Neuromechanics

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Fig. 1 - Muscles anterior labeled Muscles anterior labeled.png
Fig. 1 - Muscles anterior labeled

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. [1] [2] 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. [3] In addition to participating in reflexes, neuromechanical process may also be shaped through motor adaptation and learning. [4]

Contents

Neuromechanics underlying behavior

Walking

Fig. 2 - 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. Inverted Pendulum.png
Fig. 2 - 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.

The inverted pendulum theory of gait is a neuromechanical approach to understand how humans walk. As the name of the theory implies, a walking human is modeled as an inverted pendulum consisting of a center of mass (COM) suspended above the ground via a support leg (Fig. 2). As the inverted pendulum swings forward, ground reaction forces occur between the modeled leg and the ground. Importantly, the magnitude of the ground reaction forces depends on the COM position and size. The velocity vector of the center of mass is always perpendicular to the ground reaction force. [5]

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. [6]

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) remain constant between both groups. [7]

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. [8]

Controversy

The inverted pendulum theory directly contradicts the six determinants of gait, another theory for gait analysis. [9] The six determinants of gait predict very high energy expenditure for the sinusoidal motion of the Center of Mass during gait, while the inverted pendulum theory offers the possibility that energy expenditure can be near zero; the inverted pendulum theory predicts that little to no work is required for walking. [5]

Measuring the neural control of muscles - Electromyography

Electromyography (EMG) is a tool used to measure the electrical outputs produced by skeletal muscles upon activation. Motor nerves innervate skeletal muscles and cause contraction upon command from the central nervous system. This contraction is measured by EMG and is typically measured on the scale of millivolts (mV). Another form of EMG data that is analyzed is integrated EMG (iEMG) data. iEMG measures the area under the EMG signal which corresponds to the overall muscle effort rather than the effort at a specific instant.

Equipment

There are four instrumentation components used to detect these signals: (1) the signal source, (2) the transducer used to detect the signal, (3) the amplifier, and (4) the signal processing circuit. [10] The signal source refers to the location at which the EMG electrode is place. EMG signal acquisition is dependent on distance from the electrode to the muscle fiber, so placement is imperative. The transducer used to detect the signal is an EMG electrode than transforms the bioelectric signal from the muscle to a readable electric signal. [10] The amplifier reproduces an undistorted bioelectric signal and also allows for noise reduction in the signal. [10] Signal processing involves taking the recorded electrical impulses, filtering them, and enveloping the data. [10]

Latency

Latency is a measure of the time span between the activation of a muscle and its peak EMG value. Latency is used as a means to diagnose disorders of the nervous system such as a herniated disc, amyotrophic lateral sclerosis (ALS), or myasthenia gravis (MG). [11] These disorders may cause a disruption of the signal at the muscle, the nerve, or the junction between the muscle and the nerve.

The use of EMG to identify nervous systems disorders is known as a nerve conduction study (NCS). Nerve conduction studies can only diagnose diseases on the muscular and nerve level. They cannot detect disease in the spinal cord or the brain. In most disorders of the muscle, nerve, or neuromuscular junction, the latency time is increased. [12] This is a result of decreased nerve conduction or electrical stimulation at the site of the muscle. In 50% of patients with cerebral atrophy cases, the M3 spinal reflex latency, was increased and on occasion separated from the M2 spinal reflex response. [13] [14] The separation between the M2 and M3 spinal reflex responses is typically 20 milliseconds, but in patients with cerebral atrophy, the separation was increased to 50 ms. In some cases, however, other muscles can compensate for the muscle suffering from decreased electrical stimulation. In the compensatory muscle, the latency time is actually decreased in order to substitute for the function of the diseased muscle. [15] These kinds of studies are used in neuromechanics to identify motor disorders and their effects on a cellular and electrical level rather than a system motion level.

