This unit discusses Descriptions of most commonly employed biofeedback modalities: SEMG (III-A 1-2) and Muscle anatomy and physiology; antagonistic and synergistic muscle groups (IV-A).
This unit covers Three Types of Muscles, the Skeletal Muscle System, Motor Units, Muscle Action Potentials, Types of Skeletal Muscle Fibers, the EMG Signal, Skeletal Muscle Contraction, the Stretch Reflex, the Tendon Reflex, Muscle Action, Skeletal Muscles of Clinical Interest (Face), and Skeletal Muscles of Clinical Interest (Front and Back).
Please click on the podcast icon below to hear a full-length lecture.
Three Types of Muscles
The human body contains three types of muscles: skeletal, smooth, and
cardiac.
Most muscles are also controlled involuntarily. We are often unaware of
the rhythmic movement of the diaphragm and the intercostal muscles that
allow us to breathe. The monosynaptic stretch reflex, which maintains
postural muscle tone and stabilizes limb position, also operates
unconsciously.
There are two kinds of smooth muscle: single-unit and
multiunit smooth muscle (Tortora & Derrickson, 2021).
Single-unit smooth muscle comprises part of the walls of small
arteries and veins and the stomach, intestines, uterus, and urinary
bladder.
Multiunit smooth muscle is found in the walls of large arteries, airways
to the lungs, the arrector pili muscles that move hair follicles, the
muscle rings of the iris, and the ciliary body that focuses the lens.
Compared with skeletal muscles, smooth muscle contraction starts more
gradually and persists longer. Smooth muscles can shorten and stretch
more than other kinds of muscles. Smooth
muscles usually operate involuntarily, and some tissues have an intrinsic
rhythm (autorhythmicity). They are jointly controlled by the autonomic
branch of the peripheral nervous system and the endocrine system.
Before biofeedback training, most patients could not voluntarily control
smooth muscles since their contraction does not provide sufficient
feedback. For example, blood vessel dilation or constriction in the
fingers produces such "faint" feedback that we are ordinarily unaware of
these changes. Biofeedback converts smooth muscle activity into a digital display, colored bar, or tone that a patient can use to voluntarily warm or cool the fingers.
Below is a
BioGraph ® Infiniti display of peripheral blood flow that simultaneously shows blood volume pulse and
temperature detected from the hand.
Temperature biofeedback illustrates a crucial concept. We can teach
patients to control "involuntary" processes by supplementing inadequate
natural feedback.
Cardiac muscle contraction lasts 10-15 times longer than skeletal muscle
contraction due to the gradual movement of calcium ions into the sarcoplasm
(muscle fiber cytoplasm).
Skeletal muscle contraction only occurs when a motor neuron
releases acetylcholine. In contrast, the sinoatrial and atrioventricular node pacemaker cells
generate the heart rhythm. The
autonomic branch of the peripheral nervous system and the endocrine
system jointly adjust heart rate by acting on these pacemakers.
As with smooth muscle, most patients cannot voluntarily control the heart
rhythm before biofeedback training. Biofeedback supplements cardiac
muscle feedback to teach patients to slow their heart rate or reduce the
frequency of abnormal rhythms like premature ventricular contractions
(PVCs) when they are distressed (Tortora & Derrickson, 2021).
Skeletal Muscles Are Endocrine Glands
Human skeletal muscles are increasingly recognized as endocrine organs that secrete a variety of bioactive molecules known as exerkines during physical activity. These exerkines play significant roles in inter-organ communication and systemic physiological regulation. For instance, Growth and Differentiation Factor 15 (GDF15) is identified as a novel exerkine secreted by skeletal muscle during exercise, which promotes lipolysis in human adipose tissue, highlighting its role in energy metabolism (Laurens et al., 2020).
Additionally, skeletal muscles secrete other exerkines such as thymosin beta-4 (TMSB4X), which is upregulated during muscle contraction and has been implicated in cellular crosstalk, although its effects on metabolic disorders remain inconclusive (Gonzalez-Franquesa et al., 2021).
Furthermore, skeletal muscle-derived exerkines like apelin, kynurenic acid, and lactate have been shown to influence the progression of neurodegenerative diseases by mediating the muscle-brain axis, thus underscoring their therapeutic potential (Bian et al., 2024). This evidence collectively supports the concept of skeletal muscles functioning as glands that secrete exerkines, contributing to the regulation of various physiological processes and disease states.
Skeletal Muscle System
The skeletal muscle system consists of extrafusal muscle fibers and
connective tissue. Important units of distance, signal frequency and strength, and time are summarized below.
Skeletal muscle fibers are striated (striped) due to
alternating light and dark bands. These fibers run from 10-100 micrometers in
diameter and 10 to 30 centimeters in length. The number of skeletal muscle fibers is determined before birth, and most survive across our lifespan (Tortora & Derrickson, 2021),
The sarcolemma, a plasma membrane,
encloses individual fibers. Muscular tissue shares four properties: electrical excitability (producing muscle action potentials), contractility (contracting forcefully), extensibility (stretching), and elasticity (returning to its initial length and shape; Tortora & Derrickson, 2021).
