Muscle Actions 

Muscle actions are categorized as concentric, eccentric, or isometric depending on whether the muscle shortens, lengthens, or remains the same.  The change in muscle length depends on the relationship between the relative strengths of the contractile force generated by the muscle and the resistant force of gravity and additional weight.  

 
 

Concentric

Shortening

Contractile force is greater than resistant foce

Examples

  • Biceps Brachii: lifting phase of bicep curl
  • Gastrocnemius and Soleus: jumping into the air

Eccentric

Lengthening

Resistant force is greater than contractile force

The greatest muscle force is is produced during this muscle action

Examples:

  • Bicep Brachii: lowering phase of bicep curl
  • Gastrocnemius and Soleus: landing from a jump

Isometric

Static, non-moving

Contractile and resistant forces are equal

Examples:

  • All working muscles during a plank hold or wall sit