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Neuromechanical strategies of legged locomotion

Neuromechanical strategies of legged locomotion refers to the principles by which the nervous system coordinates neurophysiological activities and resulting behavior at the musculoskeletal level for terrestrial locomotion using limbs.

Neural control of locomotion involves complex interaction among neuromechanical elements at multiple hierarchical levels: higher centers for planning and integrating sensory information (e.g. visual, vestibular, proprioceptive, cutaneous), spinal circuit for regulating rhythmic activity (e.g. central pattern generator), and local feedback such as stretch reflex. The outcome of these processes is manifested at the execution level in muscle activities, where forces exerted by activating muscles results in net moment at the joints which subsequently produce joint movement or limb endpoint forces.

However, because the number of muscles exceeds the number of joints to be controlled, any movement can be produced with multiple muscle coordination patterns. Therefore, specific muscle pattern to produce a given movement must be selected from an infinite number of possible solutions. Nevertheless, the nervous system uses not just any solution randomly chosen from the pool for a given motor task, but rather coordinates the muscles in a specific manner, exhibiting consistent and robust functional behavior.

There are multiple suggested strategies that the nervous system may use to coordinate activities in the muscles for legged locomotion in animals.

Passive dynamics
One simple strategy is to make the most of the dynamics of the body itself. The viability of this strategy has been demonstrated in many passive-dynamic walkers, where minimum or no actuation and control is required to produce human-like walking. This strategy in particular takes advantage in energetics because most of the motion can be accounted for by relying on passive dynamics of the limbs. Even for active-dynamic walking, minimal energy input is required for powering necessary to compensate for mechanical energy dissipation at the end of each step on heel-strike. Although such “passive” walking is not normally observed in human gait, people behave more like the passive-dynamic walkers when consciously walking down-hill in “relaxed” condition, suggesting that it might actually be a useful gait strategy when energetic efficiency is in concern.

Intrinsic muscle mechanics
Muscles are not merely viscoelastic force-generating actuators; rather, its intrinsic mechanics provides a way how nervous system can intervene for control, e.g. mediate maneuverability or stability while running in cockroaches, or  gunea fowls. Many of the animal locomotive behaviors such as running or hopping, can be modeled simply as a spring–mass system, where the nervous system can adjusts the stiffness of the limb by tuning muscle activities to govern the overall control of the center of mass.

Physiological differences across muscles also play an important role in context-dependent recruitment of muscles in various tasks. For example, coordination of two synergistic ankle extensors, medial gastrocnemius (MG) and soleus (SOL) in cats during walking, trotting and jumping depends on muscle architecture. MG is comprised most of fast-twitching fibers and pennate into the tendon at an angle which results in greater cross sectional area. Thus, MG is useful in generating large forces, fast. In contrast, SOL is a muscle comprised mostly of slow-twitching fibers, with its fibers aligned parallel to the tendon. Thus, SOL is more useful in standing or slow locomotion where it is fatigue-resistant withstanding pre-longed contraction.

Morphology is also reflected in a proximal to distal gradient (or difference) in neuromechanical function in legged locomotion. In animal running, large proximal muscles primarily controls the cyclic movement of leg and contribute to large portion of mechanical work required for the task, whereas small distal muscles are fast in adjusting force and work thus contributes to maintaining stability.

Optimizing the cost
Acquiring a better solution for legged locomotion in animals can be an outcome of an evolving process, working at various time scales such as development, learning and adaptation. The criteria to be optimized can be quantified with specific cost functions to minimize or maximize and has demonstrated such goal-directed strategies for selecting muscle patterns in both experiments and theoretical models.

Energetics
Making an energetically efficient movement is found to be a primary goal for locomotion in vertebrates. Humans walk at speed with minimal cost of transport, where step frequency, length, or width are tuned to minimize mechanical energetics.

One popular criterion in resolving musculoskeletal redundancy in biomechanical models with optimization is minimizing muscle stress, which is also a form of muscular effort. This has been widely applied to predict muscle coordination in walking simulations. However, measured muscle activity often varies from these predictions, suggesting that the choice of the nervous system for given task should be a good enough solution, but not necessarily the best for one case.

Stability
In some cases, maintaining balance during locomotion is in particular of most importance, e.g. older adults or patients with gait disorder. Through the selection of muscle activation patterns producing equivalent motor output, local stability of a musculoskeletal system can be tuned. The linearized dynamics conferred by the active stiffness and viscosity of muscles and the rigid-body mechanics can be quantified by the eigenvalues of the system. Interestingly, predicted optimal solution using minimum muscle stress criteria often yields unstable limb dynamics, suggesting that there might be a trade-off between effort and stability. How the nervous system opts to stay stable, at the cost of effort-wise expenditure, in unstable environment or task can actually be observed from increase in co-activation of muscles across joints.

Muscle synergy hypothesis
Studying spatiotemporal coordination of muscles in many different motor tasks and species has revealed a modular organization where muscles are activated synchronously in small number of groups, called muscle synergies. Studies suggest that the nervous system may use a few sets of solutions, reducing the number of independent degrees of freedom that must be controlled. This hypothesis has been mainly supported by identification of a small number of groups of muscles with fixed ratios of activation called muscle synergies which can account for the large number of electromyographic (EMG) activities examined during motor behaviors.

In cats, muscle synergies serve as flexible substrate for the control of intralimb coordination during locomotion. Muscle synergies found in human walking can be commonly used for multiple motor behaviors that involve different neural pathways: spinal circuit for locomotion, possibly brainstem for postural balance, and motor cortex for voluntary modifications to walking. Also, muscle synergies analysis has potential use for assessing functional performance of locomotive deficiency, e.g. in post-stroke hemiparesis or Parkinson’s disease