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.

https://science.sciencemag.org/content/sci/288/5463/100.full.pdf

How Animals Move: An Integrative View

The sections in this article are:


1
Biomechanical and Electromyographical Information
1.1
Single Limb During Locomotion

1.2
Interlimb Coordination

1.3
Treadmill Versus Overground Locomotion

1.4
Trunk Movements During Locomotion

1.5
Pathological Gaits

1.6
Summary




2
Neural Generation of “Basic Locomotor Synergy”
2.1
Central Versus Peripheral

2.2
Parts of CNS of Primary Importance for Neural Control of Basic Locomotor Synergy

2.3
Spinal Centers for Locomotion—Behavioral Results

2.4
Reflex Control of Basic Locomotor Synergy

2.5
Activity in Certain Spinal, Cerebellar, and Brain Stem Neurons and in Certain Reflex Pathways During Locomotion

2.6
Cerebellum and Locomotion

2.7
Initiation of Locomotion—Brain Stem Circuitry

2.8
Central Organization of Spinal Pattern Generation

2.9
Possible Rhythm-Generating Mechanisms and Models—Facts and Fiction

2.10
Developmental Aspects

2.11
Summary




3
Adapting Basic Locomotor Synergy to Animal's Needs
3.1
Changing Speed

3.2
Goal-Directed Locomotion—Turning and Walking Along Curvatures

3.3
Modifications of “Locomotor Posture”

3.4
Positioning of Limb in Each Step

3.5
Reflex Adaptation of Step

3.6
Summary




4
Concluding Remarks

Control of Locomotion in Bipeds, Tetrapods, and Fish

From the mechanical point of view the spinal system is highly complex, containing a multitude of components, passive and active. In fact, even if the active components (the muscles) were exchanged by passive springs, the total number of elements considerably exceeds the minimum needed to maintain static equilibrium. In other words, the system is statically highly indeterminate. The particular role of the active components at static equilibrium is to enable a virtually arbitrary choice of posture, independent of the distribution and magnitude of the outer load albeit within physiological limits. Simultaneously this implies that ordinary procedures known from the analysis of mechanical systems with passive components cannot be applied. Hence the distribution of the forces over the different elements is not uniquely determined. Consequently nervous control of the force distribution over the muscles is needed, but little is known about how this achieved. This lack of knowledge implies great difficulties at numerical simulation of equilibrium states of the spinal system. These difficulties remain even if considerable reductions are made, such as the assumption that the thoracic cage behaves like a rigid body. A particularly useful point of view about the main principles of the force distributions appears to be the distinction between a local and a global system of muscles engaged in the equilibrium of the lumbar spine. The local system consists of muscles with insertion or origin (or both) at lumbar vertebrae, whereas the global system consists of muscles with origin on the pelvis and insertions on the thoracic cage. Given the posture of the lumbar spine, the force distribution over the local system appears to be essentially independent of the outer load of the body (though the force magnitudes are, of course, dependent on the magnitude of this load). Instead different distributions of the outer load on the body are met by different distributions of the forces in the global system. Thus, roughly speaking, the global system appears to take care of different distributions of outer forces on the body, whereas the local system performs an action, which is essentially locally determined (i.e. by the posture of the lumbar spine). The present work focuses on the upright standing posture with different degree of lumbar lordosis. The outer load is assumed to consist of weights carried on the shoulders. By reduction of the number of unknown forces, which is done by using a few different principles, a unique determination of the total force distributions at static equilibrium is obtained.(ABSTRACT TRUNCATED AT 400 WORDS)

