Skeletal muscle performance is attributed to work producing crossbridge interactions between the contractile proteins, actin and myosin. Myosin kinetics are through to underpin the isotonic force velocity relationship observed in skeletal muscle. Variation in maximum shortening velocity (Vmax ) is attributed to variation in myosin isoform and is thought to dictate organismal performance. However, Vmax  varies in ways that cannot be explained by actin-myosin interactions, decreasing with decreasing muscle recruitment irrespective of the myosin isoform recruited. This effect of recruitment on Vmax  has been suggested to be due to the physical properties of the muscle, such as mass, viscosity and elasticity, limiting its capacity to accelerate and so reach a myosin-defined Vmax . Here we determined the effect of recruitment on Vmax  in 2 anuran plantaris muscles (cane toad and tree frog), with an ~40-fold difference in muscle mass, in vitro. If mass is an important determinant of Vmax , then we would expect a greater effect of recruitment in the larger muscle.  There was a significant effect of recruitment on Vmax  in both muscles (p< .005), and this effect was significantly greater in the larger cane toad muscle (p< .005). This has implications for the scaling of locomotor performance as the ratio between the force available for acceleration and mass to be accelerated systematically declines with increasing body size. Hence, larger species may be limited in their locomotor performance irrespective of the myosin isoform expressed, potentially explaining effects such as the scaling of maximum running speed.

The effects of muscle mass on maximum shortening velocity and the implications for scaling of locomotor performance

Muscle is the prime mover of animal locomotion, but muscle force is not usually equal to output force. Instead, the instantaneous ratio between output and input force (the mechanical advantage, MA) is modulated by intervening skeletal elements. Traditional functional analysis, focussing on the instantaneous action of gearing, suggests that smaller MA favours speed, while larger MA favours force. However, mounting evidence suggests that this perspective is too simplistic when considering the outcome of a dynamic muscle contraction. We approach this problem systematically by introducing an analytical framework to study the effect of gearing. This framework reveals two mechanisms by which the output speed of a contraction is affected by gearing. First, gearing can limit muscle work output, because it is bound not only by muscle displacement, but also by muscle shortening velocity. Second, it controls how each unit of muscle work is partitioned into system energies (e.g. kinetic energy vs heat). Consequently, smaller gear ratios do not always result in larger output speeds. No single optimal gear ratio exists for any organism, but rather the optimum depends on internal muscle properties and contraction dynamics, as well as external forces. The analytical framework we propose provides a blueprint to determine optimality conditions across muscle physiology, anatomy, mechanical environment and size, and so to investigate form-function relationships in musculoskeletal gearing.

Musculoskeletal gearing for maximum speed: analytic optimization across mechanical environment and size.

Silicon (Si) is increasingly being recognised as an essential element for plant health to tackle abiotic stresses, and it may also deter herbivores from plant-feeding, as a result of tooth wear caused by silica phytoliths. However, direct quantitative evidence for Si-induced wear is scarce. In this study, we subjected pristine Atta vollenweideri ant mandibles to multiple wear treatments on Hordeum vulgare barley leaves, grown with either 0-, 2- or 4-mM Si-supplemented hydroponic media, resulting in varying Si concentrations in the leaves. Pristine mandibles were collected from freshly eclosed ants that had never engaged in foraging, and wear treatments were conducted by driving isolated mandibles through leaf lamina at a constant speed using custom-built clamps attached to a motor stage. To quantify the effects of mandible wear, before, in between, and after each treatment, the forces required to cut standardised PDMS sheets were measured using a custom-built fibre optic force transducer; after 300 mm barley cut length, the cutting forces on PDMS increased significantly across all Si conditions; this force increase, however, was not consistent across Si concentrations, such that the change in cutting force per leaf length cut was 174% larger on 4 mM compared to 0 mM leaves. These results indicate that mandible abrasion may be facilitated by the mechanical contact between cutting edge and silica phytoliths. As increased mandible wear is likely of significant detriment to lifetime foraging efficiency, targeted Si-supplementation may be an effective yet ‘green’ agricultural strategy to provide protection against insect herbivores.

Abrasive silica phytoliths significantly increase mandible wear in leaf-cutter ants

Many small vertebrates and arthropods are able to run with sticky feet, and must hence be able to rapidly switch between strong attachment and effortless detachment. The timescale over which contacts are formed and broken precludes chemical control. Instead, terrestrial climbing animals appear to rely on mechanical `switches’. Recently, strong evidence has emerged that at least some of these switches are universal: Whether adhesive pads are `hairy’, `smooth’, `wet’ or `dry’ pads, there appears to exist a linear relationship between the shear force, applied in parallel to the surface, and the adhesive force, normal to the surface, required to detach them. This ‘shear-sensitivity’ provides a rapid mechanical switch, enables larger animals to maintain approximately constant safety factors despite smaller surface-to-volume ratios, and has implications for optimal step coordination patterns during climbing on vertical and inverted substrates. The control of adhesion via shear forces is deeply integrated with the anatomy and locomotion of climbing animals, and involves both active neuro-muscular control, and rapid passive responses. The resulting dynamic adhesive systems are robust, reliable, versatile and nevertheless remarkably simple. They may thus serve as suitable inspiration for the design of climbing robots.

David Labonte

4

6

SHORT BIO

Stay up to date with the latest Underline news!

PRESENTATIONS

CONFERENCES

COMPANY

RESOURCES