Aerodynamic corrections for the flight of birds and bats in wind tunnels

Few wind tunnel studies of animal flight have controlled or corrected for distortions to behaviour, physiology or flight aerodynamics representing the difference between flight in the tunnel and flight in free air. Aerodynamic correction factors are derived based on lifting-line theory and the method of images for an animal flying freely within closed- and open-section wind tunnels; the method is very similar to that used to model flight in ground effect, and as in ground effect the corrections to induced drag may be substantial. These correction factors are used to estimate bound wing circulation, drag and mechanical power for comparison with free flight, and to derive testable predictions of optimum flight strategies for an animal in a tunnel. In an open-section tunnel, mechanical power is increased compared to free flight, and the animal should fly at the tunnel centre. In a closed tunnel mechanical power is usually reduced, and substantial savings are available, particularly at low speeds, if the animal flies close to the tunnel roof. Anecdotal observations confirm that birds and bats adopt this strategy. The mechanical power-speed curve in a closed tunnel is flatter than the curve for free flight, and this may explain the flat metabolic power-speed curves for birds and bats obtained in some measurements.     

Aerodynamic Ground Effect in Fruitfly Sized Insect Takeoff

Aerodynamic ground effect in flapping-wing insect flight is of importance to comparative morphologies and of interest to the micro-air-vehicle (MAV) community. Recent studies, however, show apparently contradictory results of either some significant extra lift or power savings, or zero ground effect. Here we present a numerical study of fruitfly sized insect takeoff with a specific focus on the significance of leg thrust and wing kinematics. Flapping-wing takeoff is studied using numerical modelling and high performance computing. The aerodynamic forces are calculated using a three-dimensional Navier–Stokes solver based on a pseudo-spectral method with volume penalization. It is coupled with a flight dynamics solver that accounts for the body weight, inertia and the leg thrust, while only having two degrees of freedom: the vertical and the longitudinal horizontal displacement. The natural voluntary takeoff of a fruitfly is considered as reference. The parameters of the model are then varied to explore possible effects of interaction between the flapping-wing model and the ground plane. These modified takeoffs include cases with decreased leg thrust parameter, and/or with periodic wing kinematics, constant body pitch angle. The results show that the ground effect during natural voluntary takeoff is negligible. In the modified takeoffs, when the rate of climb is slow, the difference in the aerodynamic forces due to the interaction with the ground is up to 6%. Surprisingly, depending on the kinematics, the difference is either positive or negative, in contrast to the intuition based on the helicopter theory, which suggests positive excess lift. This effect is attributed to unsteady wing-wake interactions. A similar effect is found during hovering.     

Mechanics of gliding in birds with special reference to the influence of the ground effect

A model of the mechanics of gliding without loss of altitude (horizontal gliding) is developed. The model can be employed to assess the influence of the principal drag components (induced, profile and parasite drag), choice of initial and final glide velocities and height above the ground on glide distance. For birds gliding near to the ground the ground effect acts to decrease the induced drag and increase the lift to drag ratio of the wings. Minimum drag speed is reduced for birds gliding near to the ground. The model is applied to the gliding flight of the black skimmer (Rhyncops nigra). Glide distances for given initial and final velocities are significantly increased in the influence of the ground effect over out of ground effect values.     

Effects on the crank torque profile when changing pedalling cadence in level ground and uphill road cycling

Despite the importance of uphill cycling performance during cycling competitions, there is very little research investigating uphill cycling, particularly concerning field studies. The lack of research is partly due to the difficulties in obtaining data in the field. The aim of this study was to analyse the crank torque in road cycling on level and uphill using different pedalling cadences in the seated position. Seven male cyclists performed four tests in the seated position (1) on level ground at 80 and 100 rpm, and (2) on uphill road cycling (9.25% grade) at 60 and 80 rpm.The cyclists exercised for 1 min at their maximal aerobic power. The bicycle was equipped with the SRM Training System (Schoberer, Germany) for the measurement of power output (W), torque (Nm), pedalling cadence (rpm), and cycling velocity (km h(-1)). The most important finding of this study indicated that at maximal aerobic power the crank torque profile (relationship between torque and crank angle) varied substantially according to the pedalling cadence and with a minor effect according to the terrain. At the same power output and pedalling cadence (80 rpm) the torque at a 45 degrees crank angle tended (p < 0.06) to be higher (+26%) during uphill cycling compared to level cycling. During uphill cycling at 60 rpm the peak torque was increased by 42% compared with level ground cycling at 100 rpm.When the pedalling cadence was modified, most of the variations in the crank torque profile were localised in the power output sector (45 degrees to 135 degrees).     

