Skeletal and Muscular Systems

• Motor unit


  Made up of the motor neuron and the muscle fibres it innervates
• Muscle contraction


  1. Nerve impulse conducted down the axon

  2. Acetylcholine released into synaptic cleft

  3. If electrical charge reaches threshold, muscle fibre contracts
• All-or-none law


  If stimulus reaches threshold, all muscle fibres in motor unit contract with maximum force, otherwise no contraction
• Motor units have around 70 per cent of slow, low-intensity, long-duration muscle fibres
• Nerve impulse conduction in a motor unit

  1. Nerve impulse initiated in motor neuron cell body

  2. Nerve impulse conducted down axon

  3. Nerve impulse reaches synaptic cleft

  4. Acetylcholine released into synaptic cleft

  5. Electrical charge reaches threshold in muscle fibre

  6. Muscle fibre contracts
• If the electrical charge does not reach the threshold, none of the muscle fibres will contract
• This is known as the all-or-none law
• Slow oxidative muscle fibres

  • Contract with overall low force
  • Available for recruitment in sub-maximal aerobic work
  • Resist fatigue well
• Fast oxidative glycolytic muscle fibres


  • Contract with high force
  • Supplement slow oxidative fibres at higher workloads
  • Fatigue moderately
• Fast glycolytic muscle fibres


  • Contract with high force
  • Recruited only at maximum workloads or exhaustion
  • Fatigue quickly
• Recruiting muscle fibre types


  Start with slow oxidative, then add fast oxidative glycolytic, then fast glycolytic as workload increases
• Small motor neurons stimulate few small muscle fibres, producing small slow force over long time, resisting fatigue well
• Large motor neurons stimulate many large muscle fibres, producing large rapid force but fatiguing quickly
• Aerobic training work-to-rest ratios

  1. 1 or 1:0.5 for slow oxidative fibres
• Maximal weight training work-to-rest ratios

  Very high, with 3-5 minutes rest between sets of 2-6 reps to target fast glycolytic fibres
• Fast glycolytic fibres take 4-10 days to recover after being used to exhaustion
• Maximal weight training sessions should leave a minimum of 48 hours before using the same muscle group again to allow fast glycolytic fibres to recover
• Skeletal system


  Provides support, protection, movement, storage for minerals and lipids, blood cell production, and energy storage
• Skeletal system


  • Composed of bones, cortilage, ligaments and tendons
• Human skeleton


  • 206 bones
  • Axial skeleton (80 bones)
  • Appendicular skeleton (126 bones)
• Bone types


  • Long (e.g. femur)
  • Short (e.g. carpals)
  • Flat (e.g. scapula)
  • Irregular (e.g. vertebrae)
• Joint


  Any point where two bones meet
• Joint structures


  • Fibrous (joined by dense connective tissue)
  • Cartilaginous (joined by cortilage)
  • Synovial (not directly joined)
• Synovial joints


  • Enclosed by fibrous articular capsule lined with synovial membrane
  • Contain synovial fluid to lubricate the joint
  • Include hinge, pivot, saddle, plane, candyloid, and ball and socket joints
• Joint movements


  • Flexion and extension
  • Abduction and adduction
  • Rotation
  • Circumduction
  • Pronation and supination
• Muscular system


  Responsible for producing movement, maintaining posture, stablising joints, and generating heat
• Muscle types


  • Skeletal
  • Cardiac
  • Smooth
• Skeletal muscles


  Voluntary muscles, which contract and relax to move bones
• Agonist and antagonist muscles


  Agonist is the primary muscle responsible for movement, while the antagonist opposes the agonist to provide balance
• Muscle components


  • Muscle fibres
  • Myofibrils
  • Actin and myosin filaments
  • Sarcoplasmic reticulum
  • Sarcolemma
• These muscle components are all critical in the process of muscular contraction
• Functional Roles of Muscles

  • Producing movement
  • Maintaining posture
  • Stabilising joints
  • Generating heat
• Production of movement

  Contraction and relaxation of muscles working alongside the skeletal system
• Posture maintenance


  Ability of muscles to keep the body in positions that allow us to perform daily activities
• Joint stabilisation


  Muscles providing tension around the joints, which also aids in preventing injuries
• Heat generation


  Through the process of muscular contraction, muscles are able to generate heat to help maintain normal body temperature
• Types of Muscle Contraction

  Isotonic and isometric contractions
• Isotonic contractions


  Muscle length changes as it contracts, subdivided into concentric and eccentric contractions
• Concentric contractions


  Muscle shortens as it contracts, e.g. lifting phase of a bicep curl
• Eccentric contractions


  Muscle lengthens as it contracts, e.g. lowering phase of a bicep curl