LINKING MUSCLE FORCE, WORK, AND METABOLIC COST

Researchers have long sought to link muscle force and work with the energetics of terrestrial locomotion. The laws of mechanics and thermodynamics provide the necessary framework, yet our understanding of how multiple muscles function together during locomotion remains incomplete. Measurements of organismal energy consumption have been paired with biomechanical analyses by both present authors (20,23,32) and several other research groups (e.g., (3,7,9,29)), providing important information on general links between locomotor mechanics and energetics. However, these techniques lack the resolution necessary to establish these relations at the muscular level. In addition to the challenges associated with quantifying muscle energy use, the other major impediments to progress on this front are the difficulties in measuring muscle force and work in individual muscles during locomotion. There are some limited situations where a computed joint moment or power can be related uniquely to muscle force or power, as when a single muscle is known to be the only major contributor to a net joint moment (e.g., (24)). However, whole-body, segment, or joint measurements usually do not provide information on the mechanics of individual muscles because of the redundancy of the musculoskeletal system. Computational models and direct measurements both provide a means for bridging this gap.

Musculoskeletal models can be used to generate simulations that replicate locomotor movements, yielding force and displacement histories for each modeled muscle. Modeling studies have advanced our understanding of the mechanical energetics of locomotion (17,36), yet they usually reveal little about the metabolic cost of the associated muscle actions (17). Several investigators have used muscle energetics models in generating simulations of locomotion (e.g., (1,16,31,34)); however, there usually has not been a strong focus on directly relating mechanics and energetics of individual muscles. Prime examples of the potential of this approach can be found in two recent studies on triceps surae tendon compliance (11,26). Lichtwark and Wilson (11) combined muscle models with experimental measurements to study medial gastrocnemius efficiency (ratio of muscle work to muscle heat+work) during walking and running in humans. The measured stiffness of the Achilles tendon was found to be nearly optimal for maximizing gastrocnemius efficiency for both walking and running because of favorable fascicle shortening velocities in both gait forms. Sellers et al. (26) used a whole-body musculoskeletal model to generate energy-optimal simulations of running and found that the beneficial effects of storage and recovery of elastic energy on both speed and economy were due mostly to the mechanical properties of the Achilles tendon, with all other muscles combined making a smaller contribution. These studies reinforce the notion that the human Achilles tendon is a critical elastic energy store that is highly tuned to benefit both force production and energy consumption in locomotion.

Blood flow analyses in guinea fowl also have provided important insights into the links between muscle force, work, and energy use. Rubenson and Marsh (24) recently combined, for the first time, an inverse dynamic analysis with muscle blood flow and organismal energetics to assess the mechanical efficiency (ratio of mechanical power to metabolic power) of both the combined limb-swing muscles and a single limb-swing muscle (tibialis cranialis, an ankle flexor muscle). For both the combined limb-swing muscles (Fig. 6) and the tibialis cranialis, efficiency was quite low for walking (<5%) and increased considerably with speed. This led to the conclusion that the mechanical work required to accelerate the limb was likely not the major determinant of energy use, except possibly at fast running speeds. Other mechanical functions, such as work done against antagonist muscles or isometric force production, also may be important determinants of limb-swing energy use (3,24). The efforts to assess efficiency of gastrocnemius (11) and tibialis cranialis (24) in vivo provide good examples of investigating the energetic function of an individual muscle with respect to its mechanical role (exit point C in Fig. 1).

Although the estimates of muscle efficiency described in the preceding paragraph were based on individual muscle energy use, the assessment of muscle-specific force and work was limited. Measurements of individual muscle force and work are possible in animal studies (e.g., (2)), and there have been some limited applications in humans (e.g., (8)). Buckle transducers, strain gauges, and fiber optic cables have been used to directly measure tendon forces, whereas sonomicrometry crystals attached to fascicles and ultrasonography have been adopted to assess muscle strain. These techniques have contributed substantially to our understanding of the mechanics of muscle function in locomotion, such as how fascicles and tendons interact with different muscle architectures, and under different locomotor conditions (2,8). We see future studies pairing direct muscle force and work measurements with data on muscle blood flow as a powerful approach for linking the mechanics and energetics of locomotion at the muscular level. However, direct simultaneous measurements of muscle force and energy use likely will be limited to animal studies for the near future. Although measurement of tendon force in humans is possible, blood flow is difficult to measure in humans during gait and is restricted to certain muscles, leaving the modeling approach as the best available option.

Fonte: http://journals.lww.com/acsm-essr/Fulltext/2011/04000/Understanding_Muscle_Energetics_in_Locomotion__New.2.aspx