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On Training and Tacit Knowledge
Tacit knowledge refers to knowledge that can only be gained through experience, and which cannot easily be described in words or numbers. It is very difficult to convey, although it can be learned. Often, the only way to successfully teach it is experientially, the way a master imparts skills to an apprentice. When people say, “learn by doing,” they are often referring to the learning of tacit knowledge.
Most forms of tacit knowledge are either physical or social. There is the intuition a practiced mechanic develops which allows him to understand how a new machine works when he first encounters it, before he has read a manual or been taught by someone else. There is the instinct of a skilled goalie, who knows where the ball is going before she can explain its trajectory. She may never have studied physics, aerodynamics, or kinesiology, but when the opposing forward kicks the ball, she knows where to be to stop it.
Athletes therefore develop an awareness of their body through practice, which allows those who perform at the highest levels to quickly pick up entirely new movements and techniques. They can see an exercise performed by someone else and understand what that person is doing without ever having seen it before. Crucially, they understand how they can do it, too.
Athletes also understand their sport at the so-called a gut level. Sometimes, what appears to be superstition is actually intuition developed through experience. A pitcher may instinctively adapt his strategy to different weather conditions without knowing why. He may not even be consciously aware that he is doing so.
It may surprise some readers, but all exercise requires a high degree of this tacit knowledge. The longer you lift weights and the more you try to learn about the human body, the more you become aware of just how complex human movement is. You realize how little you used to know. Exercises that once seemed simple now feel complicated in ways that you find difficult to convey. You begin to come to terms with the limits of your own knowledge and awareness, and you grasp a hint of just how much more there is to know. Even at this late date, there is much that we do not understand about the body.
There are those who find weight training boring. It appears repetitive, often performed inside buildings without aesthetic interiors, and mindless. But then there are others, including many athletes and sports professionals, who don’t find it mindless. To them, it engages the mind at a very high level. A movement such as the deadlift, for instance, appears simple, but it requires a great degree of coordination between most of the major (and many minor) muscle groups within the body. It is technically and mechanically interesting. If you deadlift very heavy, you can’t let your mind wander. You must be focused on what you are doing. You feel different muscles within your body engage, and you can tell if something fires improperly or if you relax too early.
For an athlete, and especially for a trainer, every workout is an opportunity to gain this type of knowledge. Not only is each workout designed to stimulate key physiological parameters, but it is also an opportunity to practice. That could mean practicing new movements or techniques, or it could mean mastering the same skill one has honed for years. It could mean trying to develop new tweaks and innovations, or it could mean focusing on deepening one’s instinct for physical movement. It is a deliberate form of practice, a deep practice, one that is anything but mindless.
Physical exertion can concentrate the mind very effectively. But surprisingly, the mind doesn’t always concentrate where you might want it to. It takes work to focus on the carriage of the arms, the rhythm of the breathing, the sensation of the footfalls, the cadence of the run. Running is deceptive. It is something “everyone knows how to do,” and yet most people don’t do it nearly as efficiently as they could. There are thousands of little variables that go into it.
Barbell lifts are similar. There are more possible cues than any lifter can keep at the forefront of the mind during any single repetition. This can be intimidating, because a lot can go wrong in a heavy lift. And if something goes wrong, it can go wrong very badly.
Even less-intense lifts contain a surprising amount of information. Take a basic step-up. A deceptively simple exercise. A hip hinge that isolates a single leg. Everyone knows how to step up onto a step. Right? But to perform a step-up properly, an athlete must ensure multiple muscles are firing correctly, and multiple joints are coordinating and moving properly. For anyone wishing to develop tacit knowledge, a simple exercise like this is a perfect opportunity. Concentrate on the action. Ask yourself: what are my hamstrings doing? My glutes? Where do I begin to lose control on the descent? How is the posture in my upper body? Is my pelvis tilting forward? Is there movement in my ankle? Where am I unstable? And so on.
Experienced powerlifters often retain a large amount of tacit knowledge about the basic lifts they perform. Men and women who have practiced heavy back squats weekly for years will talk about feeling “off.” They come into the gym, warm up, start to do it, and stop. They feel off. Something is wrong. They may not be able to explain exactly what is off that day, or why something is off. Maybe their ankles are too tight, limiting their range of motion and forcing them to compensate elsewhere. Maybe their back is tight from sitting all day, and that tightness is limiting the external rotation in their hips. Maybe their glutes aren’t firing properly. Maybe a combination of all of the above, or perhaps a combination of unrelated factors.
But how exactly is it that athletes know something is “off” in their bodies? And how can someone tell if their hip is hinging properly or if their glute is misfiring?
As children, we were taught that there are five senses. But most athletes and coaches are aware that there are two more: proprioception and interoception. These two senses are what give athletes knowledge of what their body is doing at any given time.
Proprioception is the awareness of one’s body as it moves in space. This includes perception of balance and knowledge of what each limb and joint is doing at any particular moment. The more you exercise, the more you develop this sense. You can feel when you are performing an exercise properly or not, even if you don’t feel any pain or discomfort. It isn’t through touching objects in your surroundings, or listening very carefully, or even looking at yourself in a mirror, that you sense where your foot is, or your arm, or how your knee is moving. It is through proprioception.
