The Science of Punch Power II

[Le Professeur]

As for the muscle itself, three constraints shape the entire striking technique: it generates less force when it shortens rapidly; it takes time to develop its maximum force; and it slows itself down when it is tense. An effective strike is one that overcomes these three limitations.

The first is the trade-off between force and speed, long described by the physiologist Hill: the faster a muscle shortens, the less force it can exert; and mechanical power – the product of force and speed – is greatest at intermediate levels of force and speed, at around one-third of their respective maximums. For a punch, this means that the load moved at the end of the arm is light – and therefore operates at speed – whilst the lower body pushes a much greater mass, operating under force. This is yet another argument in favour of the kinetic chain: the large muscles of the legs and trunk provide the force, whilst the distal segments provide the speed.

The second constraint is the time it takes for force to build up. A muscle takes several hundred milliseconds – around three to four hundred – to reach its maximum force, whilst the propulsive phase of a strike lasts only a few hundred milliseconds at most. We therefore never reach our muscles’ maximum force during a stroke: what matters is the rate at which the force builds up, not just its maximum level. This is why training focused on speed and ballistic movements is justified.

The third factor relates to the stretch-shortening cycle I mentioned: a muscle that is stretched just before contracting produces more force, because the elastic energy stored in the tendons and the stretch reflex are added to the voluntary contraction. For this benefit to occur, the transition between stretching and shortening must be very brief. In a punch, this stretch comes from the separation between the pelvis and shoulders, the slight prior flexion of the supporting leg, and the withdrawal of the opposite arm. A visible pause before the strike negates the benefit, and this is also what telegraphs the blow. Research also indicates that when the trunk and lower body are fatigued, striking power decreases for all strikes.

Next comes the question of relaxation. When a motor muscle contracts, its antagonist also contracts to some extent: this is co-contraction, which stabilises the joint but slows down the movement. The rule is therefore not to use more muscle tension than the technique requires, as tense muscles do not move quickly, and not to clench the fist before impact. High-energy strikes are loose and fast, and the flick of the arm – known as the ‘snap’ – helps to keep the muscles relaxed. This gives rise to a classic piece of advice: strike through the target. When you aim for the surface, you anticipate resistance and tense your muscles just before impact, which slows the fist down. When attention is focused on a point beyond the target, the muscles remain relaxed and the fist reaches its maximum speed. The argument is plausible and aligns with what motor learning research says about the effects of directing attention outside the body, but it has not been established by measurements in this specific context. It is a useful mental cue for eliminating anticipatory braking, not a law of physics.

Breathing completes this picture. Two facts are well established. Firstly, a brief, forceful exhalation against a stationary ribcage creates pressure in the abdomen which stiffens the torso at the moment of contact – that continuous, rigid path between the fist and the centre of mass that I mentioned – and this is the function of vocalised breathing in combat sports. Secondly, breathing influences the speed of the movement: one cannot move faster than one’s exhalation, and inhalation is a moment of vulnerability, with a longer reaction time and reduced resistance to impact. The Chinese internal arts advocate a reverse breathing technique, in which one inhales through the nose whilst drawing the abdomen in slightly without expanding the chest, then exhales forcefully through the mouth whilst expanding the lower abdomen, the flanks and the lower back, with approximately two-thirds of the breath being expelled at the moment of the strike. It is also advised to execute an entire sequence on a single exhalation, because each inhalation between strikes creates a moment of weakness. More far-fetched claims, such as those of doubled strength, an ‘iron shirt’ or energy circulation, are not supported by any evidence. In practical terms, simply breathe in before the strike whilst remaining relaxed; begin exhaling the moment the rear leg pushes off and the pelvis rotates; keep the exhalation audible and the glottis open rather than holding your breath, as holding your breath whilst fully contracted raises blood pressure and causes rapid fatigue; and keep about a third of your air in reserve to remain stable after impact.

Let us now turn to the impact itself, for at the moment of contact it is no longer the speed of the fist alone that counts, but the way in which mass, duration and contact area combine. Measurements taken from Olympic boxers across several weight categories, striking an instrumented dummy at jaw level with a rear straight punch, yielded an average force of approximately three thousand four hundred newtons over eighteen strikes, a hand speed of nine metres per second and an effective punch mass of less than three kilograms. The acceleration of the dummy’s head averaged fifty-eight times the force of gravity. The most instructive finding is that the force was higher in the heavier weight classes, mainly due to a greater effective mass rather than greater speed: the middleweight boxer, the fastest in the series at nearly twelve metres per second, actually had the lowest force, whilst the super-heavyweights exceeded four thousand three hundred newtons. These data are based on small samples, using gloves and a dummy, and are only valid as orders of magnitude and as a general trend. Other studies fall within the same range: among elite boxers, approximately 3,150 newtons for the rear straight punch, 3,000 for the hook and 3,200 for the uppercut – roughly three times the figures recorded for junior boxers, a discrepancy that persists even when force is normalised for body mass and thus points to technical quality. A more recent study of trained amateur boxers, using wearable sensors, measured a peak of over 3,000 newtons and nearly 10 metres per second for the straight right following a conditioning protocol involving dumbbell throws.