Influence of leg stiffness and its effect on myodynamic jumping performance.

The purposes of this study are: a) to examine the possibility of influencing the leg stiffness through instructions given to the subjects and b) to determine the effect of the leg stiffness on the mechanical power and take-off velocity during the drop jumps. A total of 15 athletes performed a series of drop jumps from heights of 20, 40 and 60 cm. The instructions given to the subjects were a) "jump as high as you can" and b) "jump high a little faster than your previous jump". The jumps were performed at each height until the athlete could not achieve a shorter ground contact time. The ground reaction forces were measured using a "Kistler" force plate (1000 Hz). The athletes body positions were recorded using a high speed (250 Hz) video camera. EMG was used to measure muscle activity in five leg muscles. The data was divided into 5 groups where group 1 was made up of the longest ground contact times of each athlete and group 5 the shortest. The leg and ankle stiffness values were higher when the contact times were shorter. This means that by influencing contact time through verbal instructions it is possible to control leg stiffness. Maximum center of mass take-off velocity the can be achieved with different levels of leg stiffness. The mechanical power acting on the human body during the positive phase of the drop jumps had the highest values in group 3. This means that there is an optimum stiffness value for the lower extremities to maximize mechanical power.     

Effect of Footwear Modifications on Oscillations at the Achilles Tendon during Running on a Treadmill and Over Ground: A Cross-Sectional Study

Background

Achilles tendon injuries are known to commonly occur in runners. During running repeated impacts are transferred in axial direction along the lower leg, therefore possibly affecting the oscillation behavior of the Achilles tendon. The purpose of the present study was to explore the effects of different footwear modifications and different ground conditions (over ground versus treadmill) on oscillations at the Achilles tendon.

Methods

Oscillations were measured in 20 male runners using two tri-axial accelerometers. Participants ran in three different shoe types on a treadmill and over ground. Data analysis was limited to stance phase and performed in time and frequency space. Statistical comparison was conducted between oscillations in vertical and horizontal direction, between running shoes and between ground conditions (treadmill versus over ground running).

Results

Differences in the oscillation behavior could be detected between measurement directions with peak accelerations in the vertical being lower than those in the horizontal direction, p < 0.01. Peak accelerations occurred earlier at the distal accelerometer than at the proximal one, p < 0.01. Average normalized power differed between running shoes (p < 0.01) with harder damping material resulting in higher power values. Little to no power attenuation was found between the two accelerometers. Oscillation behavior of the Achilles tendon is not influenced by ground condition.

Conclusion

Differences in shoe configurations may lead to variations in running technique and impact forces and therefore result in alterations of the vibration behavior at the Achilles tendon. The absence of power attenuation may have been caused by either a short distance between the two accelerometers or high stiffness of the tendon. High stiffness of the tendon will lead to complete transmission of the signal along the Achilles tendon and therefore no attenuation occurs.     

Avoidance of headwinds or exploitation of ground effect—why do birds fly low?

