Running perturbations reveal general strategies for step frequency selection

Recent research has suggested that energy minimization in human walking involves both a fast preprogrammed process and a slow optimization process. Here, we studied human running to test whether these two processes represent control mechanisms specific to walking or a more general strategy for minimizing energetic cost in human locomotion. To accomplish this, we used free response experiments to enforce step frequency with a metronome at values above and below preferred step frequency and then determined the response times for the return to preferred steady-state step frequency when the auditory constraint was suddenly removed. In forced response experiments, we applied rapid changes in treadmill speed and examined response times for the processes involved in the consequent adjustments to step frequency. We then compared the dynamics of step frequency adjustments resulting from the two different perturbations to each other and to previous results found in walking. Despite the distinct perturbations applied in the two experiments, both responses were dominated by a fast process with a response time of 1.47 ± 0.05 s with fine-tuning provided by a slow process with a response time of 34.33 ± 0.50 s. The dynamics of the processes underlying step frequency adjustments in running match those found previously in walking, both in magnitude and relative importance. Our results suggest that the underlying mechanisms are fundamental strategies for minimizing energetic cost in human locomotion.     

Afferent feedback in the control of human gait.

Sensory activity contributes to motor control in two fundamentally different ways. It may mediate 'error signals' following sudden external perturbations and it may contribute to the pre-programmed motoneuronal drive. Here we review data, which illustrate these two functions of sensory feedback in relation to human walking. When ankle plantarflexors are unloaded in the stance phase there is a sudden decrease in the sensory activity in muscle and tendon afferents from the active muscles. This decrease in sensory activity results in a drop in EMG activity recorded from the soleus muscle, which demonstrates that the sensory activity contributes importantly to the activation of the muscles. Data suggests that a spinal pathway from gr. II muscle afferents is responsible for this positive feedback contribution to the motoneuronal drive during walking.When cutaneous nerves from the foot are stimulated in the early swing phase of walking a late reflex response may be observed in the tibialis anterior muscle. This reflex may help to ensure that the foot is lifted effectively over an obstacle. Data suggest that this reflex response is at least partly mediated by a transcortical reflex pathway. It seems to be important that reactions to external perturbations are integrated at a supraspinal level during human walking.     

Predicted threshold against forward and backward loss of balance for perturbed walking

The biomechanical mechanisms of loss of balance have been studied before for slip condition but have not been investigated for arbitrary perturbation profiles under non-slip conditions in sagittal plane. This study aimed to determine the thresholds of center of mass (COM) velocity and position relative to the base of support (BOS) that predict forward and backward loss of balance during walking with a range of BOS perturbations. Perturbations were modeled as sinusoidal BOS motions in the vertical or anterior-posterior direction or as sagittal rotation. The human body was modeled using a seven-link model. Forward dynamics alongside with dynamic optimization were used to find the thresholds of initial COM velocity for each initial COM position that would predict forward or backward loss of balance. The effects of perturbation frequency and amplitude on these thresholds were modeled based on the simulation data. Experimental data were collected from 15 able-bodied individuals and three individuals with disability during perturbed walking. The simulation results showed similarity with the stability region reported for slip and non-slip conditions. The feasible stability region shrank when the perturbation frequency and amplitude increased, especially for larger initial COM velocities. 89.5% (70.9%) and 82.4% (68.2%) of the measured COM position and velocity combinations during low (high) perturbations were located inside the simulated limits of the stability region, for able-bodied and disabled individuals, respectively. The simulation results demonstrated the effects of different perturbation levels on the stability region. The obtained stability region can be used for developing rehabilitative programs in interactive environments.     

