Phasic vs. Tonic Muscles

To stretch or not to stretch? That is not the question. Not really. Though there are numerous studies debating the merits of stretching, the ones which find no benefit to the athlete are typically flawed. The authors researching the efficacy of stretching inevitably apply a general stretching protocol to the subjects in their study with a one-size-fits-all mentality. But different activities cause different responses in different muscles. This is simple to understand when one considers that not all muscles are created equal. For the purpose of this discussion, I will focus on the difference between Phasic muscles and Tonic muscles.

Phasic Muscles are composed of at least 51% fast-twitch muscle fibers. These are powerful muscles, but they fatigue more easily than do tonic muscles. Kind of a shame, too, as these muscles are primarily responsible for movement. The gluteals are good examples of phasic muscles.

Tonic Muscles are slow-twitch dominant, composed of at least 51% slow-twitch muscle fibers. As such, they are highly resistant to fatigue and have a greater propensity for work. The iliopsoas is an example of a tonic muscle group.

One of the major differences between phasic and tonic muscles that is of particular interest to triathletes is how these muscles respond to faulty loading. Loading is the resistance which the muscles of the body must overcome. In the gym, it may be a dumbbell. In life, it’s gravity. Thus, even if the only weight room you’ve ever spent time in is the wait room at your doctor’s office, it’s fair to say we all experience loading in our lives. Faulty loading can take the form of under-use, misuse, or disuse. But as triathletes, who swim, bike, and run for up to seventeen hours all in the same day, the form of faulty loading we are typically concerned with is overuse.

Tonic muscles respond to faulty loading by shortening and tightening. With a lower threshold for stimulation, tonic muscles need very little encouragement to turn on. This can, and often does, result in hyperactivity of a tonic muscle, limiting the motion at the joint(s) over which that muscle crosses. As mentioned in Chapter One of Holistic Strength Training for Triathlon ( , this lack of flexibility (or more specifically, this lack of mobility) results in all the biomotor abilities being adversely affected.

Phasic musculature does the exact opposite. It tends to lengthen and weaken in relation to its relative antagonist(s) or opposing muscle (group). The problem is then magnified by the fact that muscles which are short and tight will hold their antagonists in a lengthened position. This can lead to what is commonly termed stretch weakness. Stretch weakness is defined by Florence Kendall in her book entitled Muscles: Testing and Function with Posture and Pain as

weakness that results from muscles remaining in an elongated condition, however slight, beyond the neutral physiological rest position, but not beyond the normal range of muscle length.

She goes on to say that “the concept relates to the duration of the faulty alignment rather than the severity of it” (italics mine). So is it any surprise that the aspiring triathlete, who may spend up to seven hours at a time hunched over the bike with a rounded back, has increased thoracic kyphosis and can’t stand up straight? Brick that with a swim where the pectorals and medial shoulder rotators get overworked during the course of an hour-and-a-half-pool session, and the source of the typical triathlete’s faulty posture becomes clear. Now the lengthened muscles of the thoracic spine are being pulled by the tight muscles of the chest, shoulders, and lats. This results in even more thoracic kyphosis.

Maybe you should just run, you’re thinking. Well, the increased lumbar curvature created by the tight, overworked quads and hip flexors of the average runner causes a compensation in the thoracic spine leading to… say it with me… increased thoracic kyphosis. So much for the benefits of cross training, right? Instead of one source for our orthopedic and postural aberrations, we triathletes have three. I guess we’re just S.O.L.

But no, we’re not out of luck. We just can’t rely on dumb luck when it comes to our stretching program. We can’t just do random stretches for every part of the body and expect our sport-specific muscle imbalances to be addressed. We need a specific course of stretching which actively targets the muscles we abuse when we swim, bike, and run.

The question then isn’t if to stretch, but when to stretch and how? If you perform stretches for every part of the body, you haven’t done anything to alleviate the muscle imbalance caused by your triathlon training. The tight muscles are still tighter than the loose ones. Your body is still out of alignment. And a body that’s not properly aligned moves and functions less efficiently, increasing its susceptibility to fatigue and, ultimately, to injury.