Lateral thin filaments are 8 nm in diameter, 1-2 μ in length, and comprise the protein actin. They span the I band and transit part of the A band. Central thick filaments are 16 nm in diameter, 1-2 μ in length, and comprise the protein myosin. They connect at a sarcomere's M-line (M for middle). There are two thin for every thick filament where thin and thick filaments overlap. The filaments that comprise a myofibril do not extend a muscle fiber's length. Instead, they are stacked in compartments called sarcomeres separated by plate-shaped zones of dense protein called Z discs. Z-discs anchor thin filaments. Sarcomeres extend from Z disc to Z disc and are a muscle fiber's smallest contractile unit. Sarcomeres comprising a myofibril are aligned like boxcars (Marieb & Hoehn, 2019; Tortora & Derrickson, 2021).
A sarcomere consists of A, I, and H bands, and the M line.
An A band runs the length of thick filaments and appears as a sarcomere's darker middle region. Thin and thick filaments are almost perfectly aligned and overlap at the ends of each A band. The lighter, less dense I band contains the remaining thin and no thick filaments. Z disks intersect its center. The interleaving of dark A and light I bands produces the banding visible in myofibrils and skeletal muscle fibers (Marieb & Hoehn, 2019; Tortora & Derrickson, 2021).
Titin, a structural protein called connectin, links a Z disc (also called Z line) to the M line in the center of a
sarcomere to stabilize the thick filament's position. The segment of the titin molecule attached to the Z disc
can stretch more than four times its resting length.
Muscle fibers
generate force by pulling Z discs together, shortening the sarcomeres.
This force, called active tension, moves bones at joints and
resists active stretching due to external forces like gravity. Check out the YouTube video Muscle Contraction Process: Molecular Mechanism [3D Animation].
The Role of Connective Tissue
Connective tissue packages muscle fibers together and connects muscle
fibers to bone (as well as skin, muscle, and deep fascia).
Fascia, fibrous
connective tissue, divides muscles into functional groups, aids muscle movement, transmits force to bones,
encloses and supports blood vessels and nerve fibers, and secures organs in their place (Tortora & Derrickson, 2021).
Fascicle-tendon connections can assume five shapes: circular, fusiform (narrow at the ends and wide in the center), parallel, pennate (feather-shaped, unipennate, bipennate, and multipennate), or triangular (convergent; Tortora & Derrickson, 2021). Graphic courtesy of Wikimedia.
Tendons are dense fasciae that connect
muscles to other structures. Connective tissue provides the skeletal
muscle system's elasticity. Connective tissue elasticity produces passive
tension, allowing muscle fibers to generate,
sum, and transmit force
(Tortora & Derrickson, 2021).
Motor Units
Skeletal muscle fibers are organized into motor units consisting of an
alpha motor neuron and the extrafusal muscle fibers it controls.
In the diagram below, a motor neuron's axon branches to simultaneously innervate four extrafusal muscle fibers. This motor unit is designed for precise control
instead of generating force.
Alpha motor neurons, which conduct messages from the central nervous
system to skeletal muscles, are called efferent nerves because they leave
the central nervous system.
A motor unit contains from 3 to 3000 muscle fibers that contract
completely or not at all. The average motor unit contains 150 fibers (Buchthal & Schmalbruch, 1980; Tortora & Derrickson, 2021). All motor unit muscle fibers share the same composition.
Smaller motor units perform precise movements. For example, the larynx, which controls speech, contains motor units with 1-2 muscle fibers. The extraocular muscles that move the eyes contain motor units with 10-20 muscle fibers. Larger motor units are designed to deliver power instead of precise control. The gastrocnemius muscle located in the calf contains
whose motor units with 2000-3000 muscle fibers (Marieb & Hoehn, 2019).
SEMG biofeedback uses an electromyograph to monitor the total electrical output in microvolts (millionths of a volt) from all the motor units firing near surface electrodes. Contraction strength
and SEMG signal voltage are proportional to the number of recruited motor
units.
The muscles comprising each motor unit are protected against exhaustion
by a refractory period of about 1 millisecond, during which they lose their
excitability. Cardiac muscle has a 250-millisecond refractory period (Tortora & Derrickson, 2021). Motor units
control adjoining
bundles of fibers to produce coordinated muscle contraction.
A skeletal muscle contains motor units that differ in the number and
size of fibers. Motor unit firing is rotated to prevent fatigue and
produce smooth movement. Motor units are recruited in order of size.
Smaller units, which are highly excitable, are recruited first since they gradually build up tension. These units provide the precise motor
control required in tasks like writing. Larger motor units are normally recruited last as additional force is needed to perform a bench press. In life-threatening emergencies, many motor units may be recruited
at once as part of the fight-or-flight response (Tortora & Derrickson, 2021).
While the muscle fibers comprising an individual motor unit show an all-or-none response, a skeletal muscle can
produce graded responses by activating different sets of motor units (Tortora & Derrickson, 2021).
When an action potential reaches the terminal branches, calcium ions move inside them. Vesicles containing ACh expel their contents into the NMJ. Each motor endplate contains 30-40 million acetylcholine (ACh) receptors. ACh release depolarizes the motor end plate, producing an end plate potential (EPP).
