Flex Time: Extend Workout Perks With Progressive Stretches

This aspect of exercise programming is often sacrificed to make the most of workout time—here’s why every minute of stretching is worthwhile.

by Kyle Stull, DHSC, MS

Are you flexible? Perform a quick test: Back up against a wall and try to touch your toes. Could you? If not, do you think this means you simply need to stretch more? As you likely know (or have deduced), flexibility is a bit more complicated than that. Yes, muscles and tissues must be able to extend, but the body must also feel stable and be able to coordinate movement.

To touch your toes from a standing position takes considerable core stability and coordination. Older but still valid research found that safely touching the toes requires a 9-degree posterior shift to change the angle of the upper leg (Kippers & Parker 1987). Without that shift, the body may sense it is about to fall over, so it may attempt to stabilize the pelvis by engaging whatever muscles it can recruit. Since the hamstrings attach to the pelvis and assist other core muscles with optimal stability, it is common for them to contract (tighten) before others.

This article does not elaborate further on this issue, but the point is this: Flexibility does not work in isolation; it is part of the full, integrated human condition. Thus, appropriate stability training must complement all flexibility programs.

Here, we will dive into the theories of flexibility and extensibility; the flexibility continuum and its relation to the NASM Optimum Performance Training™ model; and some strategies to help you create flexibility programming appropriate to each client’s unique needs (and your own!).

The NASM OPT™ Model and Flexibility Training

In NASM Essentials of Personal Fitness Training, FLEXIBILITY is introduced as the normal extensibility of all soft tissues that allows full range of motion at a joint and optimum neuromuscular efficiency during movement (NASM 2018). To demonstrate flexibility, the tendons, ligaments, fascia and muscle must all be able to move and slide across one another to allow smooth joint motion. This requires the interplay of various muscle groups—some of which must produce or reduce force during the motion, while others stabilize the joint.

Flexibility training is such an important component of fitness programming that the NASM OPT model recommends addressing it with a client before diving into cardiorespiratory, core or resistance training. That’s because most of the people you work with will require some amount of flexibility training before they are capable of performing core, cardio and strength exercises safely and effectively (NASM 2018).

4 Theories on Flexibility

There are several theories about flexibility and extensibility. In the literature, four leading theories have emerged to explain the changes in muscle extensibility that occur when a muscle is stretched:

  • viscoelastic deformation
  • plastic deformation
  • changes in sarcomeres
  • sensory changes


Muscles and other soft tissues in the body demonstrate what are known as VISCOELASTIC PROPERTIES. To provide some context, a viscous substance is something like a liquid that can change shape based on the amount of force applied to it. Elastic properties refer to a substance’s tendency to return to its original state after a force is removed.

Some researchers believe that immediate increases in muscle length may derive from the ability of tissues to change when force is applied and then to revert to their original state when force is removed (Chan, Hong & Robinson 2001; De Weijer, Gorniak & Shamus 2003).

Furthermore, the theory suggests that how much viscoelastic deformation occurs may depend upon the duration and intensity of the force applied (Xu, Li & Zhang 2013). For example, applying greater force for a longer period of time may lead to greater deformation.


A second theory suggests that PLASTIC DEFORMATION explains the immediate and often permanent elongation that occurs when a stretch has taken the muscle beyond the elastic limit (NASM 2019). According to this theory, plastic deformation of muscle occurs when enough force is applied for long enough (Jacobs & Sciascia 2011) to lead to viscoelastic changes (Chang, Hong & Robinson 2001). However, taking the muscle “too far” past its elastic limit is also associated with substantial strain and discomfort. Thus, it’s important to know “how far to push” in order to lengthen muscles without inducing injury.


A third possible explanation for why muscles may lengthen is the addition of sarcomeres. Animal studies have suggested that when a muscle is held in a fully lengthened position for an extended amount of time (up to periods of several days in some cases), the body adapts by producing more sarcomeres.

A SARCOMERE is the “functional unit of the muscle, much like the neuron is for the nervous system” (NASM 2018). It is the point where a muscle shortens or lengthens. Thus, a muscle with more sarcomeres should demonstrate more length.