ABSTRACT Birds often fly close to the ground or water. Wind shear theory predicts that wind speeds decline with proximity to the substratum, so birds might be expected to fly lower when flying upwind than when flying downwind. We tested this prediction and found that the wind shear equation is valid at heights below 4 m, with wind speed over a smooth surface ～40% lower at a height of 0.08 m than at 4 m. Birds that fly close enough to smooth substrata can also benefit energetically from ground effect, where vortices generated by their flight interact with the ground or water. This suggests that birds should use ground effect more when flying upwind than when flying downwind. We determined the percent time spent flying in ground effect by 21 species of passerine and non‐passerine birds flying in sheltered coastal aquatic and nearby terrestrial areas of County Cork, Ireland. We found that use of ground effect was uncommon for passerines, but common for a variety of non‐passerine waterbirds. However, phylogenetic analysis indicates no linkage between phylogeny and incidence of ground effect use and it is probable that incidence of use is determined by ecology rather than phylogeny. Great Cormorants (Phalacrocorax carbo) used ground effect most frequently over water (59.4% of time in flight). Over land, Barn Swallows (Hirundo rustica) used ground effect most often (19.8% of time). Phylogenetic contrasts regression analysis showed no significant relationship between use of ground effect and either wing aspect ratio or wing loading for 18 of our focal species, though simple linear regression analysis indicated that birds with greater wing loading used ground effect slightly (but significantly) more often. We found that 95% of Great Cormorants flying upwind used ground effect whereas only 35% did so when flying downwind. Few Black‐headed Gulls (Chroicocephalus ridibundus) used ground effect (probably because they fly high to locate prey), but still showed greater use when flying upwind (25%) than downwind (2.5%). When flying upwind in ground effect at the wind speeds encountered in our study, the velocity for minimum power (V_mp) for Great Cormorants was exceeded, suggesting theoretical benefits of about 14.3%. Our study indicates that several species exploit both wind shear and ground effect to minimize energy expenditure during commuting and foraging, but that others do not, because of either complexity of habitat morphology or the demands of their foraging ecology.     

Whole body movements in altered G environments

Davies and Rennie determined that the instantaneous peak and mean power transferred to the center-of-mass of a subject during jumping could be calculated from analyzing the ground reaction force. Since Davies and Rennie's report, vertical jumping has been used to measure the peak and mean power transferred to the center-of-mass and to assess human performance. It has also been suggested that the external load a subject experiences effects the power production. However, it is difficult to determine if the difference in power production as defined in these studies is due to the external loading, or is an effect of changing the dynamics of the movement. In this paper, we describe a study which allows us to more clearly define the power developed during jumping under different weights, and the relationship between the power developed and jump height.     

Emergent effects of ground beetles size diversity on the strength of prey suppression

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The ground beetle fauna of ancient and recent woodlands in the lowlands of north-west Germany (Coleoptera, Carabidae)

In north-west Germany the ground beetle fauna was investigated using pitfall traps at 79 sampling sites in ancient woodlands and recent woodlands (with and without direct contact to old stands). Two woodland types were considered: The Quercion robori-petraea-woodlands (oak-beech-type) on mainly sandy soils and the Stellario-Carpinetum-woodlands (hornbeam-type) on mainly loamy soils. The number of recorded ground beetle species inhabiting exclusively or predominantly woodlands in the investigation area is significantly higher in ancient stands of both woodland types than in recent ones. No statistically substantiated relation between habitat size (both about 1800 and in 1990) and the number of characteristic woodland ground beetle species could be ascertained. Carabus glabratus and Abax parallelus show a distinct focus in ancient woodlands. Significantly more records of Carabus violaceus and Abax parallelepipedus are known from ancient woodlands than from recent ones. Twelve of the 16 ground beetle species, for which no difference in the colonisation of ancient and recent woodlands was ascertained, are macropterous. Half of the eight brachypterous woodland species is exclusively or predominantly found in ancient woodlands, suggesting that power of dispersal is an important factor which determines the species number in woodland fragments of different age.     

The effect of Knee-Ankle-Foot orthosis stiffness on the parameters of walking

The purpose of this simulation study was to analyze the effect of variation in Knee-Ankle-Foot-Orthosis stiffness on the joint power and the energy cost of walking. The effect of contractile tissue was simulated using linear elastic spring and viscous dampers in knee and ankle joints. Then, joint angles, ground reaction force, were collected from Twenty chronic hemiparesis subjects (15 males and 5 females) and twenty control subjects (14 males and 6 females), and spring stiffness were considered as the inputs. In this new study, the generated muscle torques were optimized by changing the stiffness as the desired output in the mathematical model attained by the MATLAB SimMechanics toolbox. Finally, the simulated mathematical model was introduced as an appropriate substitute in obtaining the optimized stiffness with a more convenient and efficient designed orthosis.     