As the methods differ, these values cannot be compared to the nearest newton.

The effective mass is that which, if propelled at the measured speed of the hand, would result in the amount of momentum actually transferred during contact. Averaging less than three kilograms, it represents only a few per cent of a boxer’s mass – a far cry from the ‘half’ figure often heard in some gyms. The two figures do not contradict each other, but the more rigid, anchored and aligned the body is at the moment of contact, the closer the effective mass comes to the mass set in motion, and this is precisely what the body structure I mentioned earlier is all about.

The duration of contact deserves a final word. The force curve of a punch comprises a brief initial peak followed by a weaker tail as long as the puncher continues to push. We can formulate a principle: the raw power of a punch is greater the shorter the time the fist remains in contact. A strike that makes contact too early, whilst the elbow is still very bent, turns into a push. Both components have their uses, as the energy peak causes local damage and the trailing push adds momentum. A follow-through – that is, a continuation of the force after impact through the extension of the limbs and the push from the rear foot – thus generates additional momentum. This leads to a practical test: if a heavy bag is sent flying a long way under your strike, it means you have pushed; whereas a well-focused strike causes it to sway only slightly and shakes it more or less on the spot. This is an empirical criterion rather than a quantitative one, but it is consistent with the distinction between energy and momentum.

Traditional teachings distinguish between four main types of impact. A focused impact corresponds to most punches: the fist comes to an abrupt halt on contact and the force is transmitted through the target. A piercing impact concentrates the shock on a fine point, such as a strike with the fingers or second knuckle against a soft surface, and the small contact area exerts high pressure. The snap-back impact, typical of a strike with the back of the fist against a hard or bony surface, delivers a strong impulse during very brief contact followed by a rapid return. Finally, the crushing impact characterises the boxer’s wide fist, the forearm strike or circular kicks, involving a great deal of momentum and little focalisation. We also distinguish between three types of target surface based on anatomy: soft, such as the abdomen; hard, such as the head, wrists or fingers; and hard-soft, such as the thighs or forearms, as the same blow does not have the same effect on each. The term ‘hydrostatic shock’ is sometimes used to describe the effect of a focused impact on a body composed of approximately sixty per cent water. The analogy is qualitative and borrowed from ballistics; in head biomechanics, however, it is linear and, above all, angular accelerations that predict injuries.

The contact area affects the pressure, which is force divided by area. Paint-transfer measurements of fist impressions yield approximately eleven square centimetres for the first two knuckles on a hard surface, fifty-seven for the entire fist on a soft target, sixty-six for a free-fighting glove and one hundred and fifty-two for a boxing glove. Assuming a force of three thousand newtons distributed uniformly – which is a simplifying assumption – this gives an average pressure of approximately two and a half megapascals for the two knuckles, compared with five tenths for the whole fist, four tenths for a free-fighting glove and two tenths for a boxing glove. Area has no effect on momentum, but it determines the concentration of energy: a glove reduces cuts and bruises without reducing the transfer of momentum to the head, which means it does not protect against diffuse head trauma. The bare fist, on the other hand, concentrates the pressure more on the target and on the hand, leading to a behavioural effect: protected hands become invulnerable and allow one to strike much harder on hard targets such as the forehead, whereas with bare hands, the fragility of the hand naturally limits one’s willingness to engage. As for the added mass of a glove, it has a noticeable effect only on strikes where the bulk of the movement comes from the arm: in rapid sequences of strikes in the Wing Chun style, a drop from seven strikes per second to around five has been measured when using boxing gloves, whilst the rhythm of strikes delivered through the rotation of the torso or the whole body remains unchanged. A weight of a few hundred grams slows down an isolated arm, not the whole body.

