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Overactive / Underactive Muscles. Everything you need to know.

Updated: Aug 26

Overactive muscle: A state of having disrupted neuromuscular recruitment patterns that lead a muscle to be more active during a joint action.

*Overactive muscles are shortened, tight, and strong (also called hypertonic).


Underactive muscle: A state of having disrupted neuromuscular recruitment patterns that lead a muscle to be relatively less active during a joint action.

*Underactive muscles are lengthened, inhibited, and weak (also called hypotonic).


Muscle Imbalance is the alteration of muscle length surrounding a joint.

Think of it like a Tug-of-War between opposing muscles (agonist / antagonist). The overactive (strong) muscle wins and pulls the limb or body part into an altered (unwanted) position.

Example: If a persons knees cave in (knee valgus) during a squat assessment the adductors are one of the suspected overactive muscles because they are responsible for pulling the legs / knees together (think "adding" together to remember the adductors function). The glutes and abduction muscles would be suspected weak which would counter act the unwanted movement (knees caving in).

Knee valgus video and associated muscle imbalances


Quick Note: Aspiring trainers interested in becoming certified can check out CPT Prep Study Guides on Amazon and Audio Prep on Audible for additional information related to the exams along with practice questions similar to the actual tests!


Muscle imbalances can be caused in a variety of ways:

  • Postural stress (bad posture).

  • Repetitive movement or positions. *People who sit a lot tend to have overactive (tight) hip flexors.

  • Cumulative trauma.

  • Poor training technique.

  • Lack of core strength.

  • Lack of neuromuscular efficiency.

Length-Tension Relationship (LTR): The resting length of a muscle and the tension the muscle can produce at that resting length. LTR describes the relationship between the contractile proteins (actin & myosin) of a sarcomere and their force-generating capacity. When these contractile proteins are ideally aligned with the most cross-bridging, they can produce the greatest amount of force. Shortening or lengthening minimizes the cross-bridges reducing the muscle's ability to produce optimal force. *Visualize a fighter trying to throw a punch into a heavy bag, but they are too close or too far away to land it with optimal force.


Altered Reciprocal Inhibition (altered length-tension): Process by which an overactive muscle (shortened, tight, and strong), and/or myofascial adhesions in the muscle cause decreased neural drive of its function antagonist. *Tight hip flexors decrease neural drive to the gluteus maximus inhibiting its function. This causes synergistic dominance where the synergist (hamstrings) take over to perform the movement.

Synergistic Dominance (altered force-couples): When synergists take over function for an underactive (lengthened, inhibited, and weak) prime movers. *Understanding and identifying synergistic dominance will help you cue clients into better form.


Muscle Fiber Cross Section

Muscle Cross Sections

Identifying Muscular Imbalances


The first step to identifying muscular imbalances is to perform a Static Posture assessment. Paying attention to the (5) kinetic chain checkpoints (Feet/Ankles, Knees, Hips, Shoulders, & Head) and following the protocol listed in your personal training manual. If an imbalance is suspected it should be noted and further assessed during the next phase which is Movement assessments (Overhead Squat, Bend-and-Lift, Push-Pull assessments, Thomas Test (hip flexors), Passive straight leg raise (hamstings), Hurdle-step test).


Important Tip #1: Our bodies take the shape of the positions we are in most of the time. Speak with your client about their daily routine and habits to identify possible areas that are causing dysfunction. Making the client aware of a muscular imbalance and giving them options to counteract it in their daily lives will help them correct the imbalance, maintain alignment, and regain their functional capacity.


Example: If a person has a desk job and sits for the majority of their day (driving to work, at work, at home on the couch in the evening) there is going to be an uphill battle in restoring proper function and balance in their muscles. This should not be done solely during the 30 minute to 1 hour training sessions they have during the week. You can make them aware of what the excessive sitting is doing to their muscles and give them options to counteract that position such as: 10 - 15 minutes of daily mobility work, getting a standing desk, setting a daily alert on their phones to remind them to correct their posture, get up to stretch, or walk around for a few minutes frequently throughout their day, take phone calls standing up, evening walks with their family, foam rolling while watching TV at night, etc. The daily habits and activities make the biggest difference in muscle balance and function.


Important Tip #2: Look for imbalances and improper movement patterns at every training session with your client. The initial assessments are just to get a baseline and know where to begin. The goal is to restore proper posture and functional movement throughout the training program.


Once imbalances are identified, overactive muscles should be lengthened, and underactive muscles strengthened to restore proper length-tension relationships. This is accomplished through the design of a corrective exercise program for the individual. Remember the phrase "Straighten the body before you Strengthen it."


Corrective Exercise Protocol

  1. Inhibit / Lengthen the overactive muscles. The first step is Self-Myofascial Release (SMR) using a foam roller or other tools such as a lacross ball, softball, back-buddy, etc. This inhibits the muscle and breaks up any tension and adhesions in the fascia that may be present. The second step is to use stretching techniques (PNF, contract / relax, static) on the overactive muscles which helps to lengthen the shortened muscle.

  2. Activate / Strengthen the underactive muscles by performing isolated exercises that target the specific muscles that are weakened and elongated.

  3. Integrate functional movement patterns once the length-tension relationships have been restored.


Tips to remember common Overactive and Underactive muscles


1. Learn the major muscles of the body (prime movers) and their functions (see anatomy websites below). To remember a muscles function visualize what happens when the muscle contracts. *The biceps contract to flex the elbow joint / Triceps contract to extend the elbow joint.


Anatomy Websites to Learn Muscles and Locations

  • Zygote Body: 3D image of the human body that you can rotate and peel away layers to see muscle locations and what they are connected to.

  • Inner Body: Detailed description of the muscular system with clickable illustration of muscle locations.

  • Get Body Smart: Muscles of the human body - actions, attachments, locations, and nerve supply.

  • Muscles & Motion: Great illustrated videos and descriptions of muscular mechanics involved in any movement of the human body. Also shows the difference between improper and proper form of various movements.

  • Poke a Muscle: Game to help memorize muscle locations

2. Once you know what the muscles are and what they do take yourself, friends, or family members through the same process that you will take a client through starting with static posture. First stand with good posture aligning the kinetic chain checkpoints. Then put your self in each of the deviated positions (Lordosis, Kyphosis, Sway back, Flat back). Visualize the tug-of-war going on with the muscles and which ones are winning. *Hip flexors and Erector spinae are "winning" in the Lordosis position (Anterior / front pelvic tilt).

3. Perform the movement assessments on yourself, friends or family members. You may find that you have an imbalance yourself while doing these assessments. Do the same as above, first perform the movements properly and then incorrectly using the common compensations (arms falling forward, feet turning out, knees caving in). Visualize the tug-of-war going on. Below is a sample of the overactive and underactive muscles involved with common compensations and distortion patterns.




Helpful Links with additional information


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~ CPT Exam Prep Team

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