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Design of an Ergonomic Guitar Aid
3 Biomechanical Analysis In order to gain a deeper understanding of the mechanism of injury i.e. why are guitar players prone to musculoskeletal problems, a biomechanical analysis of the act of playing is required. By observing users during the act of playing, hypothesis were made about instances that may result in these injuries which were then correlated with literature for validation. A summary of these results is presented in the following section. Other issues with the guitar that cause discomfort to the users are also reported. It should be noted that the instrument used for the observational research was a Jackson Dinky model, which has the most common body shape known as SuperStrat. Different guitars have different bodies which means they will fit the user differently and may result in slightly different observations, but the core issues are the same amongst all popular guitars. These observations were made throughout the course of the project to test and validate various hypothesis.
Gaurav Genani/Master's Thesis/Integrated Product Design
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Design of an Ergonomic Guitar Aid
3.1. Introduction A biomechanical analysis should describe the interaction of forces during the act of playing. Two kinds of forces are involved : Internal forces and external forces. Internal forces are the forces used to maintain body posture. External forces, in this case, refers to the forces generated to counteract external loads i.e. grip force required for fretting. A detailed analysis of posture and grip is presented below. Due to lack of literature on biomechanics of playing guitar, literature relevant to hands, wrist, fingers and neck was referred. Observational research was done with various users throughout the project to study how literature correlates with the real scenario.
3.2 Posture Posture is the way muscles and skeleton hold the body erect against gravity. For a good posture, the segments of the spine should be properly aligned to allow its natural curvature without any twist. A good posture also requires symmetry - when the left and right side of the body are balanced with respect to each other. The same applies for maintaining a good posture while playing a musical instrument. A pianist, drummer or horn player strive to keep their back straight, neck vertical and the left and right sides balanced while playing to perform optimally. For a guitar player, it is difficult to do so due to the design of the instrument as described below. While sitting, the way an electric guitar rests on the lap of the user when not supported with the arms is shown in Fig 3.1 (a). To bring the instrument in a desired playing position, the right arm presses the guitar against the body shown in Fig 3.1 (b).
Fig 3.1. a) How the guitar rests without using the right arm b) by placing the right arm on the edge of the body, guitar is in desired playing position
Gaurav Genani/Master's Thesis/Integrated Product Design
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Design of an Ergonomic Guitar Aid
Depending on the guitar, the arm is supported somewhere below the elbow in shear. This may negatively impact blood flow to the muscles and tendons of the picking hand.
Fig 3.2 Right arm rests on the edge of the body Compared to classical guitar, the body of electric guitar is much thinner. As a result, the fretboard and picking area are much closer to the body. Because of this, rotation about the shoulder is required to position the hands at the right location. Due to this, guitarists prefer to have the fretboard pointing away from the body to varying degree, as this reduces the rotation of the left arm which is unsupported, but puts more strain on the right shoulder. They do so by pivoting the guitar with the right arm..
Fig 3.3 Right arm is used to pivot the guitar neck away from the body a) Neutral position of the right arm , left elbow sticking out from the back b) Fretboard turned outwards, right arm rotated backwards
Gaurav Genani/Master's Thesis/Integrated Product Design
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Design of an Ergonomic Guitar Aid
Some users prefer to rest the guitar on their left leg as it allows a more comfortable position for the right hand – in front of the belly. However, the neck is further away from the body and the torso is turned to the left. Moreover, the guitar tends to slip down the leg in doing so. To prevent this slipping, the leg is raised up by resting the foot on a support.
Fig 3.4. User with guitar resting on the left leg a) right arm in front of the belly, guitar tends to slip in this position b) raising the left leg holds the guitar in position
It can be seen that when the guitar is resting on the user’s lap, where the guitar neck is located is irrespective of the height of the torso. The location of the guitar neck is a determining factor for wrist angle, and unnecessary flexion or extension should be avoided as discussed earlier. When the guitar is resting on the lap, the guitar neck is more or less horizontal. But if the neck is rotated upwards, a lot of wrist flexion can be avoided. However, there is always a compromise between elbow flexion and wrist flexion. The guitar neck is kept at about 45 deg in the recommended classical guitar posture.
Fig 3.5 Wrist flexion when the guitar is a) horizontal b) at 45 degree elevation, while holding the same chord
Gaurav Genani/Master's Thesis/Integrated Product Design
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Design of an Ergonomic Guitar Aid
Moreover, users tend to slouch to be able to see the fretboard when the neck rests too low i.e. when it is horizontal. Some users also use a guitar strap while sitting to get a desired neck angle. One problem with this is that guitar neck is too close to the body, as the strap works by pushing the instrument against the body to secure the instrument. This requires rotation of the shoulder backwards. Moreover, a load on the shoulder creates an imbalance between the left and right sides of the body.
