Increased Duty Factor Research Articles

Exploring running styles in the field through cadence and duty factor modulation

According to the dual-axis model, running styles can be defined by cadence and duty factor, variables that have been associated with running performance, economy and injury risk. To guide runners in exploring different running styles, effective instructions to modulate cadence and duty factor are needed. Such instructions have been established for treadmill running, but not for overground running, during which speed can be varied. In this study, five participants completed eight field training sessions over a 4-week training period with acoustic instructions to modulate cadence, duty factor, and, in combination, running style. Instructions were provided via audio files. Running data were collected with sports watches. Participants’ experiences with guided-exploration training were evaluated with the user experience questionnaire. Data analysis revealed acoustic pacing and verbal instructions to be effective in respectively modulating cadence and duty factor, albeit with co-varying effects on speed and the non-targeted variable (i.e. duty factor or cadence). Combining acoustic pacing and verbal instructions mitigated these co-varying effects considerably, allowing for running-style modulations in intended directions (particularly towards the styles with increased cadence and increased duty factor). User experience of this form of guided-exploration training was overall positive, but could be improved in terms of autonomy (dependability). In conclusion, combining acoustic pacing and verbal instructions for running-style modulation is effective in overground running.

Effects of particle size and thickness of quartz sand on the webbed foot kinematics of mallard (Anas platyrhynchos).

The webbed foot structure of mallards (Anas platyrhynchos) exhibits effective anti-subsidence properties when walking on soft ground. To investigate the effects of quartz sand particle size and thickness on joint angles and the movement patterns of webbed feet, we created a testing substrate with quartz sand and utilized high-speed cameras and kinematic analysis tools for data acquisition. Mallards mainly adjusted the tarsometatarso-phalangeal joint (TMTPJ) during touch-down and lift-off stages in response to increasing particle size or enhanced ground roughness. Conversely, adjustments to the intertarsal joint (ITJ) predominantly took place during mid-stance. Conversely, mallards predominantly adjusted the ITJ during touch-down and lift-off when coping with increased quartz sand thickness, with TMTPJ adjustments mainly occurring at touch-down. As quartz sand particle size increased, the TMTPJ angle increased, the ITJ angle decreased, toe closure advanced, and the duty factor decreased throughout the entire stride cycle. In contrast, increasing quartz sand thickness led to more delayed TMTPJ adjustments, slower webbed foot closure, and an increased duty factor throughout the stride cycle. Mallards modify their leg posture to notably decrease the touch-down foot angle upon encountering sandy terrain. This action subsequently forms a depression beneath their feet, contributing to sand consolidation and limiting flow. During the stance phase, the mallard's weight is distributed across the webbed foot, generating minimal pressure and preventing significant subsidence while walking on sandy ground.

The repeated bout effect influences lower-extremity biomechanics during a 30-min downhill run

ABSTRACT The repeated bout effect in eccentric-biased exercises is a well-known phenomenon, wherein a second bout of exercise results in attenuated strength loss and soreness compared to the first bout. We sought to determine if the repeated bout effect influences changes in lower-extremity biomechanics over the course of a 30-min downhill run. Eleven male participants completed two bouts of 30-min downhill running (DR1 and DR2) at 2.8 m.s−1 and −11.3° on an instrumented treadmill. Three-dimensional kinematics and ground reaction forces were recorded and used to quantify changes in spatiotemporal parameters, external work, leg stiffness, and lower extremity joint-quasi-stiffness throughout the 30-min run. Maximum voluntary isometric contraction (MVIC) and perceived quadriceps pain were assessed before-after, and throughout the run, respectively. DR2 resulted in attenuated loss of MVIC (P = 0.004), and perceived quadriceps pain (P < 0.001) compared to DR1. In general, participants ran with an increased duty factor towards the end of each running bout; however, increases in duty factor during DR2 (+5.4%) were less than during DR1 (+8.8%, P < 0.035). Significant reductions in leg stiffness (−11.7%, P = 0.002) and joint quasi-stiffness (up to −25.4%, all P < 0.001) were observed during DR1 but not during DR2. Furthermore, DR2 was associated with less energy absorption and energy generation than DR1 (P < 0.004). To summarize, the repeated bout effect significantly influenced lower-extremity biomechanics over the course of a downhill run. Although the mechanism(s) underlying these observations remain(s) speculative, strength loss and/or perceived muscle pain are likely to play a key role. Highlights A 30-min downhill running bout increased contact time and reduced flight time transitioning to an increased duty factor. Lower-extremity stiffness also decreased and mechanical energy absorption increased over the course of the first 30-min downhill running bout. When the same bout of 30-min downhill running was performed three weeks later, the observed changes to lower extremity biomechanics were significantly attenuated. The findings from this study demonstrated, for this first time, a repeated bout effect for lower extremity biomechanics associated with downhill running.