Mechanical power in running: a comparison of different approaches

The purposes of the present study were: (1) to compare four different methods of calculating mechanical power in running on the basis of comparable data over a wide range of running velocity; (2) to examine the linearity of the relation between mechanical power as calculated with the four methods and running velocity. Eight runners participated in the investigation (height: 1.82 +/- 0.03 m, body mass: 81.05 +/- 4.69 kg). A Kistler force platform registered all components of the ground reaction force (1000 Hz) during one foot ground contact, which was additionally video taped using two high-speed video cameras running at 120 Hz. Four different methods were used to calculate mechanical power. Two methods determined the mechanical power due to the work done on the athletes' center of mass and two were calculated from the motion of the athletes' segments. The four different methods provided different relations between mechanical power and running velocity. The calculations on the basis of kinematic data cannot be recommended to determine efficiency of movement. The methods based on ground reaction force measurements revealed significant linear relations (r = 0.90, r2 = 0.84) between running velocity and mechanical power.     

The automated multiparameter analyzer for cells (AMAC) IIA, a true bridge circuit Coulter-type electronic cell volume transducer.

A true bridge Coulter effect (electronic cell volume) transducer has been developed. All resistances of this bridge are now the result of current flow through saline channels. Contamination by electrode products including gas bubbles has been completely eliminated since both power electrodes are now remote from the flow chamber. Since the orifice is in series with an approximately 10 K ohm resistance generated by a gel-filled capillary and a displacement rheostat, it floats electrically, at virtual ground. The other side of the bridge consists of a fluid side-wire. Removing the power electrode from the orifice outlet makes possible downward flow and the use of a single outer sheath, and eliminates noise generated by gas bubbles which could possibly be trapped. It should now be possible to combine this design with that of the AMAC III square orifice, to produce an electro-optical sorter where all parameters are measured simultaneously. This true bridge circuit possesses the further advantage that noise due both to the power supply and to overvoltage at the power electrodes is common-mode rejected, and any drift due to changes in electrode polarization is eliminated. Preliminary experiments confirm results with the AMAC II that hemoglobinopathies can be recognized by the increased coefficient of variation (CV) of the erythrocyte spectra.     

Comparisons of peak ground reaction force and rate of force development during variations of the power clean

The aim of this investigation was to determine the differences in vertical ground reaction forces and rate of force development (RFD) during variations of the power clean. Elite rugby league players (n = 11; age 21 ± 1.63 years; height 181.56 ± 2.61 cm; body mass 93.65 ± 6.84 kg) performed 1 set of 3 repetitions of the power clean, hang-power clean, midthigh power clean, or midthigh clean pull, using 60% of 1-repetition maximum power clean, in a randomized order, while standing on a force platform. Differences in peak vertical ground reaction forces (F(z)) and instantaneous RFD between lifts were analyzed via 1-way analysis of variance and Bonferroni post hoc analysis. Statistical analysis revealed a significantly (p < 0.001) greater peak F(z) during the midthigh power clean (2,801.7 ± 195.4 N) and the midthigh clean pull (2,880.2 ± 236.2 N) compared to both the power clean (2,306.24 ± 240.47 N) and the hang-power clean (2,442.9 ± 293.2 N). The midthigh power clean (14,655.8 ± 4,535.1 N·s?1) and the midthigh clean pull (15,320.6 ± 3,533.3 N·s?1) also demonstrated significantly (p < 0.001) greater instantaneous RFD when compared to both the power clean (8,839.7 ± 2,940.4 N·s?1) and the hang-power clean (9,768.9 ± 4,012.4 N·s?1). From the findings of this study, when training to maximize peak F(z) and RFD the midthigh power clean and midthigh clean pull appear to be the most advantageous variations of the power clean to perform.     

