Motor Activity – Anatomy, Types, Functions

Motor Activity – Anatomy, Types, Functions

The motor activity involves movement quality and quantity that both influence and are influenced by states of arousal. Imbedded in activity levels are the qualitative aspects of movement that include muscle tone, posture, coordination, symmetry, strength, purposefulness, and planning, or praxis.

Motor activity is represented by several behaviors (e.g., ambulation, grooming, rearing, sniffing) that involve coordinated involvement of sensory, motor, and associative processes. Motor activity testing often is performed in a novel environment using an automated detection system. Rodents may exhibit a substantial diurnal cyclicity in their level of spontaneous motor activity that must be considered when designing test batteries and interpreting data. Motor activity changes may result from CNS and/or PNS damage.

Types of motor skills

Gross motor skills require the use of large muscle groups to perform tasks like walking, balancing, and crawling. The skill required is not extensive and therefore are usually associated with continuous tasks. Much of the development of these skills occurs during early childhood. The performance level of gross motor skills remains unchanged after periods of non-use.[rx] Gross motor skills can be further divided into two subgroups: oculomotor skills, such as running, jumping, sliding, and swimming; and object-control skills such as throwing, catching, and kicking. Motor skills are movements and actions of the muscles. Typically, they are categorized into eighteen groups:

  • Fine motor skills – requires the use of smaller muscle groups to perform smaller movements with the wrists, hands, fingers, and feet, and toes. These tasks are precise in nature, like playing the piano, writing carefully, and blinking. Generally, there is a retention loss of fine motor skills over a period of non-use. Discrete tasks usually require more fine motor skills than gross motor skills.[rx] Fine motor skills can become impaired. Some reasons for impairment could be an injury, illness, stroke, congenital deformities, cerebral palsy, and developmental disabilities. Problems with the brain, spinal cord, peripheral nerves, muscles, or joints can also have an effect on fine motor skills, and decrease control.[rx]


Motor skills develop in different parts of a body along three principles:

  • Cephalocaudal – development from head to foot. The head develops earlier than the hand. Similarly, hand coordination develops before the coordination of the legs and feet. For example, an infant is able to follow something with their eyes before they can touch or grab it.[rx]
  • Proximodistal – the movement of limbs that are closer to the body develop before the parts that are further away, such as a baby learns to control the upper arm before the hands or fingers. Fine movements of the fingers are the last to develop in the body.[rx]
  • Gross to specific – a pattern in which larger muscle movements develop before finer movements. For example, a child only being able to pick up large objects, to then picking up an object that is small between the thumb and fingers. The earlier movements involve larger groups of muscles, but as the child grows finer movements become possible and specific things can be achieved.[rx]

In children, a critical period for the acquisition of motor skills is preschool years (ages 3–5), as fundamental neuroanatomic structure shows significant development, elaboration, and myelination over the course of this period.[rx] Many factors contribute to the rate at which children develop their motor skills. Unless afflicted with a severe disability, children are expected to develop a wide range of basic movement abilities and motor skills.[rx] Motor development progresses in seven stages throughout an individual’s life: reflexive, rudimentary, fundamental, sports skill, growth and refinement, peak performance, and regression. Development is age-related but is not age-dependent. In regard to age, it is seen that typical developments are expected to attain gross motor skills used for postural control and vertical mobility by 5 years of age.[rx]

There are six aspects of development:

  • Qualitative – changes in movement-process results in changes in movement-outcome.
  • Sequential – certain motor patterns precede others.
  • Cumulative – current movements are built on previous ones.
  • Directional – cephalocaudal or proximodistal
  • Multifactorial – numerous-factors impact
  • Individual – dependent on each person

In the childhood stages of development, gender differences can greatly influence motor skills. In the article “An Investigation of Age and Gender Differences in Preschool Children’s Specific Motor Skills”, girls scored significantly higher than boys on visual motor and graphomotor tasks. The results from this study suggest that girls attain manual dexterity earlier than boys.[rx] Variability of results in the tests can be attributed towards the multiplicity of different assessment tools used.[rx] Furthermore, gender differences in motor skills are seen to be affected by environmental factors. In essence, “parents and teachers often encourage girls to engage in [quiet] activities requiring fine motor skills, while they promote boys’ participation in dynamic movement actions”.[rx] In the journal article “Gender Differences in Motor Skill Proficiency From Childhood to Adolescence” by Lisa Barrett, the evidence for gender-based motor skills is apparent. In general, boys are more skillful in object control and object manipulation skills. These tasks include throwing, kicking, and catching skills. These skills were tested and concluded that boys perform better with these tasks. There was no evidence for the difference in locomotor skill between the genders, but both are improved in the intervention of physical activity. Overall, the predominance of development was on balance skills (gross motor) in boys and manual skills (fine motor) in girls.[rx]

