Tension is a partial, temporary hyperactivity that decreases with distance and time, going back to normal after serving its function. Body tension is a must for every organism to move on its own, without support by other organisms or natural forces. Lifeless objects too need tension to transform into a kinetic energy.
Thus humans, as part of nature, can't/needn't be tense for long times. Tension is mostly needed for certain occasions/parts (the critical few) requiring max. energy, e.g. full body alert. It takes great effort yet achieves great change, physically, mentally & socially; otherwise, it's painful and energy-wasting.
We tense up or relax voluntarily & involuntarily, as there are muscles and organs we can control and some we cannot, although the line between these two gets finer, from one person to the other, when applying self-control techniques. Voluntary tension is that of muscles, tendons and joints. Involuntary tension is that of inner muscles, heart, lungs, intestines, as well as body chemicals: blood, hormones, neurons, etc. (their molecular density and speed).
Both tension and relaxation are forms of inner speed, where we move inward rather than outward, with no need to leave our place. Tense movements happen faster than relaxed ones, thus relaxation always takes longer to reach than tension. When we loosen up, we move the same tense muscles in the opposite direction, away from joints, valves and centers of movement. Contrarily, when we tense up we move directly toward those centers of tension. When we relax we move closer to stillness and further from motion; whereas, when we are tense it's difficult to stay still. The outward movement of a muscle is naturally slow and soothing, taking longer to reach, as opposed to tensing up that can be instant and painful. Figuratively, relaxation is walking, tension is running, inwards.
With practice, we can tense up or loosen up on demand, as easily as taking a step or raising a hand, even to control involuntary muscles and inner organs. Through the cycle of tension & relief, we direct more energy inside, toward muscles and inner organs; rather than outside, toward "joints" causing actual movement from place to place.
Mixing tension, speed & stimulation together achieves maximum alertness, as each completes and leads to the other. Physically, they induce the body to release adrenalin which in turn increases heart-beats, breathing, perspiration, pupil dilation, etc. Stimulation (of "senses & nerves") is the easiest, safest alert tool, causing minimal tension and speed, mostly needed when the latter two aren't available or desired (consuming more energy & causing aftermath effects taking long to settle). There are many ways to stimulate the mind, e.g. provocative thoughts, inducing fear or desire; and to stimulate the senses, e.g. exposure to light, water, coldness, coarse surfaces ... and assuming certain postures: upright, lotus, Burmese, upside-down, etc.
However, all the above alert tools are temporarily effective. For long-term alertness, they need to be preceded by sufficient SLEEP and resting first.
• Health: The tension used in building muscular tissue is vital for bone protection, supporting spinal cord, fast metabolism, increasing stamina, etc. as in compulsory physiotherapy treating weak limbs, overweight problems, etc. The very endorphins released after exercise help improve sleep afterwards. Even involuntary inner muscles need some "enjoyable" tension for better performance: swallowing, excretion, labor, lactation, etc.
• Strength: Technology spared modern humans much of the physical effort their ancestors relied on. Yet, because technology is not always available, efficient, safe, or cheap, we may resort to our natural body-machine for help, being more PRACTICAL. Without tensing up we can't do activities requiring physical strength: pulling/pushing/carrying heavy objects "more & faster"; sustaining strenuous positions; and many sports, jobs, chores, or even games and arts, requiring various degrees of effort.
Such strength is vital for SECURITY sometimes: saving/lifting a loved one (trapped/injured/unconscious/disabled/weak/slow); sustaining a life-threatening position (accidentally hanging from a building/tree/cliff) or movement (walking/running/swimming "away" from danger); overpowering an attacker (wisely, human/animal); removing a sudden obstacle (big rock/fallen tree); pushing a heavy cart, stopped vehicle, etc.
• Control: The "abdominal muscles" are the place to achieve maximum body-control when they tense up, thanks to their central position in the human body and mutual support of the spinal cord (the latter being also useful in mind-control, acting as a messenger between the brain and the rest of the body). We should pay extra attention to our abs. by doing them the favor of exercise, not by burdening them with carrying extra fats, having a full stomach, bladder or rectum, wearing uncomfortable clothes, sitting in wrong postures (harming the spine supporting them), or whatever affects the sensitivity of such dear muscles.
