1.1 Performance and health
Ergonomics is the science of performance and health. Wherever humans interact with equipment to perform a task, ergonomics will try to adjust the equipment to the human, rather than the other way around. The goal is not only to get the best performance, but also to reduce the effort and to avoid that human limitations are neglected. While playing guitar, the limits are approached in the physical domain in terms of left hand dexterity, finger power, right hand motion frequency, speed-accuracy trade-off, right-left hand coordination, etc. But also perceptually some limits are near, such as sensitivity for tuning and timing error and the sometimes immense cognitive complexity of playing together.
Challenging the human limits involves training effects. Humans do not wear from training, but recover after every training session and often to a slightly higher performance capacity than before, known as super compensation. There is a natural, continuous decay of skill during inactivity, which is counteracted by the super compensation. For any training intensity a floating performance level results, higher as the training is more intense. This is one of the reasons that professional musicians practice for hours each day.
The constant challenge of limits is at the brink of affecting health. As long as the human body is in a regulatory range, the damage of the training (tiredness, soreness, immune response) will be repaired. A condition for the regulatory range is that sufficient relaxation interrupts activity. One important barrier for recovery is when joints are loaded during motion, resulting in Repetitive Strain Injury. This is a potential career stopper for musicians. Awkward hand positions, mental stress (which increases constant muscle tension) and repetitive motion do not go together well. Neither do static loads and extended times. Picture a tense guitarist, standing slightly bent over a not well balanced guitar and forcing his hand to a difficult chord and you have the prototype of one who will be in trouble sooner or later. Genani et al (2013) concluded that damage to the motion system is more prevalent in guitarists than players of any other instrument, with posture and balance as key factors.
While designing a guitar much can be done to ease the guitarist. That starts with a good balance and a guitar shape which accommodates the body, both standing and seated. That will relax static forces which otherwise are applied to balance the guitar, competing with the freedom of the fretting hand to move. Next is the contact area, which should comfortably touch the right arm and assure stability of position. It should offer a stable base for the right pinky while finger picking. Next, the left hand should be relieved from wrist bending, associated with playing at either end of the fretboard. And it should avoid excessive force, such as when the left thumb presses the lower string or when a barre chord is held. Finally, playability parameters like string tension, string spacing and action must be controlled, depending on hand anthropometry.
1.2 Wrist position
The wrist should be almost straight to be relaxed. It can be observed that the best guitarists in all styles have their wrists indeed straight or elegantly slightly bent. The wrist bending is the result of a couple of coherent factors. The most important factor is the desire to look at the fretboard. Also the very best guitarists do that, but there is a difference between the experienced and inexperienced players. The inexperienced want to look from the top, demanding either that the head is brought forward, looking down, or that the guitar is turned with the top a little upward. Both have their problems. Head bending increases the tension on the back (and offer an introvert picture to the audience), while turning the guitar demands extra bending of the wrist. The experienced players look from the side. That makes a lot of difference for the attitude, as their backs can be straight, but the view on the fretboard is limited. Apparently, experience compensates for the missing view.
Another difference is that classically trained players hold the guitar with the neck up and close to their bodies (Fig 1.2.1, left). The upright position is the key factor in left wrist release. They make little use of the left thumb to grab a bass note, which would be awkward, and play with their fingers overhand to the bass strings. The fingers run over the full length and width of the fretboard. The thumb is used at the far end to increase the grip on the neck, but near the neck joint the hand position makes that more difficult. For that reason the guitar is put on the left leg. This shift turns the left hand in an easier position. But it also positions the right forearm over the widest part of the lower bout. A sharp edge on the guitar body may pinch off the large veins of the arm. In an attempt to solve this, the edge is sometimes beveled or rounded. Slightly tilting the guitar face-up helps as well, as the forearm then runs parallel to the top, but must be traded against detrimental left wrist bending. Also, the right forearm is not supposed to touch the top, on the penalty of absorbing resonances.
