Muscle Contractions Flashcards
Cardiac Muscle
Striated
Involuntary
Skeletal Muscle
Striated
Voluntary
Smooth Muscle
Non-Striated
Involuntary
Skeletal Muscle Properties
- Extensibility
- Elasticity
- Excitability
- Conductibility
- Contractility
Skeletal Muscle Functions
- Produces movement
- Maintains posture
- Stabilizes joints
- Generates heat
- Additional functions
Major events of muscle contraction
- Nerve impulse reaches the end of a motoneuronà release of the neurotransmitter acetylcholine (ACh)
- ACh diffuses rapidly across the gap of the neuromuscular junction and binds to ACh receptors on the motor endplate of the muscle fiber
- Stimulation of ACh receptors initiates impulses that travels along the sarcolemma, through the T tubules, to the sacs of the SR.
- Ca2+ is released from the SR into the sarcoplasm, where it binds to troponin molecules in the thin myofilaments.
- Tropomyosin molecules in the thin myofilaments shift.
- Actin’s active sites are exposed.
- Energized myosin cross bridges of the thick myofilaments bind to actin and use their energy to pull the thin myofilaments toward the center of each sarcomere.
- This cycle repeats itself many times per second, as long as adenosine triphosphate (ATP) is available.
- As the filaments slide past the thick myofilaments, the entire muscle fiber shortens.
Sliding Filament Theory
- Distance between the two Z-disks shortens
- This arrangement and interactions between actin and myosin allows for the shortening of the sarcomeres which generates force!
Sensory Receptors: Classification
- By the type of stimulus they detect
- By their body location
- By their structural complexity
Type of Stimulus Detected by Receptors
- Mechanoreceptors
- Thermoreceptors
- Photoreceptors
- Chemoreceptors
- Nociceptors
Receptor Body Location
- Exteroceptors
- Interoceptors
- Proprioceptors
Receptor Structural Complexity
- Simple receptors
- Complex receptors
Motor Unit
Motor neuron and all the skeletal muscles it innervates.
Axon of motor neurons extend from the spinal chord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle.
Most Important Characteristic of a Muscle
- Ability to develop tension (force) Tension can be: * Active * Passive - Exert a force on the bony lever
Active Tension
Tension developed by the contractile elements of the muscle.
- Initiated by cross-bridge formation and movement of the actin and myosin.
- Length of the muscle affects the ability of the muscle to create tension.
Passive Tension
Refers to the tension developed in the elastic component of the muscle.
- Does not contribute to movements in the middle of range of motion but contribute when muscle is lengthened.
- Added to the active tension when the muscle is lengthened.
- Become slack when the muscle is shortened.
Tendon Properties
When a tendon is elongated, it develops tension.
From a tension/elongation curve, four different regions can be identified: first, second, third and fourth.
- Tendon stiffness can be measured in the linear region of the force/elongation curve and is calculated as the ratio between the applied force and the elongation of the tendon.
First Region of Tension/Elongation Curve of the Tendon
Toe region: non-damaging forces are applied, where the resting crimp angle collagen fibers are reduced causing fiber stretching.
Second Region of Tension/Elongation Curve of the Tendon
Linear
Loading induces stretching of the aligned fibers
Third Region of Tension/Elongation Curve of the Tendon
Appears after further elongation and fibers failure occurs in an unpredictable manner.
Fourth Region of Tension/Elongation Curve of the Tendon
Appears when the tendon breaks so under complete failure.
Isometric Contraction
Contraction with no limb movement, without a change of muscle length. No change in joint angle.
- Used in therapeutic exercises, daily activities and sport.
- Advantage of strengthening without too much stress on the injured structures
- Very important in retarding muscle atrophy and muscle weakness
Why does the force decrease during isometric contraction?
Because energy is required to maintain the contraction (ATP) and the SR is not able to restore Ca2+ during a muscle contraction.
Concentric vs. Eccentric
- Shortening (concentric) and lengthening (eccentric) contractions of the skeletal muscle.
- Different mechanisms of force generation
- Difference in maximal force production
- Difference in energy cost
Eccentric contractions can produce greater force than concentric contractions through different mechanisms of force generation.
Concentric Contractions
Decrease in joint angle, muscle shortening.
- Will shorten (contract) if a sufficient number of sarcomeres actively shorten and if either one or both ends of the muscle fiber is free to move.
- H-zone and I-band shorten, thin filaments are pulled over the thick filaments and exert a force which is transmitted to the joint via the tendon.
- Initiates or accelerates movement to overcome some external resistance (gravity).
Eccentric Contraction
Increase in joint angle, lengthening of the muscle.
- Thin filaments are pulled away from the thick filaments, and cross-bridges are broken and reformed as the muscle lengthens through the H-zone and I-band while generating force.
- Tension is generated by the muscle as cross-bridges are reformed.
- It exerts a braking action against a downward movement and to maintain balance.
Which characteristics of natural locomotion is eccentric contraction responsible for?
- Allows dissipation of mechanical energy during body deceleration.
- Allow the conversion of kinetic energy into elastic energy of tendon.
Such energy is then regained during limb support, resulting in less muscle work and energy required during locomotion.
Isotonic during concentric contractions
- If velocity is high → decrease in muscle force
- Producing high levels of force induces a slower muscle shortening.
- Ex: lifting 5kg weight vs. 40kg → maximum speed 5kg>40kg
- Takes more energy to perform
Isotonic during eccentric contractions
- Opposite relationship
- If velocity is high → increase in muscle force
- Non-contractile elements provide additional forces
- Produce from 20 to 40% more force
- Ex: lifting 5kg weight vs. 40kg → maximum speed 40kg >5kg
Isotonic during isometric contraction
Velocity = 0
Kinetic Chain
- Can be open or closed.
- Combination of several successively arranged joints constituting a complex motor unit.
- If both ends of this system are fixed such that no
movement can occur at either end, the application of an external force causes each segment to receive and transfer force to the adjacent segment, generating a chain reaction.
- If both ends of this system are fixed such that no
- Used in a wide variety of clinical conditions, including musculoskeletal, sports medicine, neuro-rehabilitation as well as prosthetics and orthotics.
Open Kinetic Chain (OKC)
The segment furthest away from the body (e.g. foot) is free and not fixed to an object.
Allow for a motion in the distal segment while movement is restricted to a single joint.
Typically includes exercises to improve strength and range of motion.
Closed Kinetic Chain (CKC)
The distal aspect of the extremity is fixed to an object that is stationary.
With the distal part fixed, movement at any one joint in the kinetic chain requires motion as well at the other joints in the kinetic chain, both proximal and distal parts receive resistance training at the same time.
OKC vs CKC: Clinical Application
A therapeutic exercise program should include a combination of OKC and CKC exercises for optimal results.
OKC vs CKC: ACL Reconstruction
- CKC techniques used for rehabilitation to improve functional outcomes
- Less pain and laxity compared to OKC
OKC vs CKC: Patellofemoral Pain Syndrome
- OKC and CKC have different effects in rehabilitation: combination of both recommended.
OKC vs CKC: Shoulder Pain
- Progressive rehabilitation from CKC to OKC
OKC vs CKC: Spinal Cord Injury
CKC: Reduce Pressure