Effects of perturbation magnitude on dynamic stability when walking in destabilizing environments

External perturbations applied to the walking surface or visual field can challenge an individual's ability to maintain stability during walking. Accurately quantifying and predicting changes in stability during walking will further our understanding of how individuals respond to challenges encountered during daily life and guide the development of assessments and rehabilitation interventions for individuals at increased risk of falling. This study is the first to determine how orbital and local dynamic stability metrics, including maximum Floquet multipliers and local divergence exponents, change in response to continuous mediolateral visual and surface perturbations of different amplitudes. Eleven healthy individuals walked in a fully immersive virtual environment. Participants completed two 3-min walking trials each under the following nine conditions: no perturbations, surface perturbations at each of 3 amplitudes, and visual perturbations at each of 5 amplitudes. All perturbations were applied as continuous pseudo-random oscillations. During both surface and visual perturbations, individuals were significantly more orbitally and locally unstable compared to un-perturbed walking. As walking surface perturbation amplitudes increased, individuals were more orbitally (but not locally) unstable. As visual perturbation amplitudes increased, individuals were more locally (but not orbitally) unstable between lower and higher amplitudes. Overall, these dynamic stability metrics were much less sensitive to changes in perturbation amplitudes than to differences between un-perturbed and perturbed walking, or to differences between mechanical and visual perturbations. This suggests that the type of perturbation(s) applied has a far greater impact than the magnitude of those perturbations in determining the response that will be elicited.     

Muscle pump function during locomotion: mechanical coupling of stride frequency and muscle blood flow

We imposed opposing oscillations in treadmill speed and grade on nine rats to test for direct mechanical coupling between stride frequency and hindlimb blood flow. Resting hindlimb blood flow was 15.5 +/- 1.7 ml/min. For 90 s at 7.5 m/min, rats alternated walking at -10 degrees for 10 s and +10 degrees for 10 s. This elicited oscillations in hindlimb blood flow having an amplitude of 4.1 +/- 0.5 ml/min (18% of mean flow) with a delay presumably due to metabolic vasodilation. Similar oscillations in speed (5.5-9.5 m/min) elicited oscillations in hindlimb blood flow (amplitude 3.4 +/- 0.5 ml/min, 15% of mean flow) with less of a delay, possibly due to changes in vasodilation and muscle pump function. We then simultaneously imposed these speed and grade oscillations out of phase (slow uphill, fast downhill). The rationale was that the oscillations in vasodilation evoked by the opposing oscillations in speed and grade would cancel each other, thereby testing the degree to which stride frequency affects hindlimb blood flow directly (i.e., muscle pumping). Opposing oscillations in speed and grade evoked oscillations in hindlimb blood flow having an amplitude of 3.3 +/- 0.6 ml/min (16% of mean flow) with no delay and directly in phase with the changes in speed and stride frequency. The finding that hindlimb blood flow changes directly with speed (when vasodilation caused by changes in speed and grade oppose each other) indicates that there is a direct coupling of stride frequency and hindlimb blood flow (i.e., muscle pumping).     

Energetic costs of producing muscle work and force in a cyclical human bouncing task

Muscles expend energy to perform active work during locomotion, but they may also expend significant energy to produce force, for example when tendons perform much of the work passively. The relative contributions of work and force to overall energy expenditure are unknown. We therefore measured the mechanics and energetics of a cyclical bouncing task, designed to control for work and force. We hypothesized that near bouncing resonance, little work would be performed actively by muscle, but the cyclical production of force would cost substantial metabolic energy. Human subjects (n = 9) bounced vertically about the ankles at inversely proportional frequencies (1-4 Hz) and amplitudes (15-4 mm), such that the overall rate of work performed on the body remained approximately constant (0.30 ± 0.06 W/kg), but the forces varied considerably. We used parameter identification to estimate series elasticity of the triceps surae tendon, as well as the work performed actively by muscle and passively by tendon. Net metabolic energy expenditure for bouncing at 1 Hz was 1.15 ± 0.31 W/kg, attributable mainly to active muscle work with an efficiency of 24 ± 3%. But at 3 Hz (near resonance), most of the work was performed passively, so that active muscle work could account for only 40% of the net metabolic rate of 0.76 ± 0.28 W/kg. Near resonance, a cost for cyclical force that increased with both amplitude and frequency of force accounted for at least as much of the total energy expenditure as a cost for work. Series elasticity reduces the need for active work, but energy must still be expended for force production.     