The more popular position is with the guitar on the right leg and the neck almost horizontal. Due to the shift to the right, chords can be easily grabbed, both with the thumb on the bass string (Fig 1.2.1, right) or as a barre chord.
Fig 1.2.1. Wiktoria in classical position (with elbow brace), left, and Chet in popular position with a thin body guitar, bending over (right). The support point is indicated by a ¤. Note that in the classical position the guitar sits farther to the right of the player.
Playing treble notes gets harder since the left hand is in front of the body, with the index much further from the fretboard than the little finger. Trying to rotate the hand counterclockwise causes high stress on the wrist. Lifting the whole guitar makes the positions of both left wrist and right forearm more comfortable (Fig 1.2.2, left). The left arm bends in the elbow, a remainder of the classical position. The right upper arm rests on the side of the guitar. Another frequently used solution is to push the neck away from the player, with the guitar slightly aside, which has a similar effect: to position the forearm perpendicular to the neck (Fig 1.2.2 right). As for the popular style going up to the 18th fret is not unusual, a cutaway may be required. A full acoustic guitar gives less freedom than the thinner solid bodies found on electric guitars and thin body acousticals.
Fig 1.2.2. Paco lifts the guitar (left), sitting with a curved spine, while Al holds the guitar more aside (right) and with the neck almost down, and his neck also.
For fast playing ultimate freedom is required. Two ways to unleash the right arm are playing standing with a sling (Fig 1.2.3, left) or lifting the right arm when seated (right). Since the right arm has little reference now, support for the little finger or ring finger is helpful. Also here, note that putting the finger on the resonating top is not good for the sound.
In the pictures the support location is marked with a little sun. If this location is different from the center of mass of the guitar, a moment results which must continuously be compensated.
Fig 1.2.3. Two fast players with both arms free. Tommy does it standing (left) while Sahid is twisting to the extreme (right).
Attempts have been made to relieve stress with a twisted guitar neck, rotating the surface of the fretboard 20 degrees clockwise at the headstock (Torzal natural twist) while keeping it flat at the heel. It turns the face of the fretboard away from the player’s gaze at the headstock. If the provocation of forward bending is resisted, it seems indeed to compensate for the natural hand rotation while reaching up and down the fretboard. Maybe it would for the position of both hands and for the gaze even be better to start with a flat position at the headstock and rotate the neck towards the player at the heel, but this would also involve a tilted bridge, with a lot of acoustical complications.
An interesting playing style is demonstrated by Andy McKee in ‘Drifting’, playing with the left hand from above, which puts all analysis, literally and as a figure of speech, upside down.
1.3 Balance
Stability of the guitar is a priority in any position. Fortunately an acoustic guitar is considerably lighter than a solid body (1.5 – 2 kg for acoustic, 3.5 for solid body). Nevertheless the guitar must be kept in position with the left hand free and also the right arm free during fast rhythms. While playing, the left hand is constantly correcting the neck position without exerting too much force. The guitar must thus be supported under the center of mass or hung from above the center of mass. Figs 1.2.1 to 1.2.3 show that the grip varies greatly with playing styles and that the support may be somewhere from well below the soundhole to well above. In fact in section 21.7 it is shown that the center of mass is about 44 cm from the guitar bottom, suggesting that Wiktoria and Chet hold their guitars in balance (Fig 1.2.1), while the others support their guitars slightly below the CoM, thus with some of the weight carried by the left hand. It may be doubted if the players are aware of this but it may nevertheless influence their technique. Some may prefer a light but consistent load over a load which flips from one side to the other. Think of your own head, which hangs a little forward, loading the back muscles a bit. This allows the muscles a tighter control over the head position than a balanced head for which the muscles continuously alternate relaxed and stressed conditions. Note that the sling is actually supporting the guitar at two points, so that the seemingly unbalance does not harm. Tommy has two hands free for optimal fast playing.