Are they arboreal? Climbing abilities and mechanics in the red-backed salamander (Plethodon cinereus).

While red-backed salamanders (Plethodon cinereus) are most often observed in terrestrial forested areas, several studies report arboreal substrate use and climbing behavior. However, salamanders do not have any of the anatomical features commonly observed in specialized climbing species (e.g., claws, setae, suction cups). Instead, salamanders cling to surfaces using the shear and adhesive properties of their mucous layer. In this study, we explore the capabilities and spatiotemporal gait patterns of arboreal locomotion in the red-backed salamander as they move across twelve substrate conditions ranging in diameter, orientation, and roughness. On arboreal substrates, red-backed salamanders decreased locomotor speed, stride frequency, phase and stride length, and increased duty factor and stride duration. Such responses have been observed in other non-salamander species and are posited to increase arboreal stability. Furthermore, these findings indicate that amphibian locomotion, or at least the locomotor behavior of the red-backed salamander, is not stereotyped and that some locomotor plasticity is possible in response to the demands of the external environment. However, red-backed salamanders were unable to locomote on any small-diameter or vertically-oriented coarse substrates. This finding provides strong evidence that the climbing abilities of red-backed salamanders are attributable solely to mucous adhesion and that this species is unable to produce the necessary external "gripping" forces to achieve fine-branch arboreal locomotion or scale substrates where adhesion is not possible. The red-backed salamander provides an ideal model for arboreal locomotor performance of anatomically-unspecialized amphibians and offers insight into transitionary stages in the evolution of arborealism in this lineage.

Straight Gait Research of a Small Electric Hexapod Robot

Gait is an important research content of hexapod robots. To better improve the motion performance of hexapod robots, many researchers have adopted some high-cost sensors or complex gait control algorithms. This paper studies the straight gait of a small electric hexapod robot with a low cost, which can be used in daily life. The strategy of “increasing duty factor” is put forward in the gait planning, which aims to reduce foot sliding and attitude fluctuation in robot motion. The straight gaits of the robot include tripod gait, quadrangular gait, and pentagonal gait, which can be described conveniently by discretization and a time sequence diagram. In order to facilitate the user to control the robot to achieve all kinds of motion, an online gait transformation algorithm based on the adjustment of foot positions is proposed. In addition, according to the feedback of the actual attitude information, a yaw angle correction algorithm based on kinematics analysis and PD controller is designed to reduce the motion error of the robot. The experiments show that the designed gait planning scheme and control algorithm are effective, and the robot can achieve the expected motion. The RMSE of the row, pitch, and yaw angle was reduced by 35%, 25%, and 12%, respectively, using the “increasing duty factor” strategy, and the yaw angle was limited in the range −3°~3° using the yaw angle correction algorithm. Finally, the comparison with related works and the limitations are discussed.

The effect of shoe type and fatigue on strike index and spatiotemporal parameters of running

We aimed to observe differences in running style parameters and the stride-to-stride coefficient of variation and correlative patterns using detrended fluctuation analysis (DFA) between conventional and first-time minimalistic shoe use. We also aimed to study the effect of fatigue on these parameters. 26 recreational runners were tested using a pressure insole device on a treadmill whilst wearing conventional (CONV) and minimalistic (MIN) shoes. They then performed a prolonged running bout simulating a fatiguing training session, before being tested a second time in both shoe types. Average values of strike index (initial ground contact point on the footsole expressed as a percentage of total sole length) were not significantly different between CONV [25.7±14.6% (unfatigued), 23.1±11.1% (fatigued)] and MIN [28.9±19.1% (unfatigued), 26.7±17.6% (fatigued)] (p=0.501). The fatigued state also yielded a similar strike index compared to the unfatigued state (p=0.661). An overall trend in decreased inter-stride correlative patterns of strike index was observed in MIN compared to CONV (p=0.075). No differences in contact time, flight time, stride time, duty factor, stride length and stride frequency were found between shoe types. A trend in reduced flight time (p=0.078) and therefore increased duty factor (p=0.053) was observed due to fatigue. We conclude that in recreational runners, no meaningful, acute adaptation in running style occurs as a result of first-time MIN use. Similarly, runners were able to maintain their running style after a prolonged running bout.