Energy expenditure during tennis play: a preliminary video analysis and metabolic model approach

The aim of this study was to estimate, using video analysis, what proportion of the total energy expenditure during a tennis match is accounted for by aerobic and anaerobic metabolism, respectively. The method proposed involved estimating the metabolic power (MP) of 5 activities, which are inherent to tennis: walking, running, hitting the ball, serving, and sitting down to rest. The energy expenditure concerned was calculated by sequencing the activity by video analysis. A bioenergetic model calculated the aerobic energy expenditure (EEO2mod) in terms of MP, and the anaerobic energy expenditure was calculated by subtracting this (MP - EEO2mod). Eight tennis players took part in the experiment as subjects (mean ± SD: age 25.2 ± 1.9 years, weight 79.3 ± 10.8 kg, VO2max 54.4 ± 5.1 ml·kg(-1)·min(-1)). The players started off by participating in 2 games while wearing the K4b2, with their activity profile measured by the video analysis system, and then by playing a set without equipment but with video analysis. There was no significant difference between calculated and measured oxygen consumptions over the 16 games (p = 0.763), and these data were strongly related (r = 0.93, p < 0.0001). The EEO2mod was quite weak over all the games (49.4 ± 4.8% VO2max), whereas the MP during points was up to 2 or 3 times the VO2max. Anaerobic metabolism reached 32% of the total energy expenditure across all the games 67% for points and 95% for hitting the ball. This method provided a good estimation of aerobic energy expenditure and made it possible to calculate the anaerobic energy expenditure. This could make it possible to estimate the metabolic intensity of training sessions and matches using video analysis.     

Perturbations both trigger and delay seizures due to generic properties of slow-fast relaxation oscillators

The mechanisms underlying the emergence of seizures are one of the most important unresolved issues in epilepsy research. In this paper, we study how perturbations, exogenous or endogenous, may promote or delay seizure emergence. To this aim, due to the increasingly adopted view of epileptic dynamics in terms of slow-fast systems, we perform a theoretical analysis of the phase response of a generic relaxation oscillator. As relaxation oscillators are effectively bistable systems at the fast time scale, it is intuitive that perturbations of the non-seizing state with a suitable direction and amplitude may cause an immediate transition to seizure. By contrast, and perhaps less intuitively, smaller amplitude perturbations have been found to delay the spontaneous seizure initiation. By studying the isochrons of relaxation oscillators, we show that this is a generic phenomenon, with the size of such delay depending on the slow flow component. Therefore, depending on perturbation amplitudes, frequency and timing, a train of perturbations causes an occurrence increase, decrease or complete suppression of seizures. This dependence lends itself to analysis and mechanistic understanding through methods outlined in this paper. We illustrate this methodology by computing the isochrons, phase response curves and the response to perturbations in several epileptic models possessing different slow vector fields. While our theoretical results are applicable to any planar relaxation oscillator, in the motivating context of epilepsy they elucidate mechanisms of triggering and abating seizures, thus suggesting stimulation strategies with effects ranging from mere delaying to full suppression of seizures.     

Two Independent Contributions to Step Variability during Over-Ground Human Walking

Human walking exhibits small variations in both step length and step width, some of which may be related to active balance control. Lateral balance is thought to require integrative sensorimotor control through adjustment of step width rather than length, contributing to greater variability in step width. Here we propose that step length variations are largely explained by the typical human preference for step length to increase with walking speed, which itself normally exhibits some slow and spontaneous fluctuation. In contrast, step width variations should have little relation to speed if they are produced more for lateral balance. As a test, we examined hundreds of overground walking steps by healthy young adults (N = 14, age < 40 yrs.). We found that slow fluctuations in self-selected walking speed (2.3% coefficient of variation) could explain most of the variance in step length (59%, P < 0.01). The residual variability not explained by speed was small (1.5% coefficient of variation), suggesting that step length is actually quite precise if not for the slow speed fluctuations. Step width varied over faster time scales and was independent of speed fluctuations, with variance 4.3 times greater than that for step length (P < 0.01) after accounting for the speed effect. That difference was further magnified by walking with eyes closed, which appears detrimental to control of lateral balance. Humans appear to modulate fore-aft foot placement in precise accordance with slow fluctuations in walking speed, whereas the variability of lateral foot placement appears more closely related to balance. Step variability is separable in both direction and time scale into balance- and speed-related components. The separation of factors not related to balance may reveal which aspects of walking are most critical for the nervous system to control.     