Burrell shapes the guitar after the human body to increase the contact surface area. Apart from tone effects this is a reasonable approach from the ergonomics point of view. The shape of the guitar box volume is acoustically almost irrelevant as the guitar body is a pressure box for the basses that depend on it. For higher pitches the form of the top is at stake, not the volume. Wedge guitar body form, narrow at the bass side, wide at the treble side, would do no harm and is indeed used. It turns the top a little to the eye and makes access by hands easier. More visually appealing is a slight wedge form with a narrow shoulder and a fat bottom and this is done more often. However, Burrell is twisting the soundboards and that must be studied carefully for its sound effects as it affects stiffness. Other stability means are a foldable leg rest and a sling.
1.4 Finger posture
Human hands vary greatly in size and build. In the small inventory of https://www.classicalguitardelcamp.com/viewtopic.php?t=64592 the span from pinky to thumb (Fig 1.4.1, left) varies between 210 and 250 mm (230 +/- 9%) and the stack of pinky, ring finger and middle finger (Fig 1.4.1, right) between 34 and 47 mm (40.5 +/- 16%). The relative variation in scale length is with 64 cm +/- 3% smaller than relative variation in hand span. Variation in nut width compares better to relative finger stack width. Since each fretboard shoulder is 3 to 4 mm, the nut width of 41 to 52 mm translates into string spacing from 7.0 to 9.5 mm (8.3 +/- 16%).
There is a typical anthropometric issue here: what is a relevant measurement to compare hand and neck? Grabbing a chord is a three dimensional action, involving both hand size and flexibility of the hand. It is not obvious that two dimensional measurements can represent the hand capability well. Moreover, so many joints are included and so much soft tissue, that taking reproduceable anthropometric measurements is difficult. Hand span and finger stack width seem to represent hand size more than flexibility. Measuring the capability of a particular hand to grab a series of chords on a particular neck would be more to the point, but such data have not been taken or published. It is likely that the outcome is also dependent on the neck thickness, although thin necks are not always considered an advantage.
Fig 1.4.1. Measurement of hand span and finger stack.
Interestingly, if a capo is put on the first fret, the effective scale is reduced to about 61.5 cm, which is the shortest scale in use on full size guitars. It may thus be assumed that hand span issues focus on chords which involve pressing of the first fret. Using the capo while tuning the strings down to the standard tuning may help. It also reduces the fretting force because the action is lower now. However, the tone changes because the strings are now a little heavy for the scale. Hand span issues on higher frets cannot be resolved by buying a different guitar but must come from alternative finger positions.
Sources on the internet show no clear relationship between hand size and neck dimensions. The inventory mentioned above finally produced ‘a R squared correlation of 0.6’, which is a puzzling statement because R squared is not a correlation but a measure of explained variance. If it is indeed variance the correlation between hand span and preferred scale length must have been .77, a reasonably useful score (https://www.classicalguitardelcamp.com/viewtopic.php?t=53237&start=30). The correlation pertains to a linearized relationship, implying an increase in preferred scale length of 9.2 mm for each 20 mm increase in hand span:
Preferred scale length = .46*hand span + 547 (mm)
This finding proves that the simple logic ‘larger hands thus longer scale’ is not valid. Following this reasoning the scale should grow in proportion to the hand but a 10% increase in hand size calls for a just 1.5% longer scale. Most adaptation is due to unknown factors, with arm-hand motion as a candidate. This is very typical for correlations: the mechanism involved is not revealed.
The relationship above was not completely linear, because in a table the data are roughly summarized by 6-15 mm increase in scale length for each 20 mm in hand span, the highest value for the smallest hands. Apparently the acoustical requirements set a bottom and a ceiling to scale length, where the bottom is most challenging to small hands. These are found in children. The table has been adopted by playableguitar.com (http://www.playableguitar.com/hand-size.html), which recommends smaller guitars for children, but from hand span 170 mm on, a small hand for an adult, full size guitars are proposed. Players with small hands use workarounds to emulate difficult chords. Some sources (for instance Smith, 2005) provide data on hand anthropometry, which indicate that the largest hands and arms are 30% larger than the smallest, both for men and women. The male/female difference is smaller than that with just over 10% size difference. The populations thus overlap for a major part and individual variation dominated the gender difference. Smith was scaling electric guitars for children, basing scale length more on arm reach than on hand span.