Let us move on to the technique itself, as all punches share the same logic of support, rotation, extension and structure on impact, and differ only in their trajectory and the dominant limb. First, a few common principles. There must be no visible preparation: winding up the punch, raising the elbow or clenching the fist before the attack gives away your intention, and a good exercise is to practise in front of a mirror to eliminate any tell-tale signs – a foot that moves, a shoulder that drops, a jaw that clenches. The fist is twisted a quarter or half turn just before impact, which causes the force to spiral and produces a final acceleration, whereas twisting too early slows the fist down before contact. You strike with the first two knuckles and the right wrist, whilst the elbow remains slightly bent on impact, with around five centimetres of extension still available; if it is too close, the strike becomes a push. The fist then returns along the same trajectory, without dropping the hand, and you exhale once per strike or per short combination.

The straight punch from the rear arm – the cross – is the most powerful strike in the boxing stance because it engages the longest chain of movement, from the rear foot to the shoulder via the leg, the hips and the torso. It is delivered from a closed guard, with weight slightly forward, without the arm or shoulder pulling back. The back foot pushes against the ground, the back leg extends, the back hip moves forwards, and the back knee follows the direction of the strike without twisting inwards too much. The hips rotate first, the torso follows with a slight delay, the front leg plants firmly and acts as a pivot, and the head moves slightly forwards and to the opposite side to shift the weight behind the punch. The shoulder then moves forward; the elbow remains in line with the shoulder without opening out; the arm straightens; and the fist closes at the very last moment. On impact, a continuous line connects the fist to the back foot: fist, wrist, elbow, shoulder, torso, leg – the right wrist, the elbow slightly bent, the abdomen tight. The fist then returns along the same path and the guard closes. The mistakes that ruin this strike are always the same: a raised or open elbow through which power escapes, a bent wrist, a raised shoulder, a torso that leans forwards, a rear heel lifted off the ground without rotation of the pelvis, or a strike delivered using only the arm.

The straight punch from the front arm, the jab, is less powerful for the same reason as the front kick: less acceleration distance and less mass engaged. It is mainly used to disorient the opponent and gauge distance, but if executed poorly, using only the arm, it has no power. It is delivered from the rear hand near the jaw, the front hand near the centre line and the elbow tucked in, with a push from the rear leg that causes the front hip to snap slightly, the hand moving before or at the same time as the foot. The fist rotates during its trajectory and strikes with the first two knuckles, the body forming a straight line between the fist, the back and the rear foot upon impact. Some instructors teach a stiff straight punch, with the arm locked for a fraction of a second to feel the impact travel down to the back foot; however, this should be practised with caution when unarmed, as a locked elbow leaves one vulnerable to hyperextension.

The hook is a strike generated by rotating the torso, not the arm. The torso rotates first via a shoulder roll, the front foot pivots, the hips turn, and the arm follows. The elbow is positioned in line behind the hand, level with the fist at the moment of contact, as raising the elbow too early gives the strike away. The fist should ideally be vertical, with the palm facing inwards, to strike with the first two knuckles; bear in mind that with a horizontal fist, the hand risks breaking if it is not bandaged, whilst the optimal orientation remains a matter of debate: it is best to try both under supervision. Your weight should follow the rotation: backwards when you step back, forwards when you step forward. An effective exercise involves alternating a deep rear straight punch with a hook, hundreds of times, whilst aiming to maximise the difference between the two final positions: the rear straight punch, in fact, builds up the body’s torsion, which powers the hook.

The uppercut is a close-range punch that relies on the stacking of the body segments – foot, knee, hip, elbow, fist – in a line that is as straight as possible upwards. In line with the upward motion, the hips are slightly lower at the start and then rise straight up in time with the punch, without straightening the legs or lifting the chin. A variation aimed at the body, the shovel hook, starts from the centre of the body with an inward trajectory of about thirty degrees and minimal arm movement. As for the lunge punch, it is considered the most powerful because it moves the greatest amount of mass: a squeeze of the adductors brings the back leg sharply forwards, then the back foot pushes off and the punch is delivered, with the striking hip rotating by approximately forty-five degrees. The risk is a loss of balance and, consequently, as we have seen, the generation of momentum without a firm base. Leaning the upper body forward, lifting the back heel and looking down are the three mistakes to avoid.

The open hand also offers more surface area than a clenched fist, which is a hard, small surface—therefore powerful for the target but demanding on the hand. The palm, with a surface area of approximately sixty-five square centimetres—significantly larger than that of the knuckles—reduces the pressure on the hand whilst maintaining the same force. The hammer strike and the back of the fist are strikes with a curved or whipping trajectory; the back of the fist corresponds to the ‘snap-back’ impact, involving very brief contact and a strong impulse on bony surfaces, thanks to the whipping action of the elbow and a rapid return of the hip. The edge of the hand and the spear-hand strike at narrow areas of the throat or unprotected zones; as it is virtually impossible for a finger to literally penetrate the flesh, the intended effect is localised pressure.