Fig 3.6 (left) For some users, the guitar rests too low and they have to slouch to be able to see the guitar neck. Fig 3.7 (Right) User with guitar strap while sitting to lift the neck up
A guitar strap is used to fasten the instrument to the body when playing during standing. This involves raising the instrument above the head, and putting the strap around the neck, which is rather inconvenient. Moreover, changing the length of the strap is also an arduous process. A slide is used for length adjustment most commonly (shown in Fig 3.8) and it is located at the back of the user when the instrument is on the player. This means the guitar has to be taken off for adjustment. There is also no way of knowing if the instrument will now be in the right position after adjustment and it is kind of a trial and error method.
Fig 3.8. User putting on the guitar with the strap
Gaurav Genani/Master's Thesis/Integrated Product Design
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Design of an Ergonomic Guitar Aid
While standing, the position and the angle of the neck is determined by the length of the strap. For a particular length of the strap, there is only one stable position for the guitar. This is one of the issues with a guitar strap - the guitar tends to slip into its ‘stable’ state where the neck rotates down and the user needs to keep adjusting it to his desired angle.
Fig 3.9 User with the guitar while standing; various positions of guitar according to different strap lengths; area in yellow shows the possible location of the fretting hand
This can be explained by the following free body diagram. When the guitar is suspended with a strap, the instrument is in equilibrium about the center of gravity when the moments from both ends of the strap which are connected to the hook are balanced.
Fig 3.10. Free body diagram of the guitar and strap, guitar will be in equilibrium when R1xa = R2xb
Hence, rather than the instrument being in the right position, the user has to position his arms to place his fingers on the fretboard as the guitar strap only allows vertical adjustment of the guitar. The weight of the instrument is borne by the collar bone and the moment is transferred on the
Gaurav Genani/Master's Thesis/Integrated Product Design
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Design of an Ergonomic Guitar Aid
back. Those who play live for long duration often complain of back ache, which can be attributed to the imbalance of forces with the guitar supported on one side. As shown in Fig 3.7, some users tilt the head downwards which allows a better view of the fretboard while playing. This creates a forward bending moment on the spine, which is counteracted by the passive connective tissue structures such as the joint capsules and ligaments, to keep the head in a stable position (Harms-Ringdahl, 1986, adopted by Frankel-Nordin). It can be seen that even slight neck flexion creates a bending moment three times greater than the neutral position at the C7-T1 joint. As hypothesised earlier, an injury or inflammation in this region can produce a myriad of symptoms in the upper limbs, as the origin of the brachial plexus, which also includes the ulnar nerve, is somewhere in this region.
Fig 3.11. Neck Flexion moments around the axes of motion of the atlanto-occipital (OCC-C1) joint and the C7-T1 motion segment (marked with x’s)are presented for three positions of the head. Values shown are the median for seven subjects (Adapted from Harms – Ringdahl, 1986)
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3.3. Grip How one grips the fretboard is not just a matter of personal preference, but also an indicator of one’s style of playing and if that person is prone to musculoskeletal injuries. Although the grip is not constant, but keeps changing as one plays different passages at various locations of the fretboard, most players predominantly use one of the following three grips : a) classical b) pinch and c) thumb around (Fig 3.12). The difference between these grips is how the force of fretting applied on the fingerboard is counterbalanced by providing a support at the back of the fretboard. This support is needed otherwise the neck will keep swaying each time a note is fretted; it also provides a stable point of reference for the fingers to know where the frets are located. To some extent, the act of gripping also supports the weight of the left arm, but this may vary between individuals. While playing guitar, the user alters his grip on the neck dynamically depending on what is being played i.e. the thumb position constantly varies between the ones shown in the image. The locations of fingers vary depending on the note being fretted, and the location of thumb varies accordingly to balance the force of fretting which decides the shape of the grip.
Gaurav Genani/Master's Thesis/Integrated Product Design
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Design of an Ergonomic Guitar Aid
1. Classical Grip Thumb padding rests at the back of the fretboard
F Fretting
2. Pinch Grip Fretboard is pinched between the thum padding and right below the Metacarpophalangeal Joint of the index finger
Fig 3.16. Various grips and interaction of forces; extracted from a videos taken while user was asked to play one of his favourite musical pieces , grip alters dynamically during a single passage
Fig 3.17. Gripping with the palm puts wrist in extension, thumb wrapped around the fretboard
F Fretting
3. Thumb Around Grip The whole area between the thumb Interphalengeal Joint and the Metacarpophalangela Joint of index finger is used to wrap around the neck
F Fretting
One scenario is when the thumb wraps around the fretboard sticking out on the top (Fig 3.17). In this position, the wrist is mostly in extension, which allows a greater overall gripping force (Volz et al 1980). Blues guitarists, who bend the strings a lot to alter the pitch of a note, prefer this grip as string bending requires a lot of force. However, the amount of stretch one can get in flexion is limited. On the fretboard, it relates to how far along horizontally the fretboard one can reach between the index finger and the pinky finger. Moreover, the fact that a larger surface area is in contact while gripping makes it difficult to move up or down the fretboard. In general, this approach
Gaurav Genani/Master's Thesis/Integrated Product Design
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Design of an Ergonomic Guitar Aid
provides higher overall stability and force, but restricts playability. On the other hand, classical and heavy metal players mostly use only the padding of the thumb at the back of the fretboard to support the fingers. This keeps the wrist is in flexion, which allows more flexibility for the fingers and greater reach but limits the maximum force that can be transferred through the fingers.