Although finger fatness seems directly related to string spacing, there is no free choice of string spacing because neck width is bound to guitar type. That does not need to be, but it is, suggesting that training will solve the difficulties. It helps to keep the left lower arm and hand square to the neck, so that the crease under the fingerjoints is parallel to the neck. Fingers are a bit wider than thick and in this way the thickness prevails.
The cross profile of the fingerboard is usually flat (classic) or slightly convex (modern, radius 250-400 mm). At close observation the best shape depends on the playing style. Radiused fretboards make it somewhat easier to grab a barre chord and also to grab the low e string with the thumb. The main driver though, seems to be easier string bending on a curved fret. Because of the cylindrical configuration of the strings, there is more access to the strings for fingerpicking. Strumming is harder, however.
There are more options to shape the fretboard optimally for a particular musical style, although these have not been explored as yet. Let’s do some forward thinking. One important preference is how chords are grabbed. For a barred chord a convex shape demands less force and assures a clear sound better than a flat shape. However, for pressing strings with the left hand, while not touching neighbours, approaching the string from above with the fingertip is better than an oblique approach. This calls for a concave shape, except for the thumb which must curl over the fretboard edge. The advantage is not only the steeper angle between finger and fretboard, but also that the fingers need not reach so far. The thumb has then more length to get around the edge. Using the side of the thumb is easier with a convex bass side, however, resulting in an S-shape (Fig 1.4.2).
At the bridge side, the shape is controlled by the strum versus fingerstyle attack. Full chords are strummed better on a flat arrangement, while strumming selected strings, up to three or four, is easier in a slightly convex arrangement. For picking the lower three strings, which are mostly played by the thumb, the arrangement could better be concave, because the thump moves easy in a circle. For the fingers a convex configuration is better. Such a string arrangement thus runs from an S at the headstock to an inverted S at the bridge, with a flat arrangement somewhere in between. That this is an unknown design must mean that a safe compromise between playing styles is the usual solution or that the production complexity is not worth the gains. There is no technical objection in the fret positions, as these may follow the cross sectional shapes, but the irregular shape of the saddle may cast a difficulty, as it involves differentiation in bridge height per string. For an electric guitar bridge height is no issue, but for an acoustic guitar it is important for the energy of the note. So far for the free roaming of the mind.
Fig 1.4.2. A curved fretboard, optimized for left hand finger placement and right hand fingerstyle (shape exaggerated for clarity). The fretboard is nearly flat in the middle.
Guitarists have different preferences for the profile of the neck at the backside, ranging from wide and square to slender and round, maybe depending on the size and dexterity of their hands. For long hands with fine fingers a narrow string spacing is allowed and thus a narrow neck. For them the neck profile is not critical, as they can easily wrap their fingers around the neck. For strong hands, with short, thick fingers a wide string spacing may be desired, but that means also a wide neck, for which they may lack the hand size. This cohort of the population, for which no evidence was found that the members would be lesser musically gifted than the long handed, would deserve a thin and wide design that enables them to play the guitar as well. The S-shaped nut is a useful starting point for a neck with short circumference and wide string spacing. Obviously, such a neck profile demands training to use it well. Genani et al (2013) experimented with trapezoid neck profiles and found those more comfortable than C-profiles, but their number of subjects was insufficient for conclusions. Guitars are offered with great variety in neck profile, usually named C, D, V and U, after the similarity with the shape of the letter. Also asymmetrical C is incidentally applied, tapered towards the treble side. Experimentation with three neck profiles, trapezoid, D and asymmetrical, provides the following:
- For free dexterity the treble edge must be placed low at the first phalanx of the index. Only then the fingertip can approach the fret from above (Fig 1.4.3, top-left).