长期围封对疏勒河源区高寒草甸地表节肢动物多样性的影响

围封会促进退化高寒草甸植被和土壤环境恢复,长期围封也会导致生物多样性及其功能下降,影响高寒草甸生态系统的稳定,但这种影响会随着季节和生境条件变化而异。为了探究不同退化程度高寒草甸地表节肢动物群落变化对围封禁牧的响应,利用陷阱法调查了疏勒河源区沼泽化草甸、草甸和草原化草甸3种不同退化梯度高寒草甸围封禁牧和自由放牧处理下地表节肢动物群落结构变化。结果表明：围封禁牧对高寒草甸地表节肢动物群落组成及多样性的影响随生境条件不同而异。禁牧降低了沼泽化草甸地表节肢动物的物种丰富度,而提高了草甸和草原化草甸地表节肢动物的物种丰富度;围封禁牧对沼泽化草甸地表节肢动物群落结构影响较小,显著降低了草甸生境地表节肢动物活动密度、提高了地表节肢动物多样性和均匀度,相反,禁牧显著提高了草原化草甸生境地表节肢动物活动密度、降低其多样性和均匀度;豹蛛属1种是高寒草甸主要的地表节肢动物类群（相对多度为67.0%）,高寒草甸土壤水分有效性等生境条件不同影响了豹蛛属1种对围封禁牧的响应模式,进而影响了地表节肢动物群落结构变化。总之,高寒草甸退化程度影响了地表节肢动物多样性对禁牧和放牧的响应模式,沼泽化草甸适度放牧有利于提高地表节肢动物多样性及其功能。     

The effect that side dominance has on barbell power symmetry during the hang power clean

The aim of this study was to examine whether ground reaction force (GRF) side differences were transmitted and related to bar end power output asymmetries during hang power clean (HPC) performance and whether progressive loading would intensify this effect. Differences between the dominant (D) and nondominant (ND) side average GRFs (AGRFs) of both feet and average bar end power outputs were recorded simultaneously from 15 recreationally trained male volunteers at 30, 60, and 90% 1RM using 2 force platforms and 3 high-speed digital cameras, quantifying side dominance from perceived handedness (left- or right-side dominance [LRSD]), GRF side dominance (force side dominance [FSD]), and bar end power output side dominance (barbell side dominance [BSD]). With the exception of the LRSD condition, differences between the D and ND side AGRFs were significant (FSD: 1.8-4.3%; BSD: 5.1-6.4%, p < 0.05). Bar end power output side differences were significant for all conditions (LRSD: 1.5-5.4%; FSD: 0.5-3.4%; BSD: 3.9-5.6%, p < 0.05). Progressive loading did not significantly affect GRF side differences or the FSD average bar power side differences. However, during the LRSD and BSD conditions, the 60 and 90% side average bar power side differences were >the 30% equivalents. Average GRF side differences were not related to bar end power output side differences. Because of the consistent side difference of 4-6% investigators and strength and conditioning practitioners should exercise caution when interpreting changes in bar end power output.     

Effect of a Wide Stance on Block Start Performance in Sprint Running

This study aimed to clarify the effect of widened stance width at the set position during the block start phase in sprint running on kinematics and kinetics at the hip joint and block-induced power. Fourteen male sprinters volunteered to participate in this study. They performed three block-start trials with a normal stance width (25 ± 1 cm, normal condition) and a widened stance width (45 ± 2 cm, widened condition) at the set position. The block start movements were recorded at 250 Hz with high-speed cameras and the ground reaction forces at 1250 Hz with force plates. During the block phase in the widened condition, the hip abduction and external rotation angles in both legs were significantly larger and smaller, respectively, than those in the normal condition. The positive peak value of the hip power in the rear leg was significantly greater in the widened condition than that in the normal condition. However, no significant difference was seen in the normalized block-induced power between the widened and normal conditions. We conclude that a widened stance width at the set position affects the hip-joint kinematics and rear hip power generation during the block start phase, but no effect on the block-induced power when considering sprinting performance during the whole block start phase.     