The EPP triggers a depolarizing impulse called a muscle action potential (MAP) that travels sequentially along the sarcolemma into the network of T tubules, causing the terminal cisterns of the sarcoplasmic reticulum to release Ca2+ ions into the sarcoplasm to initiate muscle contraction (Tortora & Derrickson, 2021).
Skeletal muscle
depolarization is the source of the SEMG signal and initiates muscle
contraction.
The only way to accurately record a muscle's unique activity is to place electrodes along its striations. Recording across a muscle's striations produces invalid measurements since
it averages the activity of several muscles (Sherman, 2010).
When two ACh molecules bind to a nicotinic ACh receptor on the motor end
plate, a sodium channel opens, allowing sodium ions to enter the muscle
fiber and then potassium ions to leave the fiber's interior. The inflow
of positive sodium ions shifts the end plate's internal voltage from -80 to -90 millivolts to +50 to +75 millivolts producing a muscle action potential (MAP).
This skeletal muscle fiber depolarization (positive shift in membrane
potential) triggers muscle contraction (Hall & Hall, 2020).
Below is a NeXus-10 ® BioTrace+ display of the raw and integrated 100-500 Hz SEMG signal generously provided by John S. Anderson.
Below is a
BioGraph ® Infiniti SEMG display.
The enzyme acetylcholine esterase (AChE) deactivates ACh to allow
muscle fiber relaxation. The sodium-potassium pump restores the muscle
fiber to a resting negative voltage so it can be depolarized again,
resulting in a new contraction (Tortora & Derrickson, 2021).
Types of Skeletal Muscle Fibers
Skeletal muscle fibers vary in structure and function. Three fiber
categories have been identified: slow oxidative, fast oxidative, and fast
glycolytic. Skeletal muscles are composed of different proportions of
these fibers depending on muscle action. However, the motor units that
make up a muscle contain the same kind of fiber.
Slow oxidative (SO) fibers are small red fibers are rich in myoglobin, mitochondria, and capillaries.
Their capacity to produce ATP through oxidative metabolism is high. Since
SO fibers split ATP at a slow rate, they contract slowly, work for extended periods, and strongly resist fatigue.
Postural muscles contain a
high proportion of SO fibers that enables continuous isometric
contraction to resist gravity. SO fibers contribute frequencies from 20-90 Hz (Bolek et al., 2016).
SO fibers contain fewer skeletal muscles in their motor units than FOG and FG fibers, are used more frequently in daily activities, and are recruited earlier (Fox & Rompolski, 2022).
The frequency range for surface recording is 2-1,000 Hz.
Tissues
(skin,
subcutaneous fat, muscle, and connective tissue) absorb the higher
frequencies that can be detected within the muscle fibers by inserted electrodes (Stern, Ray, & Quigley, 2001).
Below is a BioGraph ® Infiniti SEMG FFT display of a frontales placement.
Lower frequency amplitude increases
with stronger muscle contraction and the energy distribution shifts toward the higher frequencies.
The amplitude or strength of an electromyogram reflects the number of
active motor units, their firing rate, and distance from the electrodes.
A bandpass of 20-200 Hz covers the most critical SEMG frequencies
(Andreassi, 2007). Slow-twitch fibers generate frequencies from 10-90 Hz, and fast-twitch fibers generate
frequencies from 90-500 Hz.
The greatest concentration of power in a resting muscle lies
between 10 and 150 Hz (Stern et al., 2001).
Strong muscle contraction shifts the distribution of
SEMG power
upwards and to the right toward higher frequencies.
Fatigue decreases motor unit firing rate and shifts the distribution of SEMG power to lower frequencies, lowering the median frequency. Thought Technology Ltd. generously shared the illustration below.
A wide bandpass is desirable because it better monitors a muscle's dynamic range. Failure to use the correct bandpass may make a muscle appear more relaxed than it is or miss fatigue entirely (Sherman, 2010).
Muscle Action Potentials Are Briefer Than The Muscle Contraction Period
Skeletal muscle fiber depolarization (which produces the SEMG signal)
lasts 1 to 2 ms and ends before a muscle fiber starts to contract. The
SEMG reflects muscle depolarization, not muscle contraction. The period of
mechanical contraction is considerably longer and lasts from 10-100 ms.
Calcium binds to troponin, myosin-actin
cross-bridges form, and peak muscle fiber tension develops. The relaxation phase also lasts 10-100 ms. Calcium is actively transported back to the sarcoplasmic reticulum, myosin heads detach from actin, and tension declines (Tortora & Derrickson, 2021)
A muscle action potential depolarizes the interior of the muscle fiber.
This membrane potential change releases stored calcium ions from the sarcoplasmic reticulum. The
presence of calcium ions allows actin (thin) and myosin (thick)
filaments to bind to each other forming cross-bridges. Simple contact
will not shorten a muscle. Myosin must bind to actin, move it inward,
break contact, and bind again. These power strokes use stored energy
released by splitting ATP.
Muscle contraction may be isometric or isotonic. Muscles produce tension
with minimal fiber shortening during isometric contraction. For example, muscles do not appreciably shorten when you perform a plank due to the resistance of gravity. Posture results from
continuous isometric contraction. SO fibers are
specialized for this contraction due to their fatigue resistance.