Fig 3.18 a) When only padding of thumb is resting on the fretboard, wrist is in flexion; b) this allows greater stretch which can be increased further when combined with ulnar deviation
It can be seen from Fig 3.16 that the most efficient way of gripping, solely from a mechanics perspective, is the classical grip as it directly counterbalances the force of fretting. The other two grips employ a greater surface area of contact, and work much like the Roman Arch, where all vertical components of forces cancel each other out, and the resultant horizontal counterbalances the fretting force. The question then arises, why do some users prefer the pinch or thumb around grip. The answer can be found by considering the force of fretting at the top and bottom of the finger board i.e. while fretting strings at the lower and upper extremities of the fingerboard. As one frets the first string, for example, a moment is generated and due to the convex shaped thumb support, the fretboard tends to rotate around the thumb (Fig 3.19). This means the thumb constantly needs to
Fig 2. As a lower string is fretted, a clockwise moment tends to rotate the fretboard around the thumb, requiring an adjustment
F Thumb
Fig 3. Rick Toone’s Trapezoidal Profile solves this problem to some extent by providing flat top, middle and bottom Fig 3.19 The thumb surfaces
M F Fretting
Gaurav Genani/Master's Thesis/Integrated Product Design
constantly needs to adjust to balance forces while fretting due to the profile of the guitar neck
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Design of an Ergonomic Guitar Aid
adjust the direction of force applied to counter-act the moment, which may feel unstable for some users. Since the pinch and thumb grip have components of counterbalancing force in other directions as well, the moments are cancelled naturally. Radial and ulnar deviation – Whether the wrist is in radial or ulnar deviation plays a key role in little finger flexibility and stretch. When the wrist is in ulnar deviation, the little finger has much more freedom to move and can exert a higher force. Mastering control of the little finger requires a lot of practice but results in complete control of the fretboard as the little finger has a greater range of motion than any other finger (Matheson et al 1970). Keeping the wrist in ulnar deviation and slight flexion provides the most beneficial posture, as it provides the little finger with maximum freedom. The power of ulnar deviation can be increased by bending the MCP joint to 135 deg. However, according to the ergonomic criteria discussed earlier, ulnar deviation of the wrist should be avoided, but it can be seen that it is a very common amongst guitarists.
3.4 Conclusions The existing design has some flaws as it puts the body in an awkward posture while playing. While sitting, the instrument needs to be held in the playing position by using the right arm. The right arm rests on the edge of the instrument which could negatively influence fluid flow in the region. The picking area and the fretboard are close to the body which requires either one or sometimes both shoulders to be rotated backward for both hands to have access to play. Users use various configurations of the body to hold the guitar neck in desired playing position. Having the fretboard at an angle rather than horizontal allows a reduction in wrist flexion. Some users use the guitar strap while sitting to get the desired neck angle. This creates an imbalance in the torso and restricts its freedom of motion. While sitting, some users slouch to have a clear view of the fretboard. This creates unnecessary moment in the neck and lower back. While standing, the weight of the guitar rests on one side of the body affecting balance. Having a desired neck angle with a strap while standing is difficult as it tends to slip into its ‘stable state’. Putting on the guitar with a strap is also difficult. The issue with the guitar strap is that it can only be in tension and cannot take moment, so the guitar tends to swing in the transverse plane to counteract moment by pushing against the body and the neck tends to rotate in the coronal plane to achieve static equilibrium about the center of gravity. In its natural equilibrium position, the guitar neck is close to horizontal which increases instance of wrist flexion in some users. As a result, the user needs to hold the instrument in desired playing position. Users alter their grip dynamically during playing. The grip varies by the location of the thumb at the back of the fretboard and the location of fretting. Some grips put the wrist in extension and some in flexion. Grips that put the wrist in flexion allow a bigger stretch of fingers. Flexion can be reduced by increasing the neck angle away from horizontal.
Gaurav Genani/Master's Thesis/Integrated Product Design