While much of the seminal research has been completed on animals, and their muscles were held, lengthened or shortened using plaster casts and denervation (Tabary et al. 1972; Goldspink et al. 1974), the results may have relevance for humans, this theory suggests. The findings imply that regular stretching practices would need to be consistent and long term to effectively increase the number of sarcomeres.


Another theory about changes in flexibility focuses on the effect that stretching may have on the sensory system. If tension is quickly applied to muscle, the nervous system may initiate a quick reflex, causing a contraction to remove the tissue from a force that could potentially overload it. NASM (2018) describes such a response by suggesting that when a muscle is lengthened too rapidly, specialized mechanoreceptors (muscle spindles) produce a quick contraction. However, if force is applied slowly enough so as not to activate the muscle spindles and then the force is held for a time (30–45 seconds in most cases), another mechanoreceptor (the Golgi tendon organ) will signal the muscle to relax. This process is known as AUTOGENIC INHIBITION, and the theory holds that, over time, muscles will become more responsive to stretching forces and demonstrate more overall extensibility.

"Most of the people you work with will require some amount of flexibility training before they are capable of performing core, cardio and strength exercises safely and effectively."

Assessments of Flexibility

As trainers, we cannot determine what a client needs to stretch by simply asking what feels tight. We need to perform assessments that measure flexibility, stability and strength. One assessment that is easy, requires no equipment and has been found to yield useful information is the OVERHEAD SQUAT ASSESSMENT (OHSA).

Overhead Squat Assessment

The Integrated Flexibility Continuum

As you know, the NASM OPT model follows a systematic progression that enables exercise participants to advance gradually and safely along a continuum, beginning at their current level of fitness. This is true, too, of flexibility training.

The INTEGRATED FLEXIBILITY CONTINUUM covers the full range of flexibility over time, including foam rolling as well as static, active and dynamic stretching (NASM 2018). Within the NASM OPT model, there are three phases of flexibility training: corrective, active and functional.

A proper flexibility program must begin with a corrective component: an effort to improve movement patterns. If the overhead squat is used for baseline testing, then the goal should initially be to improve the squat, where appropriate. Keep in mind that the OHSA includes multiple checkpoints on the kinetic chain, including the feet, knees, lumbo-pelvic-hip complex and shoulder complex. Therefore, a well-designed flexibility program will begin with including static stretching to inhibit muscles identified as being overactive around a joint.

Once movement improves, clients should progress to active-isolated stretching, which will improve ROM by using other muscles around joints.

Then, as movement improves further and clients become stronger and more coordinated, they can progress to dynamic stretching. NASM has paired each of these three phases with foam rolling and stretching, which should be performed instead of (or prior to) traditional cardiorespiratory warmup exercises, which may exacerbate dysfunctional movement patterns. See the sample program (below) for examples and photos of each type of flexibility exercise.


Corrective flexibility is designed to improve muscle imbalances, poor joint motion and postural dysfunctions (NASM 2018). Corrective flexibility includes self-myofascial release (such as foam rolling) followed by static stretching, which can effectively increase the length of tight muscles (Skarabot, Beardsley & Stirn 2015).

FOAM ROLLING. In this phase, foam rolling should be used instead of a traditional cardio warmup, as typical cardio exercises may exacerbate faulty movement patterns. Using a foam roller in a corrective flexibility program has two purposes:

  1. Holding pressure reduces tension in the muscle.
  2. The pressure from the roller helps to rehydrate and mobilize many of the sliding surfaces. (See “Fascia, Fluids and Flexibility,” for more on the role of hydration in flexibility.)

STATIC STRETCHING. It is often argued that static stretching should not be used before activity. However, it is important to recognize how a static stretch can be an effective part of corrective flexibility. In this context, the stretch is used to reduce overactivity in a muscle and restore it to its ideal length, thus leading to overall improvements in performance over time. Furthermore, Sandberg et al. (2012) found that jump performance increased when static stretches were performed on the opposing muscles (i.e., tibialis anterior, hamstrings and hip flexors). Thus, when used correctly, static stretching prior to an activity may indeed improve performance.