Coordinated movements enabled through muscle synergies

Three-tiered hierarchy of the muscle synergy hypothesis with m synergies and n effector muscles. Muscle Synergies.png
Three-tiered hierarchy of the muscle synergy hypothesis with m synergies and n effector muscles.

A muscle synergy is a group of synergistic muscles and agonists that work together to perform a motor task. A muscle synergy is composed of agonist and synergistic muscles. An agonist muscle is a muscle that contracts individually, and it can cause a cascade of motion in neighboring muscles. Synergistic muscles aid the agonist muscles in motor control tasks, but they act against excess motion that the agonists may create.

Muscle synergy hypothesis

The muscle synergy hypothesis is based on the assumption that the central nervous system controls muscle groups independently rather than individual muscles . [16] [17] The muscle synergy hypothesis presents motor control as a three-tiered hierarchy. In tier one, a motor task vector is created by the central nervous system. The central nervous system then transforms the muscle vector to act upon a group of muscle synergies in tier two. Then in tier three, muscle synergies define a specific ratio of the motor task for each muscle and assign it to its respective muscle to act upon the joint to perform the motor task.

Redundancy

Redundancy plays a large role in muscle synergy. Muscle redundancy is a degrees of freedom problem on the muscular level. [18] The central nervous system is presented with the opportunity to coordinate muscle movements, and it must choose one out of many. The muscle redundancy problem is a result of more muscle vectors than dimensions in the task space. Muscles can only generate tension by pulling, not pushing. This results in many muscle force vectors in multiple directions rather than a push and pull in the same direction.

One debate on muscle synergies is between the prime mover strategy and the cooperation strategy. [18] The prime mover strategy arises when a muscle's vector can act in the same direction as the mechanical action vector, the vector of the limb's motion. The cooperation strategy, however, takes place when no muscle can act directly in the vector direction of the mechanical action resulting in a coordination of multiple muscles to achieve the task. The prime mover strategy over time has declined in popularity as it has been found through electromyography studies that no one muscle consistently provides more force than other muscles that are acting to move about a joint. [19]

Criticisms

The muscle synergy theory is difficult to falsify. [20] Though experimentation has shown that groups of muscles indeed work together to control motor tasks, neural connections allow for individual muscles to be activated. Though individual muscle activation may contradict muscle synergy, it also obscures it. Activation of individual muscles may override or block the input from and overall effect of muscle synergies. [20]

Motor adaptation

Ankle-foot-orthosis Ankle-foot-orthosis.png
Ankle-foot-orthosis

Adaptation in the neuromechanical sense is the body's ability to change an action to better suit the situation or environment in which it is acting. Adaptation can be a result of injury, fatigue, or practice. Adaptation can be measured in a variety of ways: electromyography, three-dimensional reconstruction of joints, and changes in other variables pertaining to the specific adaptation being studied.

Injury

Injury can cause adaptation in a number of ways. Compensation is a large factor in injury adaptation. Compensation is a result of one or more weakened muscles. The brain is given the task to perform a certain motor task, and once a muscle has been weakened, the brain computes energy ratios to send to other muscles to perform the original task in the desired fashion. Change in muscle contribution is not the only byproduct of a muscle-related injury. Change in loading of the joint is another result which, if prolonged, can be harmful for the individual. [21]

Fatigue

Muscle fatigue is the neuromuscular adaptation to challenges over a period of time. The use of motor units over a period of time can result in changes in the motor command from the brain. Since the force of contraction cannot be changed, the brain instead recruits more motor units to achieve maximal muscle contraction. [22] Recruitment of motor units varies from muscle to muscle depending on the upper limit of motor recruitment in the muscle. [22]

Practice

Adaptation due to practice can be a result of intended practice such as sports or unintended practice such as wearing an orthosis. In athletes, repetition results in muscle memory. The motor task becomes a long-term memory that can be repeated without much conscious effort. This allows the athlete to focus on fine-tuning their motor task strategy. Resistance to fatigue also comes with practice as the muscle is strengthened, but the speed at which an athlete can complete a motor task is also increased with practice. [23] Volleyball players compared to non-jumpers show more repeatable control of muscles surrounding the knee that is controlled by co-activation in the single jump condition. [23] In the repeated jump condition, both volleyball players and non-jumpers have a linear decrease in normalized jump flight time. [23] Though the normalized linear decrease is the same for athletes and non-athletes, athletes consistently have higher flight times.

