Short Communication: Sarcomere Length Changes During Fish Swimming

In vivo imaging of skeletal muscle form and function: 50 years of insight

Skeletal muscle form and function has fascinated scientists for centuries. Our understanding of muscle function has long been driven by advancements in imaging techniques. For example, the sliding filament theory of muscle, which is now widely leveraged in biomechanics research, stemmed from observations made possible by scanning electron microscopy. Over the last 50 years, advancing in medical imaging, combined with ingenuity and creativity of biomechanists, have provide a wealth of new and important insights into in vivo human muscle function. Incorporation of in vivo imaging has also advanced computational modeling and allowed our research to have an impact in many clinical populations. While this review does not provide a comprehensive or meta-analysis of the all the in vivo muscle imaging work over the last five decades, it provides a narrative about the past, present, and future of in vivo muscle imaging.

Corn Snakes Show Consistent Sarcomere Length Ranges Across Muscle Groups and Ontogeny

The force-generating capacity of muscle depends upon many factors including the actin-myosin filament overlap due to the relative length of the sarcomere. Consequently, the force output of a muscle may vary throughout its range of motion, and the body posture allowing maximum force generation may differ even in otherwise similar species. We hypothesized that corn snakes would show an ontogenetic shift in sarcomere length range from being centered on the plateau of the length-tension curve in small individuals to being on the descending limb in adults. Sarcomere lengths across the plateau would be advantageous for locomotion, while the descending limb would be advantageous for constriction due to the increase in force as the coil tightens around the prey. To test this hypothesis, we collected sarcomere lengths from freshly euthanized corn snakes, preserving segments in straight and maximally curved postures, and quantifying sarcomere length via light microscopy. We dissected 7 muscles (spinalis, semispinalis, multifidus, longissimus dorsi, iliocostalis (dorsal and ventral), and levator costae) in an ontogenetic series of corn snakes (mass = 80–335 g) at multiple regions along the body (anterior, middle, and posterior). Our data shows all of the muscles analyzed are on the descending limb of the length-tension curve at rest across all masses, regions, and muscles analyzed, with muscles shortening onto or past the plateau when flexed. While these results are consistent with being advantageous for constriction at all sizes, there could also be unknown benefits of this sarcomere arrangement for locomotion or striking.

Skeletal muscle design to meet functional demands

Skeletal muscles are length- and velocity-sensitive force producers, constructed of a vast array of sarcomeres. Muscles come in a variety of sizes and shapes to accomplish a wide variety of tasks. How does muscle design match task performance? In this review, we outline muscle's basic properties and strategies that are used to produce movement. Several examples are provided, primarily for human muscles, in which skeletal muscle architecture and moment arms are tailored to a particular performance requirement. In addition, the concept that muscles may have a preferred sarcomere length operating range is also introduced. Taken together, the case is made that muscles can be fine-tuned to perform specific tasks that require actuators with a wide range of properties.

Muscle performance during frog jumping: influence of elasticity on muscle operating lengths

A fundamental feature of vertebrate muscle is that maximal force can be generated only over a limited range of lengths. It has been proposed that locomotor muscles operate over this range of lengths in order to maximize force production during movement. However, locomotor behaviours like jumping may require muscles to shorten substantially in order to generate the mechanical work necessary to propel the body. Thus, the muscles that power jumping may need to shorten to lengths where force production is submaximal. Here we use direct measurements of muscle length in vivo and muscle force-length relationships in vitro to determine the operating lengths of the plantaris muscle in bullfrogs (Rana catesbeiana) during jumping. We find that the plantaris muscle operates primarily on the descending limb of the force-length curve, resting at long initial lengths (1.3 +/- 0.06 L(o)) before shortening to muscle's optimal length (1.03 +/- 0.05 L(o)). We also compare passive force-length curves from frogs with literature values for mammalian muscle, and demonstrate that frog muscles must be stretched to much longer lengths before generating passive force. The relatively compliant passive properties of frog muscles may be a critical feature of the system, because it allows muscles to operate at long lengths and improves muscles' capacity for force production during a jump.