Components of development

  • Growth – increase in the size of the body or its parts as the individual progresses toward maturity (quantitative structural changes)
  • Maturation – refers to qualitative changes that enable one to progress to higher levels of functioning; it is primarily innate
  • Experience or learning – refers to factors within the environment that may alter or modify the appearance of various developmental characteristics through the process of learning
  • Adaptation – refers to the complex interplay or interaction between forces within the individual (nature) and the environment (nurture)
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Influences on development

  • Stress and arousal – stress and anxiety is the result of an imbalance between demand and the capacity of the individual. In this context, arousal defines the amount of interest in the skill. The optimal performance level is moderate stress or arousal.[rx] An example of an insufficient arousal state is an overqualified worker performing repetitive jobs. An example of an excessive stress level is an anxious pianist at a recital. The “Practice-Specificity-Based Model of Arousal” (Movahedi, 2007) holds that, for best and peak performances to occur, motor task performers need only to create an arousal level similar to the one they have experienced throughout training sessions. For peak performance, performers do not need to have high or low arousal levels. It is important that they create the same level of arousal throughout training sessions and competition. In other words, high levels of arousal can be beneficial if athletes experience such heightened levels of arousal during some consecutive training sessions. Similarly, low levels of arousal can be beneficial if athletes experience such low levels of arousal during some consecutive training sessions.[rx]
  • Fatigue – the deterioration of performance when a stressful task is continued for a long time, similar to the muscular fatigue experienced when exercising rapidly or over a long period. Fatigue is caused by over-arousal. Fatigue impacts an individual in many ways: perceptual changes in which visual acuity or awareness drops, slowing of performance (reaction times or movement speed), irregularity of timing, and disorganization of performance.
  • Vigilance – the effect of the loss of vigilance is the same as fatigue, but is instead caused by a lack of arousal. Some tasks include actions that require little work and high attention.[rx]
  • Gender – gender plays an important role in the development of the child. Girls are more likely to be seen performing fine stationary visual motor skills, whereas boys predominantly exercise object-manipulation skills. While researching motor development in preschool-aged children, girls were more likely to be seen performing skills such as skipping, hopping, or skills with the use of hands only. Boys were seen to perform gross skills such as kicking or throwing a ball or swinging a bat. There are gender-specific differences in qualitative throwing performance, but not necessarily in quantitative throwing performance. Male and female athletes demonstrated similar movement patterns in humerus and forearm actions but differed in trunk, stepping, and backswing actions.

Stages of motor learning

Motor learning is a change, resulting from practice. It often involves improving the accuracy of movements both simple and complex as one’s environment changes. Motor learning is a relatively permanent skill as the capability to respond appropriately is acquired and retained.[rx]

The stages of motor learning are the cognitive phase, the associative phase, and the autonomous phase.

  • Cognitive phase – When a learner is new to a specific task, the primary thought process starts with, “What needs to be done?” Considerable cognitive activity is required so that the learner can determine appropriate strategies to adequately reflect the desired goal. Good strategies are retained and inefficient strategies are discarded. The performance is greatly improved in a short amount of time.
  • Associative phase – The learner has determined the most effective way to do the task and starts to make subtle adjustments in performance. Improvements are more gradual and movements become more consistent. This phase can last for a long time. The skills in this phase are fluent, efficient, and aesthetically pleasing.
  • Autonomous phase – This phase may take several months to years to reach. The phase is dubbed “autonomous” because the performer can now “automatically” complete the task without having to pay any attention to performing it. Examples include walking and talking or sight-reading while doing simple arithmetic.[rx]

Peripheral Motor Endings

A neuromuscular junction exists between the axon terminal and the motor endplate of a muscle fiber where neurotransmitters are released.