Active flexibility can improve soft-tissue extensibility in all planes of motion through RECIPROCAL INHIBITION, or “the simultaneous contraction of one muscle and the relaxation of its antagonist” (NASM 2018).

Active flexibility is used after clients improve movement patterns and begin to include more strength training. Again, foam rolling takes the place of a traditional warmup. Here, the roller can still be used on commonly overactive muscles, or it can simply be used to move fluid and mobilize tissues before applying active-isolated stretching techniques.

ACTIVE-ISOLATED STRETCHES are performed very similarly to static stretching, with the notable exception of holding the end range for only 1–2 seconds.


Functional flexibility is a technique used to improve multiplanar soft-tissue extensibility and provide control throughout a full ROM while performing a functional movement (NASM 2018). Foam rolling and dynamic stretching can both serve as functional flexibility techniques.

As in active flexibility, foam rolling should replace a regular warmup as a means to improve fluid flow and get muscles moving. DYNAMIC STRETCHES, which incorporate movement patterns similar to those in the main workout, use body weight and momentum to take joints through a full ROM.

Common dynamic stretches are squats, lunges, lunge with rotation and pushup variations. It is important to note that dynamic flexibility comes at the end of the continuum because clients must be able to demonstrate optimal movement patterns before attempting many of the dynamic stretches. Take the common lunge to rotation, for example:

To execute this correctly, clients must demonstrate full hip flexor ROM and adequate thoracic rotation mobility. Without proper motion in these essential areas, a client could cause more harm to the lumbar spine than benefit to the rest of the body.

Stretch Your Mental Muscle

Flexibility is an often-overlooked component of training, in both individual and group exercise, but it’s clear that it deserves the attention of you and your clients. Incorporate self-myofascial release and various types of stretches (static, active-isolated and dynamic) both before and after an activity, for benefits far beyond improvements in flexibility (NASM 2018). By including progressive, systematic and assessment-driven flexibility training in your exercise programming, you can reduce the risk of muscle imbalances, compensations, joint dysfunctions and overuse injuries.


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Chaitow, L., & DeLany, J. 2008. Clinical Application of Neuromuscular Techniques. Volume 1: The Upper Body. Philadelphia: Elsevier.

Chan, S.P., Hong, Y., & Robinson, P.D. 2001. Flexibility and passive resistance of the hamstrings of young adults using two different static stretching protocols. Scandinavian Journal of Medical Science, 11, 81–86.

De Weijer, V.C., Gorniak, G.C., & Shamus, E. 2003. The effect of static stretch and warm-up exercise on hamstring length over the course of 24 hours. Journal of Orthopedic Sports Physical Therapy, 33 (12), 727–33.

Fascia Research Congress. 2018. What is fascia? ­Accessed Apr. 24, 2018: fasciacongress.org/congress/about-fascia/.

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NASM. 2019. NASM Essentials of Sports Performance Training (2nd ed.). Burlington, MA: Jones & Bartlett Learning.

Sandberg, J.B., et al. 2012. Acute effects of antagonist stretching on jump height, torque, and electromyography of agonist musculature. Journal of Strength and Conditioning Research, 26 (5), 1249–56.

Skarabot, J., Beardsley, C., & Stirn, I. 2015. Comparing the effects of self-myofascial release with static stretching on ankle range-of-motion in adolescent athletes. The International Journal of Sports Physical Therapy, 10 (2), 203–12.

Tabary, J.C., et al. 1972. Physiological and structural changes in the cat’s soleus muscle due to immobilization at different lengths by plaster casts. The Journal of Physiology, 224 (1), 231–44. Xu, B., Li, H., & Zhang, Y. 2013. Understanding the viscoelastic behavior of collagen matrices through relaxation time distribution spectrum. Biomatter, 3 (3), e224651.

Meet our experts

AFM-Author-Stull Kyle Stull, MS, LMT, NASM-CES, Kyle Stull is the Senior Master Trainer for TriggerPoint Performance Therapy, a Master Instructor for NASM and for Equinox Fitness Clubs and an international presenter.

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