Airway caliber and the work of breathing in humans

Mechanical work rate of breathing was measured in five normal subjects during voluntary eucapnic hyperventilation at rates of approximately 10, 20, 40, 60, and 80 l/min before and after inhalation of 1 mg of ipratropium bromide, an anticholinergic agent. Chest wall recoil pressure was measured over a range of lung volumes in each subject and was used as the reference pressure in the calculation of work rate. There was little change in elastic or resistive work rate at rest when vagal tone was reduced by ipratropium. The mean work at 40, 60, and 80 l/min was 8.9, 17.2, and 34.0 cmH2O.l-1.s before and 5.6, 12.4 and 25.8 cmH2O.l-1.s after ipratropium. This suggests that vagal tone significantly influences the work of breathing at high ventilatory rates, such as occur during strenuous exercise.     

Lactate accumulation during incremental exercise with varied inspired oxygen fractions

Six subjects pedaled a stationary cycle ergometer to exhaustion on three separate occasions while breathing gas mixtures of 17, 21, or 60% O2 in N2. Each subject rode for 3 min at work rates of 60, 90, 105 W, followed by 15-W increases every 3 min until exhaustion. Inspired and expired gas fractions, ventilation (V), heart rate, and blood lactate were measured. O2 uptake (VO2) and CO2 output (VCO2) were calculated for the last minute of each work rate; blood samples were drawn during the last 5 s. "Break points" for lactate, V, VCO2, V/VO2, and expired oxygen fraction (FEO2) were mathematically determined. VO2 was not significantly different at any work rate among the three different conditions. Nor did maximal VO2 differ significantly among the three treatments (P greater than 0.05). Lactate concentrations were significantly lower during hyperoxia and significantly higher during hypoxia compared with normoxia. Lactate values at exhaustion were not significantly different among the three treatments. Four subjects were able to work for a longer period of time during hyperoxic breathing. The variations in lactate accumulation as reported in this study cannot be explained on the basis of differences in VO2. The results of this research lend support to the hypothesis that differences in the performance of subjects breathing altered fractions of inspired oxygen may be caused by differences in lactate (or H+) accumulation.     

Diurnal variations in cycling kinematics

Physiological and biomechanical constraints as well as their fluctuations throughout the day must be considered when studying determinant factors in the preferred pedaling rate of elite cyclists. The aim of this study was to monitor the diurnal variation of spontaneous pedaling rate and movement kinematics over the crank cycle. Twelve male competitive cyclists performed a submaximal exercise on a cycle ergometer for 15 min at 50% of their W(max). Two test sessions were performed at 06:00 and 18:00 h on two separate days to assess diurnal variation in the study variables. For each test session, the exercise bout was divided into three equivalent 5-min periods during which subjects were requested to use different pedal rates (spontaneous cadence, 70 and 90 rev min(-1)). Pedal rate and kinematics data (instantaneous pedal velocity and angle of the ankle) were collected. The results show a higher spontaneous pedal rate in the late afternoon than in the early morning (p < 0.001). For a given pedal rate condition, there was a less variation in pedal velocity during a crank cycle in the morning than in the late afternoon. Moreover, diurnal variations were observed in ankle mobility across the crank cycle, the mean plantar flexion observed throughout the crank cycle being greater in the 18:00 h test session (p < 0.001). These results suggest that muscular activation patterns during a cyclical movement could be under the influence of circadian fluctuations.     

Tracking the scapula using the scapula locator with and without feedback from pressure-sensors: A comparative study

BackgroundThe scapula locator method has associated intra-observer and inter-observer errors caused by the dependency on the observer to locate the scapular landmarks. The potential effect of the pressures applied by the observer on the measured scapular kinematics when this method is used has also been overlooked so far. The aim of this study was to investigate the effect of using feedback on the pressures applied on the scapula using the locator on the intra-observer and inter-observer reliabilities of the method as well as on the kinematics obtained using this method.MethodsThree observers tracked the scapular motion of the dominant shoulder of each subject using the locator with no reference to pressure-feedback for three trials of bilateral elevation in the scapular plane and using the locator with pressure-feedback for three other trials. Variations between the measurements obtained were used to calculate the intra-observer errors and variations between the measurements obtained by the three observers for the same subject were used to calculate inter-observer errors. Repeated-measures ANOVA tests were used to look at differences between the two methods in terms of intra-observer and inter-observer errors and scapular kinematics.FindingsUsing pressure-feedback reduced the intra-observer errors but had no effect on the inter-observer errors. Different scapular kinematics was measured using the two methods.InterpretationsPressure-feedback improves the reliability of the scapula locator method. Differences in the scapular kinematics suggest that unregulated pressures have an effect on the physiological scapular motion.     