Muscle fiber angle, segment bulging and architectural gear ratio in segmented musculature

The anatomical complexity of myomeres and myosepta has made it difficult to develop a comprehensive understanding of the relationship between muscle fiber architecture, connective tissue mechanics, and locomotor function of segmented axial musculature in fishes. The lateral hypaxial musculature (LHM) of salamanders is less anatomically complex and therefore a good system for exploring the basic mechanics of segmented musculature. Here, we derive a mathematical model of the LHM and test our model using sonomicrometry and electromyography during steady swimming in an aquatic salamander, Siren lacertina. The model predicts longitudinal segment strain well, with predicted and measured values differing by less than 5% strain over most of the range. Deviations between predicted and measured results are unbiased and probably result from the salamanders performing slight turns with associated body torsion in our unconstrained trackway swimming experiments. Model simulations of muscle fiber contraction and segment shortening indicate that longitudinal segment strain, for a given amount of muscle fiber strain, increases with increasing initial fiber angle. This increase in architectural gear ratio (AGR = longitudinal strain/fiber strain) is mediated by muscle fiber rotation; the higher the initial fiber angle, the more the fibers rotate during contraction and the higher the AGR. Muscle fiber rotation is additionally impacted by bulging in the dorsoventral (DV) and/or mediolateral (ML) dimensions during longitudinal segment shortening. In segments with obliquely oriented muscle fibers, DV bulging increases muscle fiber rotation, thereby increasing the AGR. Extending the model to include force and work indicates that force decreases with increasing initial muscle fiber angle and increasing DV bulging and that both longitudinal shortening and DV bulging must be included for accurate calculation of segment work.

Force-length characteristics of in vivo human skeletal muscle

In the present study, the in vivo force-length relations of the human soleus (SOL) and tibialis anterior (TA) muscles were estimated. Measurements were taken in six men at ankle angles from 30 degrees of dorsiflexion to 45 degrees of plantarflexion in steps of 15 degrees, and involved dynamometry, electrical stimulation, ultrasonography and magnetic resonance imaging (MRI). For each muscle and ankle angle studied the following three measurements were carried out: (1) dynamometry-based measurement of maximal voltage tetanic moment, (2) ultrasound-based measurement of pennation angle and fibre length and (3) MRI-based measurement of tendon moment arm length. Tendon forces were calculated dividing moments by moment arm lengths, and muscle forces were calculated dividing tendon forces by the cosine of pennation angles. In the transition from 30 degrees of dorsiflexion to 45 degrees of plantarflexion the SOL muscle fibre length decreased from 3.8 to 2.4 cm and its force decreased from 3330 to 290 N. Over the same range of ankle angles the TA muscle fibre length increased from 3.7 to 6 cm and its force increased from 157 to 644 N. Over the longest muscle fibre lengths reached the force of both muscles remained approximately constant. These results indicate that the intact human SOL and TA muscles operate in the ascending limb and plateau region of the force-length relationship. Similar conclusions were reached when calculating the theoretical operating range of the two muscle sarcomeres in the study.

Sarcomere length operating range of vertebrate muscles during movement

The force generated by skeletal muscle varies with sarcomere length and velocity. An understanding of the sarcomere length changes that occur during movement provides insights into the physiological importance of this relationship and may provide insights into the design of certain muscle/joint combinations. The purpose of this review is to summarize and analyze the available literature regarding published sarcomere length operating ranges reported for various species. Our secondary purpose is to apply analytical techniques to determine whether generalizations can be made regarding the “normal” sarcomere length operating range of skeletal muscle. The analysis suggests that many muscles operate over a narrow range of sarcomere lengths, covering 94+/−13 % of optimal sarcomere length. Sarcomere length measurements are found to be systematically influenced by the rigor state and methods used to make these measurements.

How animals move: an integrative view

Recent advances in integrative studies of locomotion have revealed several general principles. Energy storage and exchange mechanisms discovered in walking and running bipeds apply to multilegged locomotion and even to flying and swimming. Nonpropulsive lateral forces can be sizable, but they may benefit stability, maneuverability, or other criteria that become apparent in natural environments. Locomotor control systems combine rapid mechanical preflexes with multimodal sensory feedback and feedforward commands. Muscles have a surprising variety of functions in locomotion, serving as motors, brakes, springs, and struts. Integrative approaches reveal not only how each component within a locomotor system operates but how they function as a collective whole.

