Biomechanical Force Instruments

Instruments that measure force have had widespread use, having been extensively used to provide insights into the mechanics of locomotion, for example, in understanding the jumping behavior of animals [67] and in gait and foot conformation studies [68]. Furthermore, force transducers have been used in assessing maximal bite force in lizards [69], turtles [70], and bats [55], and also for determining the magnitude of resistance to fracture of individual prey [69]. In ruminants, bite force has been assessed for goats, sheep, deer, and cattle [25,26,44,52,66,71]. Portable force plates used in studies with ruminants have had load transducers mounted underneath a flat surface, and the forces and moments acting on the top surface of the plate using strain gauge technology were recorded. The plates avoid many of the constraints associated with indirect assessment of bite force. Further, they exhibit a high capability to record frequent and swift movements with accuracy, and the forces and moments can be broken down into three vectors acting along the three coordinate system axes: x, y, and z. The plates can be positioned in a pit [66], or alternatively animals graze from an elevated platform [44] with swards secured to the plate such that the animals' feet are level with the base of the sward, simulating a natural grazing angle.

The threading of individual leaves through holes arranged in wooden modules created the sward board pioneered by Black and Kenney [72]. In an interesting development, Hongo and co-workers [25,26] have transformed the sward board into a force plate where the leaves threaded through each hole are attached to a transducer and the force exerted per load-cell can be recorded. The sward board offers control over determination of the number of leaves removed per bite and the force per leaf compared with studies that present animals with boxed swards of herbage secured to a force plate. Boxed swards, however, present a natural sward that is not confounded by structured spaces between clumps of leaves, the number of potential bites is greater, and there are not the time limitations on board (module) preparation that are common to hand-constructed sward boards.

The greatest limitation to using biomechanical plates to measure force lies with the maximum available herbage that can be grazed at any single time, limiting grazing sessions to shorter than 2 to 3 min. Daily intake has been shown to be of some magnitude lower than that predicted from scaling up the instantaneous rate of intake by daily grazing time, and the inability to predict whether animals maintain instantaneous bite forces across temporal scales or lower the exerted force to maintain a biting momentum is a limitation with such instrumentation. Nevertheless, force boards and plates avoid the confounding issues associated with one-dimensional laboratory-based instrumentation and allow for an objective measure of the force exerted by the animal. However, with the recent attention given to quantification of bite force in ruminants, it has become apparent that there are issues surrounding output interpretation that need resolving. Output generated from force plates has illustrated very clearly the incidence of multiple peaks, with at least two and four peaks recorded for sheep and cattle, respectively [25,26]. The incidence of multiple peaks would be expected to correlate with capture rate since a faster capture rate allows for accurate representation of the dynamics of the force being applied to the forage material, and likewise a slow capture rate will see the force accumulated into fewer data points. This raises concerns as to how bite force should be determined. A more representative assessment might be to describe force in terms of the number of peaks rather than force per bite, but in many instances, this may only be relevant for bites removed from dense and/or mature swards. Calculating a reward to cost ratio using the peak bite force recording can lead to erroneous predictions, which can only leave us wondering whether peak bite force is the parameter we seek.

On vegetative swards where the upper stratum offers little impediment to the prehension of leaf, the vertical vector is dominant, but with increasing sward structural complexity, the relative magnitude of the longitudinal and lateral vectors increases [44]. Unpublished data [44] from deer, sheep, and cattle suggests that the relative change in the magnitude of the force vectors, in response to the spatial arrangement of morphological organs, appears to be less significant for smaller bodied animals. There is no comparable data since the individual vectors appear to be independent in the data sets generated by Hongo and co-workers [25,26]. Nevertheless, clarification of the direction of head movement and dominant force vector across animal body size could provide insights into understanding bite procurement. Descriptive observations in the literature suggest that cattle and sheep sever plant material using a backward jerk of the head [73,74], and deer tear herbage with an upward and downward jerking action [75], although these observations may be confounded by the physical structure of the vegetation that animals were grazing. Hongo and Akimoto [25] suggested that the observation that cattle moved their heads in a backward direction when grazing mixtures of perennial ryegrass and reed canary grass culms was a strategy to minimize the ingestion of stems because the jerking action in a backward plane permitted stiff stems to spring back from the clamped material, and these authors described the foraging action as a "comb-out" strategy.

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