The energy expenditure of snowshoeing in packed vs. unpacked snow at low-level walking speeds

Snowshoeing is currently ranked as one of the top 20 participatory sports in the United States, and the number of participants almost tripled, from 440,000 to 1.2 million in 1998. Despite this large increase in participation, no scientific evidence exists to quantify any physiologic response to the activity. Therefore, the purpose of this investigation was to assess the energy expenditure of snowshoeing at selected low-level speeds and evaluate its acceptability as a form of aerobic conditioning exercise. Ten habitually active subjects (7 men, 3 women, mean age = 24 +/- 3.9 years, mass = 76.6 +/- 14.5 kg, height = 173.7 +/- 9.6 cm) were recruited. Steady state heart rate data were determined from 2 treadmill tests at 4 and 6 mph. Steady state heart rates at 4 mph and 6 mph from treadmill speeds were then reproduced outdoors under 2 snow conditions, packed, and unpacked snow, while caloric expenditure and speed were determined. Expired gases were collected in Douglas bags for both snowshoe and treadmill trials and then analyzed and corrected indoors for the fractional concentrations of carbon dioxide and oxygen. Data analyses indicate that caloric expenditure during snowshoeing may be considerably higher than previously reported. Snowshoeing on packed snow at 2.95 mph elicited a similar heart rate and energy expenditure response as walking on a treadmill at 4 mph or snowshoeing in unpacked snow at 2.04 mph (Vo(2) = 18.18 +/- 0.8 ml x kg(-1) x min(-1)). Snowshoeing on packed snow at 3.97 mph elicited the same heart rate and energy expenditure response as walking on a treadmill at 6 mph or snowshoeing on unpacked snow at 2.87 mph (Vo(2) = 36.72 +/- 0.8 ml x kg(-1) x min(-1)). Furthermore, increasing walking speed on snow by just 1 mph at slow speeds (2 and 3 mph) resulted in approximately twice the energy expenditure. Our data indicate that current estimates of energy expenditure while snowshoeing underestimate by greater than 50%. Apparently the energy expenditure during snowshoeing is much higher than previously considered and varies considerably because of snow terrain. Furthermore, energy expenditure levels similar to walking can be achieved on snowshoes at much slower speeds. This study represents an original investigation into energy expenditure during snowshoeing.     

Energy optimization is a major objective in the real-time control of step width in human walking

People prefer to move in energetically optimal ways during walking. We recently found that this preference arises not just through evolution and development, but that the nervous system will continuously optimize step frequency in response to new energetic cost landscapes. Here we tested whether energy optimization is also a major objective in the nervous system’s real-time control of step width using a device that can reshape the relationship between step width and energetic cost, shifting people’s energy optimal step width. We accomplished this by changing the walking incline to apply an energetic penalty as a function of step width. We found that people didn’t spontaneously initiate energy optimization, but instead required experience with a lower energetic cost step width. After initiating optimization, people adapted, on average, 3.5 standard deviations of their natural step width variability towards the new energy optimal width. Within hundreds of steps, they updated this as their new preferred width and rapidly returned to it when perturbed away. This new preferred width reduced energetic cost by roughly 14%, however, it was slightly narrower than the energetically optimal width, possibly due to non-energy objectives that may contribute to the nervous system’s control of step width. Collectively, these findings suggest that the nervous systems of able-bodied people can continuously optimize energetic cost to determine preferred step width.     