Key Points

A neuromuscular junction is a junction between the axon terminal of a motor neuron and the plasma membrane of the motor endplate of a muscle fiber.

With the arrival of an action potential to the axon terminal, voltage-dependent calcium channels open, and calcium infuses into the cell. The influx of calcium ions causes the docking of acetylcholine-containing vesicles at the plasma membrane of the neuron and exocytosis into the synaptic cleft.

Acetylcholine is a neurotransmitter contained in the vesicles of the pre-synaptic neuron. It is released into the synaptic cleft and activates nicotinic acetylcholine receptors on the motor endplate, and causes local motor endplate depolarization, also known as the endplate potential (EPP).

The endplate potential propagates across the surface of the muscle fiber, causing the fiber to contract and continuing the process of excitation-contraction coupling.

Key Terms

axon: A nerve fiber that is a long, slender projection of a nerve cell that conducts nerve impulses away from the body of the cell to a synapse.

voltage-dependent calcium channels: A group of voltage-gated ion channels found in excitable cells (e.g., muscle, glial cells, neurons, etc. ) with permeability to the ion Ca2+.

presynaptic neuron: The neuron that releases neurotransmitters into the synaptic cleft.

nicotinic acetylcholine receptor: These are cholinergic receptors that form ligand-gated ion channels in the plasma membranes of certain neurons and on the postsynaptic side of the neuromuscular junction.

synaptic cleft: A small space between neurons.

excitation-contraction coupling: This process is fundamental to muscle physiology, whereby the electrical stimulus is usually an action potential and the mechanical response is a contraction.

A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, as shown in Figures 1 and 2. The highly excitable region of muscle fiber plasma membrane is responsible for initiation of action potentials across the muscle’s surface, ultimately causing the muscle to contract.

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Invertebrates, the signal passes through the neuromuscular junction via the neurotransmitter acetylcholine.

This is a detailed view of a neuromuscular junction. The drawing identifies these parts in the junction: 1) Presynaptic terminal; 2) Sarcolemma; 3) Synaptic vesicle; 4) Nicotinic acetylcholine receptor; 5) Mitochondrion. 

Figure 1. Detailed view of a neuromuscular junction: Detailed view of a neuromuscular junction: 1) Presynaptic terminal; 2) Sarcolemma; 3) Synaptic vesicle; 4) Nicotinic acetylcholine receptor; 5) Mitochondrion.

This is an electron micrograph that shows a cross section through the neuromuscular junction. T labels the axon terminal and M labels the muscle fiber. The arrow shows junctional folds with basal lamina. Postsynaptic densities are visible on the tips between the folds. The scale is 0.3 µm. 

Figure 2. Neuromuscular junction: Electron micrograph showing a cross section through the neuromuscular junction. T is the axon terminal and M is the muscle fiber. The arrow shows junctional folds with basal lamina. Postsynaptic densities are visible on the tips between the folds. Scale is 0.3 µm.

Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca2+ ions flow from the extracellular fluid into the presynaptic neuron’s cytosol. This influx of Ca2+ causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron’s cell membrane, which results in the emptying of the vesicle’s contents (acetylcholine) into the synaptic cleft; this process is known as exocytosis.

Acetylcholine diffuses into the synaptic cleft and binds to the nicotinic acetylcholine receptors located on the motor endplate.

These receptors open to allow sodium ions to flow in and potassium ions to flow out of the muscle’s cytosol, producing a local depolarization of the motor endplate, known as an end-plate potential (EPP). This depolarization spreads across the surface of the muscle fiber and continues the excitation-contraction coupling to contract the muscle.

The action potential spreads through the muscle fiber’s network of T-tubules, depolarizing the inner portion of the muscle fiber. The depolarization activates L-type, voltage-dependent calcium channels (dihydropyridine receptors) in the T-tubule membrane, which are in close proximity to calcium-release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum.

As intracellular calcium levels rise, the motor proteins responsible for the contractile response are able to interact, as shown in Figure 3, to form cross-bridges and undergo shortening.

CLINICAL EXAMPLE of Motor Activity

Myasthenia gravis is an autoimmune disorder in which circulating antibodies block the nicotinic acetylcholine receptors on the motor endplate of the neuromuscular junction. This blockage of acetylcholine receptors causes muscle weakness, often first exhibiting drooping eyelids and expanding to include overall muscle weakness and fatigue.