That leaves the matter of training. The power of a strike is developed by working on four qualities, in this order of priority: technique and sequence, lower-body strength and power, rotational power of the torso, and finally the strength of the hands and wrists. Studies agree on this point: it is recommended to develop leg power to facilitate the sequence of movement from the ground to the fist, and the speed of the bench press at eighty per cent of one’s maximum has been found to correlate with fist speed, suggesting a transfer of strength training to striking, even if this is a correlation rather than proof of causality. Furthermore, one can possess strength without knowing how to use it; this is why exercises must be chosen for their relevance to the movement. The following are general guidelines for physical preparation, to be adapted in consultation with a coach, and do not replace the advice of a sports doctor.

A few key principles apply. For power, training should be short, fast and performed whilst fresh: few repetitions per set, a focus on maximum speed, and a full rest between sets – particularly as fatigue in the core and legs reduces the power of every strike. A foundation of maximum strength, achieved using heavy loads, underpins the power generated with light loads and speed, in line with the relationship between strength and speed. Power is generated from the ground through the core, so a powerful rotation is required, along with a core capable of remaining rigid on impact. Hands and wrists adapt more slowly than muscles, so the number and intensity of strikes are increased in stages. And one must never strike a rigid, stationary object, as a wall, a tree or a post will reflect all the force back into the joint.

For the lower body, squats and deadlifts, performed in sets of three to six repetitions at seventy-five to ninety per cent of one’s maximum, build the strength that fuels the ground-to-pelvis transfer; whilst jump squats or counter-movement jumps, performed in sets of three to five jumps with no or light load and an explosive intent, develop power. Bulgarian lunges and bench step-ups, which work each leg separately, are useful as the front and rear legs have different roles. For rotation, medicine ball throws against a wall – using a ball weighing two to four kilos and four to six throws per side – replicate the sequence of legs, pelvis and trunk. For the upper body, throwing a ball or a light dumbbell, fast bench presses where the bar is propelled quickly, and explosive push-ups or fist push-ups – which also engage the wrists – develop pushing power. In the recent study I mentioned, a dumbbell throw performed before a rear straight punch improved fist speed by more than one metre per second after eight to twelve minutes’ rest. Pull-ups and rowing balance out the pushing movements and protect the shoulders, whilst the plank and anti-rotation exercises such as the Pallof press prepare a rigid core for impact, and carrying loads, the wrist roller or the traditional weighted stick strengthen the forearms, wrists and grip.

For the strike itself, the heavy bag should be worked with impact rather than a push; if it moves far away, it means you are pushing. An auditory cue helps: all power strikes must produce the same sound, and the first strikes must sound the same as the subsequent ones. When practising ‘bear paws’, the holder positions the ‘paws’ in a V-shape for most alternating strikes, and in an L-shape for the straight punch, as too acute an angle injures the striker’s wrist; the holder also pushes the ‘paw’ slightly towards the striker on impact to protect their own shoulders and elbows. A typical programme consisting of three-minute rounds begins with the straight punch, the double straight, the straight followed by a cross, the straight-cross-hook, about twenty repetitions of each or until a consistent sound is achieved, then moves on to sets of three strikes, dodges, swings and slides, a dodge followed by an uppercut-hook, with each round ending with thirty seconds of rapid-fire straight punches. The traditional striking post, or makiwara, is made from a tapered piece of wood ten centimetres in diameter, with its top at the level of the solar plexus, covered with approximately five centimetres of firm rubber, and it must bend by about fifteen centimetres when pushed firmly; to begin with, twenty to twenty-five strikes are delivered to it per hand, without starting with one’s best strike, and any wounds must be treated before resuming. Claims that it increases bone density are not based on solid evidence, and fracture prevention is better achieved through bandaging and good technique. Finally, you can test the speed and accuracy of your strike by striking as quickly as possible, stopping your fist about five centimetres from a flame to extinguish it through the movement of air alone – taking care with your choice of location and clothing – or by bursting a balloon thrown into the air with a single strike. These two tests measure speed, not impact power.

Over twelve weeks, at a rate of three or four sessions per week, the training can be organised into three phases. The first four weeks are devoted to technique and strength, with slow, aligned strikes, shadow boxing, squats and deadlifts with moderate loads, core work and initial strikes on a soft bag. Weeks five to eight mark the transition from strength to power, with heavier loads, jumps, rotational throws and footwork practised with the intention of delivering a strike. The final four weeks focus on speed and combinations, with light, rapid loads, short combinations performed in a single exhalation, and tests on the punching bag and the ‘candle’ drill. A typical week in the second phase might include a lower-body strength session followed by light technical work; a power session with jumps and rotational throws followed by strikes on a heavy bag whilst still fresh; an upper-body strength and core stability session; and, if possible, a light technical and mobility session to aid recovery. Power exercises are always performed at the start of the session, before fatigue sets in.