Diurnal Variations in Cycling Kinematics

Physiological and biomechanical constraints as well as their fluctuations throughout the day must be considered when studying determinant factors in the preferred pedaling rate of elite cyclists. The aim of this study was to monitor the diurnal variation of spontaneous pedaling rate and movement kinematics over the crank cycle. Twelve male competitive cyclists performed a submaximal exercise on a cycle ergometer for 15 min at 50% of their W_max. Two test sessions were performed at 06:00 and 18:00 h on two separate days to assess diurnal variation in the study variables. For each test session, the exercise bout was divided into three equivalent 5‐min periods during which subjects were requested to use different pedal rates (spontaneous cadence, 70 and 90 rev min^?1). Pedal rate and kinematics data (instantaneous pedal velocity and angle of the ankle) were collected. The results show a higher spontaneous pedal rate in the late afternoon than in the early morning (p < 0.001). For a given pedal rate condition, there was a less variation in pedal velocity during a crank cycle in the morning than in the late afternoon. Moreover, diurnal variations were observed in ankle mobility across the crank cycle, the mean plantar flexion observed throughout the crank cycle being greater in the 18:00 h test session (p < 0.001). These results suggest that muscular activation patterns during a cyclical movement could be under the influence of circadian fluctuations.     

Sensitivity of tibio-menisco-femoral joint contact behavior to variations in knee kinematics

Use of computational models with kinematic boundary conditions to study the knee joint contact behavior for normal and pathologic knee joints depends on an understanding of the impacts of kinematic uncertainty. We studied the sensitivities of tibio-menisco-femoral joint contact behavior to variations in knee kinematics using a finite element model (FEM) with geometry and kinematic boundary conditions derived from sequences of magnetic resonance (MR) images. The MR images were taken before and after axial compression was applied to the knee joint of a healthy subject. A design of experiments approach was used to study the impact of the variation in knee kinematics on the contact outputs. We also explored the feasibility of using supplementary hip images to improve the accuracy of knee kinematics. Variations in knee kinematics (0.25mm in medial-lateral, 0.1mm in anterior-posterior and superior-inferior translations, and 0.1 degrees in flexion-extension and varus-valgus, 0.25 degrees in external-internal rotations) caused large variations in joint contact behavior. When kinematic boundary conditions resulted in close approximations of the model-predicted joint contact force to the applied force, variations in predictions of contact parameters were also reduced. The combination of inferior-superior and medial-lateral translations accounted for over 70% of variations for all the contact parameters examined. The inclusion of hip images in kinematic calculations improved knee kinematics by matching the femoral head position. Our findings demonstrate the importance of improving the accuracy and precision of knee kinematic measurements, especially when utilized as an input for finite element models.     

Inter-individual variation in vertebral kinematics affects predictions of neck musculoskeletal models

Experimental studies have found significant variation in cervical intervertebral kinematics (IVK) among healthy subjects, but the effect of this variation on biomechanical properties, such as neck strength, has not been explored. The goal of this study was to quantify variation in model predictions of extension strength, flexion strength and gravitational demand (the ratio of gravitational load from the weight of the head to neck muscle extension strength), due to inter-subject variation in IVK. IVK were measured from sagittal radiographs of 24 subjects (14F, 10M) in five postures: maximal extension, mid-extension, neutral, mid-flexion, and maximal flexion. IVK were defined by the position (anterior-posterior and superior-inferior) of each cervical vertebra with respect to T1 and its angle with respect to horizontal, and fit with a cubic polynomial over the range of motion. The IVK of each subject were scaled and incorporated into musculoskeletal models to create models that were identical in muscle force- and moment-generating properties but had subject-specific kinematics. The effect of inter-subject variation in IVK was quantified using the coefficient of variation (COV), the ratio of the standard deviation to the mean. COV of extension strength ranged from 8% to 15% over the range of motion, but COV of flexion strength was 20–80%. Moreover, the COV of gravitational demand was 80–90%, because the gravitational demand is affected by head position as well as neck strength. These results indicate that including inter-individual variation in models is important for evaluating neck musculoskeletal biomechanical properties.     