The effects of myasthenia gravis illustrate the importance of effective and functioning neuromuscular junctions for communication between neurons and muscles to allow contraction and relaxation of muscle fibers.

This diagram shows how muscular contraction is caused at the molecular level. A skeletal muscle contracts following activation by an action potential. The binding of acetylcholine at the motor end plate leads to intracellular calcium release and interactions between myofibrils to elicit contraction. 

Figure 3. Muscle contraction and actin-myosin interactions: Skeletal muscle contracts following activation by an action potential. The binding of acetylcholine at the motor endplate leads to intracellular calcium release and interactions between myofibrils to elicit contraction.

Overview of Motor Integration

A motor unit is comprised of a single alpha-motor neuron and all the muscle fibers it innervates.

Key Points

Motor units contain muscle fibers of all the same type; these may be many muscle fibers (as in the case of quadriceps) or a few muscle fibers (as in the case of the muscles that control eye movement).

Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units that subserve a single muscle are considered a motor unit pool.

Motor units are generally recruited in order of smallest to largest (from fewest fibers to most fibers) as contraction increases. This is known as Henneman’s Size Principle.

The smaller the motor unit, the more precise the action of the muscle.

Key Terms

Henneman’s size principle: According to this principle, motor unit recruitment is always in the same order from smallest to largest motor unit. Additionally, the motor unit action potential is an all-or-none phenomenon—once the recruitment threshold (the stimulus intensity at which a motor unit begins to fire) is reached, it fires fully.

alpha motor neuron: Alpha motor neurons (α-MNs) are large, lower motor neurons of the brainstem and spinal cord. They innervate the extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate the intrafusal muscle fibers of muscle spindles.

motor unit: A neuron with its associated muscle fibers.

This is an anatomical drawing of the rectus femoris muscle. It is one of the four quadriceps muscles of the human body. As can be seen, these muscles may have as many as a thousand fibers in each motor unit. 

Rectus femoris: The rectus femoris muscle is one of the four quadriceps muscles of the human body. These muscles may have as many as a thousand fibers in each motor unit.

A motor unit consists of a single alpha motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type (either fast twitch or slow-twitch).

When a motor unit is activated, all of its fibers contract. Groups of motor units often work together to coordinate the contractions of a single muscle. All of the motor units that subserve a single muscle are considered a motor unit pool.

The number of muscle fibers within each unit can vary. Thigh muscles, for example, can have a thousand fibers in each unit, eye muscles might have ten. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle’s need for refined motion.

The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers.

Motor unit recruitment is the progressive activation of a muscle by the successive recruitment of motor units to accomplish increasing gradations of contractile strength. The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction.

Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle. It is therefore a measure of how many muscle fibers of that muscle are activated. The higher the recruitment, the stronger the muscle contraction will be.

Motor units are generally recruited in order of smallest to largest (from fewest fibers to most fibers) as contraction increases. This is known as Henneman’s Size Principle.

This is an anatomical drawing of the orbicularis oris (eye) muscle. These small motor units may contain only 10 fibers per motor unit. The more precise the action of the muscle, the fewer fibers innervated. 

The orbicularis oris (eye) muscle: These small motor units may contain only 10 fibers per motor unit. The more precise the action of the muscle, the fewer fibers innervated.

Motor Unit Categories

Motor units are generally categorized based upon the similarities between several factors such as:

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Contraction speed in isometric contractions:

  • Rate of rise of force.
  • Time to peak of a twitch contraction (response to a single nerve impulse).


Histochemical (the oldest form of biochemical fiber typing):

  • Glycolytic enzyme activity.
  • Oxidative enzyme activity.
  • Sensitivity of myosin ATPase to acid and alkali.

Immunohistochemical (a more recent form of fiber typing):

  • Myosin heavy chain (MHC).
  • Myosin light chain—alkali (MLC1).
  • Myosin light chain—regulatory (MLC2).


The quadriceps muscles contain many thousands of muscle fibers in general, both slow and fast-twitch, to produce sufficient force for body movements such as standing, walking, running, and jumping.

The eye muscles, on the other hand, contain few muscle fibers, enabling them to be more exact in movement so that vision is not jumpy, but consequently, they produce very little force.

