Muscle Physiology and Mechanics

Structure or Components of Muscle

• Two main categories:
  • Active contractile components.
  • Passive non-contractile components, which include connective tissue.

Parallel Components

• Connective tissue arranged in parallel responds similarly to contractile components.
• When contractile components shorten, parallel components also shorten.
• Conversely, when contractile components lengthen, parallel components lengthen as well.

Series Elastic Components

• Connective tissue, such as tendons, is in series with contractile components, causing them to respond oppositely.

Contraction

• Nerve impulse travels to the motor unit.
• Actin-myosin filaments bond.
• Crossbridges form.
• Sliding filament theory/crossbridge cycling occurs.

Innervation Ratio

• A motor unit comprises one alpha motor neuron and all the muscle fibers it innervates.
• Muscles vary in the number of motor units and the number of muscle fibers within each unit.

Recruitment

Definition

• Activation of motor units to create action potentials in specific muscle fibers.

Recruitment Order – Henneman’s Size Principle

• Motor units are recruited based on size and firing rate.
  • Small motor units are recruited first, and their firing rate increases.
  • Once the maximum firing rate is achieved, larger units are recruited when more strength is needed.

Fiber Type (generally)

• Type I fibers are innervated by small units, so they tend to fire first and are less prone to fatigue due to their physiological makeup.
• Type II fibers are larger motor units used for quick firing and strength.

Contraction Types

• Concentric: Shortening.
• Isometric: No change in length.
• Eccentric: Lengthening.

Concentric

• Bones (levers) are free to move, and enough muscle force is generated to produce enough angular force to overcome a load; the muscle will shorten, and both bones will be pulled together toward the center of the muscle.
• Occurs when "lifting" against gravity.
• It is a power generator (acceleration) producing positive power.
• Generally, one bone is stabilized while the other moves.
• Active shortening; also known as shortening contraction.

Isometric

• The proximal and distal ends of a muscle are fixed, and the moment produced by the muscle equals the moment of the resistances, with myofibril shortening but no visible movement in the joint.
• No change in length.

Eccentric

• Muscle force and moment produced are insufficient to overcome an opposing moment, so the muscle lengthens and acts as a brake to control movement as the two ends of the muscle move away or apart from each other.
• Occurs when "lowering" with gravity.
• Power absorption (deceleration) produces negative power.
• Again, one bone is usually stabilized as the other moves.
• Active lengthening; also known as lengthening contraction.

Examples of Contraction Types

• Concentric contraction of the triceps occurs when pulling against a spring. If the force exerted by the triceps (Ft) is greater than that of the spring (Fs), the arm moves down, and the triceps experience concentric contraction. If the spring constant is increased or the muscle is fatigued, the limb moves upward while still contracting, resulting in eccentric contraction.
• Eccentric contraction of the biceps occurs when trying to lower a heavy object in a controlled manner. The force exerted by the ball (Fb) is greater than the muscular force (Fm), resulting in the arm lowering and the biceps lengthening. When the weight becomes less or the biceps exert a stronger force, the ball is lifted, and concentric contraction occurs.

Force Transmission to Bone

• Force transmitted to bone affects the bony components → tensile force (generally) created at the tendon-bone junction.
• Tries to cause movement.

Applications/Discussion:

• Creating Load/Loading Sufficient for Movement
  • Nerve Fires
  • Nerve impulses arrive at the motor end plates
  • Crossbridge formation occurs
  • Physiological tension develops
  • Pull on tendon creates tension through the tendon

Length-Tension Relationship: Active Tension

• Developed by contractile elements of muscle.
• Depends on:
  • Frequency
  • Number and size of motor unit firing
  • Number of cross-bridges formed

Length-Tension Relationship: Passive Tension

• Tension developed in the passive elastic components.
• May add to total tension when the muscle is lengthening.
• Does not add to total tension when the muscle is shortening as the parallel elastic components become slack with muscle shortening.

Length-Tension Relationship

• The length-tension curve demonstrates that there is an optimal alignment or length of the sarcomeres that allows for the greatest number of cross-bridges to be formed → maximal isometric tension – maximal magnitude of muscle force.

Active Insufficiency

• Full ROM is actively attempted simultaneously at all joints by a two- or multi-joint muscle.
• ‘Over-shortens’ the muscle.
• Decrease in muscle tension production capability.
• A decrease in moment by change in moment arm and passive restraint of lengthened antagonists also play a role.
• Involves active/contractile tissue.