Serum leptin concentrations in response to acute exercise in postmenopausal women with and without hormone replacement therapy.

The purpose of the study was to examine the effects of acute exercise and hormone replacement therapy on serum leptin concentrations in postmenopausal women. Subjects were 15 healthy, postmenopausal women, 8 on hormone replacement therapy (HRT) and 7 not on hormone replacement therapy (NHRT). Group comparisons indicated no significant differences between HRT and NHRT groups with respect to age, height, weight, BMI, sum of skinfolds, or VO2max, and verified significant differences in estradiol and FSH concentrations. After an overnight fast, each subject completed 30 min of treadmill exercise at approximately 80% VO2max. Over 2 hr and 10 min, baseline, exercise, and recovery blood samples were collected from an intravenous catheter. A control session conducted a month later consisted of the same blood sampling protocol without exercise. Leptin concentrations declined significantly over the course of both the exercise and control sessions, gradually decreasing from baseline levels to -1.54 +/- 0.49 ng. ml-1 postexercise, and continuing to decline to a low of -2.89 +/- 0.59 ng. ml-1 at the end of the session. There was no significant difference between groups with respect to this decline. This is the first study to document that diurnal changes in leptin concentrations in postmenopausal women are not altered by acute treadmill exercise or HRT status. The study underscores the need to account for a diurnal reduction in leptin over the course of an exercise trial.     

Finger joint impedance during tapping on a computer keyswitch

We studied the dynamic behavior of finger joints during the contact period of tapping on a computer keyswitch, to characterize and parameterize joint function with a lumped-parameter impedance model. We tested the hypothesis that the metacarpophalangeal (MCP) and interphalangeal (IP) joints act similarly in terms of kinematics, torque, and energy production when tapping. Fifteen human subjects tapped with the index finger of the right hand on a computer keyswitch mounted on a two-axis force sensor, which measured forces in the vertical and sagittal planes. Miniature fiber-optic goniometers mounted across the dorsal side of each joint measured joint kinematics. Joint torques were calculated from endpoint forces and joint kinematics using an inverse dynamic algorithm. For each joint, a linear spring and damper model was fitted to joint torque, position, and velocity during the contact period of each tap (22 per subject on average). The spring-damper model could account for over 90% of the variance in torque when loading and unloading portions of the contact were separated, with model parameters comparable to those previously measured during isometric loading of the finger. The finger joints functioned differently, as illustrated by energy production during the contact period. During the loading phase of contact the MCP joint flexed and produced energy, whereas the proximal and distal IP joints extended and absorbed energy. These results suggest that the MCP joint does work on the interphalangeal joints as well as on the keyswitch.     

Perturbation-dependent selection of postural feedback gain and its scaling

In this study we examined whether the selection of postural feedback gain and its scaling is dependent on perturbation type. We compare forward pushes applied to the back of a standing subject to previous work on responses to support translation. As was done in the previous work, we quantified the subject's response in terms of perturbation-dependent feedback gains. Seven healthy young subjects (25±3 yr) experienced five different magnitudes of forward push applied by a 1.25 m-long pendulum falling from the height of 1.4m toward the center of mass of the subject's torso. The loads on the pendulum ranged from 2 to 10 kg. Impulsive force, ground reaction forces and joint kinematics were measured, and joint torques were calculated from inverse dynamics. A full-state feedback control model was used to quantify the empirical data, and the feedback gains that minimized the fitting error between the data and model simulation were identified. As in previously published feedback gains for support translation trials, gradual gain scaling with push perturbation magnitude was consistently observed, but a different feedback gain set was obtained. The results imply that the nervous system may be aware of body dynamics being subjected to various perturbation types and may select perturbation-dependent postural feedback gains that satisfy postural stability and feasible joint torque constraints.     