Proposed New Accelerator Design for Homeland Security X-Ray Applications

Two goals for security scanning of cargo and freight are the ability to determine the type of material that is being imaged, and to do so at low radiation dose. One commonly used technique to determine the effective Z of the cargo is dual-energy imaging, i.e. imaging with different x-ray energy spectra. Another technique uses the fact that the transmitted x-ray spectrum itself also depends on the effective Z. Spectroscopy is difficult because the energy of individual x rays needs to be measured in a very high count-rate environment. Typical accelerators for security applications offer large but short bursts of x-rays, suitable for current-mode integrated imaging. In order to perform x-ray spectroscopy, a new accelerator design is desired that has the following features: 1)increased duty factor in order to spread out the arrival of x-rays at the detector array over time; 2)x-ray intensitymodulation from one delivered pulse to the next by adjusting the accelerator electron beam instantaneous current so as to deliveradequate signal without saturating the spectroscopic detector; and 3)the capability to direct the (forward peaked) x-ray intensity towards high-attenuation areas in the cargo (“fan-beam-steering”). Current sources are capable of 0.1% duty factor, although usually they are operated at significantly lower duty factors (∼0.04%), but duty factors in the range 0.4-1.0% are desired. The higher duty factor can be accomplished, e.g., by moving from 300 pulses per second (pps) to 1000 pps and/or increasing the pulse duration from a typical 4μs to 10μs. This paper describes initial R&D to examine cost effective modifications that could be performed on a typical accelerator for these purposes, as well as R&D for fan-beam steering.

Electromigration damage mechanics of lead-free solder joints under pulsed DC: A computational model

A numerical method for studying migration of voids driven by pulse electric current and thermal gradient in 95.5Sn–4Ag–0.5Cu (SAC405) solder joints is developed. The theoretical model involves coupling electron wind force, chemical potential, joule heating and stress gradient driving mass diffusion processes. Entropy based damage criteria is adopted to characterize the mass transportation mechanism, which utilize irreversible entropy production rate as a measure of material degradation. The pulse current induced EM damage results were compared with DC EM response under otherwise the same conditions. It is observed that increasing duty factor, current density and frequency leads to a faster damage accumulation. A mean time to failure (MTF) equation for solder joints under pulse current loadings is proposed which incorporates both thermomigration (TM) and EM damage. The failure mechanism is verified by our experimental results. It is observed that MTF is inversely proportional to rm, fp, and jn, where duty factor exponent m equals to 1.1, frequency exponent p equals to 1.43 and current density exponent is 1.96. This equation is effective only when pulse period is below thermal relaxation time, commonly in μs range.

The metabolic cost of walking on gradients with a waddling gait

Using open-flow respirometry and video footage (25 frames s(-1)), the energy expenditure and hindlimb kinematics of barnacle geese, Branta leucopsis, were measured whilst they were exercising on a treadmill at gradients of +7 and -7 deg, and on a level surface. In agreement with previous studies, ascending a gradient incurred metabolic costs higher than those experienced on level ground at comparable speeds. The geese, however, are the first species to show an increased duty factor when ascending a gradient. This increased duty factor was accompanied by a longer stance time, which was probably to enable the additional force required for ascending to be generated. Contrary to previous findings, the geese did not experience decreased metabolic costs when descending a gradient. For a given speed, the geese took relatively shorter and quicker strides when walking downhill. This 'choppy' stride and perhaps a lack of postural plasticity (an inability to adopt a more crouched posture) may negate any energy savings gained from gravity's assistance in moving the centre of mass downhill. Also contrary to previous studies, the incremental increase in metabolic cost with increasing speed was similar for each gradient, indicating that the efficiency of locomotion (mechanical work done/chemical energy consumed) is not constant across all walking speeds. The data here suggest that there are species-specific metabolic responses to locomotion on slopes, as well as the established kinematics differences. It is likely that a suite of factors, such as ecology, posture, gait, leggedness and foot morphology, will subtly affect an organism's ability to negotiate gradients.

