1,721,049 research outputs found
Metabolic Cost Contributions of Weight and Mass in Sloped Walking
Full text linkThe metabolic power required to walk over level ground is determined by two primary mechanical tasks: body weight (BW) support and work done on the center of mass. However, it is not yet known how weight and mass contribute to metabolic power with varying uphill and downhill slopes. We hypothesized that BW and mass would each require significant, but opposing metabolic contributions to walk on uphill versus downhill slopes. We tested our hypotheses by measuring metabolic rates in 10 healthy subjects as they walked for 5 minutes under four general conditions: unaltered (UA), with reduced weight using simulated reduced gravity, added weight, and added mass alone. Participants walked under each of these conditions on level ground (0°), uphill (+3° and +6°), and downhill (-3° and -6°) slopes. We found that the percentage of net metabolic power (NMP) due to BW increased significantly from 19 ± 18.4% on level ground up to 77 ± 7.5% at +6°. Whereas the percentage of NMP due to BW, albeit not significantly different from level ground, was -5.0 ± 22.6% and 2.9 ± 37.6% at -3° and -6°, respectively. In contrast, the percentage of NMP due to mass was 29 ± 14.3% on level ground, 18 ± 12.2% at +6°, and 44 ± 17.0% at -6°. In summary, we found that at steeper uphill slopes only, the percentage of NMP due to BW significantly increased. However, the percentage of NMP due to mass was not significantly different at any slopes compared to level ground
A Re-Examination of Running Energetics in Average and Elite Distance Runners
Full text linkWe measured the gross rates of oxygen consumption (VO2, mlO2.kg-1.min-1) and energy expenditure (E, kcals.kg-1.min-1), and determined the oxygen (O2COT, mlO2.kg-1.km-1) and energetic (ECOT, kcals.kg-1.km-1) costs of transport in Average and Elite runners over a wide range of submaximal speeds. Stride frequency (SF) and length (SL) were measured at each running speed. Ten Average (10 km run time=40-60 min) and 10 Elite (10 km run tim
Could a kangaroo win the Tour de France? The effect of relative crank angle on metabolic efficiency in cycling.
Full text linkThe rapid evolution of bicycles in the 1800s increased the speed of human powered transportation ten-fold compared to walking and decreased the metabolic power required by 300%. However, the metabolic gross efficiency has hardly changed. I tested the null hypothesis that the metabolic costs of cycling at different relative crank angles would not differ. I tested ten healthy, male, recreational bicycle riders (27.8 ± 8.2 yr, mean ± SD, mass 69.8 ± 3.2 kg) on a custom, pan-loaded cycle ergometer equipped with a standard Monark flywheel. The ergometer had a Shimano Octalink® bottom bracket, which allowed us to set the relative crank arm angles at 45° increments. Each subject completed six, 5-minute trials. The first and last trials were at a relative crank angle of 180°. We randomized the order of the middle trials (135°, 90°, 45°, and 0°). We averaged V̇O2, V̇CO2, and respiratory exchange ratio (RER) for the last 2 minutes of each 5-minute trial. From the V̇O2 and V̇CO2 measurements, we calculated metabolic power. I reject my null hypothesis; crank angles other than 180° required greater metabolic power. As relative crank angle decreased from 180°, metabolic power monotonically increased by 1.6% at 135° (p<0.002) to only 8.2% greater when the relative crank angle was 0° (p<0.001). Despite radically changing the relative crank angle, metabolic efficiency decreased by only ~8%. Thus, I conclude that attempts to enhance efficiency via pedaling technique or technology are likely futile
TO RUN OR WALK UPHILL: A MATTER OF INCLINATION
Full text linkPeople prefer to walk at slow speeds and to run at fast speeds. In between, there is a speed at which people choose to transition between gaits, the Preferred Transition Speed (PTS). At slow speeds, it is metabolically cheaper to walk and at faster speeds, it is cheaper to run. Thus, there is an intermediate speed, the Energetically Optimal Transition Speed (EOTS). My goals were to determine: 1) how PTS and EOTS compare at inclines relevant to trail and mountain runners and 2) if the heart rate optimal transition speed (HROTS) can predict either the EOTS or PTS. Ten healthy, high-caliber, male trail and mountain runners participated. On day 1, data for 0° and 15° were collected and on day 2, 5° and 10°. PTS was determined by averaging the run-to-walk transition speed (RWTS) and walk-to-run transition speed (WRTS) using an incremental protocol. EOTS was determined from metabolic cost data for walking and running at three or four speeds per incline near the expected EOTS. The intersection of the walking and running linear regression equations defined EOTS. HROTS was determined using the same linear regression procedure. PTS, EOTS, and HROTS all were slower on steeper inclines. PTS was slower than EOTS at 0°, 5°, and 10°, but the two converged at 15°. PTS and EOTS were moderately correlated at best. Although EOTS correlated with HROTS, heart rate is not an accurate tool for predicting EOTS.</p