Fatigue effects on bar kinematics during the bench press

The bench press is one of the most popular weight training exercises. Although most training regimens incorporate multiple repetition sets, there are few data describing how the kinematics of a lift change during a set to failure. To examine these changes, recreational lifters (10 men and 8 women) were recruited. The maximum weight each subject could bench press (1RM) was determined. Subjects then performed as many repetitions as possible at 75% of the 1RM load. Three-dimensional kinematic data were recorded and analyzed for all lifts. Statistical analysis revealed that differences between maximal and submaximal lifts and the kinematics of a submaximal lift change as a subject approaches failure in a set. The time to lift the bar more than doubled from the first to the last repetition, causing a decrease in both mean and peak upward velocity. Furthermore, the peak upward velocity occurred much earlier in the lift phase in these later repetitions. The path the bar followed also changed, with subjects keeping the bar more directly over the shoulder during the lift. In general, most of the kinematic variables analyzed became more similar to those of the maximal lift as the subjects progressed through the set, but there was considerable variation between subjects as to which repetition was most like the maximal lift. This study shows that there are definite changes in the lifting kinematics in recreational lifters during a set to failure and suggests it may be particularly important for coaches and less-skilled lifters to focus on developing the proper bar path, rather than reaching momentary muscular failure, in the early part of a training program.     

Seasonal variations of physiological characteristics and thermal sensation under identical thermal conditions

Seasonal variations of human thermal characteristics were inspected in thermal comfort and when constantly indoors. Metabolic rate, tympanic temperature, skin temperature, body fat, body weight and thermal sensation were measured under identical thermal conditions in a chamber over the course of one year. Experiments were carried out for each subject in both summer and winter. Six subjects were measured 35 times in summer and 45 times in winter. one subject was measured weekly for 14 months. Measurements for analyses were taken 40-60 min after entrance into the chamber. Results revealed the following. 1) For all subjects, the metabolic rate, tympanic temperature and body fat were lower in summer than in winter; thigh skin temperatures were higher in summer than in winter. The averaged individual ratio of seasonal difference was 11.9% for metabolic rate, 14.9% for body fat, 1.8% for thigh temperature and 0.53% for tympanic temperature. Seasonal differences of about 10% in metabolic rate were maintained in this study. 2) Seasonal variations of the variables were examined for phase relationships against the outdoor temperature. 2-1) Metabolic rate, thermal sensation, body weight and body fat changed in reverse phase, whereas skin temperature was in-phase. 2-2) Skin temperature lagged by about one month in both summer and winter. Body fat also lagged by about one month in summer, but corresponded to the phase in winter. Metabolic rates were also in-phase in winter but led about three months in summer. Thermal sensations lagged by about three months in winter but were in-phase in summer. Body weight was in-phase in summer and winter. 2-3) Summer disorders were observed particularly in seasonal variations of metabolic rates, tympanic temperature, skin temperatures, and thermal sensation, thereby suggesting that the effect of temperature exposure was altered by air-conditioner use.     

Biomechanics of overground vs. treadmill walking in healthy individuals.

The goal of this study was to compare treadmill walking with overground walking in healthy subjects with no known gait disorders. Nineteen subjects were tested, where each subject walked on a split-belt instrumented treadmill as well as over a smooth, flat surface. Comparisons between walking conditions were made for temporal gait parameters such as step length and cadence, leg kinematics, joint moments and powers, and muscle activity. Overall, very few differences were found in temporal gait parameters or leg kinematics between treadmill and overground walking. Conversely, sagittal plane joint moments were found to be quite different, where during treadmill walking trials, subjects demonstrated less dorsiflexor moments, less knee extensor moments, and greater hip extensor moments. Joint powers in the sagittal plane were found to be similar at the ankle but quite different at the knee and hip joints. Differences in muscle activity were observed between the two walking modalities, particularly in the tibialis anterior throughout stance, and in the hamstrings, vastus medialis and adductor longus during swing. While differences were observed in muscle activation patterns, joint moments and joint powers between the two walking modalities, the overall patterns in these behaviors were quite similar. From a therapeutic perspective, this suggests that training individuals with neurological injuries on a treadmill appears to be justified.     