Kinematics of quadrupedal locomotion in sugar gliders (Petaurus breviceps): effects of age and substrate size

Arboreal mammals face unique challenges to locomotor stability. This is particularly true with respect to juveniles, who must navigate substrates similar to those traversed by adults, despite a reduced body size and neuromuscular immaturity. Kinematic differences exhibited by juveniles and adults on a given arboreal substrate could therefore be due to differences in body size relative to substrate size, to differences in neuromuscular development, or to both. We tested the effects of relative body size and age on quadrupedal kinematics in a small arboreal marsupial (the sugar glider, Petaurus breviceps; body mass range of our sample 33-97 g). Juvenile and adult P. breviceps were filmed moving across a flat board and three poles 2.5, 1.0 and 0.5 cm in diameter. Sugar gliders (regardless of age or relative speed) responded to relative decreases in substrate diameter with kinematic adjustments that promote stability; they increased duty factor, increased the average number of supporting limbs during a stride, increased relative stride length and decreased relative stride frequency. Limb phase increased when moving from the flat board to the poles, but not among poles. Compared with adults, juveniles (regardless of relative body size or speed) used lower limb phases, more pronounced limb flexion, and enhanced stability with higher duty factors and a higher average number of supporting limbs during a stride. We conclude that although substrate variation in an arboreal environment presents similar challenges to all individuals, regardless of age or absolute body size, neuromuscular immaturity confers unique problems to growing animals, requiring kinematic compensation.

Evolution of limb bone loading and body size in varanid lizards

Geometric scaling predicts that stresses on limb bones and muscles should increase with body size. Mammals counter this size-related increase in stress partially through changes in bone geometry, but largely through changes in posture, with larger species having a more erect stance. However, the ability to counter size-related stresses in this fashion may be limited to those taxa that have a parasagittal gait (such as mammals), where legs are swung underneath the body. We examined locomotor kinematics for 11 species of varanid lizards (from 0.04 to 8 kg body mass) that have a sprawling gait, to determine how they moderate size-related stresses. Posture, as indicated by femur adduction and hip heights, did not change significantly with body size, beyond that expected from geometrical scaling. Instead, lizards mitigated size-related increases in stress by increasing duty factor and possibly reducing femur rotation. Incorporating these factors in biomechanical models predicted that both bending (∝M(0.016), where M is mass) and torsional (∝M(-0.049)) stresses should be nearly independent of body size over the size range examined. However, increasing duty factor and reducing femur rotation probably have deleterious effects on speed, and this difference in kinematics with size may explain why speed scales lower for sprawling lizards than for parasagittal mammals (∝M(0.17) and ∝M(0.24), respectively). Further, paralleling conclusions for the synapsid lineage, these findings suggest that evolution from sprawling to upright posture did not occur in archosaurs as a response to larger size; rather, these archosaurs likely became upright first and larger later.

Adaptational phenomena and mechanical responses during running: effect of surface, aging and task experience

The goals of the study were to identify adaptational phenomena in running mechanics over a variety of surfaces due to age related changes in the muscle-tendon units (MTUs) capacities, to examine whether running experience is associated with adaptational effects on running mechanics over a variety of surfaces even at old age, and to investigate whether surface condition affects running mechanics. The investigation was executed on 30 old and 19 young including 29 runners and 20 non-active subjects. In a previous study we documented that the older had lower MTUs capacities. In the present study running mechanics were analysed as the same subjects ran at 2.7 m/s over three surfaces having different compliance. Surface condition did not affect centre of mass trajectory, duty factor or joint kinetics (P > 0.01). Older react to the reduced MTUs capacity by increasing duty factor and benefiting from a mechanical advantage for the triceps surae MTU and a lower rate of force generation on all surfaces (P < 0.01). Runners displayed lower average horizontal forces and a higher mechanical advantage for the quadriceps femoris MTU for all surfaces (P < 0.01). The results provided strong evidence on that running strategy remained essentially unchanged over a variety of surfaces. Adaptive improvements in running mechanics due to task experience were present for all surfaces and did not depend on age. We further concluded that older adults were able to recalibrate their running strategy to adjust the task effort to the reduced MTUs capacities in a feedforward control manner for a variety of mechanical environments.

A micromachined refreshable braille cell

A new concept for the realization of a refreshable Braille cell is presented. An electrothermally controlled microactuator that exploits the hydraulic pressure due to the volumetric expansion of melted paraffin wax is described. The paraffin wax is contained within a bulk micromachined silicon container. The container is sealed using an elastic diaphragm of silicone rubber. The container is heated using gold microheaters located on an underlying glass substrate. All the layers used to make up the containers are bonded together using an overglaze paste. The complete 3/spl times/2 dot Braille cell has air gaps between containers, to prevent unwanted actuation by means of heat leakage from adjacent containers. The prototype Braille cell measures 7/spl times/8.5/spl times/2 mm/sup 3/ and its raised dots are held in equilibrium by pulsed actuation voltages. To maintain a dot height at 50% of its maximum, a duty factor of more than 0.8 was found, with an average power of 0.30 W (PRF=0.027 Hz). The total actuation time for a dot on an up/down cycle was /spl sim/50 s. The dot height increases with an increasing duty factor with a fixed PRF, and increases with decreasing PRF with a fixed duty factor. A stable maximum dot height was achieved by reducing the cooling time.