Passive Insufficiency

• Almost always in two- or multi-joint muscles.
• ROM is prevented or lessened when a muscle (usually antagonist) demonstrates insufficient length to allow full ROM being attempted by the agonist(s).
• When the situation occurs, passive tension is developed in the muscle preventing the motion.
• Involves passive or elastic elements.
• Reaches elastic limit.

Effect of Passive and Active Insufficiency on One Joint vs. Two Joint Muscles

• A two-joint muscle is more efficient if it is shortening at one joint and lengthening at another.

Tenodesis

• The passive response of the wrist and hand to passive tension of the wrist/finger musculature when only the wrist extensors are active.
• The actual increase in passive tension from the stretching of the muscle over the wrist.
• Rehabilitation for individuals with Cervical Spinal Cord Injury (C6).

Force-Velocity Relationship

• Velocity of the internal shortening of the myofilaments.
• As the speed of shortening contraction increases, tension decreases.
• When lengthening → as the speed of lengthening increases, tension increases.
• Recruitment order of motor units (those with slower conduction generally recruited first).
• Types of muscle fibers in motor units (Type II fibers can develop max tension more rapidly).
• Length of muscle fibers (long fibers have a higher shortening velocity than short fibers).

Applications/Discussion

• Length-Tension Relationship: Need to know a muscle’s capability to develop physiological tension (muscle force) in different ranges within a joint’s ROM
• Active and Passive Insufficiency: How segments/systems (UE or LE, for example) are positioned can affect correct technique for:
  • MMT
  • ROM
  • Strengthening
• Force-Velocity: Similar to length-tension, need to know force-velocity for appropriate interventions and specific training of muscles/activities
• Need to be able to utilize the interaction between the types of contractions (concentric, eccentric, and isometric) and their force-velocity relationships in prescribing exercises for certain problems

Exercises Prescription

• Muscle action
• Load
• Repetitions and sets
• Exercise position
• Equipment
• Exercise order
• Rest periods
• Velocity
• Frequency

Velocity for:

• Strengthening
• Endurance
• Power

Muscle Morphology: Significance of Fiber Orientation

Fusiform fibers

• The line of action is nearly parallel to the tendon and length of fiber.
• Muscles of fusiform shape tend to be longer, so they have more sarcomeres.
• Force production – almost 100% of force goes to the tendon.

Pennate fibers

• The line of action is at an angle to the tendon.
• Shorter, in general, than fusiform muscles.
• Force production – about 86% goes to the tendon (pennate angle is 30 deg so cos(30) = 0.86).
• However, pennate muscles are considered to be stronger than fusiform (fusiform for speed, pennate for strength) – Why?

Three types of Pennate

-Unipennate
-Bipennate
-Multipennate

Muscle Architecture: Physiological Cross-sectional Area (PCSA)

• Area of the cross-section of a muscle perpendicular to its fibers, generally at its largest point.
• How many contractile proteins are available for force production.
• The greater the cross-sectional area → greater number of contractile elements → greater capability for force production.
• Thicker muscles produce more force than thinner muscles of the same morphology.

Fusiform vs. Pennate

Fusiform

• Advantage: sarcomeres are in series, so maximal velocity and ROM are increased.
• Disadvantage: relatively low # of parallel sarcomeres, so the force capability is low.

Pennate

• Advantage: increase # of sarcomeres in parallel, so increased PCSA and increased force capability.
• Disadvantage: decreased ROM and velocity of shortening.

Summary for Muscle Force Production

• Number & size of muscle fibers in the cross-section of muscle.
• Size of motor units.
• Number of motor units firing.
• Frequency of motor units firing.
• Sarcomere length.
• Fiber arrangement.
• Type of muscle contraction.
• Speed or velocity of active shortening or active lengthening.

Summary for Joint Moment Production

• Two factors:
  • Muscle force production as described above
  • Length of the moment arm of muscle
• In examining overall function, cannot just consider active tension – look at passive contribution and the moment arm.
• Moment arm changes as the joint moves through ROM – where muscle length may indicate decreased active tension (as it shortens), moment arm may be changing and thus be able to alter joint moment production.

Muscle Classifications

Agonist

• Aka prime mover → muscle responsible for producing the desired or specific movement at a joint.

Antagonist

• Muscles directly opposite to the desired motion → have the potential to oppose action but are usually inactive and passively elongates.
• Example:
  • Action – Extension
  • Quads = agonist
  • Hams = antagonist
  • Action – Flexion
  • Hams = agonist
  • Quads = antagonist

Muscle Classifications

Co-contraction

• When the agonist & antagonist work or contract at the same time → may be desired (stability) or may not be desired (pathological conditions).