There is also adaptation associated with use of a prosthesis or an orthosis. This operates similarly to adaptation due to fatigue; however, muscles can actually be fatigued or alter their mechanical contribution to a motor task as a result of wearing the orthosis. An ankle foot orthosis is a common solution to injury of the lower limb, specifically around the ankle joint. An ankle foot orthosis can be assistive or resistive. An assistive ankle orthosis encourages ankle movement, and a resistive ankle orthosis inhibits ankle movement. [24] Upon wearing an assistive ankle foot orthosis, individuals have decreased EMG amplitude and joint stiffness over time while the opposite occurs for resistive ankle foot orthoses. [24] Additionally, not only can electromyography readings differ, but the physical path that joints travel along can be altered as well. [25]

Related Research Articles

<span class="mw-page-title-main">Charcot–Marie–Tooth disease</span> Neuromuscular disease

Charcot–Marie–Tooth disease (CMT) is a hereditary motor and sensory neuropathy of the peripheral nervous system characterized by progressive loss of muscle tissue and touch sensation across various parts of the body. This disease is the most commonly inherited neurological disorder, affecting about one in 2,500 people. It is named after those who classically described it: the Frenchman Jean-Martin Charcot (1825–1893), his pupil Pierre Marie (1853–1940), and the Briton Howard Henry Tooth (1856–1925).

Spasticity is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles.

In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.

A motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibres. Groups of motor units often work together as a motor pool to coordinate the contractions of a single muscle. The concept was proposed by Charles Scott Sherrington.

Muscle fatigue is the decline in ability of muscles to generate force. It can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction. There are two main causes of muscle fatigue: the limitations of a nerve’s ability to generate a sustained signal ; and the reduced ability of the muscle fiber to contract.

<span class="mw-page-title-main">Electromyography</span> Electrodiagnostic medicine technique

Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyograph detects the electric potential generated by muscle cells when these cells are electrically or neurologically activated. The signals can be analyzed to detect abnormalities, activation level, or recruitment order, or to analyze the biomechanics of human or animal movement. Needle EMG is an electrodiagnostic medicine technique commonly used by neurologists. Surface EMG is a non-medical procedure used to assess muscle activation by several professionals, including physiotherapists, kinesiologists and biomedical engineers. In Computer Science, EMG is also used as middleware in gesture recognition towards allowing the input of physical action to a computer as a form of human-computer interaction.

Motor control is the regulation of movement in organisms that possess a nervous system. Motor control includes reflexes as well as directed movement.

Motor coordination is the orchestrated movement of multiple body parts as required to accomplish intended actions, like walking. This coordination is achieved by adjusting kinematic and kinetic parameters associated with each body part involved in the intended movement. The modifications of these parameters typically relies on sensory feedback from one or more sensory modalities, such as proprioception and vision.

<span class="mw-page-title-main">Muscle coactivation</span> Contraction to provide joint stability

Muscle coactivation occurs when agonist and antagonist muscles surrounding a joint contract simultaneously to provide joint stability. It is also known as muscle cocontraction, since two muscle groups are contracting at the same time. It is able to be measured using electromyography (EMG) from the contractions that occur. The general mechanism of it is still widely unknown. It is believed to be important in joint stabilization, as well as general motor control.

<span class="mw-page-title-main">Standing</span> Human position in which the body is held upright

Standing, also referred to as orthostasis, is a position in which the body is held in an erect ("orthostatic") position and supported only by the feet. Although seemingly static, the body rocks slightly back and forth from the ankle in the sagittal plane. The sagittal plane bisects the body into right and left sides. The sway of quiet standing is often likened to the motion of an inverted pendulum.

