McGowan CP, Baudinette RV, Biewener AA.
Joint work and power associated with acceleration and deceleration in tammar wallabies (Macropus eugenii). J Exp BiolJ Exp BiolJ Exp Biol. 2005;208 :41-53.
AbstractMeasurements of joint work and power were determined using inverse dynamics analysis based on ground reaction force and high-speed video recordings of tammar wallabies as they decelerated and accelerated while hopping over a force platform on level ground. Measurements were obtained over a range of accelerations ranging from -6 m s(-2) to 8 m s(-2). The goal of our study was to determine which joints are used to modulate mechanical power when tammar wallabies change speed. From these measurements, we also sought to determine which hind limb muscle groups are the most important for producing changes in mechanical work. Because our previous in vivo analyses of wallaby distal muscle function indicated that these muscle-tendon units favor elastic energy savings and perform little work during steady level and incline hopping, we hypothesized that proximal muscle groups operating at the hip and knee joint are most important for the modulation of mechanical work and power. Of the four hind limb joints examined, the ankle joint had the greatest influence on the total limb work, accounting for 89% of the variation observed with changing speed. The hip and metatarsophalageal (MP) joints also contributed to modulating whole limb work, but to a lesser degree than the ankle, accounting for 28% (energy production) and -24% (energy absorption) of the change in whole limb work versus acceleration, respectively. In contrast, the work produced at the knee joint was independent of acceleration. Based on the results of our previous in vivo studies and given that the magnitude of power produced at the ankle exceeds that which these muscles alone could produce, we conclude that the majority of power produced at the ankle joint is likely transferred from the hip and knee joints via proximal bi-articular muscles, operating in tandem with bi-articular ankle extensors, to power changes in hopping speed of tammar wallabies. Additionally, over the observed range of performance, peak joint moments at the ankle (and resulting tendon strains) did not increase significantly with acceleration, indicating that having thin tendons favoring elastic energy storage does not necessarily limit a tammar wallaby's ability to accelerate or decelerate.
Hoyt DF, Wickler SJ, Biewener AA, Cogger EA, De La Paz KL.
In vivo muscle function vs speed. I. Muscle strain in relation to length change of the muscle-tendon unit. J Exp BiolJ Exp BiolJ Exp Biol. 2005;208 :1175-90.
AbstractThe activity of muscles can be concentric (shortening), eccentric (lengthening) or isometric (constant length). When studying muscle function it is important to know what the muscle fascicles are actually doing because the performance of muscle is strongly influenced by the type of activity: force decreases as a function of shortening velocity during concentric contractions; force produced during eccentric contractions can be stronger than maximum isometric force, and force production is enhanced if a concentric contraction follows an eccentric phase. It is well known that length changes of muscle fascicles may be different from length changes of the overall muscle-tendon unit because of the compliance of the series elasticity. Consequently, fascicles of joint extensor muscles may not undergo eccentric activity even when the joint flexes, but the extent to which this occurs may vary with the compliance of the series elasticity and may differ between species: the vastus lateralis, a knee extensor, shortens when active during trotting in dogs and lengthens in rats. Previous studies of kinematics of trotting in horses have shown that during stance, the elbow extends nearly continuously with a brief period of flexion near mid-stance and the knee exhibits two phases of flexion followed by extension. The lateral triceps (an elbow extensor) has no external tendon but the vastus lateralis has a relatively long external tendon and the fascicles insert on an aponeurosis. Thus, one might expect the relation between fascicle strain and overall length change of the muscle-tendon units to be quite different in these two muscles. In the present study in horses, fascicle length changes of the lateral triceps and vastus lateralis were measured with sonomicrometry and length changes of the muscle-tendon units were estimated from muscle architecture and joint kinematics for four horses trotting on a treadmill at nine speeds. Because the focus of this study was the relation between length changes of the muscle-tendon unit (estimated from kinematics) and length changes in the muscle fascicles, we divided the stance-phase sonomicrometry records into phases that corresponded to the alternating flexion and extension of the joint as indicated by the kinematic records. During its one eccentric phase, the triceps shortened by 0.7+/-0.4% despite a predicted lengthening of 1%. Similarly, the vastus shortened by 3.7+/-1.9% when kinematics predicted 3.2% lengthening. During their concentric phases the triceps shortened by 10.6% and the vastus shortened by 8.1%. Strain in the triceps did not change with speed but it did in the vastus. Strain rate increased with speed in both muscles as did the integrated EMG, indicating an increase in the volume of muscle recruited. Thus, despite differences in their architecture and the kinematic patterns of the associated joints, these two joint extensors exhibited similar activity.
Gillis GB, Flynn JP, McGuigan P, Biewener AA.
Patterns of strain and activation in the thigh muscles of goats across gaits during level locomotion. J Exp BiolJ Exp BiolJ Exp Biol. 2005;208 :4599-611.