Leg Stiffness and the Metabolic Cost of Hopping with Different Exoskeleton Spring Stiffness Profiles in Parallel to the Legs
No full textA previous study found that when humans hop on both legs with exoskeletal springs in parallel with the legs, net metabolic power decreases compared to normal hopping. Further, they retained near constant overall vertical stiffness. Here, I quantified the biomechanics and metabolic costs of 10 subjects (3F) who hopped on both legs normally and using a passive-elastic exoskeleton with three different spring stiffness profiles in parallel to the legs at 2.4-3.0 Hz. The springs had degressive (DG – stiff then compliant), linear (LN), or progressive (PG – compliant then stiff) stiffness. Compared to normal hopping (NH) at 2.4 – 3.0 Hz, use of the exoskeleton with DG stiffness reduced net metabolic power (Pmet) by 13-24%, LN stiffness reduced Pmet by 4-12%, and PG stiffness increased Pmet by 0-8%. Pmet was significantly reduced when using the exoskeleton with DG stiffness compared to NH at 2.4-2.6 Hz (p≤0.0135). Dimensionless vertical stiffness remained invariant while hopping with an exoskeleton compared to NH, except when using the exoskeleton with DG and LN spring stiffness at 2.8 Hz (p<0.005). Peak vertical ground reaction force was 9-24% lower (p≤0.0008) and center of mass displacement was 6-12% lower (p≤0.0013) at 2.4-3.0 Hz when using the exoskeleton with DG stiffness compared to NH. Hopping with an exoskeleton with DG stiffness provided the greatest elastic energy return (EE), followed by LN and PG (p<0.001). Future designs of passive-elastic exoskeletons used for bouncing gaits should consider using DG or LN stiffness profiles rather than PG stiffness to minimize metabolic costs.</p
A Test of the Metabolic Cost of Cushioning Hypothesis in Shod and Unshod Running
No full textPrevious studies that controlled foot/shoe mass indicate that cushioning provides energetic advantages over running barefoot. Further, running in lightweight shoes has a comparable metabolic cost as running barefoot, suggesting that positive effects of shoe cushioning may counteract negative mass effects. We hypothesized: 1) unshod running would have comparable metabolic costs as running shod and 2) cushioning will lower the metabolic cost of unshod running. Eleven participants ran at 3.35 m/s with mid-foot strike patterns unshod and shod (Nike Free 3.0; ~211 g /shoe) on a rigid treadmill. Subjects also ran unshod on 10 mm and 20 mm thick slabs of ethylene-vinyl acetate (EVA) foam (same as most running shoe midsoles) mounted on the belt. Oxygen consumption and carbon dioxide production volumes quantified metabolic power. Our findings demonstrate that cushioning reduces the metabolic cost of running, and suggest an ideal amount of cushioning (e.g. < 20mm) beyond which metabolic benefits diminish
The Effects of Suspension on the Energetics and Mechanics of Riding Bicycles on Smooth Uphill Surfaces
Full text linkBicycle suspension elements smooth the vibrations generated by irregularities in the road or trail surface. However, it is unknown whether the energy put into the suspension system exacts a metabolic or mechanical cost. Here, I investigated the effects of suspension systems on the energetics and mechanics of riding bicycles on smooth uphill surfaces in both the sitting and standing positions.Chapter 1: Twelve male cyclists rode at 3.35m/s up a motorized treadmill inclined to 7% grade. All subjects used the same road bike equipped with a steering tube front suspension system. Each subject completed six 5 minute trials separated by 5-minute rest periods, with the suspension system in rigid (locked) and compliant settings. I measured their metabolic rates from oxygen consumption and carbon dioxide production. I also measured their mechanical power outputs. In the sitting position, metabolic power averaged 13.10±0.54 (rigid) and 13.21±0.54 W/kg (compliant). Mechanical power averaged 2.83±0.06 W/kg in both conditions. During standing, metabolic