Hierarchy of Movement: Flexibility
February 2025
February is here with frigid temperatures. Let’s heat things up with a conversation about flexibility. Last month we talked about mobility and the 3 F’s: moving freely and fluidly through a functional ROM. Using a cycling analogy for joint mobility, compare the feeling of pedaling with an old worn, rusty chain to a new lubricated one. The total amount of movement may be the same, but the quality of that motion is very different.
Flexibility is one component of mobility. It is defined as ability of muscle, tendon and fascia to lengthen passively (in the absence of muscle contraction) through ROM. Flexibility is often limited in muscles and their respective tendons crossing two joints (bi-articular). The hamstring muscles, crossing both the hip and knee joints, are a great illustration of flexibility impacting mobility at the knee joint. A cyclist may have excellent knee mobility, fully straightening (0° extension) and bending until the heel contacts their butt (~130° flexion). However, when placed in a supine position with the hip flexed to 90°, knee motion stops 20° short of full extension (can’t straighten to 0°). What changed? In 90° of hip flexion (think about sitting on the saddle), the hamstring muscles, tendon and fascia are put on stretch across the hip joint. If there is limited flexibility, there isn’t enough length in those tissues to also allow the knee to fully extend. “So what?”, you might ask. So… what happens when you want to really drop into a more aerodynamic position? With adequate hamstring flexibility (length), the pelvis rolls into an anterior tilt and the spine follows to maintaining alignment. If there isn’t adequate length available, the pelvis is “tethered” and the desired aerodynamic effect is achieved through spinal flexion, placing high stress on the lumbar discs. You’ll save those previous watts, but at a price.
Is stretching the answer? The efficacy of flexibility exercises (stretching) on improving ROM, injury prevention, and their negative impact on muscular performance in explosive sports has been widely discussed in the literature. Like most recommendations, “it depends”. Stretching recommendations for the hip flexors of a cyclist may be dramatically different from those of a gymnast. Stretching may be contraindicated after an acute muscle strain, but warranted after 6 weeks of an immobilization for an ankle fracture. Stretching may minimize muscular explosiveness in a 100-meter but enhance muscle function in a 100-mile road race.
What exactly are we stretching? I’m somewhat embarrassed to admit that it took me longer than it should have to appreciate stretching targets more than just the muscle, or contractile tissue. Indeed, stretching increases the distance between the actin and myosin myofilaments (muscle proteins responsible for shortening or contraction) in each sarcomere (functional contractile unit of muscle), allowing the muscle fiber to elongate. Once the fiber reaches its maximal resting length by stretching the actin and myosin and other proteins such as titin, of each sarcomere, the target becomes the connective tissue. Each muscle fiber is surrounded by connective tissue (endomysium). Groups of fibers are arranged into fascicles, also surrounded by connective tissue (perimysium). Finally, all the fascicles of a muscle are enveloped in connective tissue (epimysium). Muscles are connected to bone via connective tissue (tendon) at the musculotenidous junction. Stretching, therefore, is much to do about connective tissue.
Muscle and connective tissue
Take your time stretching. Muscle and connective tissue are viscoelastic materials, which have rate and time dependent properties. Think about silly putty. If the goal is to stretch silly putty, the rate at which you load the silly putty will be important for its deformation behavior. The more quickly you pull or stretch the silly putty, the less deformation compared to a slower rate. The take home: a slower rate of stretching is likely more effective than a rapid, bouncing type approach. Once you have the silly putty (tissue) under tension, two magical things happen with time. If you were to hold the stretched silly putty at a constant length, the material would begin to exhibit less tension (stress relaxation). You’ve likely felt this after ~20s of placing your hamstring on stretch. Without gaining any more ROM, there is a sense of less tension. Similarly, if you were to exert a constant force on the silly putty to lengthen it, the material would eventually lengthen (creep). You might have trouble getting the silly putty back into the container. Of course, silly putty isn’t the same as muscle, tendon and fascia, but it provides a framework to understand the properties of viscoelastic material and why stretching is best accomplished at a slow rate with time (~20-30 seconds) under tension.
The literature is fairly clear that acute stretching increases stretch tolerance, stress relaxation and creep. What stretching doesn’t seem to do is decrease mechanical stiffness. And that’s good! Right? The word “stiffness” used amongst engineers and clinicians may not mean the same thing, exactly.
The Merriam-Webster dictionary defines stiffness as “lack of suppleness or flexibility.” We defined flexibility as the ability of muscle, tendon and fascia to lengthen passively (in the absence of muscle contraction) through ROM. A patient might report feeling “less stiff” after performing a hamstring stretching routine. The clinician measures increased ROM (closer to full knee extension with the hip flexed to 90°) and agrees with the patient’s assessment. “Good work! Your hamstring is less stiff.” And that makes sense. Until you dig deeper and realize that stiffness means something a bit more mathematical in the engineering world. And measuring it in the clinic takes a bit more than what I was measuring with my goniometer.
An engineer or clinician measuring stiffness for research purposes would define the extent to which the tissue resists deformation (change in length) in response to an applied force. And remember that tissue is viscoelastic, so that really gets complicated (at least for my physiology brain). You can imagine my confusion when I walked into my first tissue engineering lab meeting (Ray Vanderby, PhD, one of the best mentors ever) and learned that tendon stiffness was good??? Huh? Hadn’t I been rewarding my compliant patients (no pun intended) for decreasing stiffness through stretching? Yes, stiffness as defined by loss of flexibility is something we want to improve with stretching. Tendon stiffness, as defined by stress / strain and a time dependent modulus thrown in (not what I was measuring in the clinic), allows efficient transfer of force of the muscle to move the limb in cycling and all other forms of movement is something we would like to preserve. As we age, tendon stiffness declines. Think about Tigger at 40. Or 80. Eccentric exercise and plyometrics are ways to improve tissue stiffness. Stretching does not decrease stiffness. At least that’s my understanding.
