Neuromechanics of Human Movement

Question 1

Introduction to Neuromechanics of Muscle

Answer 1

• excitable tissue serves as basis for movement
• possesses “plasticity”
• PTs must have solid biomechanical understanding of tissue and organ system

Question 2

Introduction to Neuromechanics of Skeletal Muscle

Answer 2

• most abundant tissue in human body
• represents 40-45% of total body weight
• transforms chemical energy into mechanical energy
• mechanical energy results in the generation of internal forces
• muscle is very resilient
• can be stretched or shortened at fairly high speeds
• can withstand considerable activity without damage
• skeletal muscle performance dictated by four properties: irritability, contractility, extensibility, elasticity

Question 3

Irritability

Answer 3

• ability to respond to stimulation
• skeletal muscle is one of the most sensitive and responsive tissues in the body
• only nerve tissue is more sensitive

Question 4

Contractability

Answer 4

• ability to shorten
• occurs when muscle tissue receives sufficient stimulation
• some muscles can shorten as much as 50-70% of their resting length
• shortening distance limited by its confinement in the body

Question 5

Extensibility

Answer 5

• ability to stretch or lengthen
• muscle itself cannot produce the elongation
• another muscle or force is needed
• determined by connective tissue found in: perimysium, epimysium, fascia surrounding and within

Question 6

Elasticity

Answer 6

• ability to return to resting length after stretching or lengthening
• determined by connective tissue in muscle
• a critical component in facilitating output in a shortening muscle action preceded by a stretch: aka stretch-shortening cycle

Question 7

Biomechanical Roles of Skeletal Muscle

Answer 7

• perform a variety of different functions
• all are important to performance of human body
• functions: production of movement, maintain postures and positions, stabilize joints, other functions

Question 8

Production of Movement

Answer 8

• motion is created secondary to generation of muscle tension
• tension or force transferring to bones
• resulting motion necessary for locomotion, other segmental manipulations

Question 9

Maintenance of Postures and Positions

Answer 9

• typically involves mm actions of lesser magnitude
• mm activity is often continuous
• results in small adjustments
• functional goal is to maintain head position, balance body weight over feet

Question 10

Stabilization of Joints

Answer 10

• mm action contributes significantly to joint stability
• mm tensions generated and applied across joints via tendons
• thus provide stability where they cross joints
• among primary joint stabilizers via tendons: shoulder, knee

Question 11

Other Functions of Skeletal Muscle

Answer 11

• not related directly to human movement
• support and protect visceral organs
• alter and control pressure within cavities
• maintenance of body temperature-secondary in producing heat
• control entrances and exits to body through voluntary control: swallowing, urination, defecation

Question 12

Architecture of Skeletal Muscle

Answer 12

• muscle as tissue: fascicle, muscle fiber, myofibril, sarcomere,myofilaments, SR
• review exercise phys notes

Question 13

Excitation-Contraction Coupling How Movement Starts

Answer 13

• transfer of chemical energy to mechanic energy: ATP –> Force Production
• good idea to review content from exercise phys

Question 14

Types of Muscle FIber

Answer 14

• 3 basic types of muscle fibers
• differentiate based on: predominant energy source, speed of contraction, intensity of neural stimulation
• review charts on page 6

Question 15

Fiber Typing: Clinical Example

Answer 15

• transverse abdominus and multifidi: slow oxidative, postural muscles, low recruitment threshold, low force production
• often poorly recruited in those with LBP: if so patients rely on prime movers for postural support
• PT can retrain TA and multifidi to fire correctly

Question 16

Type of Muscular Action: Static

Answer 16

• no resultant joint motion
• exerted against an immovable object
• used interchangeably with isometric action

Question 17

Type of Muscular Action: Dynamic

Answer 17

• muscular action involving joint motion
• muscle belly shortens or lengthens
• concentric action: two ends of muscle move closer together, shortening muscular action, positive work
• eccentric action: two ends of muscle move farther apart, lengthening muscular action, negative work

Question 18

Eccentric Muscular Action

Answer 18

• linked to DOMS, as a stimulus for muscular hypertrophy-microtears in muscle’s connective tissues
• exposure to extreme bouts of eccentric loading linked to rhabdomyolysis: often results in myoglobinuria, can prompt kidney failure

Question 19

Isotonic Muscular Action

Answer 19

• dynamic form of muscular activity
• external resistance remains constant through available ROM
• ex: free weights, wt cuffs, theraband
• commonly used in clinical practice

