Muscle Contraction and Muscle Tone

Muscle Contraction Types

  • Muscles contract in two ways:
    • Isotonically
    • Isometrically

Key Terms

  • Tension: Force generated when muscles contract.
    • Higher tension when muscles contract harder.
    • Lower tension when muscles contract less hard.
  • Resistance: Force the muscle contracts against (e.g., weight).

Units

  • Pounds are used as units when discussing tension and resistance.

Isotonic Contraction

  • Muscle contracts and gets shorter or lengthens.

Concentric Phase

  • Muscle generates tension and gets shorter.
  • Tension > Resistance = Muscle Shortens.
  • Example: Bicep contraction to bring a water bottle to the face.
    • Bicep shortens, flexing the elbow.
    • If the water bottle weighs 5 pounds (resistance), the bicep generates 10 pounds of tension to lift it.

Eccentric Phase

  • Muscle maintains tension while lengthening.
  • Example: Putting the water bottle back on the desk.
    • Bicep is still contracted, but lengthening.
    • Resistance > Tension = Muscle Lengthens.
    • The water bottle weighs 5 pounds (resistance), and the bicep generates only 2 pounds of tension.
  • In the gym, eccentric contraction is called the negative of the lift.

Exam Questions

  • Scenarios will be given with certain movements, and you will need to determine the type of contraction.
  • Example 1: Concentric contraction of the bicep when flexing the elbow to bring a water bottle to the face.
  • Example 2: Eccentric contraction of the bicep when extending the elbow to put the water bottle back on the desk.
  • Example 3: Concentric contraction of the tricep when extending the elbow above the head.
  • Example 4: Eccentric contraction of the bicep when extending the elbow to put a water bottle on the desk.
  • It's important to know what each basic muscle does (bicep, tricep, quadriceps, biceps femoris, deltoid, thenar).

Isometric Contraction

  • Muscle length does not change during contraction.
  • Tension = Resistance.
  • E.g., Holding a water bottle in a fixed position.

Examples

  • Lifting a water bottle: concentric, concentric, isometric.
  • Putting a water bottle down: eccentric, eccentric, isometric.
  • If movement isn't happening and the muscle is contracted, it is an isometric contraction.
  • If tension in muscle equals resistance, there is no movement.
  • If resistance > tension, there are two scenarios.
    • Pushing against an immovable object:
      • Isometric contraction (muscle length doesn't change).
    • Weight is lowered in a controlled manner:
      • Eccentric contraction (muscle is lengthening).

Contractions Working Together

  • Isotonic and isometric contractions work together for proper movement.
  • E.g., Holding a phone (isometric) while texting (isotonic).
  • Walking up a flight of steps: concentric, then isometric, then eccentric contraction of the biceps femoris muscle.

Muscle Tone

  • Definition: State of partial muscle contraction due to continuous stimulation of motor units.

Motor Unit

  • One lower motor neuron and the muscle cells it controls.
  • Bicep muscle has thousands of motor units.
  • Each axon controls about 1,000 muscle cells.
  • At any given time, some axons are generating action potentials, causing a certain number of muscle cells in a whole muscle to contract creating tension.
  • There's always a certain number of muscle cells contracting in a whole muscle, about 3%, but not enough to generate movement.

Purpose of Muscle Tone

  • Keeps muscles ready for work.
  • Muscles respond quicker if some muscle cells are already working.
  • Muscle tone is not the same as toning muscles with exercise.
  • When you go to the gym, you are making your muscles stronger and bigger, not toning.
  • Muscle tone will be the same for a bodybuilder and a couch potato.
  • To look toned, get rid of fat to see the muscle.
  • If tone goes up, something has gone wrong.

Abnormalities with Muscle Tone

Low Muscle Tone

  • Due to fewer muscle cells contracting.
Underlying Issue
  • A decreased number of muscle cells that are contracting.
Signs and Symptoms
  • Muscle weakness.
  • Diminished reflexes (due to muscle weakness).
  • Muscle atrophy.
Causes
  • Neuropathy (damage to nerves).
  • Myopathy (damage to the muscle itself, e.g., muscular dystrophy).
  • Motor neuron disease (death of lower motor neurons).

Upper and Lower Motor Neurons

  • Lower motor neuron: directly controls the muscle.
  • Upper motor neuron: indirectly controls the muscle via the lower motor neuron.
  • Upper motor neuron has a slight inhibitory effect on the lower motor neuron so that the lower motor neuron does not overexcite the muscle.

High Muscle Tone

  • Again, you don't get more muscle tone in a gym.
  • Due to overexciting the muscle.
Causes
  • Upper motor neuron dysfunction, damage, or death.
  • Disinhibition: the damaged upper motor neuron can no longer inhibit the lower motor neuron, causing it to overexcite the muscle.
Signs and Symptoms
  • Muscle weakness.
  • Brisk reflexes.
  • Stiff muscles from spasticity.
  • Muscle cramps.
Examples
  • Stroke (loss of blood flow to the brain).
  • Multiple sclerosis (demyelination in the brain).
  • Motor neuron disease (affecting upper motor neurons).
  • Cerebral palsy (damage to the motor cortex of the brain).

Muscle Cramps

  • Most common cause: dehydration, fatigue.
  • Not low potassium (though low potassium can cause a muscle cramp).
  • During fatigue, calcium cannot be pumped back into the SR due to lack of ATP, and muscles keep contracting.
  • Dehydration changes ion concentration, which changes membrane potential and overexcites neurons/muscles.

Rigor Mortis

  • State of muscle contraction after death.
  • Due to complete depletion of ATP.
  • Without ATP myosin head does not let go of the actin filament, leading to a contracted state.
  • Knowing EC coupling (step 13) helps explain why rigor mortis occurs.

Sources of ATP in Muscle

  • Anaerobic respiration (glycolysis).
    • Uses glucose.
  • Aerobic respiration (mitochondria).
    • Uses glucose, fatty acids, ketones, amino acids.
  • Phosphagen system.
    • Uses phosphocreatine (creatine phosphate).