D6 Grip and Landing Dynamometry

Topic

Equipping a perch or landing substrate with force sensors allows for the direct measurement of contact dynamics between the bird and the surface at the precise moment of landing or grasping. This method provides experimental insight into the grasping and landing phase—one of the most difficult to observe in free-flying birds—and enables a direct assessment of both the condition of the digital flexor tendon and the risk of collision-related injury during landing.

The first of the two instruments is a grip dynamometer, which directly measures the gripping force the bird exerts with its foot when holding onto the instrumented perch. This gripping force serves as a rapid indicator of the bird's neuromuscular condition, as both the digital flexor tendon and its controlling nervous system determine the amount of force the foot can generate and sustain while closing around the perch.

The second instrument is a force platform positioned beneath the perch; it records the complete force-versus-time curve throughout the entire landing process, from the initial foot contact until the bird is fully settled. Three parameters are derived from this curve: the total impulse transferred from the perch to the foot during the landing, the peak force reached at the point of maximum impact, and the duration of contact during which the force is applied.

By fitting this same force-versus-time curve to a damped oscillator model, one obtains the leg damping factor—a value that encapsulates, in a single number, how the integrated system of wing, tendon, and leg joint absorbs the impact. This leg damping factor serves as an integrated functional indicator: rather than isolating the contribution of each individual component, it reflects the collective behavior of all three during the landing. A low value for this leg-damping factor indicates that the wing-tendon-joint system absorbs the landing impact with excessive stiffness rather than damping it smoothly; this excessive stiffness can stem from two distinct sources: damage to the tendon, causing it to cease functioning as the elastic spring that cushions impact in a healthy leg, or a loss of neuromuscular control that prevents the leg's tension from being adjusted at the precise moment of contact.

By combining dynamometer grip strength with the total impulse, peak force, contact time, and leg-damping factor provided by the force platform, this method makes it possible—based on a single landing on an instrumented perch—to diagnose both the actual condition of a bird's digital flexor tendon and the risk that a high-peak-force landing could cause damage equivalent to that of a collision, without having to wait for such damage to actually occur.

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