Muscle Classifications – Synergist

• Helps agonist perform the desired motion.
• Functioning in cooperation with agonist.
• May help directly with motion – biceps aids brachialis in elbow flexion.
• May help by providing a different action (stabilizing) that supports more effective agonist action (eg wrist extension for finger flexion).
• May help by preventing unwanted movement or actions (eg extensor and flexor carpi radialis for radial deviation).

Exercise and Testing

• Isometric
• Isokinetic
• Isoinertial // Isotonic

Strength

Absolute

• PCSA

Functional

• Fiber architecture - pennate vs fusiform
• Age and gender
• Muscle size → larger the size, the stronger the muscle
• Length-tension relationships
• Leverage of the muscle
• Speed of contraction

Effects of Immobilization

• Dependent on:
  1. Immobilization position - lengthened or shortened
  2. % of fiber types within the muscle
  3. Length of the immobilization period

Too Little Loading: Immobilization in Shortened Position

• Structural changes that occur:
  • Decrease in the number of sarcomeres
  • Increase in sarcomere length
  • Increase in the amount of perimysium
  • Thickening of endomysium
  • Change in collagen orientation
  • Increase in the ratio of connective tissue to muscle fiber tissue
  • Loss of weight and atrophy

Immobilization in Lengthened Position

• Structural changes that occur:
  • Increase in the number of sarcomeres and decrease in the length of sarcomeres
  • Increased connective tissue (endomyseal and perimyseal)
  • Hypertrophy followed by atrophy

Too Much Loading: Overuse Injury / Repetitive Trauma

• Does not allow for complete repair of the tissue
• “Muscle and tendons can fatigue with repetitive submaximal loading, with rapidly applied loads, and when the active (contracting) musculotendinous unit is stretched by external forces.” (Norkin & Levangie, p. 133)
• Microtrauma leading to inflammatory response and swelling

Muscle Strain

• Usually, a high force contraction while muscle is being lengthened by external forces (e.g., BW).
• The junction between muscle and tendon is the usual site of failure.
• Results in:
  • Localized bleeding
  • Significant inflammatory response → Swelling, redness, and pain

Delayed-Onset Muscle Soreness (DOMS)

• Primarily associated with eccentric exercise
• Signs of muscle swelling, loss of active range of motion, and decreased tension generating capability are all reported signs
• Mechanical strain of muscle fibers or in their associated connective tissues
• Reaches peak 2 – 4 days after exercise

Aging

Sarcopenia

• Loss of muscle mass
• Loss of muscle fibers has been demonstrated in some muscles
• Type II fibers decreases in number and size
• Increase in the concentration of connective tissue within the muscle belly → decrease in ROM
• Decreased muscle strength → Reduced muscle power → increased risk and incidence of falls
• Resistance training has had positive effects in the elderly:
  • May increase muscle fiber size
  • Increase strength and functional performance

Electromyography (EMG)

Definition

• Study of muscle function through inquiry of the electrical signal the muscle emulates.

Use

• Examine On/Off for biofeedback
• Examine On/Off for purposes in which only want to know if/when muscle is active
• Examine timing of activity for coordination or activation patterns

Applications/Discussion

• ROM – need to consider the stresses on the tissues when examining
  • One joint vs. two joint muscles
  • Passive and active insufficiency
• MMT // Resisted static testing – need to be able to interpret results:
  • Strong and pain-free = no lesion of contractile unit
  • Strong and painful = 1st-degree strain, stretching/minimal tearing of fibers
  • Weak and painful = 2nd-degree strain, moderate tearing of fibers
  • Weak and painless = 3rd-degree strain, complete rupture, palpable deformity

Applications/Discussion

Interventions

• Need to be able to establish an appropriate exercise prescription considering:
  • Stage (acute, sub-acute, chronic)
  • Post-immobilization (beginning set and progression)
  • Overuse injuries – activity modification but then purposeful loading to rebuild/remodel the muscles to a functional status
  • Indication for exercise (tissue healing, mobility, performance, advanced)
  • Kind of exercise (isometric, isotonic, weight-bearing)
  • Need to be able to qualitatively discern and prescribe which types of contractions (concentric, eccentric, and isometric) are being utilized in certain common exercises
  • Method of delivery//type of contractions (machine, passive, manual, resistive bands, etc.)
  • Dosage (duration, sets, how often)
  • Purpose of exercise (strength, endurance, “stretching”)
  • Exercises will be presented for individuals throughout the lifespan, so knowing the effects of aging is important to set realistic objectives for you and your clients.