Blood lactate responses in older swimmers during active and passive recovery following maximal sprint swimming

The purpose of this study was to determine the effect of age on three blood lactate parameters following maximal sprint swimming. The parameters examined were maximal blood lactate concentration, time to reach maximal blood lactate concentration, and half recovery time to baseline lactate concentration. These parameters were examined in 16 male competitive masters swimmers (n = 4 for each age group: 25-35, 36-45, 46-55, and 56 plus years) during both passive and active recovery following a maximal 100 m freestyle sprint. Passive recovery consisted of 60 min sitting in a comfortable chair and active recovery consisted of a 20-min swim at a self-selected pace. Capillary blood samples were obtained every 2 min up to 10 min of recovery then at regular intervals to the end of the recovery period. Curves of blood lactate concentration against time were drawn and the three parameters determined for each condition for each subject. There were no significant differences between age groups in any of the lactate parameters examined. A significant difference (P less than 0.05) was noted in each of the parameters between active and passive recovery over all age groups. As expected, active recovery produced lower maximal blood lactate concentrations, lower time to maximal blood lactate values, and lower half recovery times. These data suggest that intensive swimming training may prevent or delay the decline with age in the physiological factors affecting blood lactate values following a maximal sprint swim. Older sprint swimmers appeared to be capable of producing and removing lactic acid at the same rate as younger swimmers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Wheelchair propulsion: Force orientation and amplitude prediction with Recurrent Neural Network

The aim of this study was to use Recurrent Neural Network (RNN) to predict the orientation and amplitude of the applied force during the push phase of manual wheelchair propulsion.Trunk and the right-upper limb kinematics data were assessed with an optoeletronic device (Qualisys) and the force applied on the handrim was recorded with an instrumented wheel (SMARTWheel®). Data acquisitions were performed at 60/80/10/120/140% of the freely chosen frequency at submaximal and maximal conditions. The final database consisted of d = 5708 push phases.The input data were the trunk and right upper-limb kinematics (joint angle, angular velocity and acceleration) and anthropometric data (height, weight, segment length) and the output data were the applied forces orientation and amplitude. A ratio of 70/15/15 was used to train, validate and test the RNN (d_train = 3996, d_validation = 856 and d_test = 856). The angle and amplitude errors between the measured and predicted force was assessed from d_test.Results showed that for most of the push phase (∼70%), the force direction prediction errors were less than 12°. The mean absolute amplitude errors were less than 8 N and the mean absolute amplitude percentage errors were less than 20% for most of the push phase (∼80%).     

Pressure-time product, flow, and oxygen cost of resistive breathing in humans

We examined the relationship between the pressure-time product (Pdt) of the inspiratory muscles and the O2 cost of breathing (VO2 resp) in five normal subjects breathing through an external inspiratory resistance with a tidal volume of 800 ml at a constant end-expiratory lung volume [functional residual capacity, (FRC)]. Each subject performed 30-40 runs, each of approximately 30 breaths, with inspiratory flow rates ranging from 0.26 +/- 0.01 to 0.89 +/- 0.04 l/s (means +/- SE) and inspiratory mouth pressures ranging from 10 +/- 1 to 68 +/- 4% of the maximum inspiratory pressure at FRC. In all subjects VO2 resp was linearly related to Pdt when mean inspiratory flow (VI) was constant, but the slope of this relationship increased with increasing VI. Therefore, Pdt is an accurate index of VO2 resp only when VI is constant. There was a linear relationship between the VO2 resp and the work rate across the external resistance (W) for all runs in each subject over the range of W 10 +/- 1 to 137 +/- 21 J/min. Thus, at a constant tidal volume the VO2 resp was related to the mean inspiratory pressure, independent of flow or inspiratory duration. If the VO2 resp were determined mainly during inspiration, then for a given rate of external work or O2 consumption, VI would be inversely related to mean inspiratory pressure. Efficiency (E) was 2.1 +/- 0.2% and constant over a large range of VI, pressure, work rate, or resistance and was not altered by the presence of a potentially fatiguing load. The constant E over such a wide range of conditions implies a complex integration of the recruitment, mechanical function, and energy consumption of the muscles utilized in breathing.     