White muscle strain in the common carp and red to white muscle gearing ratios in fish

White muscle strains were recorded using sonomicrometry techniques for 70 fast-starts in the common carp Cyprinus carpio L. High-speed cine images were recorded simultaneously for 54 of these starts, and muscle strain was calculated independently from the digitized outlines of the fish. Sonomicrometry measurements of superficial muscle strain were not significantly different from the strain as calculated from the theory of simple bending of a homogeneous material: superficial muscle strain thus varied with chordwise distance from the spine. However, white muscle strain across a transverse section of the myotome shows less variation with chordwise position than would be expected from simple bending theory. Muscle strains measured using sonomicrometry thus do not necessarily represent the more uniform strain predicted for the whole section of the fish. White muscle strain can be accurately predicted from the spine curvatures as measured from the cine images if the gearing ratio between the red and white muscle fibres is known. A model for calculating the gearing ratio from the helical muscle fibre geometry was re-evaluated using current data for the kinematics of fast-starting C. carpio. This model predicted a mean gearing ratio of 2.8 for these fast-starts. A quicker, alternative approach to estimating gearing ratio from the position of the centroid of white fibre area is proposed and results in ratios similar to those calculated from the model of helical geometry. White muscle strains in fish can thus be estimated from measurements of spine curvature and muscle distribution alone.

Muscle, the motor of movement: properties in function, experiment and modelling

The purpose of this paper is to review exemplary aspects of different views of skeletal muscle characteristics. A classical view of muscle characteristics plays a very important role in modelling of muscles and movement. However, it often also pervades concepts on which our understanding of muscle function is based. In this view length effects, velocity effects and effects of degrees of activation and recruitment are distinguished and, often implicitly, assumed to be independent effects. It will be illustrated that using the classical approach many valuable things may be learned about muscle function and adaptation. At the same time we should realize that such a classical approach is too limited for use in generating knowledge about properties of muscles during daily use. The use of scaling of force to estimate muscular properties during submaximal activity on the basis of properties during maximal activation is shown to be very inadequate. An alternative view is described and particular examples are provided of changes in length-force characteristics as a consequence of submaximal activation, previous length change, as well as the effect of short-term histories of these variables. In addition, effects of inhomogeneities of muscle in morphology as well as physiological properties are considered. It is concluded that length-velocity-force characteristics are not unique properties of a muscle, and that these characteristics are not only strongly influenced by actual effects of recruitment, firing frequency, shortening performed and actual velocity of shortening but also by the short time history of these factors. Therefore, length, velocity and activation cannot be considered as independent determinants of muscle functioning. It is also shown that we are confronted with many indications of physiological individuality regarding these phenomena.

Human wrist motors: biomechanical design and application to tendon transfers

Moment arm, muscle architecture, and tendon compliance in cadaveric human forearms were determined and used to model the wrist torque-joint angle relation (i.e. wrist torque profile). Instantaneous moment arms were calculated by differentiating tendon excursion with respect to joint rotation. Maximum isometric tension of each wrist muscle-tendon unit was predicted based on muscle physiological cross-sectional area. Muscle forces were subsequently adjusted for sarcomere length changes resulting from joint rotation and tendon strain. Torque profiles were then calculated for each prime wrist motor (i.e. muscle-tendon unit operating through the corresponding moment arm). Influences of moment arm, muscle force, and tendon compliance on the torque profile of each motor were quantified. Wrist extensor motor torque varied considerably throughout the range of motion. The contours of the extensor torque profiles were determined primarily by the moment arm-joint angle relations. In contrast, wrist flexor motors produced near-maximal torque over the entire range of motion. Flexor torque profiles were less influenced by moment arm and more dependent on muscle force variations with wrist rotation and with tendon strain. These data indicate that interactions between the joint, muscle, and tendon yield a unique torque profile for each wrist motor. This information has significant implications for biomechanical modeling and surgical tendon transfer.