AbstractUnlike homologous muscles in many vertebrates, which appear to function similarly during a particular mode of locomotion (e.g. red muscle in swimming fish, pectoralis muscle in flying birds, limb extensors in jumping and swimming frogs), a major knee extensor in mammalian quadrupeds, the vastus lateralis, appears to operate differently in different species studied to date. In rats, the vastus undergoes more stretching early in stance than shortening in later stance. In dogs, the reverse is true; more substantial shortening follows small amounts of initial stretching. And in horses, while the vastus strain trajectory is complex, it is characterized mainly by shortening during stance. In this study, we use sonomicrometry and electromyography to study the vastus lateralis and biceps femoris of goats, with three goals in mind: (1) to see how these muscles work in comparison to homologous muscles studied previously in other taxa; (2) to address how speed and gait impact muscle actions and (3) to test whether fascicles in different parts of the same muscle undergo similar length changes. Results indicate that the biceps femoris undergoes substantial shortening through much of stance, with higher strains in walking and trotting [32-33% resting length (L0)] than galloping (22% L0). These length changes occur with increasing biceps EMG intensities as animals increase speed from walking to galloping. The vastus undergoes a stretch-shorten cycle during stance. Stretching strains are higher during galloping (15% L0) than walking and trotting (9% L0). Shortening strains follow a reverse pattern and are greatest in walking (24% L0), intermediate in trotting (20% L0) and lowest during galloping (17% L0). As a result, the ratio of stretching to shortening increases from below 0.5 in walking and trotting to near 1.0 during galloping. This increasing ratio suggests that the vastus does relatively more positive work than energy absorption at the slower speeds compared with galloping, although an understanding of the timing and magnitude of force production is required to confirm this. Length-change regimes in proximal, middle and distal sites of the vastus are generally comparable, suggesting strain homogeneity through the muscle. When strain rates are compared across taxa, vastus shortening velocities exhibit the scaling pattern predicted by theoretical and empirical work: fascicles shorten relatively faster in smaller animals than larger animals (strain rates near 2 L s-1 have been reported for trotting dogs and were found here for goats, versus 0.6-0.8 L s-1 reported in horses). Interestingly, biceps shortening strain rates are very similar in both goats and rats during walking (1-1.5 L s-1) and trotting (1.5-2.5 L s-1, depending on speed of trot), suggesting that the ratio of in vivo shortening velocities (V) to maximum shortening velocities (Vmax) is smaller in small animals (because of their higher V(max)).
Biewener AA.
Biomechanical consequences of scaling. J Exp BiolJ Exp BiolJ Exp Biol. 2005;208 :1665-76.
AbstractTo function over a lifetime of use, materials and structures must be designed to have sufficient factors of safety to avoid failure. Vertebrates are generally built from materials having similar properties. Safety factors are most commonly calculated based on the ratio of a structure's failure stress to its peak operating stress. However, yield stress is a more likely limit, and work of fracture relative to energy absorption is likely the most relevant measure of a structure's safety factor, particularly under impact loading conditions characteristic of locomotion. Yet, it is also the most difficult to obtain. For repeated loading, fatigue damage and eventual failure may be critical to the design of biological structures and will result in lower safety factors. Although area:volume scaling predicts that stresses will increase with size, interspecific comparisons of mammals and birds show that skeletal allometry is modest, with most groups scaling (l proportional, variant d0.89) closer to geometric similarity (isometry: l proportional, variant d1.0) than to elastic similarity (l proportional, variant d0.67) or stress similarity (l proportional, variant d0.5). To maintain similar peak bone and muscle stresses, terrestrial mammals change posture when running, with larger mammals becoming more erect. More erect limbs increases their limb muscle mechanical advantage (EMA) or ratio of ground impulse to muscle impulse (r/R= integral G/integral Fm). The increase in limb EMA with body weight (proportional, variant W0.25) allows larger mammals to match changes in bone and muscle area (proportional, variant W0.72-0.80) to changes in muscle force generating requirements (proportional, variantW0.75), keeping bone and muscle stresses fairly constant across a size range 0.04-300 kg. Above this size, extremely large mammals exhibit more pronounced skeletal allometry and reduced locomotor ability. Patterns of ontogenetic scaling during skeletal growth need not follow broader interspecific scaling patterns. Instead, negative allometric growth (becoming more slender) is often observed and may relate to maturation of the skeleton's properties or the need for younger animals to move at faster speeds compared with adults. In contrast to bone and muscle stress patterns, selection for uniform safety factors in tendons does not appear to occur. In addition to providing elastic energy savings, tendons transmit force for control of motion of more distal limb segments. Their role in elastic savings requires that some tendons operate at high stresses (and strains), which compromises their safety factor. Other 'low stress' tendons have larger safety factors, indicating that their primary design is for stiffness to reduce the amount of stretch that their muscles must overcome when contracting to control movement.