Certain mistakes recur time and again and result in a loss of power. Striking with the arm alone shortens the acceleration distance and reduces the effective mass, whereas the movement should originate from the foot, the hips and the core. Leaning forwards or backwards causes a loss of balance and directs the force back towards oneself. Lifting the back heel without rotation, or locking the back leg, causes the ankle or knee to absorb the recoil. Opening or lowering the elbow, raising the shoulder, bending the wrist, clenching the fist too early, locking the elbow on impact or rotating too far away from the hips causes the force to dissipate, creates dangerous moments of flexion or transforms an impact into a push. Visibly winding up a strike gives it away; holding your breath or inhaling at the wrong moment weakens the core; and relying on a single ‘best’ strike makes you predictable.

Much of what is said about striking does not stand up to scrutiny, and now is the time to sort the wheat from the chaff, for the universe takes no account of the rank or authority of the speaker. The idea that a peak value sums up the force of a strike is misleading, since force is a curve and the energy and momentum transferred tell us far more. The claim that one puts fifty to sixty per cent of one’s body mass into a strike is imprecise: the actual measured mass is in the region of three kilograms. On the other hand, the fact that power comes mainly from the legs and trunk is confirmed by analyses of force and kinetics. The role of the opposite arm is plausible, explained by the counter-rotation of the torso rather than by a transfer of force. The belief that gloves reduce the risk of brain injury is false or questionable, as they reduce local injuries but do not prevent the transfer of momentum to the head. As for explanations involving the dantian and qi, these are unsubstantiated and constitute pseudoscience in the strict sense, even though the area below the navel does indeed correspond to the body’s centre of mass, which lends a physical meaning to the instruction. Accounts of strikes that pass through the body and leave black handprints are anecdotal; the idea that striking a post increases bone density and thus strength is unsubstantiated, even though the principle of bone remodelling is plausible; the claim that reverse breathing doubles one’s strength is not quantified, even though the effect of stiffening the torso is real; and the famous ‘one-thumb strike’, presented as capable of producing an impact of several thousand newtons, is more likely to be a push.

That leaves the protection of the hand and wrist, which is the specific constraint of bare-handed striking. The classic injury is a fracture of the neck of the fourth or fifth metacarpal, near the knuckle. There is less and less talk of a ‘boxer’s fracture’ and more and more of a ‘brawler’s fracture’, because professional boxers, with their hands bandaged and gloved, rarely suffer from it. A hand that is compressed and stable before impact sustains fewer localised injuries, hence the importance of bandages and technique. When fighting bare-handed, the forehead and hard bones are the most dangerous targets for the knuckles, as the energy is concentrated over a small area: it is better to use the palm or a wider surface against bony targets, and to reserve the knuckles for softer targets or a perfectly aligned fist. A cut to the knuckles caused by a tooth is a wound with a high risk of infection, which must be seen by a doctor promptly. Progression must be gradual, with no full-force strikes against hard surfaces from day one, and training should be stopped as soon as any joint pain arises. As regards the head, since neither gloves nor helmets reduce the transfer of momentum, they do not protect against diffuse head trauma, and the best prevention remains limiting repeated full-force blows to the head. Finally, from a legal perspective, self-defence is lawful whilst fighting is not, with rules varying from country to country: this text deals with physics and technique, not the law.

A powerful punch is a sequenced transfer of energy and momentum, which originates from a stable base, travels through an aligned structure and becomes rigid on contact, with the maximum effective mass and the shortest possible duration of contact.

None of these concepts are secret: they can be verified in the laboratory as well as in the simple tests described above. If we were to summarise the key points, we would say that one must strike with the body and not with the arm, since three-quarters of the force comes from the legs and the torso; one must push off the ground with a firm thrust from the driving foot without locking the knee; use the front leg as a pivot to stabilise the pelvis and transfer the rotation to the torso; rotate the pelvis before the shoulders to make the most of muscle stretch, align the fist, wrist, elbow and shoulder whilst keeping the elbow slightly bent, remain supple during acceleration then rigid on contact by closing the fist at the last moment and exhaling sharply, aim for speed as doubling the speed quadruples the energy, keep contact brief unless you want to displace your opponent; never telegraph your punch; choose your striking surface and target; prioritise quality over quantity in your training; and test each principle for yourself on the punching bag, against the pads, on the cardboard target or with a partner.