Skeletal Muscle and Synovial Joints

Lecture Objectives

• Explain the relationship between the form and function of a muscle.
• Describe how skeletal muscles can influence movements at a synovial joint.
• Describe some major muscles of the upper and lower limbs and their roles.

Recap: Synovial Joints

• Synovial joints are formed by bones, a joint capsule, synovial membrane/fluid, hyaline cartilage, ligaments, and sometimes fibrocartilage.
• There are 7 types of synovial joints that allow different movements.

Range of Motion (ROM)

• ROM is influenced by bone end shape, ligament location and length, body surface contact, and muscles.

Skeletal Muscle

• Skeletal muscle is made of cells, also known as fibers.
• These fibers are bundled to form fascicles, which are further bundled to form muscles.
• Skeletal muscle is under voluntary control.
• Shortening of skeletal muscles causes movement at a joint.
• Skeletal muscle attaches to bones via tendons.

Skeletal Muscle and Synovial Joints

• Skeletal muscle causes movement at synovial joints by:
  • Attaching to bone (lever)
  • Muscle structure (length, number of fibers, arrangement of fibers)
  • Type of contraction (roles/action)
  • Where it crosses a joint (location)

Anatomical Levers

• Bone acts as a lever
• Joint acts as a pivot or fulcrum
• Muscle contraction provides the applied force or pull
• Weight is what is being moved acts as the load (external or internal)
• Lever arrangement determines function

Classes of Levers

• First-class levers: stabilize joint position, with the fulcrum between the force and load, examples include see-saws and scissors.
• Second-class levers: effective at overcoming heavy loads, with the load between the fulcrum and force, examples include wheelbarrows and bottle openers.
• Third-class levers: allow a large range of movement and speed, with the force between the fulcrum and load, examples include tweezers and fishing rods.

Muscle Form and Function

• Muscle form determines function, dependent on the length of muscle fibers, the number of muscle fibers, and the arrangement of muscle fibers.

Length of Muscle Fibers

• Muscle fibers can shorten up to 50% of their resting length.
• Long muscle fibres are needed if a large range of motion (ROM) is required.

Number of Muscle Fibers (CSA)

• Tension is directly proportional to the cross-sectional area (CSA) of the muscle.
• A greater number of fibers results in a greater CSA and greater tension.

Arrangement of Muscle Fibers

• Parallel: Fibres are arranged vertically between muscle tendons/attachments resulting in smaller CSA, but greater shortening
• Pennate: Fibres are arranged obliquely between muscle tendons/attachments resulting in greater CSA, but lesser shortening

Types of Muscle Action

• Concentric: The muscle is active and develops tension, tension is greater than the load, the muscle shortens, joint position changes.
• Eccentric: The muscle is active and develops tension, tension is less than the load, the muscle elongates, joint position changes.
• Isometric: The muscle is active and develops tension, tension does not outweigh the load, the muscle length and joint position do not change.

Types of Muscle Roles

• Agonist: Creates movement.
• Antagonist: Opposes/controls movement.
• Stabiliser: Holds a joint still.
• Neutraliser: Stops unwanted movement.

Agonists and Antagonists

• Agonists: Act concentrically to create a movement at a joint, biceps brachii shortens during flexion.
• Antagonists: Act eccentrically to oppose and control the movement, triceps brachii lengthens during flexion.

Stabilizers

• Stabilizers hold a joint still and prevent movement.
• Holding a heavy book requires the biceps brachii to act as a stabilizer.
• The biceps brachii action during stabilization is isometric, with no change in length or movement at the elbow joint.

Neutralizers

• Neutralizers eliminate an unwanted movement caused by another muscle.
• Biceps brachii flexes the elbow and supinates the forearm.
• Pronator muscles in forearm act as neutralisers to prevent supination but still allow flexion, for example, drinking from a glass, flexion occurs, supination does not.

Key Muscles for HUBS191

• There are ~650 named skeletal muscles in the human body; focus is on 9 muscle groups.
• Four groups contain multiple muscles, totaling 16 named muscles.
• Essential knowledge for each muscle including names, locations, and concentric actions at a joint.

Concentric Actions of Muscle

• Anterior: Flexion
• Medial: Adduction
• Posterior: Extension
• Lateral: Abduction
• Applies for all joints except for the knee

Biceps Brachii

• Origin: Scapula
• Insertion: Radial tuberosity
• Movement:
  • Shoulder = flexion
  • Elbow = flexion
  • Radioulnar joints = supination

Triceps Brachii

• Origin: Scapula & humerus
• Insertion: Olecranon process of ulna
• Movement:
  • Shoulder = extension
  • Elbow = extension

Deltoid

• Origin: Scapula & clavicle
• Insertion: Deltoid tuberosity
• Movement at the shoulder:
  • Flexion (anterior fibres)
  • Abduction (lateral fibres)
  • Extension (posterior fibres)

Iliopsoas

• Consists of the iliacus and psoas major muscles
• Origin: iliac fossa and lumbar vertebrae
• Insertion: femur
• Movement: Hip = flexion

Gluteus Maximus

• Origin: Ilium & sacrum
• Insertion: Femur
• Movement: Hip = extension

Quadriceps Femoris

• Origin:
  • Rectus femoris = ilium
  • Vastus medialis, intermedius and lateralis = femur
• Insertion: Tibial tuberosity
• Movement:
  • Hip = flexion (rectus femoris only)
  • Knee = extension

Hamstrings

• Origin: Ischium (+ femur for biceps femoris)
• Insertion:
  • Tibia (semimembranosus and semitendinosus)
  • Fibula (biceps femoris)
• Movement:
  • Hip = extension
  • Knee = flexion + rotation (when knee is flexed)

Tibialis Anterior

• Origin: Tibia
• Insertion: Tarsals
• Movement: Ankle = dorsiflexion

Triceps Surae

• Consists of the Gastrocnemius & Soleus
• Origin:
  • Condyles of femur (Gastrocnemius)
  • Tibia & fibula (Soleus)
• Insertion: Calcaneus via calcaneal/Achilles tendon
• Movement:
  • Knee = Flexion (Gastrocnemius only)
  • Ankle = plantarflexion

Preparation for Gait Cycle

• Hip flexion: Agonists are Iliopsoas & rectus femoris, antagonists are Gluteus maximus & hamstrings

Summary

• Muscle structure, location, and contraction type influence the movements at a joint.
• Muscle action and role changes depending on the movement.
• Need to learn 16 key muscles.