"In the motions and the things that you say
It all will fall, fall right into place
As fruit drops, flesh it sags
Everything will fall right into place
When we die some sink and some lay
But at least I don't see you float away"
- Gravity Rides Everything, by Modest Mouse
Have you ever taken a long car ride and ended up exhausted afterward? How about a long journey by train or by plane? Maybe instead of staying still, you were at your desk for six, eight, maybe ten hours. Maybe you weren't reading or studying, but you may have been paying attention to information in front of you - holding a gaze, keeping your eye on things. You've done little more than sit in a chair all day, but when you get home you're still exhausted. Even worse is that the exhaustion is more than mental - it's an almost total-body exhaustion that sits in your arms, legs and back. How is this possible? How is it possible to feel use and discomfort in parts of your body you haven't actively been engaging in tasks?
Sexy vs. Unsexy
The truth lies in how the body works. In a broad sense, the body is a machine composed of a biochemical laboratory that extracts nutrients from food for energy, contained within a set of muscles and bones arranged in such a way so that when muscles pull on bones, movement is produced. Simply put, muscles move bones. More comprehensively put, muscles move bones through the work of units of muscles. Movement is rarely, if ever, produced by the work of one muscle alone - if one muscle was responsible for one movement only, the human body would be much less energetically and physically efficient than it is now. Simple movements like the bending of an elbow and the turn of a wrist are produced as a result of many muscles moving together. This action is typically driven by one muscle called the prime mover, and refined by muscles around the joint that act in synergy, while being stabilised by the gentle action of a muscle acting in opposition, almost like a safety switch. The combined action of many muscles to produce complex movements is best seen in the use of joints that produce complex or dextrous movements, like the shoulder or the wrist.
That doesn't answer the question, though - why do we feel exhaustion even when we haven't been doing physical work, and when we've just been sitting still?
The answer to that question lies in an important distinction - big and small movements. A man flexing his bicep produces easily identifiable movement in the muscle - the belly of the bicep swells and rounds out, to a greater or lesser extent depending on the individual's level of training. However, what we don't see is the action of the forearm, which subtly rotates inward due to the pull of the bicep against the bone of the radius. Nor do we see the action of the brachialis, the brachioradialis, the coracobrachialis or the supinator - all of which work alongside the bicep to produce the movement. The bicep steals the show from these other actors in the game, all of which play a contributing action, and without which the bicep would have a much harder time of things doing it alone.
This principle is applicable to every single other movement the human body produces. Movement of a joint, bearing a load or engaging in any activity that requires repositioning of the body is only possible due to the combined synergistic action of unseen muscular players alongside the main driver. These members of the anatomical chorus assist and refine the movements produced by big-volume muscles like the bicep, the muscles of the quadriceps, the chest, and the shoulder as well as many more, and allow us to engage with the world in a manner that is both purposeful and effective. The distinction is pretty obvious - sexy muscles like the biceps, the pectorals, the quadriceps and the calves can only do their work thanks to an arrangement of unsexy muscles that lie hidden beneath the bellies of their more burly brethren, silently doing their work to keep things running.
We still haven't answered the question, but don't worry, we're getting there.
Inside the Situation
We've talked about how muscles act to produce movement, and now it's time to talk about how muscles act to maintain position. When sustaining posture, position, or orientation of part of the body, the body needs to maintain a base level of muscular activation at all times. This baseline level of activation is called muscle tonus - a constant engagement required to engage with the world at all times. Some muscles within the body need to maintain a higher level of tonus at rest relative to others, because of the joints they cross. The shoulder joint is a good example - it is a highly mobile, very dynamic joint made up of the rounded head of the humerus sitting within the shallow bony cup of the scapula. This loose bony configuration is reinforced by a scaffold of muscles that run over, under, and around the shoulder blade. If those muscles should lose all tonus, the head of the shoulder will slip out of its joint and sag downward, pulled away from its original seating due to the action of gravity. The muscles of the shoulder are constantly on, constantly working, every day of our lives and even in our sleep; first to ensure that the joint stays stable and then to ensure that it can work appropriately.
Every muscle runs at a baseline level of tension, regardless of if you're lying down, standing, or sitting. In fact, in those postures, the level of tension increases. During sitting, for example, the legs no longer do the work of bearing the weight of the body. That work is instead done by the pelvis, which interacts with the surface on which we sit directly with a little cushioning from the backside and the muscles of the hamstrings. Normally, the muscles supporting and coming from the pelvis are responsible for managing the load of the upper body in coordination with the lower limbs, but now the legs have been largely removed from participation. Instead of the muscles of the legs, the muscles of the lower back; the multifidus, the paraspinals, the quadratus muscles, and the erectors all need to balance the ribcage, the shoulders, the skull, and the arms, all of which may be doing their own thing depending on the movements being performed or the conformation of the body. Every single muscle is under tension. Every single muscle is working. We just don't feel the strain in the immediate moment because when you're inside the situation, it's not easy to notice all the extra work you're doing to maintain the status quo.
The above example assumes that no external forces are acting on you while you're working. Driving a car is a tough job because you need to keep your hands on the wheel and your eyes on the road. The mechanical process of holding a gaze is one that requires tiny engagements of the muscles that direct the movement of your eyes, alongside the adjustments of the muscles holding your skull in place, bracing your shoulders, holding posture, and maintaining pressure on the accelerator, the brake, or God forbid, the clutch. As humans, we have a very poor sense of what is going on within our bodies, especially when we attend to things beyond the immediate milieu of our engagement. Our attention isn't like a radar that scans constantly and picks up things within a radius, it's more like a spotlight that has a very narrow beam outside of which items of interest may not receive appropriate illumination. There's all types of work that we think of as low-engagement that in fact requires spending energy. Concentration, decision making, thinking, writing, reading, all require extra work to stay in place.
Existing needs Energy
As organisms that live within the world, we are obliged to do certain things like feed and sustain ourselves. However, the act of sustaining ourselves is itself an investiture of energy. For some people this is easily absorbed into the bandwidth of their lives, it doesn't take much for them to meet the physical, physiological, or psychological demands of the world around them. For others, those with neurodivergent presentations, those in nonstandard social states or those folks who just might be having a rougher time than others, the necessary investiture can become overwhelming. The physical act of living incurs a charge that has to be paid daily - in attentiveness to our own needs, in self-repair and self-sustaining activities and reflection, and in using those scarce dregs of spare time we have to engage in whatever activities we might think of to slake the thirst of our souls. Being imposes demands - living is an active process and so existence needs energy. We feel the debit eventually, whether it's in our bank account or our brain. It's on us to be our own task manager and assess ourselves - to see what's coming up short and how well we can address the needs of our daily lives, because the first and greatest obligation we have is to ourselves and our health. With that out of the way, it makes it that much easier for the rest of it to fall into place.