Preflexes

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Preflexes are the latent capacities in the musculoskeletal system that auto-stabilize movements through the use of the nonlinear visco-elastic properties of muscles when they contract. [1] [2] The term "preflex" for such a zero-delay, intrinsic feedback loop was coined by Loeb. [3] Unlike stabilization methods using neurons, such as reflexes and higher brain control, a preflex happens with minimal time delay; however, it only stabilizes the main movements of the musculoskeletal system.[ citation needed ]

Contents

Visco-elastic correction

Muscles possess nonlinear visco-elastic properties when they contract. [4] [5] [6] 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. [4] 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. [7]

Evolutionary opportunity

Muscles contain many different systems on which the evolutionary selection of preflex stabilization can operate. The deltoid muscle, for example, consists of at least seven segments with different bone attachments and neural control. [8] 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, and a fascicle of active contractile and passive elements. [4] 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. [9] 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 birds walk 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. [10]

Leg wiping

It is the intrinsic musculoskeletal properties of a frog's leg, not neurally mediated spinal reflexes, that stabilize its wiping movements at irritants when the leg movement is instigated. [11]

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. [6]

Related Research Articles

<span class="mw-page-title-main">Jumping</span> Form of movement in which an organism or mechanical system propels itself into the air

Jumping or leaping is a form of locomotion or movement in which an organism or non-living mechanical system propels itself through the air along a ballistic trajectory. Jumping can be distinguished from running, galloping and other gaits where the entire body is temporarily airborne, by the relatively long duration of the aerial phase and high angle of initial launch.

<span class="mw-page-title-main">Muscle contraction</span> Activation of tension-generating sites in muscle

Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.

In biomechanics, Hill's muscle model refers to the 3-element model consisting of a contractile element (CE) in series with a lightly-damped elastic spring element (SE) and in parallel with lightly-damped elastic parallel element (PE). Within this model, the estimated force-velocity relation for the CE element is usually modeled by what is commonly called Hill's equation, which was based on careful experiments involving tetanized muscle contraction where various muscle loads and associated velocities were measured. They were derived by the famous physiologist Archibald Vivian Hill, who by 1938 when he introduced this model and equation had already won the Nobel Prize for Physiology. He continued to publish in this area through 1970. There are many forms of the basic "Hill-based" or "Hill-type" models, with hundreds of publications having used this model structure for experimental and simulation studies. Most major musculoskeletal simulation packages make use of this model.

Central pattern generators (CPGs) are self-organizing biological neural circuits that produce rhythmic outputs in the absence of rhythmic input. They are the source of the tightly-coupled patterns of neural activity that drive rhythmic and stereotyped motor behaviors like walking, swimming, breathing, or chewing. The ability to function without input from higher brain areas still requires modulatory inputs, and their outputs are not fixed. Flexibility in response to sensory input is a fundamental quality of CPG-driven behavior. To be classified as a rhythmic generator, a CPG requires:

  1. "two or more processes that interact such that each process sequentially increases and decreases, and
  2. that, as a result of this interaction, the system repeatedly returns to its starting condition."

Hypertonia is a term sometimes used synonymously with spasticity and rigidity in the literature surrounding damage to the central nervous system, namely upper motor neuron lesions. Impaired ability of damaged motor neurons to regulate descending pathways gives rise to disordered spinal reflexes, increased excitability of muscle spindles, and decreased synaptic inhibition. These consequences result in abnormally increased muscle tone of symptomatic muscles. Some authors suggest that the current definition for spasticity, the velocity-dependent over-activity of the stretch reflex, is not sufficient as it fails to take into account patients exhibiting increased muscle tone in the absence of stretch reflex over-activity. They instead suggest that "reversible hypertonia" is more appropriate and represents a treatable condition that is responsive to various therapy modalities like drug or physical therapy.

A functional spinal unit (FSU) is the smallest physiological motion unit of the spine to exhibit biomechanical characteristics similar to those of the entire spine.

<span class="mw-page-title-main">Proprioception</span> Sense of self-movement, force, and body position

Proprioception is the sense of self-movement, force, and body position.

<span class="mw-page-title-main">Undulatory locomotion</span>

Undulatory locomotion is the type of motion characterized by wave-like movement patterns that act to propel an animal forward. Examples of this type of gait include crawling in snakes, or swimming in the lamprey. Although this is typically the type of gait utilized by limbless animals, some creatures with limbs, such as the salamander, forgo use of their legs in certain environments and exhibit undulatory locomotion. In robotics this movement strategy is studied in order to create novel robotic devices capable of traversing a variety of environments.

<span class="mw-page-title-main">Work loop</span>

The work loop technique is used in muscle physiology to evaluate the mechanical work and power output of skeletal or cardiac muscle contractions via in vitro muscle testing of whole muscles, fiber bundles or single muscle fibers. This technique is primarily used for cyclical contractions such as cockroach walking., the rhythmic flapping of bird wings or the beating of heart ventricular muscle.

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<span class="mw-page-title-main">Architectural gear ratio</span> Ratio between muscle-shortening velocity and fiber-shortening velocity

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<span class="mw-page-title-main">Neuromechanics</span> Interdisciplinary field

Neuromechanics is an interdisciplinary field that combines biomechanics and neuroscience to understand how the nervous system interacts with the skeletal and muscular systems to enable animals to move. In a motor task, like reaching for an object, neural commands are sent to motor neurons to activate a set of muscles, called muscle synergies. Given which muscles are activated and how they are connected to the skeleton, there will be a corresponding and specific movement of the body. In addition to participating in reflexes, neuromechanical process may also be shaped through motor adaptation and learning.

Robotic prosthesis control is a method for controlling a prosthesis in such a way that the controlled robotic prosthesis restores a biologically accurate gait to a person with a loss of limb. This is a special branch of control that has an emphasis on the interaction between humans and robotics.