Muscles groups work together to perform actions like the series of flips shown below.
You
contract at least nine muscle groups in a specific sequence to open your
mouth. You activate a different sequence to close your mouth. SEMG biofeedback records the electrical activity of
surface muscles.
The SEMG usually monitors the activity of muscle groups instead of
individual muscles.
Muscle spindles are stretch receptors
that lie in parallel with skeletal muscle fibers. Muscle spindles allow us to restore muscle length when muscle fibers lengthen. The image below from Dr. J. H. H. Scott of Leicester University
is from the first dorsal interosseus muscle of the hand.
The graphic below that depicts a muscle spindle was retrieved from irunfar.com.
Tendon Reflex
Golgi tendon organs are force
detectors that lie in series with skeletal muscle fibers. They protect muscles and tendons from injury due to forceful contractions. Graphic retrieved from firstclassmed.com.
When excessive
contraction threatens to damage muscle and tendon, they inhibit the
responsible alpha motor neurons to prevent injury. This protective mechanism
is called the tendon reflex (Breedlove & Watson,
2020). The graphic below that depicts the tendon reflex was retrieved from irunfar.com.
A spring on a door provides a good analogy. The end of the spring attached to the frame is the origin; the end attached to the door is the insertion. The origin is usually proximal; the insertion is distal (Tortora & Derrickson, 2021).
The bone where a muscle originates can move when the insertion is fixed. For example, when the tibia/foot is off the ground (open chain), the tibia moves on the femur. When the tibia/foot is fixed to the ground (closed chain), the femur moves on the tibia. Think of a knee extension machine versus performing a squat (Jadali, 2021).
Muscles that operate joints are arranged in opposing prime mover-antagonist
pairs, often located on the opposite side of a bone or joint. A prime mover (agonist) contracts to perform an action while its antagonist stretches to allow the action. For example,
when the
biceps brachii (prime mover) contracts to flex the
forearm at the elbow joint, the triceps brachii (antagonist) must relax,
or else flexion will be prevented (Tortora & Derrickson, 2021).
When muscles are arranged in opposing pairs, the prime mover and antagonist can switch roles to perform different movements. For example, when you extend the forearm against a resistance, the triceps brachii becomes the prime mover and the biceps, the antagonist.
In the lower extremities, the extensor chain muscles are generally stronger than their flexor counterparts due to their role in propulsion during the gait cycle and combatting gravity (Jadali, 2021).
When prime mover and antagonist muscles
contract concurrently in stroke patients, this can produce spasms and
rigidity. The table below was adapted from Thought Technology Ltd.'s (2009) Basics of Surface Electromyography
Applied to Physical Rehabilitation and Biomechanics.
The next section describes synergists, fixators, flexors and extensors, abductors and adductors, levators and depressors, supinators, and pronators, dorsiflexors and plantar flexors, inverters and evertors, tensors, and rotators.
Prime movers may cross other joints before attaching to the joint where it exerts its primary action. Synergists aid prime movers by stabilizing intermediate joints to prevent co-contraction (e.g., flexing your fingers without flexing the wrist) and unwanted movement. They are usually near the prime mover. For example, the deltoid and pectoralis
major anchor both the arm and
shoulder when the bicepsbrachii
(agonist) contracts to flex the forearm (Tortora & Derrickson, 2021).
Muscles that act as fixators stabilize a prime mover's origin for more efficient action. By steadying a limb's proximal end, fixators support movements at the distal end. For example, the mobile scapula is the origin of several arm muscles and must be steadied for actions like abduction. Depending on their movement, the same muscles may act as prime movers, antagonists, synergists, or fixators (Tortora & Derrickson, 2021).
The quadriceps and tibialis anterior
muscles of your leg
contract together to dorsiflex (bend the foot upwards) during the forward
swing phase of walking.
Flexors decrease the angle between two bones.
Biceps brachii flexes and supinates (turns up) the forearm. Weightlifters flex forearms, not muscles.
Extensors increase the angle between two bones. Triceps brachii
extends
the forearm. The motor control system coordinates synergist, flexor, and
extensor contraction to produce the needed limb position.
Abductors move a limb away from the center of the trunk or a body part.
The deltoid abducts the arm.
Adductors move a limb toward the center of the trunk or a body part. Pectoralis
major adducts the arm.
Levators produce upward movement. Levator scapulae elevates the
shoulder
blades.
Depressors produce downward movement. Latissiumus dorsi is the primary shoulder girdle and scapular (shoulder blade) depressor. Scapular depression is mainly due to the failure of scapular elevators and upward rotators.
Supinators turn the palm upward (anteriorly). Supinator exposes
the
anterior side of the forearm.
Pronators turn the palm downward (posteriorly). Pronator teres
exposes
the posterior side of the forearm.
Dorsiflexors point the toes toward the shin (superiorly) through flexion
at the ankle joint. Tibialis anterior dorsiflexes and inverts the foot
during the swing phase of walking. Dorsiflexion is disrupted during a
neuromuscular disorder called foot drop in which the toes drag on the
ground.
