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.

Active glycerinated rabbit psoas fibers were stretched at constant velocity (0.1-3.0 lengths/s) under sarcomere length control. As observed by previous investigators, force rose in two phases: an initial rapid increase over a small stretch (phase I), and a slower, more modest rise over the remainder of the stretch (phase II). The transition between the two phases occurred at a critical stretch (LC) of 7.7 +/- 0.1 nm/half-sarcomere that is independent of velocity. The force at critical stretch (PC) increased with velocity up to 1 length/s, then was constant at 3.26 +/- 0.06 times isometric force. The decay of the force response to a small step stretch was much faster during stretch than in isometric fibers. The addition of 3 mM vanadate reduced isometric tension to 0.08 +/- 0.01 times control isometric tension (P0), but only reduced PC to 0.82 +/- 0.06 times P0, demonstrating that prepowerstroke states contribute to force rise during stretch. The data can be explained by a model in which actin-attached cross-bridges in a prepowerstroke state are stretched into regions of high force and detach very rapidly when stretched beyond this region. The prepowerstroke state acts as a mechanical rectifier, producing large forces during stretch but small forces during shortening.

Anticipatory postural adjustments (APA) during bimanual actions can be observed when participants hold an object in one hand and then lift it with the other hand. The postural force used to hold the object is reduced in anticipation of unloading, indicating an accurate prediction of the change in load. We examined patients with unilateral or bilateral cerebellar damage as well as two individuals lacking the corpus callosum on the bimanual unloading task. The acallosal patients showed an intact APA, suggesting subcortical integration of motor signals for anticipatory adjustments during bimanual actions. Contrary to the hypothesis that the cerebellum is critical for predicting and compensating for the consequences of our actions, we found that the well-learned APA in this task was largely intact in cerebellar patients. However, cerebellar damage abolished short-term adaptation of the APA, and the patients were unable to acquire an APA in a similar but previously untrained situation. These results indicate that while over-learned anticipatory adjustments are preserved after cerebellar lesions, adaptation of this response and the acquisition of a novel coordination requires the cerebellum ipsilateral to the postural hand. Furthermore, this structure appears to be essential for the accurate timing of previously learned behaviors. The patients with cerebellar damage showed poorly timed adjustments with the APA beginning earlier than in healthy participants.

We describe the isotopic exchange of lactate and pyruvate after arm vein infusion of [3-(13)C]lactate in men during rest and exercise. We tested the hypothesis that working muscle (limb net lactate and pyruvate exchange) is the source of the elevated systemic lactate-to-pyruvate concentration ratio (L/P) during exercise. We also hypothesized that the isotopic equilibration between lactate and pyruvate would decrease in arterial blood as glycolytic flux, as determined by relative exercise intensity, increased. Nine men were studied at rest and during exercise before and after 9 wk of endurance training. Although during exercise arterial pyruvate concentration decreased to below rest values (P < 0.05), pyruvate net release from working muscle was as large as lactate net release under all exercise conditions. Exogenous (arterial) lactate was the predominant origin of pyruvate released from working muscle. With no significant effect of exercise intensity or training, arterial isotopic equilibration [(IE(pyruvate)/IE(lactate)).100%, where IE is isotopic enrichment] decreased significantly (P < 0.05) from 60 +/- 3.1% at rest to an average value of 12 +/- 2.7% during exercise, and there were no changes in femoral venous isotopic equilibration. These data show that 1). the isotopic equilibration between lactate and pyruvate in arterial blood decreases significantly during exercise; 2). working muscle is not solely responsible for the decreased arterial isotopic equilibration or elevated arterial L/P occurring during exercise; 3). working muscle releases similar amounts of lactate and pyruvate, the predominant source of the latter being arterial lactate; 4). pyruvate clearance from blood occurs extensively outside of working muscle; and 5). working muscle also releases alanine, but alanine release is an order of magnitude smaller than lactate or pyruvate release. These results portray the complexity of metabolic integration among diverse tissue beds in vivo.

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We have performed tests to find the mechanical properties of the hand and muscles driving wrist flexion and extension, and have identified parameters of a model. The hand acts as a nearly pure inertial load over most of its range of motion. It can be approximated as a rigid body rotating about a single axis. Viscosity of the wrist joint is negligible. Passive elastic torques are also small, except at extreme wrist angles. We measured torque as a function of wrist angle for maximum voluntary contractions, and angular velocity as a function of load. The torque/velocity curves for shortening muscles are well approximated by a Hill equation. To measure the "series elasticity" of the muscle equivalents, we imposed step changes in torque. The series stiffness is a monotonically increasing function of the preload, or "active state", in the Hill sense. We discuss the relationship of the measured parameters to properties of isolated muscles. To see the implications of the model structure for the "inverse problem" of identifying motor control signals, we simulated four models of different complexities, and found best fits to movement data, assuming simple pulse-shaped inputs. Inferred inputs depend strongly on model complexity. Finally, we compared the best fit control signals to recorded electromyograms.