Preflexes are the latent capacities in the

musculoskeletal system The human musculoskeletal system (also known as the human locomotor system, and previously the activity system) is an organ system that gives humans the ability to move using their Muscular system, muscular and Human skeleton, skeletal systems. ...

that auto-stabilize movements through the use of the

nonlinear In mathematics and science, a nonlinear system (or a non-linear system) is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathe ...

visco-elastic properties of

muscles Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

when they

contract A contract is an agreement that specifies certain legally enforceable rights and obligations pertaining to two or more parties. A contract typically involves consent to transfer of goods, services, money, or promise to transfer any of thos ...

. The term "preflex" for such a zero-delay, intrinsic feedback loop was coined by Loeb. Unlike stabilization methods using

neurons A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

, such as

reflexes In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a Stimulus (physiology), stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous s ...

and higher brain control, a preflex happens with minimal time delay; however, it only stabilizes the main movements of the musculoskeletal system.

Visco-elastic correction

Muscles possess nonlinear visco-elastic properties when they contract. This property can autocorrect movements when a muscle is forced to change its length, and at a velocity different from that with which it was originally commanded. Such automatic correction is useful when a commanded action is perturbated, for example, if a step goes into a hole as this causes the foot to unexpectedly stretch down. The nonlinear visco-elastic properties of muscles interact with these perturbation induced velocity and length differences such that they counteract directly, as they happen, the effects upon the body of the perturbation. Part of the resistance to perturbation is passive, by means of the nonlinear increase in passive tension and joint torques produced by muscular and other soft tissues. Tissue prestress is a preflexive property that constitutes a basal level of passive tension which, due to its presence in antagonistic tissues of a joint, increases joint passive stiffness and stability.

Evolutionary opportunity

Muscles contain many different systems on which the

evolutionary selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...

of preflex stabilization can operate. The

deltoid muscle The deltoid muscle is the muscle forming the rounded contour of the shoulder, human shoulder. It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic cat. Anatomically, the deltoid muscle is made up o ...

, for example, consists of at least seven segments with different bone attachments and neural control. Within each muscle segment, there exists a complex internal structure that goes down to one in which each muscle unit consists of a tendon,

aponeurosis An aponeurosis (; : aponeuroses) is a flattened tendon by which muscle attaches to bone or fascia. Aponeuroses exhibit an ordered arrangement of collagen fibres, thus attaining high tensile strength in a particular direction while being vulnerable ...

, and a fascicle of active contractile and passive elements. Another source of variation is in the internal architecture of the fiber orientation relative to a muscle's line of action, for example, as found in pennate muscles. The complexities of the different visco-elastic length- and velocity-force relationships of these subparts provides the opportunity for the adaptive selection of structurally complex muscle biocomposites with highly task-tuned nonlinear visco-elastic length- velocity- force relationships. This nature of muscles to be composite structures thus provides the adaptive opportunity for evolution to modify the visco-elastic reactions of the musculoskeletal system so they counteract perturbations without the need for spinal or higher levels of control.

Examples

Leg step recovery

Helmeted guineafowl like many other

bipedal Bipedalism is a form of terrestrial locomotion where an animal moves by means of its two rear (or lower) limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' ...

birds

walk Walking (also known as ambulation) is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined as an "inverted pendulum" gait in which the body vaults over ...

upon rough ground. When a guineafowl's leg steps into a hole (a common disruption against which evolution has tuned the nonlinear visco-elastic properties of its musculoskeletal system), a momentarily uncommanded velocity and length change in the muscles that span its leg joints occurs. This length/velocity discrepancy interacts with the nonlinear length and velocity-force relationships that have evolved in response to such a disruption with the result that the leg extends further into the hole, and thus keeps the bird's body stable and upright.

Leg wiping

It is the intrinsic musculoskeletal properties of a

frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

's leg, not neurally mediated spinal reflexes, that stabilize its wiping movements at irritants when the leg movement is instigated.

Squat jumps

A human example of a preflex stabilization occurs when a person explosively jumps up from a squat position, and the leg muscles act to provide a minimal time delay against perturbations from the vertical.

References

{{Reflist, 2 Animal physiology Reflexes Muscular system Tissues (biology) Motor skills Musculoskeletal system