A comparison of static and dynamic optimization muscle force predictions during wheelchair propulsion

The primary purpose of this study was to compare static and dynamic optimization muscle force and work predictions during the push phase of wheelchair propulsion. A secondary purpose was to compare the differences in predicted shoulder and elbow kinetics and kinematics and handrim forces. The forward dynamics simulation minimized differences between simulated and experimental data (obtained from 10 manual wheelchair users) and muscle co-contraction. For direct comparison between models, the shoulder and elbow muscle moment arms and net joint moments from the dynamic optimization were used as inputs into the static optimization routine. RMS errors between model predictions were calculated to quantify model agreement. There was a wide range of individual muscle force agreement that spanned from poor (26.4% F_max error in the middle deltoid) to good (6.4% F_max error in the anterior deltoid) in the prime movers of the shoulder. The predicted muscle forces from the static optimization were sufficient to create the appropriate motion and joint moments at the shoulder for the push phase of wheelchair propulsion, but showed deviations in the elbow moment, pronation–supination motion and hand rim forces. These results suggest the static approach does not produce results similar enough to be a replacement for forward dynamics simulations, and care should be taken in choosing the appropriate method for a specific task and set of constraints. Dynamic optimization modeling approaches may be required for motions that are greatly influenced by muscle activation dynamics or that require significant co-contraction.     

Three-dimensional kinematic analysis of the snatch technique for lifting different barbell weights

The purpose of this study was to investigate the effects of increased barbell loads on barbell and body kinematics of the snatch lifts at 60, 80, and 100% of 1 repetition maximum and to evaluate the biomechanics of snatch technique. The study was performed on 7 elite male weightlifters of the Turkish national team. Four cameras operating at 50 fields per second were used to record the lifts. For 3D kinematic analysis of center of gravity (CG) and barbell movement, the points on the body and the barbell were digitized by using an Ariel Performance Analysis System. There were significant differences between the vertical work values (p < 0.05). The power values of the 3 snatch lifts were also found to be significantly different (p < 0.05). Another significant difference (p < 0.05) was observed between maximum vertical displacement of the barbell, maximum vertical velocity of the barbell, maximum vertical displacement of CG, the vertical velocity of CG during the turnover under the barbell. The results demonstrated that vertical and horizontal kinematics of the barbell and body decreased at the pull phase of the snatch technique as the barbell load increased. The power output during the second pull increased although the work done did not change, whereas work and power output increased during the first pull phase depending on the increase in the barbell weight. The finding of this study suggested that weightlifters had to perform the turnover under the barbell and the catch phase faster, because when the barbell weight was increased at snatch lift, vertical kinematics of the barbell decreased.     

Intensity-dependent tolerance to exercise after attaining V(O2) max in humans.

The tolerable duration of high-intensity, constant-load cycle ergometry is a hyperbolic function of power, with an asymptote termed critical power (CP) and a curvature constant (W') with units of work. It has been suggested that continued exercise after exhaustion may only be performed below CP, where predominantly aerobic energy transfer can occur and W' can be partially replenished. To test this hypothesis, six volunteers each performed cycle-ergometer exercise with breath-by-breath determination of ventilatory and pulmonary gas exchange variables. Initially, four exercise tests to exhaustion were made: 1). a ramp-incremental and 2). three high-intensity constant-load bouts at different work rates, to estimate lactate (theta(L)) and CP thresholds, W', and maximum oxygen uptake (Vo2 max). Subsequently, subjects cycled to the limit of tolerance (for approximately 360 s) on three occasions, each followed by a work rate reduction to 1). 110% CP, 2). 90% CP, and 3). 80% theta(L) for a 20-min target. W' averaged 20.9 +/- 2.35 kJ or 246 +/- 30 J/kg. After initial fatigue, 110% CP was tolerated for only 30 +/- 12 s. Each subject completed 20 min at 80% theta(L), but only two sustained 20 min at 90% CP; the remaining four subjects fatigued at 577 +/- 306 s, with oxygen consumption at 89 +/- 8% Vo2 max. The results support the suggestion that replenishing W' after fatigue necessitates a sub-CP work rate. The variation in subjects' responses during 90% CP was unexpected but consistent with mechanisms such as reduced CP consequent to prior high-intensity exercise, variation in lactate handling, and/or regional depletion of energy substrates, e.g., muscle glycogen.