• Balance: Although a tense/stiff movement is generally painful, we can always be tense partially and safely, by shifting tension between different body parts, which helps us remember this great survival tool we constantly have. Some tension is always needed for:
• Alertness: Muscle tension is a quick shortcut to physical alertness (esp. the "jaw muscle," the strongest muscle in the human body, e.g. by "teeth-gritting"), increasing blood supply to all body organs, more importantly increasing the oxygen going to the brain. A bit of adrenalin you control does the same job of the caffeine in a cup of coffee. Mixing body tension with body speed and sense stimulation maximizes alertness.
• Self-Confidence: Stiffening up temporarily boosts confidence to endure physical and mental pain, as tension relatively "suppresses emotions, curbs movements, dulls senses, and minimizes (focuses) thoughts." Suppression is good "sometimes" to face an enemy/danger/crowd, instantly overcoming shyness, hesitance, diffidence, sadness, boredom, etc. It only helps for a short time, after which we can allow ourselves more freedom to move, think, feel and enjoy. Long tension causes pain, irritability and apathy.
Tension exercises can dramatically change and improve APPEARANCE, boosting self-confidence, socially and sexually. This is almost the prime motive for exercise for many, if not most, people, despite being a superficial motive, as appearances affect our life less and "appearance standards" differ from one era/person to another, esp. that technology can gradually change our looks better and faster than long tedious exercise.
• Escape: Physical stiffness is useful in circumstances where we can't release our inner energy immediately, as when stuck in a stressful/embarrassing/boring environment (with equally boring company) we don't have the power/courage to leave immediately. Then, we stiffen up and leave, physically, or at least mentally. Similarly, when forced to stay alone/indoors for long times, tension exercises can give us diversion, by "moving inside." We can "cure" mental stress by physical tension, to escape a gloomy mood/future fears or cope with a past loss. Physical tension will feel milder, by comparison, with the emotional pain.
• Energy-Saving: Because life is full of such overly-long situations, physical stiffness helps the mind in sparing us all the "dillydallying," energy, time and nerves that could be lost, therein.
• Momentum: In nature, solids are heavier and slower than liquids and gases, that one cannot speed up much while in a stiff petrified solid mode. Yet, although tension consumes inner energy that leaves less "fuel" for outer motion, it is only for a short time giving our body and mind a "spark" to start our engine.
• Privacy: Stiffness is good for keeping one's privacy. Sometimes, when we choose, voluntarily or not, not to move a foot or lift a finger or show the slightest facial expression, we can still move "in place" in a way barely noticed by others, using the two inner speeds of tension and relaxation (tenseness & looseness).
• Mood-Control: The muscle alternation between "tension and relaxation" has the benefit of indirectly controlling and changing our mood by simple unnoticeable exercises. We can instantly evoke feelings such as enthusiasm, courage, anger, etc., by the tension of certain muscles. We control anger sometimes by tightening jaw muscles (chewing imaginary chewing gum). We control boredom/drowsiness/apathy also by voluntary repeated movements to free the energy locked up inside us (e.g. shadow-boxing), that we couldn't release otherwise. We can equally evoke the opposite feelings (calmness, apathy, diversion ...) by relaxing the same or different muscles, and by other self-control techniques, such as breathing control, mantras and repetitions.
• Pleasure: Tension isn't a pleasure, yet some tension helps relieve pain ("a bit" of salt stimulates taste-buds and makes food tasty). Even relaxation exercises require applying some pressure to the aching muscles and tense areas (temple, lower-neck, lower-back, armpits, perineum, and most joints) to free the pain (bad energy) locked in there. As for very high-tension activities, e.g. power exercises, the pleasure part is mostly felt AFTERWARDS, when the body rewards us with a high rush of endorphins for enduring such tension. Shifting tension between body parts simultaneously, and using timely breaks between exercises/sets/reps help achieve this.
Those who balance using body tension at the right "time and place" are the most to enjoy it. Those who accept pain as part of life they can turn to their advantage are "emotionally intelligent" people.