https://www.tandfonline.com/doi/pdf/10.3109/17453678909154177

Stability of the lumbar spine. A study in mechanical engineering

When the hand is displaced from an equilibrium posture by an external disturbance, a force is generated to restore the original position. We developed a new experimental method to measure and represent the field of elastic forces associated with posture of the hand in the horizontal plane. While subjects maintained a given posture, small displacements of the hand along different directions were delivered by torque motors. The hand was held in the displaced positions and, at that time, we measured the corresponding restoring forces before the onset of any voluntary reaction. The stiffness in the vicinity of the hand equilibrium position was estimated by analyzing the force and displacement vectors. We chose to represent the stiffness both numerically, as a matrix, and graphically, as an ellipse characterized by three parameters: magnitude (the area), shape (the ratio of axis) and orientation (direction of the major axis). The latter representation captures the main geometrical features of the elastic force field associated with posture. We also evaluated the conservative and nonconservative components of this elastic force field. We found that the former were much larger than the latter and concluded that the behavior of the neuromuscular system of the multiarticular arm is predominantly spring-like. Our data indicated that the shape and orientation of the stiffness were invariant over subjects and over time. We also investigated the ability of our subjects to produce voluntary and adaptive changes in the stiffness. Our findings indicated that, when a disturbance acting along a fixed and predictable direction was imposed, the magnitude of the stiffness was increased but only minor changes in shape and orientation occurred. Taken together, all of these experiments represent a step toward the understanding of the interactions between geometrical and neural factors involved in maintaining hand posture and its interactions with the environment.

http://web.mit.edu/bcs/bizzilab/publications/mussa-ivaldi1985.pdf

Neural, mechanical, and geometric factors subserving arm posture in humans

This paper examines the postulate that an important function of the activity of antagonist muscle groups is to modulate mechanical impedance. Some biomechanical modeling and analyses are presented leading to a prediction of simultaneous activation of antagonist muscles in the maintenance of upright posture of the forearm and hand. An experimental observation of antagonist coactivation in this situation is presented.

Adaptive control of mechanical impedance by coactivation of antagonist muscles

1. The tension in tetanized cat soleus and lateral gastrocnemius muscles was measured during alternating lengthening and shortening movements. Sinusoidal movements were sometimes used; on other occasions the movement was at a constant velocity but with periodic reversal of direction.

2. With constant velocity movements of small amplitude the tension rose steeply during lengthening and fell during shortening in a relatively simple way. With longer movements the tension at first changed steeply as it had done with the smaller movement, but later in the movement the resistance of the muscles decreased so that the tension change became more gradual. The muscles resisted a small movement or the first part of a larger movement with a `short range stiffness' which did not persist as the movement continued.

3. So long as the constant velocity movement was not too slow the short range stiffness was independent of velocity though it lasted for more of a fast movement than of a slow one.

4. In small movements the muscle was never extended beyond its short range stiffness, and the over-all peak-to-peak tension change was therefore large compared with the amplitude of movement. When, with larger movements, the muscle was stretched into a range in which it became more compliant, the peak-to-peak force fluctuation did not increase by an equivalent amount, and over the whole course of the movement the force change per unit extension was smaller.

5. When the movement was confined to a short range, little work was expended in driving the muscle through a cycle of movement; its properties were essentially elastic. With larger amplitudes the muscle met the movement with a frictional resistance, the tension during lengthening then being greater than during shortening. A considerable amount of work had then to be done on the muscle to maintain the movement.

6. The short range stiffness was also apparent in the response to sinusoidal movements.

7. The short range stiffness was attributed to elastic properties of cross-bridges between thick and thin filaments in the myofibrils.

8. The effect of the short range stiffness on the mechanical properties of the limb is discussed.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1974.sp010613

The short range stiffness of active mammalian muscle and its effect on mechanical properties

1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation, it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.

2. After contracting isometrically the muscle was subjected to constant velocity lengthening or shortening movements.

3. During shortening the tension always fell below the isometric value. The fall in tension was usually greatest when low rates of stimulation were used.

4. The effect of lengthening on tension depended on the rate of stimulation. At high rates of stimulation the tension during lengthening always rose above the isometric tension. At lower rates of stimulation (5-15 pulses/sec) the tension rose at the beginning of an extension, but decreased later in the movement to a level that was often less than the isometric tension corresponding to that muscle length. At these stimulus rates the tension during isometric contraction was usually higher than during a sustained movement in either direction.