  • Clothing fastenings – buttons that are supposed to go into buttonholes or loops, zippers, snaps, ties, collar stays or button-down collars, and shoelaces
  • Using tableware – a knife, fork, or spoon, both for personal use and the utensils used with serving dishes
  • Opening and closing food containers – screw tops, carton spouts, plastic leftover containers, and boxes
  • Twisting doorknobs – also locks, slide chains, and keys
  • Personal care – shaving, brushing teeth, doing hair, applying makeup (especially eyeliner), putting on post-back earrings, inserting contact lenses, bathing, showering, and using the toilet
  • Handwriting – holding a pen or pencil, printing vs. cursive writing, size of individual letters, consistent size of letters, and writing in straight lines
  • Needlework – threading a needle, making the correct size and consistent stitches, casting on/off knitting, maintaining proper thread/yarn tension
  • Video gaming – thumbing the joystick, pressing keys in rapid succession, and watching the screen and operating the controller at the same time
  • Operating other electronic equipment – using a keyboard, a telephone or alarm system touchpad
  • Musical instruments – coordinating both hands to play the instrument, putting fingers in the right places on strings, over holes, or on keys

Children show fine motor coordination and the skills that go with them as they grow older and develop.

Kids develop gross motor skills at different rates. But when young kids have trouble with those skills, it can make gross motor activities like running, jumping, and throwing difficult. If your child’s gross motor skills need a little extra help, try these fun activities.

  • Trampolines – Using a trampoline is a great activity to improve balance. It can also be part of a sensory diet. Indoor trampoline parks are a fun place to socialize with other kids. But if you’re not confident your child will follow directions or if your child isn’t old enough for a trampoline park, you can also get a mini-trampoline for supervised use at home. Keep in mind that it’s important to follow safety rules, like having a jump bar.
  • Hopscotch – Hopping and jumping require strong gross motor skills, balance, and coordination. Hopscotch is a simple way to practice those skills. (As a bonus, it can help practice number skills, too!) If you don’t have a sidewalk to draw on or a playground nearby, you can set up hallway hopscotch using painter’s tape.
  • Martial arts classes – Mаrtіаl аrtѕ trаіnіng is a good way to help kids develop strength in their arms and legs. Actions like kicking, punching, and grappling work to develop those core muscle groups. It can help kids with balance and knowing where their body is in space — motor skills that can be a problem for kids with sensory issues. Martial arts can have additional benefits for kids with ADHD, too.
  • Playground play – Playing on the playground can have many benefits for kids. Swinging on a swing set can help kids develop balance. It also helps them learn how to coordinate shifting their weight and moving their legs back and forth. You may also want to encourage your child to use “unstable” playground equipment like rope ladders and wobble bridges. While they can be scary before kids get used to them, they help work trunk muscles.
  • Balloon and bubble play – Balloons and bubbles are a unique way to build gross motor skills because you can’t predict where they’re going to go. Kids can chase bubbles and try to pop as many as possible. While chasing them, they have to run, jump, zigzag, and move in ways that require sudden shifts in balance and weight. The same goes for throwing and trying to catch or kick balloons. For more structured play, you can set up a game of balloon volleyball.
  • Tricycles, scooters, and pedal cars – Some kids who struggle with gross motor skills may learn to ride a trike or bike later than their peers. But there are alternatives they can use to get places and practice balance. Some tricycles come with handles so you can push while your child practices pedaling. Or you could invest in a sturdy scooter or a pedal car. They’re all stepping stones to riding a bike. Once your child gets the hang of it, you can even set up an obstacle course or draw a track with chalk. (Just don’t forget the helmet!)
  • Dancing – Whether it’s a dance class or an indoor dance party, dancing is good gross motor practice. It helps kids develop balance, coordination, and motor sequencing skills. It also helps build your child’s awareness of rhythm. For little kids, try using songs with lyrics that add movement, like “I’m a Little Teapot” or “The Hokey Pokey.”
  • Obstacle courses – Obstacle courses get kids moving and give them a goal to accomplish. For an indoor course, use furniture, pillows, and blankets to create areas to crawl on, under, and through. Outdoors, you can use things like hula-hoops to jump in and out of, jumping jacks, belly crawling, bear walking, and other creative movements that challenge your child to balance, crawl, jump, and run.


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