During the COVID-19 pandemic we have been fully operational and look forward to speaking with you.

Muscle Spindles – What Every Personal Trainer Needs To Know

Avatar for Hadyn Luke Hadyn Luke posted this on Thursday 29th of March 2012 Hadyn Luke 29/03/2012


Muscle Spindles – What Every Personal Trainer Needs To Know

Todays fitness blog will look at the role of muscle spindles in flexibility.  Muscle spindles are sensory organs made up of intrafusal muscle fibres.  They monitor the rate and change of length in a muscle and are distributed throughout skeletal muscle (extrafusal muscle fibres) aligned to run in the direction of that muscle.

There are two types of muscle spindle:

  • those that monitor posture – Type Ia
  • and those that monitor dynamic activity such as a running gait – Type II

Both provide sensory feedback to the central nervous system  (CNS) via the peripheral nervous system  (PNS).  Their roles help the brain to understand the positioning / posture of our bodies and also monitor the safety of the speed and range of motion within which we are moving.

When a muscle lengthens or shortens the muscle spindles nuclei positioned in the centre of the spindle in a bag are excited when compressed / squashed or moved resulting in feedback to the CNS.  Depending on the feedback received the muscle will either be allowed to lengthen further or it will develop tension and come to the end of its range of movement (ROM).  An individual’s ROM can be improved via a range of developmental stretching approaches.

When we move our bodies from standing upright to walking on uneven ground, muscle spindles monitor our posture and feedback to the brain.  The brain essentially then decides whether the current length and / or rate of change in length in a muscle is of concern / dangerous.

If the brain perceives a danger, it will inhibit further lengthening by activating nerves that innervate / activate muscle actions (the alpha motor neurons) with the result a feeling of tension in a muscle.  If the rate of change is sufficiently fast/dynamic/volatile enough the brain will activate more of the same nerves and cause a stretch reflex.  This is a sudden sharp shortening of a muscle with the classic example being the knee-tap reflex test. 

If you stand and kick your foot high in the air, your muscle spindles will monitor this dynamic activity and register that the muscle is lengthening at speed. They will immediately activate the safety response, or stretch reflex, and pull the leg back down again to protect the muscle from damage.

Within the gym setting, muscle spindles are an essential physiological system, both for active training and for stretching techniques, such as PNF (Proprioceptive Neuromuscular Facilitation) stretching.

For more information on muscle spindles and personal trainer courses, visit Level 3 Certificate in Personal Training

CMS Fitness Blog would love to hear your feedback on this article.  Continue the discussion below with your peers or post your questions and we will get back to you.

Subscribe to the blog