5. At low rates of stimulation longitudinal vibratory movements of more than 0·1 mm also reduced the tension far below the isometric value, whereas the reduction was quite slight when the rate of stimulation was high.

6. The isometric tension during smooth contractions at low stimulus rates was remarkable in the following respects: it developed rather slowly, it was higher than the tension during or immediately after movements, and it was only slowly regained after movement had ceased.

7. The results are discussed in relation to the sliding filament theory of muscle contraction, which, with certain assumptions, provides an explanation for many of the findings.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1969.sp008924

The mechanical properties of cat soleus muscle during controlled lengthening and shortening movements

1. Spontaneous flexion-extension tremor of the elbow was recorded in normal human subjects while they exerted a flexing force against different loads.2. Increases in the flexing force up to (1/2) or (3/4) of the subject's maximum were accompanied by increases in the amplitude of the tremor. A further increase to very large forces was associated with a decrease in tremor.3. When the subject flexed against a spring, the frequency of the tremor was related to the properties of the spring; with increasingly stiff springs the tremor frequency increased up to about 12 Hz. With the stiffest springs, however, the tremor became irregular and very small in amplitude.4. Weights added to the limb at the wrist reduced the frequency of tremor.5. By using appropriate combinations of spring and mass, the principal tremor frequency could be adjusted between 2 and 12 Hz, higher in some subjects. Within this range the frequency(2) was approximately proportional to the spring stiffness/mass, just as it is in a spring-mass system, though the forearm provided some of this mass, and some of the spring-like resistance was in the flexor muscles.6. When by suitable loading the tremor frequency was brought between 8 and 12 Hz, and the subject forcibly flexed his elbow, the tremor became regular and large in amplitude. This was a form of limited instability in the stretch reflex which arose because a powerful reflex response to extension of the elbow acted back on the flexor muscles after a delay.7. When the limb was so loaded that its natural frequency was outside this 8-12 Hz range, it behaved as a filter, and in response to any disturbing noise it oscillated mainly at frequencies close to the natural frequencies of either the mechanical system or the stretch reflex.8. This filtering function of the limb with its stretch reflex probably plays an important part in the control of normal movement and tremor.9. The large flexing forces that were here associated with vigorous tremor normally occur when a limb supports a large mass. Such a mass would give the mechanical system within the limb a low natural frequency; inconveniently large oscillations in the 8-12 Hz range would therefore seldom occur.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1974.sp010615

The effects of load and force on tremor at the normal human elbow joint

1. The mechanical resistance of the human forearm has been measured during imposed sinusoidal flexion-extension movements of the elbow joint.2. The force required to move the limb can be divided into components required to move the mass, and components required to overcome the resistance offered by elastic and frictional properties of the muscles and other soft tissues.3. When during a vigorous flexing effort the limb was subjected to a small amplitude sinusoidal movement each extension was followed by a considerable reflex contraction of the flexor muscles. At low frequencies of movement this reflex provided an added resistance to extension, but at 8-12 Hz the delay in the reflex pathway was such that the reflex response to extension occurred after the extension phase of the movement was over and during the subsequent flexion movement. The reflex activity then assisted the movement whereas at other frequencies it impeded it.4. The reflex response to movement increased as the subject exerted a greater flexing force.5. Small movements generated a relatively larger reflex response than big ones.6. Even with large amplitudes of movement when the reflex activity was relatively small, the limb resisted extension with a high level of stiffness; this was comparable with the short range stiffness of muscles in experimental animals.7. The fact that at some frequencies the reflex response assisted the movement implies that with appropriate loading the limb could undergo a self-sustaining oscillation at those frequencies.

Peter M. H. Rack

Papers

The short range stiffness of active mammalian muscle and its effect on mechanical properties

The mechanical properties of cat soleus muscle during controlled lengthening and shortening movements

The effects of load and force on tremor at the normal human elbow joint

The forces generated at the human elbow joint in response to imposed sinusoidal movements of the forearm.

Some mechanical properties of the cat soleus muscle at various stimulus rates.