Question 20

Isokinetic Muscular Action

Answer 20

• dynamic form of muscular activity
• speed of muscular action remains constant
• aka accommodating resistance
• less commonly used

Question 21

Stretch-Shortening Cycle

Answer 21

• SSC
• plyometric muscular action
• describes concentric action immediately preceded by eccentric muscular action
• resulting concentric action produces greater force
• crucial component of human movement

Question 22

Elastic Component Contributions to the SSC

Answer 22

• stretch on muscle changes the muscle’s properties
• occurs secondary to storing of PE in SEC of muscle
• stretch on muscle produces small changes in muscle and tendon length
• maximizes accumulation of stored energy
• thus eccentric muscular action enhances recoil leads to increased force output of muscle tendon complex
• concentric muscle action also enhanced by stored elastic energy in PEC
• this contribution drops off quickly as muscle continues to shorten
• if shortening contraction occurs within reasonable time after stretch: stored energy is recovered and used 0-.9 seconds
• if stretch held too long, stored elastic energy is lost through conversion to heat

Question 23

Neural Contributions to SSC

Answer 23

• prestretch also stimulates muscle group thru reflex potentiation
• accounts for ~30% of increased subsequent concentric muscle action

Question 24

SSC and Muscular Performance

Answer 24

• SSC particularly evident in gait
• NM system may be trained to tap into this phenomenon
• net result increased muscular tension
• accounts for ~50% of total energy requirement in running

Question 25

Neural Control of Muscular Action

Answer 25

• regulated through the somatosensory nervous system
• sensation allows us to interpret the world
• motor function allows us to investigate it
• muscle action merely one component: motor unit is basic functional component of muscular action

Question 26

Somatosensory System

Answer 26

• all information from somatosensory system proceeds from receptor thru series of neurons to brain
• processing signals into meaningful information occurs in cerebrum
• interpretation known as perception may be conscious or unconscious
• motor neurons supply skeletal muscle
• cell bodies of these motor neurons are located in ventral horn
• axon is continuous: from origin in spinal cord to termination on muscle fiber
• motor axon terminates: release of ACh –> excitation and muscular action

Question 27

Forms of Sensory Information

Answer 27

• superficial or cutaneous sensory information comes from skin: including touch (superficial pressure or vibration), pain, temperature
• information from MS system includes proprioception and pain
• proprioception provides information on stretch on muscles, tension on tendons, position of joints, deep vibration
• proprioception includes both static and dynamic sense of position

Question 28

Somatic Nervous System

Answer 28

• considered to be under voluntary control but much is subconscious
• posture, balance, stereotypic movements
• motor neurons may be destroyed by disease: polio selectively affects cell bodies; ALS is characterized by progressive degeneration and death of motor neurons

Question 29

Motor Unit and Musculoskeletal Innervation

Answer 29

• motor unit is basic functional component of muscle function
• defined as alpha motor neuron and all fibers it innervates: motor neuron + all fibers it innervates, fibers range from a few to thousands, 300k in entire body
• alpha motor neuron has cell body in anterior horn of SC
• axon extends to muscle where it has few or many ranches
• terminates on the muscle fiber at the motor end plate

Question 30

Variations in Motor Units

Answer 30

• type of muscle fiber: all fibers within MU are same type; can change some characteristics with training FOG –> endurance vs. intensity
• size of cell body and axon diameter: smaller the diameter –> slower the conduction; smaller innervate SO fibers
• fibers within MU do not necessarily lie next to each other
• need for precision influences ratio of fibers innervated within MU

Question 31

All or None Law of Muscle Action

Answer 31

• all myofibrils within motor unit respond provided impulse is of sufficient duration, frequency, intensity
• results in sliding and forming of cross-bridges
• does not apply to entire muscle belly: allows us to produce graded force production

Question 32

Other Important Organs in the Somatic Nervous System

Answer 32

• alpha motor unit connected to efferent neuron: innervates extrafusal fibers, stimulates muscle action
• gamma efferent neuron: innervates intrafusal fibers: MSFs and GTOs, stimulates increased sensitivity of intrafusal fibers

Question 33

Intrafusal Fibers as Sensory Neurons

Answer 33

• MSF apparatus: attached to intrafusal fibers, run parallel to extrafusal fibers, chain fibers detect change in length, bag fibers detect changes in speed
• GTOs detects change in tension on tendons