Plantar flexors point the toes downward (inferiorly) through extension at
the ankle joint. The gastrocnemius/soleus complex, which acts on the Achilles tendon, is the body's primary plantar flexor. The grastrocnemius is considered the most important walking muscle (Jadali, 2021).
Invertors turn the sole of a foot inward. The tibialis posterior and tibialis anterior are the primary invertors. Posterior tibial tendonitis is a common orthopedic injury related to inversion (Jadali, 2021).
Evertors turn the sole of a foot outward. The fibularis longus and tertius muscles perform eversion. Peroneal tendonitis affects eversion (Jadali, 2021).
Tensors make a body part more rigid. Tensor fasciae latae
flexes and
abducts the thigh.
Rotators move a bone around a longitudinal axis. Obturator
externus laterally rotates the thigh (Tortora & Derrickson, 2021).
The complex movement sequences used in jumping rope integrate many muscle actions.
MUSCLE ACTIONS, SENSOR PLACEMENTS, AND CLINICAL APPLICATIONS
The next two sections provide a detailed description of muscle actions, sensor placements, and clinical applications. Although the BCIA Biofeedback Blueprint does not cover this content at this level of detail, it can be invaluable for applicants during their mentorship and for understanding research reports.
Skeletal Muscles of Clinical Interest (Face)
Basmajian and Blumenstein (1980) and Neblett (2006) were
the references for sensor placement. Tortora and Derrickson (2021) was the resource
for muscle action.
Action: draws the corner of the mouth upward and outward when you smile
Sensor placement: locate actives above and at approximately a 45o angle from the corner of the mouth
Clinical application: Bell's palsy and stroke
Orbicularis oculus
Action: closes the eyelids and wrinkles the forehead
Sensor placement: locate actives immediately below the center of each eye
Clinical application: Bell's palsy, blepharospasm, and stroke
Sternocleidomastoid (SCM)
Action: flexes the vertebral column and rotates the head to the opposite
side (contract left SCM and head twists to the right)
Sensor placement: center the actives 50% of the distance from the
mastoid (bulge behind the outer ear) to the medial end of the clavicle (collarbone), which places them
below the jaw along a vertical line
Clinical application: torticollis
Cervical paraspinal (semispinalis capitis and splenius capitis)
Action: extends, flexes, and rotates the head
Sensor placement: locate the actives around C2 and C5 lateral and medial to the spine
Clinical application: neck and shoulder pain and tension-type headache
Upper trapezius
Action: rotates and elevates the scapula (shoulder blade), extends, flexes, and rotates
the head and neck
Sensor placement: center the actives between C7 (seventh cervical vertebra) and the angle of the acromion
(posterior to bony triangle at the top of
the shoulder)
Clinical application: shoulder pain and tension-type headache
Bilateral monitoring of the upper trapezius with a wide bandpass may be used to assess and treat pain due to
shoulder injury (Neblett, 2012).
Bilateral monitoring of the upper trapezius with a narrow bandpass may also be used to assess and treat pain due
to shoulder injury (Neblett, 2012).
Bilateral monitoring of
the cervical and upper trapezius muscles using a narrow or wide bandpass may be used to assess and treat headache
and neck and upper back pain (Neblett, 2012).
Skeletal Muscles of Clinical Interest
Posterior
deltoid
Action: abducts, flexes, extends, and rotates the arm
Sensor placement: locate the actives behind the angle of the acromion (posterior to the bony triangle at
the top of the shoulder)
Lateral head of triceps brachii
Action: extends the elbow joint
Sensor placement: center the actives 50% of the distance between the
angle of the acromion (posterior to bony triangle at the top of the shoulder)
and the olecranon process (behind the elbow)
Biceps brachii
Action: flexes and supinates the forearm
Sensor placement: center the actives over the bulge in this muscle
Latissimus dorsi
Action: extends, adducts, rotates the arm, and moves the arm inferiorly
and posteriorly
Sensor placement: center the actives under the inferior angle of the
scapula (shoulder blade)
Extensor carpi ulnaris
Action: extends and adducts the hand at the wrist joint
Sensor placement: center the actives 1/3 of the distance between the
posterior medial epicondyle and the posterior styloid process of the
forearm
A flexor/extensor placement using a wide bandpass may be used to assess and
treat crushing injury to the arm and hand (Neblett, 2012).
A web placement using a wide bandpass may also be used to assess and treat
crushing injury to the arm and hand (Neblett, 2012).
Abductor pollicis brevis
Action: abducts the thumb and assists in opposition
Sensor placement: center the actives over the most prominent bulge at the
proximal end of the thumb
Adductor pollicis
Action: adducts the thumb
Sensor placement: center the actives within a triangle that starts at the
skin web
Vastus lateralis (VL) and vastus medialis (VMO)
Action: each muscle extends the leg at the knee joint
Sensor placement: locate the actives vertically within the oval bulges
above the patella (kneecap) for the vastus lateralis(VL) and vastus medialis
obliquus(VMO) sites,
respectively.