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The study of animal locomotion is a branch of biology that investigates and quantifies how animals move.

Proprioception refers to the sensory information relayed from muscles, tendons, and skin that allows for the perception of the body in space. This feedback allows for more fine control of movement. In the brain, proprioceptive integration occurs in the somatosensory cortex, and motor commands are generated in the motor cortex. In the spinal cord, sensory and motor signals are integrated and modulated by motor neuron pools called central pattern generators (CPGs). At the base level, sensory input is relayed by muscle spindles in the muscle and Golgi tendon organs (GTOs) in tendons, alongside cutaneous sensors in the skin.

<span class="mw-page-title-main">Interlimb coordination</span> Coordination of the left and right limbs

Interlimb coordination is the coordination of the left and right limbs. It could be classified into two types of action: bimanual coordination and hands or feet coordination. Such coordination involves various parts of the nervous system and requires a sensory feedback mechanism for the neural control of the limbs. A model can be used to visualize the basic features, the control centre of locomotor movements, and the neural control of interlimb coordination. This coordination mechanism can be altered and adapted for better performance during locomotion in adults and for the development of motor skills in infants. The adaptive feature of interlimb coordination can also be applied to the treatment for CNS damage from stroke and the Parkinson's disease in the future.

Andrew Biewener is the Charles P. Lyman Professor of Biology in the Department of Organismic and Evolutionary Biology at Harvard University and Faculty Director of the Concord Field Station. He is also a Fellow of the American Association for the Advancement of Science.

References

  1. ↑ Blickhan, R.; Seyfarth, A.; Geyer, H.; Grimmer, S.; Wagner, H.; Gunther, M. (2007). "Intelligence by mechanics". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 365 (1850): 199–220. Bibcode:2007RSPTA.365..199B. doi:10.1098/rsta.2006.1911. PMID   17148057. S2CID   1141390.
  2. ↑ Valero-Cuevas, F. J.; Yi, J. W.; Brown, D.; McNamara, R. V.; Paul, C.; Lipson, H. (2007). "The Tendon Network of the Fingers Performs Anatomical Computation at a Macroscopic Scale". IEEE Transactions on Biomedical Engineering. 54 (6): 1161–1166. CiteSeerX   10.1.1.419.1719 . doi:10.1109/TBME.2006.889200. PMID   17549909. S2CID   1869716. PDF
  3. ↑ Loeb, G. E. (1995). "Control implications of musculoskeletal mechanics". Proceedings of 17th International Conference of the Engineering in Medicine and Biology Society. Vol. 2. pp. 1393–1394. doi:10.1109/IEMBS.1995.579743. ISBN   978-0-7803-2475-6. S2CID   62579032.
  4. 1 2 3 Brown IE, Loeb GE. (2000). "A reductionist approach to creating and using neuromusculoskeletal models". In JMC Winters, P.E. (ed.). Biomechanical and neurological control of posture and movements. New York: Springer. pp. 148–63. ISBN   978-0-471-50908-0.
  5. ↑ Nishikawa, K.; Biewener, A. A.; Aerts, P.; Ahn, A. N.; Chiel, H. J.; Daley, M. A.; Daniel, T. L.; Full, R. J.; Hale, M. E.; Hedrick, T. L.; Lappin, A. K.; Nichols, T. R.; Quinn, R. D.; Satterlie, R. A.; Szymik, B. (2007). "Neuromechanics: An integrative approach for understanding motor control". Integrative and Comparative Biology. 47 (1): 16–54. doi: 10.1093/icb/icm024 . PMID   21672819.
  6. 1 2 Van Soest, A. J.; Bobbert, M. F. (1993). "The contribution of muscle properties in the control of explosive movements". Biological Cybernetics. 69 (3): 195–204. doi:10.1007/bf00198959. PMID   8373890. S2CID   16196068.
  7. ↑ Souza, T.R.; Fonseca, S.T.; Gonçalves, G.G.; Ocarino, J.M.; Mancini, M.C. (2009). "Prestress revealed by passive co-tension at the ankle joint". Journal of Biomechanics. 42 (14): 2374–2380. doi: 10.1016/j.jbiomech.2009.06.033 . PMID   19647832.
  8. ↑ Brown, J. M. M.; Wickham, J. B.; McAndrew, D. J.; Huang, X. -F. (2007). "Muscles within muscles: Coordination of 19 muscle segments within three shoulder muscles during isometric motor tasks". Journal of Electromyography and Kinesiology. 17 (1): 57–73. doi:10.1016/j.jelekin.2005.10.007. PMID   16458022.
  9. ↑ Azizi, E.; Brainerd, E. L.; Roberts, T. J. (2008). "Variable gearing in pennate muscles". Proceedings of the National Academy of Sciences. 105 (5): 1745–1750. Bibcode:2008PNAS..105.1745A. doi: 10.1073/pnas.0709212105 . PMC   2234215 . PMID   18230734.
  10. ↑ Daley, M. A.; Biewener, A. A. (2006). "Running over rough terrain reveals limb control for intrinsic stability". Proceedings of the National Academy of Sciences. 103 (42): 15681–15686. Bibcode:2006PNAS..10315681D. doi: 10.1073/pnas.0601473103 . PMC   1622881 . PMID   17032779.
  11. ↑ Richardson, A. G.; Slotine, J. J.; Bizzi, E.; Tresch, M. C. (2005). "Intrinsic Musculoskeletal Properties Stabilize Wiping Movements in the Spinalized Frog". Journal of Neuroscience. 25 (12): 3181–3191. doi: 10.1523/JNEUROSCI.4945-04.2005 . PMC   6725085 . PMID   15788775.
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