- The thickness of the neck at the treble side does not make much difference as the bend in the profile falls in soft tissue (Fig 1.4.3, top-right).
- The shape of the bend at the bass side is more critical as it may hit the thumb joint. A recessing profile makes it more comfortable (Fig 1.4.3, bottom-left) and allows better access to the low e-string.
- This recess (Fig 1.4.3, bottom-right) can provide a platform for the thumb to give counterpressure to the fingers. This is an even better way to free the hand, but only if the thumb does not play.
Fig 1.4.3. Asymmetrical and trapezoid neck profiles offer more or less freedom for the left hand.
The conclusion from this trial is that the trapezoid and asymmetrical (tapered to the bass side) neck profiles offer the most freedom. The wider area for thumb placement and the slightly higher stiffness of the asymmetrical profile give it some advantage. The trapezoid is little different from the C-profile but the thumb knows better where it is.
These profiles come in thin (19 mm thickness), chunky (23 mm), fat (25 mm) and super (28 mm) size. This thickness is at the nut. Most necks are tapered towards the nut, on average thinning 2 mm, but not for the fat and super models. That the neck is thicker near the heel has good reasons: thickness is a major factor in neck bending under string tension and neck bending there will have greater impact on string height than at the nut. The additional 2 mm may seem futile, but it increases the local stiffness with 30%.
EndurNeck came up with an asymmetrical neck profile, which changes over the length of the neck. The fretboard is conventional, but the neck is at the nut deepest at the bass side and near the heel at the treble side. The underlying assumption is that at the heel end the thumb is used to counterbalance the finger grip only, whereas at the nut the thumb is used for fretting, forcing the fingers to bend stronger, leaving less room for a deep neck. This may be correct, but the question is if players are not using the back of the neck as a rail, guiding the up and down slides of the hand. As their ‘rail’ runs diagonal, it may demand some training to avoid derailing of the finger tips. This raises another matter. Most guitar players look at their fingers and that is the reason that the dot inlays, showing where their fingers are, have a function. However, looking at the fretboard is detrimental to the body’s posture and it is healthier to learn to finger in the blind, maybe helped incidentally by dot inlays at the side of the fretboard. The guitar is then facing towards the audience and the left hand is rotated less. If the eyes are used so eagerly to guide hand position, why would not the tactile capability be exploited to monitor hand position? In Fig 1.4.4 Argentine tango guitarist Mirta Alvarez is pictured during a two octave and a minor third spanning chord, a difficult grip, not for her though. It is spectacular to see her fingers run like a spider over the fretboard, touching lightly, while her hand jumps over the full neck length without even a glance. Not all of us are so gifted as Mirta, but if any person can learn to type in the blind, a guitarist could learn to grab the neck in the blind. To support the awareness the left hand could sense the neck profile and monitor position by the change in profile, if the profile is not identical everywhere. The potential derailing could then be turned into an advantage rather than a drawback.
Fig 1.4.4. Mirta Alvarez plays with the guitar facing the audience, trusting her proprioceptor feedback (information from joints) and motor programs (motion planning) to let her hand jump over the fretboard without visual control. Note the absence of dot inlays. Picture from mirtaalvarez.com, modified.
Another fretboard variation is the fanned-fret guitar, which is a multiscale instrument. The strings have graded scales and the frets are graded accordingly, resulting in a fan of frets rather than parallel frets. Originally designed to extend the range with a 7th string at the bass side (baritone extension) or at the treble side (treble extension) with natural full or clear tones, the contemporary builder Novax claims that playing multiscale is physically less stressful. No evidence was provided for that claim. The discussion on scale versus hand size may be relevant here. Some chords may be stretched on a multiscale instrument, but on the other hand the treble strings will be shorter and possibly easier to play. The net outcome is unclear.