Interoception is the awareness of things internal to the body. You don’t taste or smell or touch hunger. You know you are hungry. You don’t touch fatigue, but you feel tired.
A lot of physical tacit knowledge is related to these two senses. Not all of it, to be sure. Certainly, getting a bad feeling about a change in the weather before a big game isn’t related to either of them. But proprioception and interoception are important skills for an athlete to hone, and beyond exhortations to “listen to your body” and “maintain spatial awareness,” there isn’t a whole lot that can be done to coach them. There is even less that can be done to quantify them.
We can describe physical sensations in words, and we can use scales like RPE (rate of perceived exertion) to attempt to capture our level of effort as a number between 0 (no effort) and 10 (maximum effort). But it is difficult for us to truly measure how hungry someone feels, or how much pain they are experiencing. One person’s 10 is another person’s 2, especially if the first person is relatively untrained. Some people will naturally rate anything that causes them to feel the “I can’t do this” pressure in their brain as a 10, whereas others will rate anything that didn’t hospitalize them as less than an 8.
What this means is that one person’s tacit knowledge about his or her own body cannot easily be transmitted to another person to replicate. Moreover, it cannot be touched or understood by smart devices, advanced metrics, complicated models, or machine learning algorithms. No artificial intelligence can comprehend what it is to move one’s body through space, to feel one’s hamstring contract, to follow one’s fist backwards past the side of one’s hip without ever looking down, to know at all times where one’s feet are landing, because an artificially intelligent device cannot experience such things.
Nor can an AI effectively train an athlete, because to do so requires extrapolating one’s own experience and knowledge of one’s own body, to the movement and improvement of another’s. A trainer or a coach needs to have tacit knowledge both of his or her own physical performance, and of the physical performance of others. Moreover, he or she must know how to effectively stimulate the proper physiological variables in the proper configuration at the proper time to elicit the best response in each particular athlete. And while our bodies all respond to the same stimuli, we don’t all respond exactly the same, and therefore each athlete will be a little different.
Which brings us to another area of tacit knowledge required of trainers and coaches: interpersonal communication.
According to some metrics, GPT-4 is able to “say the right thing” to comfort bereaved humans more often than the average human being. But it isn’t doing this because it is able to actually read the bereaved humans and understand what they are feeling and what they need to hear. It is just better at selecting the phrases most likely to comfort most humans. It isn’t even aware that it is comforting a human, because it isn’t aware that it is doing anything. It isn’t aware of anything at all.
Human beings are aware of other people. We may not always be good at reading others’ emotions, and we will always face some difficulty in knowing what is really going on inside another’s mind, but we are born with the ability to learn something about other human beings from their faces.
Trainers and coaches must be good at adapting to the nonverbal feedback our clients and athletes give us. An algorithm may be able to design a training program, and even make adjustments on the fly based on variations in resting heart rate or insulin levels or body composition or other metrics. But a human being can figure out what another human being actually wants and needs, not just what might be determined to be optimal in a particular case.
Not Easily Reproduced
Coaching, personal training, physical therapy, group exercise instruction, and similar professions both require, and provide, opportunities for the development of vast stores of tacit knowledge—about the human body, about physical training, about working with human beings, and more. This tacit knowledge is, like much of the tacit knowledge in the world, particular, local, contextual, and often found only inside the heads of individual human beings.
This makes it very difficult for technology to replace the jobs of the best coaches and trainers and physical therapists. To be sure, many people’s fitness needs can be met by apps and algorithms—just as there have always been many people who could train perfectly fine on their own using workout plans copied out of a book. Some trainers were those people before they became trainers. But (as with professionals in other industries) the fitness professionals who go above and beyond their basic tasks to emphasize the elements of their work which cannot easily be replicated by technology will always have a competitive advantage that keeps them in business.
There is a more important conclusion to consider, too. One that is not original to me, but one that is often ill-understood nonetheless. Because tacit knowledge in a variety of professions can only be learned from experience, and there is always some information that cannot be stored in a computer or externally managed or modeled with statistics or even understood at a basic level once context is stripped away (i.e., at a remove from the particulars of the situation), it is impossible to direct all of the work of those professions at scale from a single hub. Some decisions will always be left to the individuals involved in each particular situation.
All human bodies operate according to certain principles which apply universally, but those bodies are so complex—involving tens of thousands of subtle variables at play at any given time—that there is nearly infinite variation among human bodies. It may prove impossible to ever perfectly model anyone’s body (doing so would likely require reproducing it exactly), meaning that the limit of what statistical models (or algorithms, or artificial intelligence) can tell us about even one single person’s body will always be less that there theoretically is to know. We will never have a computer that can predict exactly what one person’s body is going to do next.
We should be humbled by the recognition of that fact. Perhaps one of the future roles of the fitness professional will be to remind human beings, as Friedrich Hayek wrote, of “how little [we] know about the things [we] imagine [we] can design.”
 Hayek, Friedrich. (1988) The Fatal Conceit. University of Chicago Press.
Ben Connelly a writer, runner, and personal trainer based in Virginia. Ben can be found on Twitter and on his Substack, Hardihood Books, where he publishes short fiction and essays.