End Of Downstroke Research Articles

Spring or string: does tendon elastic action influence wing muscle mechanics in bat flight?

Tendon springs influence locomotor movements in many terrestrial animals, but their roles in locomotion through fluids as well as in small-bodied mammals are less clear. We measured muscle, tendon and joint mechanics in an elbow extensor of a small fruit bat during ascending flight. At the end of downstroke, the tendon was stretched by elbow flexion as the wing was folded. At the end of upstroke, elastic energy was recovered via tendon recoil and extended the elbow, contributing to unfurling the wing for downstroke. Compared with a hypothetical 'string-like' system lacking series elastic compliance, the tendon spring conferred a 22.5% decrease in muscle fascicle strain magnitude. Our findings demonstrate tendon elastic action in a small flying mammal and expand our understanding of the occurrence and action of series elastic actuator mechanisms in fluid-based locomotion.

Interpretation of crank torque during an all-out cycling exercise at high pedal rate

The use of cycle ergometers with strain gauges bonded on to the cranks enables the evaluation of strength, velocity and power. However, the torque measured at the end of downstroke is higher at high velocity than at medium velocity in contradiction with the force–velocity relationship. Kinematic data were collected with a camera during sprints on a Lode ergometer with strain gauges. Leg mechanical energy (E Leg) and its derivative (dE Leg/dt) were calculated from the sum of the kinetic and potential energies. The opposition of P ext (output at crank level) and dE Leg/dt before bottom dead centre suggested a transformation of E Leg into external work, explaining the higher torque at the end of downstroke at peak velocity. Another explanation is that the leg muscles worked on different parts of their torque–angle relationships as the phase between knee and ankle angles was different at low and high cycling frequencies. Musculo-skeletal net power output (algebraic sum of P ext and dE Leg/dt) indicated that musculo-skeletal structures participated in the upward movement, whereas P ext was negative.

Kinematic, Aerodynamic and Anatomical Mechanisms in the Slow, Maneuvering Flight of Pigeons

ABSTRACT A high-speed (200 Hz) infrared video system was used in a three-dimensional analysis of pigeon wing and body kinematics to determine the aerodynamic and anatomical mechanisms they use to produce force asymmetries to effect a turn during slow (3 m s−1) flight. Contrary to our expectations, pigeons used downstroke velocity asymmetries, rather than angle of attack or surface area asymmetries, to produce the disparities in force needed for directional changes. To produce a bank, a velocity asymmetry is created early in the downstroke and, in the majority of cases, then reversed at the end of the same downstroke, thus arresting the rolling angular momentum. When the velocity asymmetry was not reversed at the end of downstroke, the arresting force asymmetry was produced during upstroke, with velocity asymmetries creating disparate drag forces on the wings. Rather than using subtle aerodynamic variables to produce subtle downstroke force asymmetries, pigeons constantly adjust their position using a series of large alternating and opposing forces during downstroke and upstroke. Thus, a pigeon creates a precise ‘average’ body position (e.g. bank angle) and flight path by producing a series of rapidly oscillating movements. Although the primary locomotor event (downstroke) is saltatory, maneuvering during slow flight should be considered as a product of nearly continuous, juxtaposed force generation throughout the wingbeat cycle. Further, viewing upstroke as more than stereotypical, symmetrical wing recovery alters the evolutionary and functional context of investigations into the musculoskeletal mechanisms and the associated neural control involved in this unique kinematic event.

The co-ordination between respiration and wing beats in birds

Investigations in 10 species showed that respiration of birds in flight is usually co-ordinated with wing beats, but the co-ordination is not obligatory. Respiration synchronous with wing beats (1∶1 co-ordination) was found only in pigeons and crows, the other species exhibited one of 11 other types of co-ordination. Quails, ducks and pheasants, birds with relatively high wing beat frequencies (with relatively small wings) showed a 5∶1 co-ordination. Within species, and even during a flight the type of co-ordination changed, and simultaneously there were sudden changes in the respiration frequency. For the most part, the beginning of inspiration was linked with the (end of) upstroke and the beginning of expiration with the end of downstroke.