Technical approaches for high-average-power free-electron lasers

Free-electron-laser (FEL) oscillators have only recently achieved their original promise as producers of high-power, short-wavelength, tunable radiation. Room-temperature accelerator systems have generally had limited duty factor due to excessive Ohmic losses on cavity walls. The application of superconducting radio-frequency (SRF) technology has now permitted an increase by more than two orders of magnitude in FEL average power due just to increased duty factor in continuous-wave operation. A concurrent technical development that leveraged the high efficiency of SRF linacs was the demonstration of beam energy recovery while lasing. This leads to high overall efficiency and scales favorably to systems with even higher average power. This paper will discuss the issues relating to high-average-power light sources. The planned and demonstrated performance of several FEL facilities will illustrate the sizable advantages that superconducting radio frequency offers for high average flux and output multiplexing for several simultaneous users. An important new class of light sources, energy-recovering linacs, will be introduced.

Effects of the duty factor on the surface characteristics of the plasma nitrided and diamond-like carbon coated high-speed steels

The purpose of this study is to investigate the effects of the duty factor on the surface characteristics of plasma nitrided and diamond-like carbon (DLC) coated high-speed steels. In the duplex treatment, pulsed plasma nitriding was performed with N 2 and H 2 gases at 530°C for 4 h. Duty factors were controlled within the range from 0.1 to 1 by varying both the pulse-on time and the pulse-off time. Plasma-assisted chemical vapor deposition equipment was used for the deposition of DLC films. Since the adhesion of the film to the substrate is of primary importance for the industrial application of duplex layers, the critical load was measured by the scratch test method. Properties of the nitrided and duplex treated layer were analyzed by using optical microscopy, scanning electron microscopy, X-ray diffractometry, atomic force microscopy and micro-Vickers hardness measurement. It was found that surface roughness increased with increasing duty factor for all the nitrided and duplex treated surfaces. The critical load to failure decreased with increasing substrate roughness.

Pulse train analysis in ultrasonic assisted EDM

Improved discharge characteristics with ultrasonic vibration of either electrode in EDM, are analysed for their time average effects. Records of the pulse trains obtained randomly on a transient recorder were analysed for ignition delay, microshorts, open circuit and arcing pulses and average pulse energy. Ultrasonic irradiation of the spark gap showed no significant effect on ignition delay or average pulse energy per active pulse but the inactive pulses are significantly reduced with reduced incidence of arcing. The latter would facilitate increased duty factor though in practice the machining rate showed improvement even at the same frequency and duty factor due to the improved sparking efficiency.

Materials Considerations for a High Power-Density Accelerator

A 100-mA 50-MeV H- accelerator is being designed at Los Alamos. The accelerating structures will operate at 425-MHz and will consist of a radio-frequency quadrupole (RFQ) to 2-MeV and a drift-tube linac (DTL) from 2 to 50-MeV. Design parameters have been specified to match the maximum operating capabilities of the rf system: 2-ms pulse length and 6% duty factor. The accelerating gradient in the DTL will be 4-MV/m; the maximum electric field will be approximately 1.2 times the Kilpatrick limit. These design parameters are substantially more ambitious than those of the accelerator test stand (ATS). The larger accelerating gradient and increased duty factor will increase the average power density in the DTL to approximately 25 times the design values for the ATS DTL. The increased duty factor will raise the average power density on the proposed RFQ to approximately 6 times the operating values of the ATS RFQ. The instantaneous power density on the drift tubes is predicted to be 60 W/sq cm, and the average power density is predicted to be 4 W/sq cm. These power densities are not excessive, but do represent design challenges in specific areas. Some components where thermal difficulties may be encountered include RFQ vanes, drift tubes, post couplers, slug tuners, rf seals, and tuning bars. That copper will be used on the rf-structure surfaces is not in question. Concern with residual activation and thermal management forces the investigation of materials other than the traditional carbon- and stainless-steel base materials used at Los Alamos.