Tibialis anterior
Action: dorsiflexes and inverts the foot
Sensor placement: center the actives vertically within an oval region around the
tuberosity of
the tibia (shin bone)
Clinical application: foot drop
Gastrocnemius
Action: plantar flexes the foot and flexes the knee joint
Sensor placement: center the actives vertically over the bulges of either
head of this muscle
Erector spinae (sacrospinalis)
Action: maintains the erect position of the spine and extends the vertebral
column
Sensor placement: locate active pairs lateral to the spine, above the iliac crest (rounded upper margins of
the ilium bone above the buttocks) about L4 and below the bottom of the ribs at about L2. Clinicians should
palpate these landmarks to achieve consistent lumbar placements
Clinical application: low back pain
Bilateral monitoring of
the lumbar region using a wide bandpass may be used to assess and treat middle back pain (Neblett, 2012).
Glossary
α-actinin: structural Z disc protein that connects actin to titin molecules.
A band: dark, central region of a sarcomere that runs the length of thick filaments and includes overlapping actin filaments.
abductors: muscles that move a limb away from the center of the trunk or a
body part. For example, abductor pollicis brevis moves the thumb outward.
acetylcholine (ACh): the neurotransmitter released by alpha motor neurons at
the neuromuscular junction depolarizes motor endplates producing a muscle action potential (MAP).
acetylcholine esterase (AChE): the enzyme that deactivates ACh to allow
skeletal muscle fiber relaxation.
actin: contractile protein that is the main component of thin filaments; each actin molecule contains a binding site for myosin heads which form cross-bridges during muscle contraction.
active tension: force generated by muscle fibers when myosin filaments pull Z
discs together, shortening the sarcomeres.
adductors: muscles that move a limb toward the center of the trunk or a body
part. For example, adductor pollicis moves the thumb inward.
alpha motor neuron: motoneuron that innervates the skeletal muscles fibers
that comprise its motor unit.
amplitude: the strength of an EMG signal that is measured in microvolts.
angle of the acromion: the site posterior to the bony triangle at the top of the
shoulder.
annulospiral receptors: muscle spindle length receptors that stretch as a
muscle lengthens and activate alpha motor neurons to strengthen muscle contraction to increase muscle tone.
antagonist: a muscle that opposes a prime mover's action and yields to its movement. For example, the triceps brachii (antagonist) opposes flexion by the biceps brachii (prime mover or agonist).
biceps brachii: a muscle that flexes and supinates the forearm. Active electrodes are
centered over the bulge in this muscle.
C7: the seventh cervical vertebra.
cardiac muscle: muscle fibers that comprise most of the heart wall. These
fibers have crossed striations that allow the heart to pump and contain the same actin and myosin filaments,
bands, zones, and Z discs as skeletal muscles.
clavicle: collarbone.
concentric contraction: fibers shorten as your body moves upward, for example,
a pull-up.
deltoid: synergist that along with the pectoralis major muscle anchors both
the arm and shoulder when the biceps brachii (agonist) contracts to flex the forearm. Active electrodes are located
behind the angle of the acromion (posterior to the bony triangle at the top of the shoulder) for the posterior
deltoid.
depressors: muscles that produce downward movement. For example, the latissimus dorsi lowers the shoulder blades and shoulder girdle.
dorsiflexors: muscles that point the toes toward the shin (superiorly) through
flexion at the ankle joint. For example, the tibialis anterior dorsiflexes and inverts the foot during
the swing phase of walking.
eccentric contraction: fibers lengthen as you lower yourself, for example,
when performing a squat.
efferent nerves: nerves like alpha motor neurons that leave the central
nervous system.
erector spinae (sacrospinalis): a muscle that maintains the erect position of the
spine and extends the vertebral column. Active pairs are located lateral to the spine, above the iliac crest
(rounded upper margins of the ilium bone above the buttocks) about L4, and below the bottom of the ribs at about
L2.
evertors: muscles that turn the sole of a foot outward. For example, fibularis longus/tertius are the primary eversion muscles.
exerkines: a group of signaling molecules released during exercise that plays a crucial role in mediating its systemic benefits, including improved metabolism, cardiovascular health, and cognition.
extensor carpi ulnaris: a muscle that extends and adducts the hand at the wrist
joint.
extensor digitorum longus: a muscle that dorsiflexes, extends the toes, and everts
the foot.
extensors: muscles that increase the angle between two bones. For example, the triceps brachii extends the forearm.
extrafusal fibers: skeletal muscle fibers that are striated (striped) due to
alternating light and dark bands.
fascia: fibrous connective tissue that divides muscles into functional groups,
aids muscle movement, transmits force to bones, encloses and supports blood vessels and nerve fibers, and secures
organs in their place.
fascicles: muscle fiber bundles.
fast glycolytic (FG) fibers: white fibers are poor in myoglobin, mitochondria,
and capillaries but contain extensive stores of glycogen. FG fibers produce ATP using anaerobic metabolism
that cannot continuously supply needed ATP, causing these fibers to fatigue easily. They are also called fast-twitch A fibers.
fast oxidative-glycolytic (FOG) fibers: red fibers are rich in myoglobin,
mitochondria, and capillaries. Their capacity to produce ATP through oxidative metabolism is high, and FOG fibers
split ATP rapidly producing high contraction velocities. They show less resistance to fatigue than SO fibers. They
are also called fast-twitch B fibers.
flexor digitorum longus: a muscle that plantar flexes, flexes toes, and inverts
the foot.
flexors: muscles that decrease the angle between two bones. For example, the biceps brachii flexes and supinates (turns up) the forearm.
frontales: a muscle that draws the scalp forward, raises eyebrows, and wrinkles
the forehead. The actives are located between the eyebrows and hairline.
gastrocnemius: a muscle that plantar flexes the foot and flexes the knee joint.