Question 34

Muscle Spindle Fibers

Answer 34

• contribute to proprioception
• sensory ending respond to changes in muscle length, velocity of change in length
• afferent sensory arise from intrafusal fibers: type Ia respond to quick and tonic tension, type II respond to tonic tension
• type Ia and II sensory fibers are activated with stretch
• small efferent fibers adjust spindle fiber so its responsive throughout muscles’ physiologic range

Question 35

GTO’s

Answer 35

• also contribute to proprioception
• encapsulated at junction of extrafusal fibers and tendons: woven among collagen strands in tendon, located near musculotendinous junction
• attached in series
• sensitive to changes in tension: passive stretch, active muscular stretch
• sensitive to very slight change in tension
• serves to prevent overactivity of alpha motor neurons
• GTO fires with stimulation > threshold: prolonged stretch, prolonged isometric action
• can override input from MSF
• thus allows relaxation –> elongation

Question 36

Clinical Applications: Stretching

Answer 36

• static stretching: sustained, load is constant, most common clinical form
• ballistic stretching: high impulse, integral to plyometric training, important in return to most sport/fitness
• PNF: originally used in neuro rehab, autogenic or reciprocal inhibition used to reset sensors in muscle and increase AROM and PROM, has both stretching and strengthening components

Question 37

Static Stretching

Answer 37

• muscle is slowly elongated
• positioned to tolerance and maintained
• individual feels in this position: mild tension, mild discomfort
• extreme pain or discomfort should be avoided
• slow, prolonged stretch helps to: circumvent muscle spindle reflex, facilitates muscle inhibition via GTOs
• thus combined neuro effect is to minimize influence of muscle spindle fibers, maximize influence of GTOs

Question 38

Autogenic Inhibition

Answer 38

• stimulation of muscle causing its neurologic relaxation
• occurs with activation of GTO
• serves as basis for static stretching and some PNF stretching techniques

Question 39

Reciprocal Inhibition

Answer 39

• stimulation of muscle causing neurologic relaxation of its antagonist
• helps ensure ability to move through available ROM
• serves as basis for some PNF stretching techniques

Question 40

Factors Affecting Force of Muscular Action

Answer 40

• fiber stimulation
• diameter of muscle
• fibers/motor unit
• number of motor units fired
• length of muscle when it’s recruited
• length of muscle when it’s recruited
• force-velocity curve
• angle of muscular attachement

Question 41

Frequency of Stimulation

Answer 41

• AP –> series of events
• series of events: last ~100 msec; results in a single twitch
• if AP’s are far enough apart –> twitch, relax, return to resting state
• if APs are close together –> does not return to resting point
• this 2nd phenomenon is known as temporal summation aka graded response
• no resting period –> temporal summation
• if stimulus is fast enough –> tetany (sustained contraction)

Question 42

Diameter of the Muscle

Answer 42

• force is directly proportional to cross-sectional area

- bigger muscle, bigger force

Question 43

Number of Fibers/Motor Unit

Answer 43

• motor unit: alpha motor neuron; all fibers it innervates

- number varies from a few fibers to thousands: 5-10 in eyelids but over 1000 in gluteus maximus

Question 44

Number of Motor Units Fired

Answer 44

• aka recruitment principle
• each motor unit has summation threshold
• larger the motor unit, higher the threshold
• smaller units are fired first at low tensions
• allows gradation of tension and more fine control

Question 45

Length of Muscle When Stimulated

Answer 45

• CE=contractile element: actin and myosin
• PEC=parallel elastic component: connective tissue
• SEC=series elastic component: tendon
• force depends on length
• optimal length exists for force/tension
• above or below, tension decreases

Question 46

Force-Velocity Curve

Answer 46

• lightest load at max velocity
• max load at slowest velocity
• at highest velocity: concentric results in smallest tension and eccentric results in highest tension

Question 47

Practical Applications

Answer 47

• muscle position when stimulated
• muscle length pathology
• look at slides in notes on page 19

Question 48

Angle of Muscular Attachment

Answer 48

• not all force produced is put to use in generating segmental rotation
• depending on angle of insertion: some force directed to joint stabilization or destabilization
• thus pulls bone into, or away from, the joint
• even while muscle tension may be maintained during joint movement
• the rotary component and torque will vary depending on angle of insertion
• many of neutral starting positions are “weak” positions mechanically: secondary to most mm force is directed along length of bone
• thus less weight can typically be lifted as compared to starting with some flexion/extension in the joint

Question 49

Angle of Muscular Attachment in Action

Answer 49

• at acute angles the parallel component of the force is highest and stabilizes the joint while the rotary force is low
• as angle increases to 90* rotary component increases to its maximum, there it hits functional “sticking point”
• as angle increases beyond 90*, rotary component diminishes, parallel component increases to produce a dislocating force