The bicycle wheel is a common analogy which effectively represents this idea. Ideally, thirty-two spokes running from the rim to the hub are tensioned appropriately to keep the wheel spinning true. Logging a lot of miles on the bike, especially under harsh road conditions with bumps or potholes, can lead to a wheel which wobbles as certain spokes get tighter while others become looser. Each imperfection in the road leads to the wheel wobbling worse and worse.

During college, I worked in a bike shop in St. Petersburg, Florida. Some Mondays, guys would come in with their wheels after crashing at the weekend’s bike race to see if the wheels were salvageable. The head mechanic, a guy named Ray who worked wonders with the spoke wrench, would stick the wheel in the truing stand and spin it. The arms of the stand would tell him which spokes were in need of tightening and which should be loosened. He’d keep fine tuning the calibration of the stand—tightening a spoke half a turn here, loosening another with a quarter turn—until the wheel ran as straight and true as the day the cyclist bought it.

Some wheels, and some cyclists, weren’t so lucky. One day a guy in shredded Lycra limped into the shop carrying his mountain bike. He’d gone down pretty hard on a training ride and his front wheel was so out of true he’d had to walk the bike to the store. The guy asked us if we could fix it enough for him to ride it home. Not much for words, Ray took the wheel from the guy, went behind the counter, and held it up at eye level as if he were reading which spokes needed attention. Suddenly, and with force which could be heard over the Chili Peppers playing on the shop’s stereo, he slammed the wheel down hub-first again and again. After a few seconds, he paused, repositioned the wheel in his hands like a guy making a pizza, and slammed it down on the counter a few more times. Finally, he stopped banging the wheel and gave it back to the cyclist, who looked a bit more abused than when he’d come in. But his face changed as he spun the wheel. It still wobbled. But if he could endure a jerky ride, the wheel looked like it just might get him home.

Throughout the body, ideal length-tension relationships exist which, when altered by chronic shortening or lengthening of certain muscles, result in faulty joint kinematics. It’s a matter of physics. Forces generated by movement or loading cannot be adequately dissipated in a joint which has moved away from its instantaneous axis of rotation. The resulting premature degradation of the joint itself inevitably hastens the demise of the triathlete’s competitive career. But if you stretch the right muscles at the right time and in the right way, just like a wheel in a truing stand, your chances of maintaining your orthopedic integrity increase exponentially. And though I can’t promise you that you won’t ever have to walk your bike home, with correct stretching you should never have to limp your body home.



Yes, Cynthia. I realize who Florence Kendall is—and if you read the 6th paragraph, you’ll see how I refer to “her” book. You may then come to the conclusion that the “He” in the following paragraph is a typo and was meant to be “She”. Thank you—I will correct that.

And we are in agreement that athletes are not “S.O.L.” My book ( and, indeed, the following paragraphs go on to explain that. However, if that’s not sufficient (since the medium of print has its inherent limitations), you can work with a P.T. Or you can even hire someone like myself who is trained to consider the body from a holistic perspective, address all the relevant drivers, and make sure you can function in the real world in addition to the physical therapy clinic.


Florence Kendall was a WOMAN, not a man as you mention. She was one of the most distinguished Physical Therapists in the world. Her book, Muscles: Testing and Function with Posture and Pain, is an excellent guide for postural alignment assessment.

Athletes are NOT S.O.L. In fact, you have an industry of trained professional physical therapists, knowledgeable in biomechanical principles, skeletal and muscle symmetry, strength training, and helping you achieve your goals. You can go to the American Physical Therapy website to find a specialist in your area.


Exactly, Jeremy. Indeed, Guy Voyer teaches aspects of segmental strengthening and stretching where one uses specific positions/motions to target the more distal or proximal portion(s) of the muscle depending on which area needs to be addressed. We can get real deep and complex if necessary. But the majority of people would probably best be served by starting with the basics—just moving their bodies consistently and following the flexibility before stability before strength before power concept in the development of an athlete/individual.


And then of course there is the possibility that musculature that is quite broad and crosses multiple joints, such as the iliopsoas and quadratus lumborum, have mixed fiber types. The aspect of the muscle that is closest to the joint(s) would have the best force production and mechanical advantage to produce a stabizing contraction whereas the aspect of the muscle further from the joint would have force production more suitable to producing a torque (movement of the joint). Since a single motor neuron produces an action potential only at the muscle fibers ennervated and there are many neurons ennervating a whole muscle belly, there could feasibly be different types of contractions from the same muscle.

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