Multi-scaling has impact on the sound of particular notes, such as G and B.
1.5 Free neck length
On modern guitars, playing the high frets has become part of the style and the 12 frets to the shoulder of classical acoustic guitars is in folk guitars stretched to 14. That is a practical limitation, because the 65 cm scale and the 52 cm long body of a large guitar leave no room for more free neck length, at least not with the common build. A switch from 12 to 14 frets demands a considerable body shape change, with a reduced upper bout. Jazz and electric guitars, which are not bound by an acoustic body, may have 16 frets to the shoulder and still there is a need for a cutaway to access higher frets. The cutaway is not found on classical acoustic guitars and not on most folk guitars. On semi-acoustic guitars it is common though. The neck join is still in the way of the wrist while fretting beyond fret 18. Thin body guitars are a bit more relaxed, but another neck construction, without the heel, delivers another gain of two accessible frets. The neck is then protruding through the body, in the body cave.
An unresolved issue is if even higher notes are not desirable in acoustic musical styles or that they are avoided since they are not available. One can only observe that if they come available, like in semi-acoustic and electric instruments, they are being used. Electric guitarists use up to 24 frets.
For the builder such long free neck ends cast a challenge. A long neck with a small cross section is a recipe for instability if the strings start pulling at it. One could resort to necks made from high tensile materials to counteract this instability. Most guitars have steel or carbon inlays in a wooden neck.
1.6 Action and playability
A higher action requires more finger lifting and higher pressure during fretting. A low action involves the risk of fingering noises and buzzing of the string on the next fret. For the finger this casts a force-speed-accuracy problem. Force is not only dependent on action, but on fret position and string gauge as well. These factors increase the applied force considerably, from less than 1 to over 5 N. For the a-string the fretting force is as in Table 1.6.a.
Table 1.6.a. Force (in Newton) required to fret the a-string with medium heavy grip.
The accuracy is more difficult to quantify. Fitts (1954) found a relationship between speed and accuracy in movements which has been confirmed in many other conditions. There is a linear relationship between movement time and the difficulty of the move, expressed as the logarithm of the accuracy A/W, where A is the jump and W the margin in landing. Gorniak et al (2008) found that this is also true when the jump is a change in force rather than a change in distance. The movement time increases with increasing place difficulty, but for the same place difficulty also with the magnitude of the force change. Gorniak applied relatively low forces (10-20% of maximum contraction) and still found significant increases in movement time with force, in particular for low place accuracy. This is the typical guitar condition, where the distance margin is half a fret on the scale and the force margin anything above the string contact threshold. There is thus some evidence that high force is detrimental to the speed of playing. Average males can finger press 80 N and females half of that, so the values in Table 1.6.a are below 3% and 6% of the maximum, respectively. This compares to the range in Gorniak’s experiment. Since the slowdown is supposedly caused by motor planning in the brain, it may be that experienced players overcome the limitations. It is hard to believe, however, that higher forces do not affect ease of playing and indeed most players agree that light strings and low action improve playability (see for example music.stackexchange.com/questions/4593/how-does-string-gauge-affect-a guitars-sound-and-playability).
Another training issue is the formation of calluses. The force is applied on a tiny skin surface, leading to excessive local pressure. The skin can only maintain health and (intermittent) blood perfusion if the pressure is spread over a larger area, as effectuated by a firm layer of callus.
The idea to adjust action by raising or lowering the bridge is a crude solution. It moderates action most for the higher frets. It takes a more detailed analysis to adjust action to a safe minimum for every spot on the fretboard. The analysis must take both playability and tone purity into account, because any action is always associated with a slight pitch shift. This is the subject of Chapters 15 and 16. Raising or lowering the bridge is common on archtops, but not on flattops, because a higher bridge is increasing the stress on the top and that is a matter of delicate balance (see section 2.8).