Active electrodes are centered vertically over the bulges of either head of this muscle.
Golgi tendon organs: force detectors that lie in series with skeletal muscle
fibers. When excessive contraction threatens to damage muscle and tendon, they inhibit the responsible alpha motor
neurons to prevent injury. This protective mechanism is called the tendon reflex.
H bands: narrow central region in each A band containing thick filaments.
I bands: lighter, less dense region of a sarcomere that contains the remaining thin but no thick filaments and is bisected by a Z disc.
iliac crest: rounded upper margins of the ilium bone above the buttocks.
ilium: bone located above the buttocks.
insertion: the more movable bone at a joint.
invertors: muscles that turn the sole of a foot inward. For example, the tibialis posterior and tibialis anterior are the primary invertors.
isometric contraction: a contraction in which muscles produce tension with
minimal fiber shortening.
isotonic contraction: a contraction in which muscles produce movement by
exerting tension on an attached structure (bone).
lateral head of triceps brachii: a muscle that extends the elbow joint.
latissimus dorsi: a muscle that extends, adducts, medially rotates the arm, and moves the
arm inferiorly and posteriorly. The actives are centered actives under the inferior angle of the scapula (shoulder
blade).
levators: muscles that produce upward movement. For example, the levator
scapulae elevates the shoulder blades.
M lines: central region of an H zone, located at the center of a sarcomere, that contains the proteins that connect thick filaments.
mandible: jaw.
masseter: a muscle that elevates and protracts the mandible. The actives are
located using the angle of the jaw as a landmark.
mastoid: the bulge behind the outer ear.
medial epicondyle: the medial prominence of the bottom aspect of humerus located
on the inside of the elbow.
monosynaptic stretch reflex: a reflex that maintains postural muscle tone and
stabilizes limb position by correcting increases in muscle length.
motor unit: an alpha motor neuron and the muscle fibers it controls.
multiunit smooth muscle: muscle found in the walls of large arteries, airways
to the lungs, the arrector pili muscles that move hair follicles, the muscle rings of the iris, and the ciliary
body that focuses the lens.
muscle action: the movement produced by skeletal muscle contraction.
muscle action potential (MAP): a depolarizing impulse that travels along the sarcolemma and T tubules, producing
the EMG signal and initiating skeletal muscle contraction.
muscle spindles: stretch receptors that lie in parallel with skeletal muscle
fibers and trigger the monosynaptic stretch reflex.
myofibrils: each skeletal muscle fiber comprises hundreds to thousands of these units built from thin and thick filaments.
myofilaments: building blocks of a myofibril. Thin filaments are
primarily composed of actin, and thick filaments are mainly composed of myosin.
myomesin: structural protein that comprises a sarcomere's M line, binds to
titin, and connects adjacent thick filaments.
myosin: contractile protein that comprises thick filaments; a single molecule contains two heads that bind to sites on actin during muscle contraction.
neuromuscular junction (NMJ): the synapse between an alpha motor neuron and a skeletal muscle.
nicotinic ACh receptor: an ionotropic receptor on the skeletal muscle endplate binds two ACh molecules to allow sodium ions to enter the muscle fiber.
olecranon process: the bony projection behind the elbow.
orbicularis oculus: muscle that closes the eyelids and wrinkles the forehead.
The actives are located immediately below the center of each eye.
origin: a muscle's attachment to the more stationary bone.
passive tension: the elasticity of connective tissue allows muscle fibers to
produce, sum, and transmit force.
patella: kneecap.
peak frequency: the highest amplitude frequency in the SEMG signal.
pectoralis major: synergist that, along with the deltoid, anchors both the arm
and shoulder when the biceps brachii (agonist) contracts to flex the forearm.
plantar flexors: muscles that point the toes downward (inferiorly) through
extension at the ankle joint. For example, the gastrocnemius/soleus complex act on the Achilles tendon.
posterior deltoid: a muscle that abducts, flexes, extends, and rotates the arm.
Active electrodes are located behind the angle of the acromion (posterior to the bony triangle at the top of the
shoulder).
posterior styloid process: the rear aspect the projection from the medial and back
part of the ulna (elbow bone).
power strokes: after actin and myosin filaments form cross bridges, myosin
moves actin inward, breaks contact, binds to actin, and then repeats this process.
prime mover (agonist): a muscle directly responsible for producing a specific movement and is opposed by an antagonist. For example, when the biceps brachii (prime mover) contracts to flex the forearm at the elbow joint, the triceps brachii (antagonist) must relax, or else flexion will be prevented.
pronators: muscles that turn the palm downward (posteriorly). For example, pronator teres exposes the posterior side of the forearm.
proprioceptive feedback: information about body position and movement.
quadriceps: a muscle that, along with the tibialis anterior, dorsiflexes (bends
the foot upwards) during the forward swing phase of walking.
refractory period: 5-millisecond interval during which skeletal muscles lose their
excitability to prevent exhaustion.
reverse muscle action (RMA): the origin and insertion switch to perform the opposite movement.
rotators: muscles that move a bone around a longitudinal axis. For example,
the obturator externus laterally rotates the thigh.
sarcolemma: the skeletal muscle fiber membrane.
sarcomeres: skeletal muscle compartments separated by dense zones
called Z discs.
scapula: shoulder blade.