Mechanisms underlying the reduced performance measures from using equipment with a counterbalance weight system

Bench press throws are commonly used in the assessment of upper-body power and are often performed on a Smith machine that uses a counterbalance weight to reduce the net load on the barbell. The use of a counterbalanced Smith machine was recently shown to reduce performance measures, but the mechanisms for this reduction have not been established. The purpose of this study was to determine the underlying physiological and biomechanical causes of the reduced performance measures found when using a counterbalanced Smith machine. Twenty-four men (mean ± SE: age, 23 ± 1 years; weight, 91.0 ± 3.5 kg; height, 178.9 ± 1.2 cm) performed Smith machine bench press throws at 30% of 1-repetition maximum under 4 conditions: (a) rebound movement and counterbalance, (b) rebound movement and no counterbalance, (c) concentric-only movement and counterbalance, and (d) concentric-only movement and no counterbalance. Peak power, peak force, and peak concentric and eccentric velocities were measured using a linear accelerometer, and peak ground reaction force was measured using a force plate. The counterbalance condition produced significantly (p < 0.05) lower peak accelerometer-based force (-21.2 and -17.0% for rebound and concentric-only bench press throws, respectively) but increased peak ground reaction force (5.3 and 3.2%). The discrepancy between changes in peak accelerometer-based force and peak ground reaction force suggests that an increase in net external load occurred during the movement. For performance testing of explosive movements, the use of a counterbalance system results in an underestimation of performance capability, likely because of an increase in the net external load during the concentric phase. Therefore, a counterbalance system should not be used for explosive movement performance testing.     

Effect of rocker shoes and running speed on lower limb mechanics and soft tissue vibrations

Previous studies have shown a possible effect of running speed and the sole material of footwear on lower-limb mechanics and soft tissue vibrations, while little information has been offered concerning the influence of the shape of the footwear’s sole. The purpose of this study is to assess the effect of running speed and rocker shoes on muscular activity, ground reaction force, and soft tissue vibrations. Twenty participants performed heel-toe running with two shoes, differentiated only by their sole shape (i.e. rocker and non-rocker), at four running speeds. Ground reaction force and electromyograms of the gastrocnemius medialis and vastus lateralis were measured, and soft tissue accelerations of the same muscles were recorded with tri-axial accelerometers. A continuous wavelet transform was applied to the accelerometer’s signals to analyse them in the time-frequency domain. The rocker of the shoes did not change the muscular activations, ground reaction force, nor power of soft tissue vibrations. In opposite, increased running speed led to an augmentation of all of the measured parameters. Interestingly, running speed augmentation led to a greater increase in high frequencies component of soft tissue vibrations (25–50 Hz, 242%) than lower ones (8–25 Hz, 111%). Consequently, we indicated a 10% increase in the relative part of the high frequencies of the total power. In conclusion, although rocker shoes have shown an effect on lower-limb kinetics in some studies, no influence on soft tissue vibration is denoted. By contrast, soft tissue vibrations may be modulated by changing running speed.     

The intensity of oxidative phosphorylation and the function of the adenylate system in the liver mitochondria of active and hibernating susliks Citellus undulatus

The state of adenylate system and intensity of oxidative phosphorylation in liver mitochondria of active and hibernating ground squirrels were studied depending on the concentration of extramitochondrial Ca2+ ([Ca2+]ex). It was shown that at [Ca2+]ex.10(-7) M, the content of ATP as well as ATP/ADP ratio are slightly lower in the mitochondria of hibernating ground squirrels than in the mitochondria of active animals. The other parameters of the adenylate system under the same conditions differ insignificantly. [Ca2+]ex increase to 10(-6) M has little effect on the parameters of the adenylate system of active animals. On the contrary, the mitochondria of hibernating ground squirrels are strongly affected: the level of ATP is 1.5-fold and the ratio of ATP/ADP is almost 2-fold decreased. At both [Ca2+]ex the intensity of oxidative phosphorylation is essentially higher in the mitochondria of active ground squirrels. With increasing [Ca2+]ex the rate of ATP synthesis decreases, and in the mitochondria of hibernating animals the decrease is more pronounced than in the mitochondria of active animals. Thus, oxidative phosphorylation and adenylate system of mitochondria from hibernating ground squirrels are more sensitive to [Ca2+]ex increase than those of the mitochondria of active animals.