Skeletal Muscle Contractions – Characteristics

Skeletal Muscle Contractions – Characteristics


Characteristics include:

  • All or none law.
  • Stimulus-response relationship.
  • Frequency of stimulus.
  • Motor unit recruitment.
  • Starling’s Law.
  • Isometric contractions
  • Muscle Tone.
  • Denervation hypersensitivity.

1. “All or none law”:

  • Motor unit is “Motor neuron collectively with all its peripheral branches & innervated muscle fibers”.
  • Motor unit obey all or none law.
  • Ie., On stimulation, either all motor unit fibers will contract maximally or not contract at all.
  • This depends upon stimulus intensity (threshold/subthreshold).


  • Motor unit is “Unit of contraction”.
  • Nerve fiber is “Unit of activation”.
Concentration of motor unit:
  • 3-6 fibers in a motor unit:
  • Seen with muscles for fine, graded & precise movement.
  • Eg: Fingers & eye.

600-1000 muscle fibers per unit:

  • Seen with muscles for gross movements.
  • Eg: Leg.

2. Stimulus-Response relationship:

  • Contractile response depends on stimulus strength.
  • With increasing stimulus strength → Number of contracting motor units increases.
  • Hence, 
  • Increased no. of contracting motor units Stronger muscle contraction.

3. Frequency of stimulus:

  • Increased frequency of stimulation → Increases contraction strength & frequency of activation of motor units.

This explains, 

Simple muscle twitch:

  • Contractile response of a skeletal muscle to a single brief stimulus.


  • Due to summation of twitches.

Partial/Incomplete tetanus:

  • State of repetitive muscle contractions separated by partial relaxation, at slower frequency of stimulation.

Complete tetanus:

  • State of sustained muscle contraction, at higher frequency of stimulation.

Post-tetanic potentiation:

  • Phenomenon of “Repetitive stimulation enhancing force development due to increased intracellular Ca2+.
Mechanism behind:
  • Ca2+concentration in sarcoplasm determines muscle tension to produce tetanus.

During single twitch:

  • Ca2+ released into sarcoplasm insufficient for tetanic tension.

On rapid & successive muscle stimulation:

  • With each stimulus Ca2+ efflux into sarcoplasm.
  • Thus, progressive sarcoplasmic Ca2+ accumulation.

On maximum sarcoplasmic Ca2+ levels:

  • Muscle tension created.

For tetanic response:

  • Tetanic tension is about four times twitch tension.

4. Motor unit recruitment:

  • Start of muscle contraction – Smallest motor units contract first.
  • On insufficient power generation, larger motor units are recruited.
  • “Henneman principle”/”Size principle”
  • Order of recruitment from smaller to larger motor unit.
  • Increases contraction strength 

5. Starlings law:

Explains that, 

  • There is an optimal length at which force generated by muscle is maximal.
  • Ie., Upto a certain limit,
  • Greater the initial length/length at relaxed state ——> Greater is contraction force.

6. Isotonic Vs isometric contraction

Isotonic contraction:

  • Contraction with change of length at constant tension.
  • Tension is equal to weight lifted during muscle contraction.

Isometric contraction:

  • Contraction with increased tension & constant length.
  • Generates more contraction force.


  • Gym exercises are isotonic type.
  • Involves change in muscle length with constant tension.
  • Requires greater energy than isometric contraction.
  • Thus, isotonic exercise best increases muscle strength.

7. Tonus:

  • State of muscles in partial contraction.
  • Defined as “Muscle resistance to passive stretch”.
  • Involves γ-motor neuron activity for contraction.
  • Assessed by observing resistance offered by muscle to passive stretch.

8. Denervation hypersensitivity:

  • Destruction of skeletal muscle nerve supply causes –
  • Abnormal muscle excitability.
  • Increased sensitivity to circulating acetylcholine.


  • Fine irregular contraction of individual fiber.
  • Classically in lower motor neuron lesion.
  • Not visible grossly.
  • On motor nerve regeneration, fibrillation disappears.

Fasiculations – 

  • Jerky, visible contractions.
  • Occurs with group of muscle fibers/complete motor unit supplied by motor neuron.
9. Muscle fiber type:

Type I/slow motor units:

  • Have early recruitment.

Type II motor units:

  • Type IIa/“Fast-Fatigue Resistant” (FR) motor units.
  • Type IIb/“Fast-Fatigable” motor units.

Factors increasing force of muscle contraction:

  • Increased number of motor units.
  • Increasing frequency of stimulus (Tetanic stimulus).
  • Increasing stimulus strength.
  • Appropriate initial length.
  • Larger motor unit recruitment (Henneman principle).
  • Type II/fast unit.
  • Isometric contraction.
Exam Question


  • Characteristics of skeletal muscle contractions include:
  • All or none law.
  • Stimulus-response relationship.
  • Frequency of stimulus.
  • Motor unit recruitment.
  • Starling’s Law.
  • Isometric contractions
  • Muscle Tone.
  • Denervation hypersensitivity.
  • Motor unit obey all or none law.
  • Unit of activation – Nerve fiber.
  • Unit of contraction – Motor unit. 

3-6 fibers in a motor unit

  • Seen with muscles for fine graded & precise movement
  • Eg: Fingers & eye.
  • Contractile response depends on strength of stimulus.
  • With increasing stimulus strength → Number of contracting motor units increase.
  • Larger number of motor units contracting → Stronger muscle contraction.
  • Strength of contraction is increased by increasing frequency of stimulation.
  • This, in turn increases frequency of activation of motor units.
  • Tetanus is due to summation of twitches.
  • On stimulating muscle in rapid succession, there is progressive sarcoplasmic Ca2+ accumulation.
  • Tetanic tension is reached when sarcoplasmic Ca2+ levels reach their maximum.
  • Tetanic tension is about four times the twitch tension.
  • Larger motor units recruited on insufficient power generation.
  • “Henneman principle”/ “Size principle”:
  • Order of recruitment from smaller to larger motor unit.
  • This increases contraction strength 
  • According to Starlings law, there is an optimal length at which force generated by a muscle is maximal.
  • Isotonic contraction: Contraction in which there is change of length at constant tension.
  • Isometric contraction: Contraction in which there is constant length with increased tension.
  • Hence, generates more force of contraction/tension.
  • Muscle strength is best increased by isotonic exercise.
  • Exercises one does in the gym are isotonic exercises as muscle length changes in each step but not tension.
  • Tonus involves γ-motor neuron activity leading to muscular contraction.
  • Fine irregular contraction of individual fiber appears, referred as “Fibrillation”.
  • Fasiculations – Jerky, visible contractions of muscle group.

Type II motor units:

  • Type IIa/”Fast-Fatigue Resistant” (FR) units.
  • Type IIb/”Fast-Fatigable” units.

Factors increasing force of muscle contraction include:

  • Increased number of motor units.
  • Increasing frequency of stimulus (Tetanic stimulus)
  • Larger motor unit recruitment (Henneman principle)
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