power averaged 14.22±0.73 (rigid) and 14.17±0.81 W/kg (compliant). Mechanical power averaged 2.86±0.03 and 2.87±0.05 W/kg respectively. None of these differences were statistically significant.Chapter 2: Eight male and four female mountain bikers rode at 2.77m/s up a motorized treadmill inclined to 7% grade. Subjects rode a dual-suspension mountain bike. Each subject completed six 5 minute trials separated by 5-minute rest periods, with the suspension set to firm and soft conditions. I measured their metabolic rates from oxygen consumption and carbon dioxide production. I also measured their mechanical power outputs. In the sitting position, metabolic power averaged 11.38±0.48 (firm) and 11.44±0.49 W/kg (soft). Mechanical power averaged 2.54±0.20 W/kg in both conditions. During standing, metabolic power averaged 12.46±0.62 (firm) and 12.63±0.90 W/kg (soft). Mechanical power averaged 2.57±0.21W/kg in both conditions. None of these differences were statistically significant.In conclusion, suspension systems in both road and mountain bikes had no effect (p>0.10) on the metabolic or mechanical power required for bicycle riding on smooth uphill surfaces in either seated or standing positions
An Analysis of the Bicycle-Rider Interface Forces in Stationary Road Cycling
Full text linkTwo distinct studies were undertaken in order to examine the effects that both external weight distribution, and internal, bike-rider interface forces had on cyclists. The first section of the study looked at the bike-rider interface forces, and how they fluctuate during normal cycling; as well as how they vary with changes in rider power output, hand position, and cadence. In order to analyze these changes in isolation, three different studies were undertaken. The studies each examined 10 USAC Category 3 or better riders who were tested for 6 minute trials. Riders were tested with their hands on the tops, drops and hoods, with cadences of 60-90 RPM, and power outputs of 1-4 watts per kg. It was found that for each 1 W/kg power output increase, saddle forces decreased by 5.2 percentage points and bottom bracket forces increased by 3.3 percentage points. Cadence did not affect bike-rider interface forces. Shifting a rider's hands from the hoods to the tops and the drops increased the stem force by approximately 2 and 4 percentage points, respectively. The weight distribution study examined the effect of different bike fitting procedures on the bike-rider system, front/rear wheel, weight distribution. The study compared 13 amateur and 14 professional riders with four different fitting techniques. It was found that the Retül Fit weight distribution was 44.7%/55.3% front/rear and the Body Geometry Fit was 32.5%/61.5% front/rear. It was also found that the professional fit and the self-fit 40.4%/59.6%, and 38.5%/61.5% respectively, are similar (p=.9239)
The Biomechanics and Energetics of Skateboarding
No full textI investigated the ground reaction forces (GRF), stride kinematics and metabolic cost of skateboarding on an instrumented treadmill. Superficially, skateboarding appears to be a hybrid of walking, running, and cross-country skiing. I hypothesized that the push-foot in skateboarding would exhibit a vertical GRF peak similar in shape to running but with a lower magnitude. Further I hypothesized that the push-foot would exhibit greater propulsive GRF than braking GRF. Regarding stride kinematics, I hypothesized that skateboarders would increase their stride length (sL) at faster speeds. Finally, I hypothesized that skateboarding would have a smaller metabolic cost compared to walking and running at comparable speeds. Subjects (9 males/2 females) skateboarded on a force-instrumented treadmill at 1.00, 1.25, 1.50, 2.00, 2.50, 3.00, 3.50, and 4.00 m*sec-1, walked at 1.25m*sec-1, and ran at 3.0m*sec-1. Upon GRF analysis, I discovered two distinctly different groups of skateboarders: subjects who demonstrated a braking force (“brakers”) and subjects that did not (“non-brakers”). The peak vertical and horizontal GRF for brakers resembled running with half of the magnitude. Both groups showed decreased SF at faster speeds, but brakers used slower SF than non-brakers. Walking and skateboarding 1.25m*sec-1 had the same metabolic cost at but at 3.0m*sec-1, skateboarding required approximately half the metabolic cost of running. Skateboarding is a unique mode of locomotion, with two distinctly different forms, that allows a person to move at a running velocity with the GRF and metabolic cost of walking.</p
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