Amplitude and phasing of trunk motion is critical for the efficacy of gait training aimed at reducing ambulatory loads at the knee

The purpose of this study was to determine the contribution of changes in amplitude and phasing of medio-lateral trunk sway to a change in the knee adduction moment when walking with increased medio-lateral trunk sway. Kinematic and kinetic data of walking trials with normal and with increased trunk sway were collected for 19 healthy volunteers using a standard motion analysis system. The relationship between the change in first peak knee adduction moment (ΔKAM) and change in trunk sway amplitude (ΔSA; difference between maximum contralateral trunk lean and maximum ipsilateral trunk lean) and phasing (SP; time of heel-strike relative to time of maximum contralateral and time of maximum ipsilateral trunk lean) was determined using nonlinear regression analysis. On average, subjects increased their SA by 9.7?±?3.6 deg (P?相似文献   

An effective balancing response to lateral perturbations at pelvis level during slow walking requires control in all three planes of motion

In this study we investigated balancing responses to lateral perturbations during slow walking (0.85 m/s). A group of seven healthy individuals walked on an instrumented treadmill while being perturbed at the level of waist at left heel strike in outward and inward lateral directions. Centre of mass (COM) and centre of pressure (COP), rotation of pelvis around vertical axis, step lengths, step widths and step times were assessed. The results have shown that beside control of COP in lateral direction, facilitated by adequate step widths, control of COP in sagittal direction, slowing down movement of COM was present after commencement of lateral perturbations. Sagittal component of COM was significantly retarded as compared to unperturbed walking for both inward (4.32 ± 1.29 cm) and outward (9.75 ± 2.17 cm) perturbations. This was necessary since after an inward perturbation first step length (0.29 ± 0.04 m compared to 0.52 ± 0.02 m in unperturbed walking) and step time (0.45 ± 0.05 s compared to 0.61 ± 0.04 s in unperturbed walking) were shortened while after an outward perturbation first two step lengths (0.36 ± 0.05 m and 0.32 ± 0.11 m compared to 0.52 ± 0.03 m in unperturbed walking) were shortened that needed to be accommodated by the described modulation of COP in sagittal plane. In addition pronounced pelvis rotation assisted in bringing swing leg to new location. The results of this study show that counteracting lateral perturbations at slow walking requires adequate response in all three planes of motion.     

Old men running: mechanical work and elastic bounce

It is known that muscular force is reduced in old age. We investigate what are the effects of this phenomenon on the mechanics of running. We hypothesized that the deficit in force would result in a lower push, causing reduced amplitude of the vertical oscillation, with smaller elastic energy storage and increased step frequency. To test this hypothesis, we measured the mechanical energy of the centre of mass of the body during running in old and young subjects. The amplitude of the oscillation is indeed reduced in the old subjects, resulting in an approximately 20% smaller elastic recovery and a greater step frequency (3.7 versus 2.8 Hz, p=1.9x10(-5), at 15-17 km h(-1)). Interestingly, the greater step frequency is due to a lower aerial time, and not to a greater natural frequency of the system, which is similar in old and young subjects (3.6 versus 3.4 Hz, p=0.2). Moreover, we find that in the old subjects, the step frequency is always similar to the natural frequency, even at the highest speeds. This is at variance with young subjects who adopt a step frequency lower than the natural frequency at high speeds, to contain the aerobic energy expenditure. Finally, the external work to maintain the motion of the centre of mass is reduced in the old subjects (0.9 versus 1.2 J kg(-1) m(-1), p=5.1x10(-6)) due to the lower work done against gravity, but the higher step frequency involves a greater internal work to reset the limbs at each step. The net result is that the total work increases with speed more steeply in the old subjects than in young subjects.     

Effects of walking speed and step frequency on estimation of physical activity using accelerometers

This study evaluated the accuracy of assessing step counts and energy costs under walking conditions altered by step frequency changes at given speeds using uni- (LC) and tri-axial accelerometers (AM, ASP). Healthy young men and women (n=18) volunteered as subjects. Nine tests were designed to manipulate three step frequencies, low (-15% of normal), normal, and high (+15%), at each walking speed (55, 75, and 95 m/min). A facemask connected to a Douglas bag was attached to subjects, who wore accelerometers around their waist. LC underestimated the step counts at normal or high step frequency at 55 m/min and AM also at all step frequencies at 55 m/min, whereas ASP did not in all trials. LC underestimated metabolic equivalents (METs) at low or normal step frequency at all walking speeds. AM underestimated METs at low step frequency at all walking speeds and at high step frequency of 95 m/min. ASP gave underestimates only at low step frequency of 95 m/min. The degree of the percentage error of METs for AM and ASP was affected by step frequency. Significant interaction between step frequency and speed was found that for LC. These results suggest that LC and AM can cause errors in step-count functions at a low walking speed. Furthermore, LC may show low accuracy of the METs measurement during walking altered according to step frequency and speed, whereas AM and ASP, which are tri-axial accelerometers, are more accurate but the degree of the percentage error is affected by step frequency.     

