 4.8, 6.7 and 5.4 kg heavier (P < 0.05) for SW × BF, CH × BF, LL × BF and T × BF, respectively. However, as a result of the heavier live weight of the crossbred ewes, production efficiency (lamb output per kilogram live weight (W) and lamb output per kilogram metabolic live weight (W0.75)) was higher (P < 0.001) for LL × BF ewes only. For all ewe breeds, litter size at birth per ewe lambed, total lamb birth weight per ewe lambed and litter size at weaning increased (P < 0.001) with age up to 5 years, but decreased in 6-year-old ewes. Average lamb weaning weight and total weaned lamb output per ewe lambed increased (P < 0.001) with age up to 4 years . Production efficiency of the 6-year-old ewes was lower (P < 0.01) than the younger ewes. This study shows that adopting a flock replacement policy based on crossing BF ewes with LL, SW, T and CH sires can lead to significant improvements in the productivity of hill flocks.     

Caffeine, performance, and metabolism during repeated Wingate exercise tests

Investigations examining the ergogenicand metabolic influence of caffeine during short-term high-intensityexercise are few in number and have produced inconsistent results. Thisstudy examined the effects of caffeine on repeated bouts ofhigh-intensity exercise in recreationally active men. Subjects(n = 9) completed four 30-s Wingate(WG) sprints with 4 min of rest between each exercise bout on twoseparate occasions. One hour before exercise, either placebo (Pl;dextrose) or caffeine (Caf; 6 mg/kg) capsules were ingested. Caf ingestion did not have any effect on poweroutput (peak or average) in the first two WG tests and had a negative effect in the latter two exercise bouts. Plasmaepinephrine concentration was significantly increased 60 min after Cafingestion compared with Pl; however, this treatment effect disappearedonce exercise began. Caf ingestion had no significant effect on bloodlactate, O₂ consumption, oraerobic contribution at any time during the protocol. After the secondWingate test, plasma NH₃concentration increased significantly from the previous WG test and wassignificantly higher in the Caf trial compared with Pl. These datademonstrate no ergogenic effect of caffeine on power output duringrepeated bouts of short-term, intense exercise. Furthermore, there was no indication of increased anaerobic metabolism after Caf ingestion with the exception of an increase inNH₃ concentration.

     

Energy and water balance during torpor and hydropenia in the pigmy gerbil,Gerbillus pusillus

Summary Under ad lib. food conditions,G. pusillus used 107% of the predicted daily energy expenditure. This increased significantly with water deprivation to 116% of the allometric expected value, thereby reflecting an increase in activity in search of food with high moisture content, a shift to metabolic water for meeting the animal's water requirements and a change in the state of hydration of the animal. Economic water expenditure through efficient kidney concentrating ability, reduced pulmocutaneous evaporation, reduced faecal water loss, and tolerance to haemoconcentration, was sufficient to impart virtual independence of exogenous water. Gerbillus pusillus responded to food shortage by abandoning homeothermy. In doing so, it was able to maintain a fairly stable body mass by monitoring energy intake and ensuring that energy expenditure did not exceed intake. There was no significant difference in energy expenditure between gerbils on a restricted energy intake and ad lib. water and those gerbils on a restricted energy intake and no water. Both these experimental groups were able to maintain a positive water balance, for whilst less metabolic water was available, water loss primarily through evaporation decreased concomittantly with reduced body temperature. In addition, the volume of urine excreted when torpid accounted for approximately 5% of the volume excreted by coenothermic rodents deprived of free water. Urine, although less concentrated than that of non-torpid gerbils deprived of water, was ten times more concentrated than the plasma. Daily torpor, although uniquely suited to energy conservation, is not without cost. A reduced body temperature resulted in a decline in ‘apparent assimilation efficiency’ (AE). It seems therefore that the cost of maintaining an elevated body temperature for the full duration of digestion is impractical. The cost of employing torpor as calculated from the reduced AE of food (0.5 kJ·day⁻¹), was however insignificant in comparison to the savings accured (32.25 kJ·day⁻¹) by the use of torpor.