Chronic stretching with improvement in ROM has been attributed to the myogenic response of sarcomeres adding more functional units in series to lengthen a muscle. Said simply, a muscle shortened due to posture, injury or immobilization is not optimized. Stretching may restore tissue length to decrease strain and optimize the crossbridge interaction of the contractile proteins, myosin and actin, for force development through sarcomerogenesis. A sarcomere is the functional unit of muscle, and genesis indicates the addition of sacromeres in series to restore length. This may be particularly important following muscle injury or immobilization.
Does stretching prevent injury? Several prospective studies have failed to show an effect of stretching on injury rates. However, lack of flexibility may impact joint health by limiting ROM and therefore synovial fluid bathing of the cartilage (remember the Tin Man?), creating abnormal stress on adjacent joints, and is a risk factor for strain induced muscle damage.
If you subscribe to stretching as part of your quest to maintain or increase flexibility, how do you best go about it? There are many types of stretching, and the nomenclature can be confusing. Similar to the zone model of aerobic training, there is no agreement on one set of terms to describe stretching. For the purposes of this article, we will define stretching modes as passive or active, referring to the state of muscle contraction. The term dynamic warmup was introduced earlier to describe exercises for mobility, and therefore won’t be used to describe flexibility training.
Passive stretching implies to lengthening of the muscle, tendon and fascia without contraction. This is the most traditional form of stretching that many of us were introduced to in physical education.
Passive calf stretch, with static hold for 15-30 seconds, 2-4 repetitions.
The data suggests that passive stretching improves ROM, likely through stretch tolerance rather than tissue length, and reduces muscle soreness when performed after an exercise session. Cautionary note: most of the detrimental effects of stretching on muscle strength and performance are attributed to the static mode of stretching prior to exercise. From a physical therapy perspective, static stretching is valuable for patients recovering from muscle injury or immobilization following surgery.
Active stretching, in this context, refers to a contraction of a muscle just prior to stretching. This concept is rooted in work done by physician Dr. Herman Kabat and physical therapist Margaret Knott between 1946- 1954 called proprioceptive neuromuscular facilitation (PNF). Briefly, proprioceptors are sensors that detect changes in body position or movement. Specifically, muscle spindles and Golgi tendon organs (GTO) report changes in length and rate of stretch from within muscle or tendon, respectively, and convey these to the central nervous system. It is beyond the scope of this article (and my ability) to describe the physiology more completely. It has been suggested that there is a neurological effect, evidenced by increase in ROM in the non-PNF stretched limb due in part to proprioceptors causing autogenic and reciprocal inhibition. Stately simply, the core principle of PNF is that a muscle contraction will facilitate relaxation.
Hold-Relax (HR) and Contract-Relax (CR) are PNF techniques whereby firing the target muscle (TM) causes GTO excitation. Following contraction, there is a refractory period, termed autogenic inhibition, allowing enhanced lengthening. Similarly, with Contract-Relax Agonist Contraction (CRAC), contraction of the muscle opposite the TM (i.e., contracting the hamstrings while stretching the quadriceps) will excite the muscle spindles, causing reciprocal inhibition of the TM (the body is not wired to have antagonistic muscles firing at the same time- it would be difficult to move), facilitating stretch. Whatever the mechanism, active PNF stretching has been demonstrated (not unequivocally) to be more effective than static stretching for increasing ROM.
Anecdotally, speaking from 30 years of experience working with athletes, PNF is my go-to. Here are two examples of how to incorporate PNF techniques. Note: there are various definitions and prescriptions for HR, CR, and CRAC. The following examples may take liberties from the official PNF literature.
Hold-Relax or Contract-Relax for Hamstrings: athlete uses a strap to passively extend the lower leg until tension is felt in the hamstrings. Athlete then performs ~70% maximal hamstring contraction with no (HR) or little movement (CR) for ~10 seconds. Upon relaxation, the strap is used to further stretch the hamstring ~20 seconds. HR/CR is repeated 2-3 times.
Contract-Relax Agonist Contraction for Quadriceps: athlete holds ankle of target leg to prevent movement, and performs an isometric contraction of quadriceps muscle at or above 70% max for ~10 seconds. Upon relaxation, athlete maximally fires the hamstrings for ~5 seconds as the knee is pulled into greater flexion and held for ~20 seconds. CRAC is repeated 2-3 times.
You may be wondering where yoga falls into the realm of flexibility. Yoga is much more than flexibility training: it includes stability, balance and community. A humbling moment for me was taking my first yoga class a few years ago, and realizing that the instructor (and dear friend) knew more about stretching than I did a physical therapist and muscle physiologist. Fascia doesn’t run in a single plane. So why had I been instructing patients and athletes to stretch muscles in a single plane??? Some cyclists happily incorporate yoga into their rest and recovery day, while others balk at the idea of spending an hour exploring Warrior poses. I would encourage you to check out a class and decide for yourself (see the community partner page on my website). She has created two short 20 minute “essentials” yoga video targeting muscle groups most often plagued by repetitive motions of cycling available to those who Draft Responsibly.
