tubercles maintain areas of attached flow and delay stall to highangles of attack for enhanced maneuverability and agility. Themorphological features displayed by cetaceans for flow control canbe utilized in the biomimetic design of engineered structures forenhanced hydrodynamics.

doi:10.1016/j.cbpa.2008.04.077

A2.8Froude efficiency in human swimming

P. Zamparo (University of Verona, Italy)

Humans can be considered as “fluid machines” that obtain thethrust necessary to proceed at a given speed by using two engines: thelegs and the arms. The Froude efficiency (ηF) is the efficiency withwhich this thrust is transformed into “useful” work (the work toovercome hydrodynamic resistance).

In subjects swimming by using the leg kick, ηF can be assessed bymeans of methods usually applied to the study of undulating fish:frommeasures of the speed of the backward wave travelling along thebody (c) and of the forward speed (v): ηF=(c+v) / 2c. When swimmingbarefoot ηF is of about 0.60; the use of fins allows for an increase of ηFup to 0.65 and 0.75 (with double fins or a monofin, respectively).

The mode of propulsion of the arm stroke (in the front crawl) issimilar to the one adopted by “rowing” animals: the body can beassumed to move forward at constant speed (v) while the upper limbsmove forward and backwardwith a velocity (u) relative to the body. Inthis case: ηF ˜ v/u. In the front crawl the movement of the upper limbscan be modelled as that of a paddle wheel, a case in which the term ucan be easily calculated frommeasures of rim speed. In the arm stroke,ηF is of about 0.45 in elite male swimmers and 0.35–0.40 incompetitive swimmers of both genders; in children and masterathletes ηF can be as low as 0.25–0.30.

doi:10.1016/j.cbpa.2008.04.078

A2.9Leading edge vortices lift in bat flight

F. Muijres (Lund University); L. Johansson (Lund University); R. Barfield(Lund University); M. Wolf (Lund University); G. Spedding (Universityof Southern California); A. Hedenström (Lund University)

Small nectar-feeding bats use hovering flight when feeding fromflowers. According to quasi-steady aerodynamic theory, often used tomodel animal flight, these bats should not be able to generate enoughlift when hovering. This and previous wake measurements indicatethat additional unsteady aerodynamic lift mechanisms are required,similar to the ones used by insects. Here we show the first direct proofof the use of unsteady aerodynamics by hovering vertebrates. Wemeasured and analyzed the airflow on top of the wing of a Pallas'long-tongued bat flying freely in front of a feeder in a wind tunnel.The results show that these bats develop a Leading Edge Vortex (LEV)on top of the wing, which stays attached to the wing during thedown-stroke. It is quite remarkable that these bats can control thisLEV, since the aerodynamics around fixed wings at similar Reynoldsnumbers are notoriously sensitive to small disturbances. The circu-lation increment of the LEV is estimated, showing that the LEVcontributes significantly (40%) to the overall lift production. The

biological implications and possible biomimetic applications of theseresults are discussed.

doi:10.1016/j.cbpa.2008.04.079

A2.10The effects of length trajectory on the efficiency of mouse skeletalmuscle

N. Holt (University of Leeds)

Muscle length trajectory, the pattern of lengthening and short-ening, is a major determinant of the mechanical power output ofskeletal muscle (Askew and Marsh, 1997: J. Exp. Biol. 200, 3119 3131).Mechanical power output increases as the proportion of the cyclespent shortening increases. Whilst maximising muscle power outputcan be an important selection advantage, the metabolic energyexpenditure is also undoubtedly under selection pressure. The effectsof the muscle length trajectory on efficiency are currently unknown.

The effects of length trajectory on initial muscle efficiency, the ratioof the mechanical work output to the initial metabolic cost, wereexamined using the work loop technique and measurement of heatproduction using an antimony bismuth thermopile. Muscles weresubjected to cyclical contractions with sinusoidal and sawtooth lengthtrajectories over a range of cycle frequencies. In the sawtooth cycles theproportion of the cycle spent shortening was varied. Strain amplitudeand stimulus phase and duration were optimised to maximisemechanical power output. Data will be presented showing how initialefficiency is affected by length trajectory in mouse skeletal muscle.

doi:10.1016/j.cbpa.2008.04.080

A2.11Mechanics and energetics of load carrying in humans

N. Heglund (Université Catholique de Louvain)

Abstract not available.

doi:10.1016/j.cbpa.2008.04.081

A2.12Energetic trade-offs that determine optimal step length in humanwalking

A. Kuo (University of Michigan)

Humans prefer to walk at the energetically optimal combination ofstep length and frequency at a given speed. The optimum is difficult toexplain because it depends not only on the energetics ofmuscle but alsoon the mechanics of walking, both of which are incompletely under-stood. We used the principles of passive dynamic walking to show that,theoretically, longer steps at a given speed are mechanically costlybecause they induce higher collision losses followingheel strike, as bodytransitions from using one leg as an inverted pendulum to the other.Experiments indicate that one component of the metabolic cost ofwalking is for the mechanical work associated with those collisions,

S66 Abstracts / Comparative Biochemistry and Physiology, Part A 150 (2008) S64–S73