SEMG: surface electromyogram.
SEMG power: the energy of the surface electromyogram measured in
microvolts.
single-unit smooth muscle: smooth muscle that comprises part of the walls of
small arteries and veins, the stomach, intestines, uterus, and urinary bladder.
skeletal muscles: extrafusal muscles that move the bones of the skeleton and
are striated (striped) muscles due to alternating light (I) and dark (A) bands.
slow oxidative (SO) fibers: red fibers rich in myoglobin,
mitochondria, and capillaries with a high capacity to produce ATP through oxidative metabolism. Since SO fibers
split ATP at a slow rate, contraction velocity is slow, and these fibers are highly resistant to fatigue. Postural
muscles contain a high proportion of SO fibers. They are also called slow-twitch fibers.
smooth muscle: single-unit and multi-unit muscle fibers whose contraction
starts more gradually and persists longer than skeletal muscle contraction. Smooth muscles can shorten and stretch
more than other muscles and usually operate involuntarily. Some tissues have an intrinsic rhythm
(autorhythmicity).
sodium-potassium pump: the mechanism that exchanges sodium ions for potassium ions
to restore the muscle fiber to a resting negative voltage so that it can be depolarized anew, resulting in new
contraction.
sternocleidomastoid (SCM): a muscle that flexes the vertebral column and rotates
the head to the opposite side (contract left SCM and head twists to the right). Actives are centered 50% of the
distance from the mastoid (bulge behind the outer ear) to the medial end of the clavicle (collarbone), which
places them below the jaw along a vertical line.
striated: striped. For example, skeletal muscle fibers are striped due to
thin (light) and thick (dark) filaments.
supinators: muscles that turn the palm upward (anteriorly). For example, the supinator exposes the anterior side of the forearm.
synergists: muscles that stabilize a joint to reduce the origin's interference
with movement. For example, the deltoid and pectoralis major anchor both the arm and shoulder
when the biceps brachii (agonist) contracts to flex the forearm.
temporalis: a muscle that elevates and retracts the mandible (jaw). Clinicians place active electrodes above the zygomatic arch (horizontal bony ridge from temporomandibular joint to the cheek).
tendon: connective tissue that connects muscle to bone and transmits the force
of muscle contraction to the attached bones.
tendon reflex: Golgi tendon organs are force detectors that lie in series with skeletal muscle fibers.
When excessive contraction threatens to damage muscle and tendon, they inhibit the responsible alpha motor neurons
to prevent injury.
tensors: muscles that make a body part more rigid. For example, the tensor
fasciae latae flexes and abducts the thigh.
thick filaments: dark-colored filaments that are mainly composed of myosin.
thin filaments: light-colored filaments that are mainly composed of
actin.
tibia: shinbone.
tibialis anterior: a muscle that dorsiflexes and inverts the foot. Actives are
vertically centered within an oval region around the tuberosity of the tibia (shinbone).
titin: a structural protein that links a Z disc to the M line in the center of a sarcomere to stabilize the thick filament position and contribute to myofibril elasticity and extensibility.
triceps brachii (lateral head): a muscle that extends the elbow joint. Actives
are centered 50% of the distance between the angle of the acromion (posterior to bony triangle at the top of the
shoulder) and the olecranon process (behind the elbow).
tropomyosin: a regulatory protein component of thin filaments that prevents myosin heads from binding to actin when a muscle fiber is relaxed.
troponin: a regulatory protein component of thin filaments that changes shape when calcium ions bind, removing tropomyosin from covering myosin-binding sites to allow the formation of cross-bridges and muscle contraction.
upper trapezius: a muscle that rotates and elevates the scapula (shoulder
blade), extends, flexes, and turns the head and neck. Active electrodes are centered between C7 (seventh cervical vertebra) and the angle of the acromion (posterior to bony triangle at
the top of the shoulder).
vastus lateralis (VL) and vastus medialis (VMO): muscles that extend the leg
at the knee joint. Clinicians position active electrodes vertically within the oval bulges above the patella (kneecap) for the
vastus lateralis (VL) and vastus medialis obliquus (VMO) sites, respectively.
Z discs: thin, plate-shaped dense zones that separate sarcomeres.
zygomatic arch: the horizontal bony ridge from temporomandibular joint to the
cheek.
zygomaticus: a muscle that draws the corner of the mouth upward and outward when
you smile. Actives are located above and at approximately a 45o angle from the corner of the mouth.
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Assignment
Now that you have completed this module, identify the muscles you monitor
and train in your clinical practice using the muscle diagrams shown above.
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