Question 50

Classification of Skeletal Muscles

Answer 50

• spurt vs. shunt
• type of movement
• fiber arrangement

Question 51

Spurt vs. Shunt Muscle

Answer 51

• spurt…
• proximal point of attachment is far from join axis
• allows more rotary than stabilizing movement
• length of mm along fixed body segment greater than mm length along moving segment
• ex: biceps brachii, rectus femoris
• shunt…
• proximal attachment close to joint axis
• distal attachment far from joint axis
• allows more stabilizing than rotary movement
• ex: brachioradialis

Question 52

Fiber Arrangement

Answer 52

• fusiform (spindle): fibers arranged in spindle shape; brachialis and tibialis anterior
• parallel: fibers are parallel to each other; sartorius and rectus abdominis
• pennate: fibers are arranged in feather-like pattern; unipennate (FPL), bipennate (RF, TP), multipennate (deltoid)
• triangular: like fusiform, with more flat orientation; pectoralis, trapezius, latissimus dorsi, gluteus medius
• sphincter: fibers arranged in circular direction; orbicularis oris

Question 53

Functional Roles of Skeletal Muscles

Answer 53

• prime mover vs. assistant mover
• agonist vs. antagonist
• stabilizers and neutralizers
• one joint vs. two joint muscles

Question 54

Skeletal Muscle as Prime Mover

Answer 54

• describes muscles primarily responsible for a given movement
• ex: lats, p major shorten to produce extension

Question 55

Skeletal Muscle as Assistant Mover

Answer 55

• describes muscles recruited to help prime movers
• typically recruited when increased force is needed
• ex: triceps, teres major may help with extension

Question 56

Skeletal Muscle as Agonist

Answer 56

• describes muscles creating same joint movement

- typically describes muscles directly producing a given motion

Question 57

Skeletal Muscle as Antagonist

Answer 57

• describes muscles opposing or producing the opposite joint motion
• must relax to allow a movement to occur
• or contract concurrently with agonists to control or slow a joint motion; aka co-contraction

Question 58

Agonists vs. Antagonists

Answer 58

• look at slides on p 25
• muscle may be more susceptible to injury when playing role of antagonist
• either at attachment or in muscle fiber itself
• secondary to muscle at times being simultaneous contracted and stretched

Question 59

Skeletal Muscle as Stabilizer

Answer 59

• acting in one segment so specific motion in adjacent joint can occur
• very important in many regions: shoulder girdle during UE motion, hip region and pelvic girdle during gait, lumbar spine during many tasks
• scapular, hip, spinal/cervical are the main ones

Question 60

Skeletal Muscle as Neutralizer

Answer 60

• also described as synergizer
• muscle acts to eliminate unwanted joint action of another muscel
• g. max contributes to hip extension but also IR femur
• if ER undesired: glute min and TFL contract-produce neutralizing IR action so cancel out ER action of G max

Question 61

One- and Two-Joint Muscles

Answer 61

• most cross one joint so dominating action is at joint it crosses
• muscles crossing two joints create a multitude of different joint motions
• actions depends upon position of the body, interaction with external objects such as the ground
• two joint muscles act primarily on joint where it has largest moment arm
• look at examples on page 27
• two joint muscles decrease work required from single-joint muscles
• contribute to mechanical coupling of joints allowing for rapid release of stored energy
• may allow positive work at one joint and negative work at anotehr

Question 62

Indirect Action

Answer 62

• a muscle can even create motion in joints it does not span

- ex: soleus PF ankle actively and extends knee indirectly in standing in closed chain position

Question 63

Active Insufficiency

Answer 63

• decreased ability of mm to produce or maintain active tension: no cross bridges left when muscle activated in shortened position
• typically occurs as 2 joint muscle contracts across 2 joints simultaneously
• results in severely compromised tension

Question 64

Passive Insufficiency

Answer 64

• limitation of motion caused by elongation of muscle
• typically occurs as a 2 joint muscle is lengthened across 2 joints simultaneously: hip flexion with knee extended
• this passive tension or insufficiency may cause secondary joint motion: finger extension with active wrist flexion

Question 65

Tendoesis

Answer 65

• Movement caused by passive tension
• ex: hand movement in quadriplegics: often have wrist extension but no finger flexion; wrist flexion opens fingers, then placed hand over object; wrist extension closes fingers, exerting force on object
• functionally beneficial for these people to use joystick controller