In previous studies of attentional focus effects, investigators have measured performance outcome. Here, however, the authors used electromyography (EMG) to determine whether difference between external and internal foci would also be manifested at the neuromuscular level. In 2 experiments, participants (N=11, Experiment 1; N=12, Experiment 2) performed biceps curls while focusing on the movements of the curl bar (external focus) or on their arms (internal focus). In Experiment 1, movements were performed faster under external than under internal focus conditions. Also, integrated EMG (iEMG) activity was reduced when performers adopted an external focus. In Experiment 2, movement time was controlled through the use of a metronome, and iEMG activity was again reduced under external focus conditions. Those findings are in line with the constrained action hypothesis (G. Wulf, N. McNevin, & C. H. Shea, 2001), according to which an external focus promotes the use of more automatic control processes.
Most insects are thought to fly by creating a leading-edge vortex that remains attached to the wing as it translates through a stroke. In the species examined so far, stroke amplitude is large, and most of the aerodynamic force is produced halfway through a stroke when translation velocities are highest. Here we demonstrate that honeybees use an alternative strategy, hovering with relatively low stroke amplitude (Ϸ90°) and high wingbeat frequency (Ϸ230 Hz). When measured on a dynamically scaled robot, the kinematics of honeybee wings generate prominent force peaks during the beginning, middle, and end of each stroke, indicating the importance of additional unsteady mechanisms at stroke reversal. When challenged to fly in low-density heliox, bees responded by maintaining nearly constant wingbeat frequency while increasing stroke amplitude by nearly 50%. We examined the aerodynamic consequences of this change in wing motion by using artificial kinematic patterns in which amplitude was systematically increased in 5°increments. To separate the aerodynamic effects of stroke velocity from those due to amplitude, we performed this analysis under both constant frequency and constant velocity conditions. The results indicate that unsteady forces during stroke reversal make a large contribution to net upward force during hovering but play a diminished role as the animal increases stroke amplitude and flight power. We suggest that the peculiar kinematics of bees may reflect either a specialization for increasing load capacity or a physiological limitation of their flight muscles.bee flight ͉ flight in heliox ͉ stroke amplitude ͉ unsteady mechanisms ͉ wingbeat kinematics I n 1934, August Magnan and André Sainte-Lague (1) concluded from a simple mathematical analysis that the flight of bees was ''impossible.'' Since this time, bees have symbolized both the inadequacy of aerodynamic theory as applied to animals and the hubris with which theoreticians analyze the natural world. Although the assumptions used by Magnan and SainteLague have since proven erroneous (2), conventional fixed-wing aerodynamic theory is indeed insufficient to explain the flapping flight of bees and other small insects. In particular, the performance of insect wings, when tested under steady conditions in wind tunnels, is too low to account for the forces required to sustain flight (3). However, a number of more recent studies have demonstrated that wings perform much better when started from rest or rotated continuously around their base (4-6) due to the formation of a leading-edge vortex (LEV). Instead of shedding to initiate stall, the LEV remains attached throughout each stroke, presumably because of the transport of vorticity by span-wise flow (7-9). Whereas the delayed stall forces are greatest at midstroke, flapping wings generate additional forces during stroke reversals. These forces, which result from the rapid rotation of the wing, added mass effects, and the influence of the wake shed from previous strokes, are very sensitive to the precise p...
The purpose of the study was to investigate the characteristics of shock attenuation during high-speed running. Maximal running speed was identified for each subject [n = 8 males, 25 (SD 4.6) years; 80 (8.9) kg; 1.79 (0.06) m] as the highest speed that could be sustained for about 20 s on a treadmill. During testing, light-weight accelerometers were securely mounted to the surface of the distal antero-medial aspect of the leg and frontal aspect of the forehead. Subjects completed running conditions of 50, 60, 70, 80, 90, and 100% of their maximal speeds with each condition lasting about 20 s. Stride length, stride frequency, leg and head peak impact acceleration were recorded from the acceleration profiles. Shock attenuation was analyzed by extracting specific sections of the acceleration profiles and calculating the ratio of head to leg power spectral densities across the 10-20 Hz frequency range. Both stride length and stride frequency increased across speeds (P < 0.05) and were correlated with running speed (stride length r = 0.92, stride frequency r = 0.89). Shock attenuation increased about 20% per m x s(-1) across speeds (P< 0.05), which was similar to the 17% increase in stride length per m x s(-1). Additionally, shock attenuation was correlated with stride length (r = 0.71) but only moderately correlated with stride frequency (r = 0.40) across speeds. It was concluded that shock attenuation increased linearly with running speed and running kinematic changes were characterized primarily by stride length changes. Furthermore, the change in shock attenuation was due to increased leg not head peak impact acceleration across running speeds.
SUMMARYA critical but seldom-studied component of life history theory is how behavior and age affect whole-organism performance. To address this issue we compared the flight performance of honey bees (whose behavioral development and age can be assessed independently via simple manipulations of colony demographics) between distinct behavioral castes (in-hive nurse bees vs outof-hive foragers) and across lifespan. Variable-density gases and high-speed video were used to determine the maximum hovering flight capacity and wing kinematics of age-matched nurse bees and foragers sampled from a single-cohort colony over a period of 34 days. The transition from hive work to foraging was accompanied by a 42% decrease in body mass and a proportional increase in flight capacity (defined as the minimum gas density allowing hovering flight). The lower flight capacity of hive bees was primarily due to the fact that in air they were functioning at a near-maximal wing angular velocity due to their high body masses. Foragers were lighter and when hovering in air required a much lower wing angular velocity, which they were able to increase by 32% during maximal flight performance. Flight performance of hive bees was independent of age, but in foragers the maximal wingbeat frequency and maximal average angular velocity were lowest in precocious (7-14 day old) foragers, highest in normal-aged (15-28 day old) foragers and intermediate in foragers older than 29 days. This pattern coincides with previously described age-dependent biochemical and metabolic properties of honey bee flight muscle.
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