<span class="mw-page-title-main">Orthotics</span> Medical specialty that focuses on the design and application of orthoses

Orthotics is a medical specialty that focuses on the design and application of orthoses, or braces. An orthosis is "an externally applied device used to influence the structural and functional characteristics of the neuromuscular and skeletal system".

<span class="mw-page-title-main">Locomotor effects of shoes</span>

Locomotor effects of shoes are the way in which the physical characteristics or components of shoes influence the locomotion neuromechanics of a person. Depending on the characteristics of the shoes, the effects are various, ranging from alteration in balance and posture, muscle activity of different muscles as measured by electromyography (EMG), and the impact force. There are many different types of shoes that exist, such as running, walking, loafers, high heels, sandals, slippers, work boots, dress shoes, and many more. However, a typical shoe will be composed of an insole, midsole, outsole, and heels, if any. In an unshod condition, where one is without any shoes, the locomotor effects are primarily observed in the heel strike patterns and resulting impact forces generated on the ground.

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

Spinal locomotion results from intricate dynamic interactions between a central program in lower thoracolumbar spine and proprioceptive feedback from body in the absence of central control by brain as in complete spinal cord injury (SCI). Following SCI, the spinal circuitry below the lesion site does not become silent rather it continues to maintain active and functional neuronal properties although in a modified manner.

Normal aging movement control in humans is about the changes in the muscles, motor neurons, nerves, sensory functions, gait, fatigue, visual and manual responses, in men and women as they get older but who do not have neurological, muscular or neuromuscular disorder. With aging, neuromuscular movements are impaired, though with training or practice, some aspects may be prevented.

In neuroscience and motor control, the degrees of freedom problem or motor equivalence problem states that there are multiple ways for humans or animals to perform a movement in order to achieve the same goal. In other words, under normal circumstances, no simple one-to-one correspondence exists between a motor problem and a motor solution to the problem. The motor equivalence problem was first formulated by the Russian neurophysiologist Nikolai Bernstein: "It is clear that the basic difficulties for co-ordination consist precisely in the extreme abundance of degrees of freedom, with which the [nervous] centre is not at first in a position to deal."

Clinical Electrophysiological Testing is based on techniques derived from electrophysiology used for the clinical diagnosis of patients. There are many processes that occur in the body which produce electrical signals that can be detected. Depending on the location and the source of these signals, distinct methods and techniques have been developed to properly target them.

Neuromechanics of orthoses refers to how the human body interacts with orthoses. Millions of people in the U.S. suffer from stroke, multiple sclerosis, postpolio, spinal cord injuries, or various other ailments that benefit from the use of orthoses. Insofar as active orthoses and powered exoskeletons are concerned, the technology to build these devices is improving rapidly, but little research has been done on the human side of these human-machine interfaces.

<span class="mw-page-title-main">Proportional myoelectric control</span>

Proportional myoelectric control can be used to activate robotic lower limb exoskeletons. A proportional myoelectric control system utilizes a microcontroller or computer that inputs electromyography (EMG) signals from sensors on the leg muscle(s) and then activates the corresponding joint actuator(s) proportionally to the EMG signal.

<span class="mw-page-title-main">Cutaneous reflex in human locomotion</span>

Cutaneous, superficial, or skin reflexes, are activated by skin receptors and play a valuable role in locomotion, providing quick responses to unexpected environmental challenges. They have been shown to be important in responses to obstacles or stumbling, in preparing for visually challenging terrain, and for assistance in making adjustments when instability is introduced. In addition to the role in normal locomotion, cutaneous reflexes are being studied for their potential in enhancing rehabilitation therapy (physiotherapy) for people with gait abnormalities.

<span class="mw-page-title-main">Interlimb coordination</span>

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.

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