Dynamic stability of passive dynamic walking on an irregular surface

Falls that occur during walking are a significant health problem. One of the greatest impediments to solve this problem is that there is no single obviously "correct" way to quantify walking stability. While many people use variability as a proxy for stability, measures of variability do not quantify how the locomotor system responds to perturbations. The purpose of this study was to determine how changes in walking surface variability affect changes in both locomotor variability and stability. We modified an irreducibly simple model of walking to apply random perturbations that simulated walking over an irregular surface. Because the model's global basin of attraction remained fixed, increasing the amplitude of the applied perturbations directly increased the risk of falling in the model. We generated ten simulations of 300 consecutive strides of walking at each of six perturbation amplitudes ranging from zero (i.e., a smooth continuous surface) up to the maximum level the model could tolerate without falling over. Orbital stability defines how a system responds to small (i.e., "local") perturbations from one cycle to the next and was quantified by calculating the maximum Floquet multipliers for the model. Local stability defines how a system responds to similar perturbations in real time and was quantified by calculating short-term and long-term local exponential rates of divergence for the model. As perturbation amplitudes increased, no changes were seen in orbital stability (r(2)=2.43%; p=0.280) or long-term local instability (r(2)=1.0%; p=0.441). These measures essentially reflected the fact that the model never actually "fell" during any of our simulations. Conversely, the variability of the walker's kinematics increased exponentially (r(2)>or=99.6%; p<0.001) and short-term local instability increased linearly (r(2)=88.1%; p<0.001). These measures thus predicted the increased risk of falling exhibited by the model. For all simulated conditions, the walker remained orbitally stable, while exhibiting substantial local instability. This was because very small initial perturbations diverged away from the limit cycle, while larger initial perturbations converged toward the limit cycle. These results provide insight into how these different proposed measures of walking stability are related to each other and to risk of falling.     

Amplitude effects of medio-lateral mechanical and visual perturbations on gait

Falls during walking are a major contributor to accidental deaths and injuries that can result in debilitating hospitalization costs, lost productivity, and diminished quality of life. To reduce these losses, we must develop a more profound understanding of the characteristic responses to perturbations similar to those encountered in daily life. This study addresses this issue by building on our earlier studies that examined mechanical and visual perturbations in the same environment by applying the same continuous pseudo-random perturbations at multiple (3 mechanical, 5 visual) amplitudes. Walking variability during mechanical perturbations increased significantly with amplitude for all subjects and differences as measured by variabilities of step width, COM position, and COM velocity. These parameters were the only ones sensitive to the presence of visual perturbations, but none of them changed significantly with perturbation amplitude. Additionally, visual perturbation effects were far less consistent across participants, with several who were essentially unaffected by visual perturbations at any level. The homogeneity of the mechanical perturbation effects demonstrates that human responses to mechanical perturbations are similar because they are driven by kinetics that require similar corrections that must be made in order to maintain balance. Conversely, responses to visual perturbations are driven by the perceived need to make corrections and this perception is not accurate enough to produce amplitude-related corrections, even for a single participant, nor is this perception consistent across individuals. This latter finding is likely to be relevant to future visual perturbation studies and the diagnosis and rehabilitation of gait and balance disorders.     

