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Neuromechanics

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Human leg bones labeled

Lower limb neuromechanics is an interdisciplinary field which combines neuroscience and biomechanics to study human movement and its relation to the brain, specifically in limbs below the waist. Lower limbs consist of four parts: hip bone girdle, thigh, lower leg, and foot[1] .
Muscles signals stimulated by neurological impulses are collected using electromyography (EMG). Leg muscles that are commonly recorded through EMG are the gluteus maximus, quadriceps femoris, rectus femoris, vastus medialis, soleus, lateral gastrocnemius, medial gastrocnemius, and tibialis anterior.

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Background

Neuroscience

The components of the nervous system are outlined in the graph.

Neuroscience is the study of the nervous system. The nervous system is comprised of two sub-systems: the peripheral nervous system and the central nervous system[2].

The peripheral nervous system is then composed of three sub-systems: the somatic nervous system, the autonomic nervous system, and the visceral nervous system[3] . The autonomic nervous system is also broken down into the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. The nervous system responsible for voluntary motion, including lower limb motion, is the somatic nervous system[4]. Though the somatic nervous system is part of the peripheral nervous system, motion also involves use of elements of the central nervous system: the brain and the spinal cord[4] .

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Biomechanics

The graph shows the vertical ground reaction force of the left leg for a person walking. Stance phase is the first 60% of the gait cycle, and swing phase is the last 40% of the gait cycle.

Biomechanics is the study of the structure and function of living systems such as humans, animals, and other organisms by means of mechanics. Lower limb biomechanics is concerned with the interactions of forces at the hip, knee, and ankle joints as well as the shear force and torsion on bone segments.

The gait cycle is a repetitive event that consists of one full step from heel-strike to the next heel-strike in the same foot. It can be divided into two phases: stance phase and swing phase[5]. Stance phase consists of the time during which the heel strikes the ground to the point in time at which the toe leaves the ground[5]. Swing phase consists of the rest of the gait cycle, the time between the toe leaving the ground to the next heel strike[5].

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Inverted Pendulum Theory

Center of mass on a massless leg travelling along the trunk trajectory path in inverted pendulum theory. Velocity vectors are shown perpendicular to the ground reaction force at time 1 and time 2.

In the inverted pendulum theory, the weight of the body is reduced to a center of mass resting on a massless leg at a single support. The ground reaction force travels from the center of pressure at the bottom of the massless leg to the center of mass at the top of the massless leg. The velocity vector of the center of mass is always perpendicular to the ground reaction force[6] .

Walking consists of alternating single-support and double-support phases. The single-support phase occurs when one leg is in contact with the ground while the double-support phase occurs when two legs are in contact with the ground[7]

Motion equations of the inverted pendulum

Neurological influences

The inverted pendulum is stabilized by constant feedback from the brain and can operate even in the presence of sensory loss. In animals who have lost all sensory input to the moving limb, the variables produced by gait (center of mass acceleration, velocity of animal, and position of the animal) remained constant between both groups[8] .

During postural control, delayed feedback mechanisms are used in the temporal reproduction of task-level functions such as walking. The nervous system takes into account feedback from the center of mass acceleration, velocity, and position of an individual and utilizes the information to predict and plan future movements. Center of mass acceleration is essential in the feedback mechanism as this feedback takes place before any significant displacement data can be determined[9] .

Criticisms

The inverted pendulum theory directly contradicts the six determinants of gait, another theory for gait analysis[10] . The six determinants of gait predict very high energy expenditure while the inverted pendulum theory offers the possibility that energy expenditure can be zero[6] . The theory must be considered incomplete as it describes a walking strategy not taken by humans and does not explain the energy cost involved in walking.

Electromyography

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Muscle Synergies

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Adaptation

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References

  1. ^ O'Rahilly, Ronan (1982). Basic Human Anatomy: A Regional Study of Human Structure. W B Saunders Co. ISBN 0721669905.
  2. ^ "What Are the Parts of the Nervous System?". National Institute of Health. Retrieved 25 September 2013.
  3. ^ Costanzo, Linda (2010). Physiology. McGraw Hill. ISBN 9781416062165.
  4. ^ a b Noback, Charles (2005). The Human Nervous System: Structure and Function. Springer. ISBN 1588290395.
  5. ^ a b c Perry, Jacqueline (2010). Gait Analysis: Normal and Pathological Function. Slack Incorporated. ISBN 1556427662.
  6. ^ a b Kuo, Arthur (6). "The six determinants of gait and the inverted pendulum analogy: A dynamic walking perspective" (PDF). Human Movement Science. 26 (4): 617–656. Retrieved 6 November 2013. {{cite journal}}: Check date values in: |date= and |year= / |date= mismatch (help); Unknown parameter |month= ignored (help)
  7. ^ Kuo, Arthur (2005). "Energetic Consequences of Walking Like an Inverted Pendulum: Step-to-Step Transitions" (PDF). Exercise Sport Science Review. 33 (2): 88-97. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ Lockhart, Daniel (16). "Optimal sensorimotor transformations for balance" (PDF). Nature Neuroscience. 10: 1329-1336. doi:10.1038/nn1986. Retrieved 6 November 2013. {{cite journal}}: Check date values in: |date= and |year= / |date= mismatch (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  9. ^ Welch, Torrence (19). "A Feedback Model Reproduces Muscle Activity During Human Postural Responses to Support-Surface Transitions". Journal of Neurophysiology. 99: 1032–1038. doi:10.1152/jn.01110.2007. Retrieved 6 November 2013. {{cite journal}}: Check date values in: |date= and |year= / |date= mismatch (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  10. ^ Cuccurullo, Sara (2009). Physical Medicine and Rehabilitation Board Review. Demos Medical Publishing. pp. 457–462. ISBN 1933864184.
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