Stick insects walking on a wheel: Perturbations induced by obstruction of leg protraction

Summary Stick insects (Carausius morosus) walking on a wheel were perturbed by restricting the forward protraction of individual legs. A barrier placed before a single middle or rear leg prevented that leg from reaching its normal protraction endpoint but allowed it unimpeded retraction. Upon striking the barrier, the protracting leg attempted to get past it and thereby prolonged protraction. This prolongation increased with the extent to which the obstruction infringed upon the leg's normal step range. Barriers placed near the midpoint of this range elicited large perturbations: the blocked leg often continued its protraction throughout many step cycles of the other legs (Fig. 1 E, F). For the most part walking was irregular and smooth forward progression was disrupted. Nevertheless, the infrequent steps by the affected leg usually were coordinated with those of the adjacent ipsilateral legs.More rostral barrier positions elicited smaller perturbations: the blocked leg usually made one step in each step cycle of the other legs (Fig. 1 B, C, D, G). Measurements for these regular step sequences showed quantitatively that protraction duration increased in proportion to the severity of the infringement on normal leg movement (Figs. 3, 4). The fraction of the step period occupied by protraction increased from ca. 10% for normal walking to ca. 50% for caudal barrier positions. This proportionality is interpreted to show the importance of spatial components of the walking program.When one leg was obstructed, its extended protraction influenced the stepping of the three adjacent legs as follows. First, the ipsilateral rostral leg showed the largest change: its protraction onset was regularly delayed for the duration of the extended protraction (Figs. 4, 7, 8), demonstrating a strong, centrally mediated inhibition. The presence of a further delay of up to 100 to 140 ms suggests that peripheral input from the protracting leg may be important for releasing this inhibition. Second, steps by an adjacent caudal leg were not measurably affected. However, the method may not have sufficed to reveal such effects because during regular walking middle leg protractions rarely lasted long enough to conflict with subsequent steps by the ipsilateral rear leg. Third, contralateral effects differed between middle and rear leg obstructions. If the obstructed leg was a middle leg, its extended protraction had little effect upon stepping by the contralateral middle leg: the latter leg frequently protracted while the blocked leg continued its protraction and there was no consistent change in the phase relation of these two legs (Table 1). In contrast, if the obstructed leg was a rear leg, protractions by the contralateral rear leg tended to be delayed (Table 1).     

Differential regulation of sympathetic burst frequency and amplitude following acute hypoxia in humans

Current evidence suggests that the persistent sympathetic nerve activity (SNA), commonly observed after exposure to hypoxia (HX), is mediated by chemoreceptor sensitization and or baroreflex resetting. Evidence in humans and animals suggests that these reflexes may independently regulate the frequency (gating) and amplitude (neuronal recruitment) of SNA bursts. In humans (n = 7), we examined the regulation of SNA following acute isocapnic HX (5 min; end-tidal Po(2) = 45 Torr) and euoxic hypercapnia (HC; 5 min; end-tidal Pco(2) = +10 from baseline). HX increased SNA burst frequency (21 ± 7 to 28 ± 8 bursts/min, P < 0.05) and amplitude (99 ± 10 to 125 ± 19 au, P < 0.05) as did HC (14 ± 6 to 22 ± 10 bursts/min, P < 0.05 and 100 ± 12 to 133 ± 29 au, P < 0.05, respectively). Burst frequency (26 ± 7 bursts/min, P < 0.05), but not amplitude (97 ± 12 au), remained elevated 10 min post-HX. The change in burst amplitude (but not frequency) was significantly related to the measured change in ventilation (r(2) = 0.527, P < 0.001). Both frequency and amplitude decreased during recovery following HC. These data indicate the differential regulation of pattern and magnitude of sympathetic outflow in humans with sympathetic persistence following HX being specific to burst frequency and not amplitude.     

Predicting metabolic cost of level walking

Energy expenditure in walking is usually expressed as a function of walking speed. However, this relationship applies only to freely adopted step length-step rate patterns. Both the step length and the step rate must be used to preduct the energy expenditure for any combination of step length and step rate. Evidence on seven subjects indicates that the energy demand for such a combination can be determined by conducting two experiments. In the first, the subject is allowed to freely choose his own walking pattern to achieve a set of prescribed speeds. In the second, the speed is kept constant but the subject is forced to adopt a range of prescribed step rates. The results of the two experiments combined yield enough data to make possible the determination of the energy equation of the pattern, encompassing both "free" and "forced" gaits. Results show that the freely chosen step rate requires the least oxygen consumption at any given speed. Any other forced step rate at the same speed increases the oxygen cost over that required for the "free" step rate.