Basic Grip Explanation
The cylindrical power grip generates the maximum gripping force possible by hand (Hedge 2013). It is an important movement that is performed daily when grasping heavy cylindrical objects or anything that has a handle like a bar or hammer as can be seen in figure 1.
Detailed Anatomy of the Grip
As can be deduced from figure 1 the grip is characterised by the flexion of digits 2-5 and flexion and adduction of the thumb around the object but in opposing directions, i.e. if digits 2-5 wrap around the object anti-clockwise the 1st digit (thumb) will wrap clockwise.
The thumb is set at an angle to the plane of the palm and its movements are described differently to those of the other digits (O’Rahilly, Catlin & Lyons 2008). This difference in the orientation (or plane of movement) of the thumb compared to digits 2-5 is a crucial anatomical feature of the hand that allows for this opposing direction and a much stronger grip (Napier 1956).
Major Movers: Extrinsic Hand Muscles
As is the case in most power grips the extrinsic muscles play a key role in the movement providing the major force (O’Rahilly et al. 2008). There are three extrinsic hand muscles that provide the vast majority of power generated in the cylindrical grip. These are all long flexors in the anterior compartment of the forearm and are Flexor Digitorum Superficialis, Flexor Digitorum Profundus and Flexor Pollicis Longus.
Flexor Digitorum Profundus (FDP) (figure 6 below) powerfully draws the digits 2-5 against the object being grasped pressing it against the palm of the hand. FDP provides the greatest portion of power and tension to the grip due to flexion of the distal interphalangeal joints around the object (Landsmeer 1962).
Grip a handle as tight as you can. Now relax and extend the distal interphalangeal joint and feel how much you grip weakens!
Flexor Pollicis Longus (FPL) as can be seen in figure 6 below, attaches to the distal phalanx of the 1st digit (thumb) much like that of FDP on digits 2-5 albeit on a different plane. This muscle allows the thumb wrap around the object creating a closed and much tighter grip.
Flexor Digitorum Superficialis (FDS) (figure 7 below) has its distal attachment on the shaft of the middle phalanx of digits 2-5 and flexes the proximal interphalangeal joints of those digits. This muscle assists FDP to provide greater tension in the flexed position.
Minor Movers: Intrinsic Hand Muscles
There are some intrinsic hand muscles that assist in the positioning of the digits through action on the metacarpophalangeal and some carpometacarpal joints. The positioning adopted gives the optimal power to the grip.
Firstly, the thumb is adducted at both the metacarpophalangeal and carpometacarpal joints. This is in direct contrast to precision movements where the result is the pad of the thumb in contact with the pad of digits 2 and 3 (Landsmeer 1962). Adductor Pollicis (figure 8 below) is the muscle responsible for this adduction of the thumb.
Secondly, not only do the fingers flex to the size of the object held and press the object against the palm of the hand, they are also laterally rotated and incline towards the ulnar side of the hand (Landsmeer, 1962). The 4th Dorsal Interossei brings the metacarpophalangeal joints of digit 4 to a state of maximum lateral rotation and Abductor Digit Minimi does the same for digit 5. These muscles can only contribute to contribute rotation when fingers are partially flexed.
Table 1 below summarises their respective roles in the cylindrical grip.
|Table 1: Extrinsic and Intrinsic Hand Muscles Important for Cylindrical Power Grip|
|Muscle||Relevance to the Cylindrical Power Grip|
|Flexor Digitorum Profundus||Powerfully flexes the distal interphalangeal joints of digits 2-5. Provides the greatest proportion of power to the grip.|
|Flexor Digitorum Superficialis||Flexes the proximal interphalangeal joints of digits 2-5. Assists Flexor Digitorum Profundus.|
|Flexor Pollicis Longus (FPL)||Flexes distal phalanx of digit 1. Allows the thumb to wrap around the object for a tighter grip.|
|Adductor Pollicis||Adducts digit 1 toward the lateral border of the palm and provides the angle for counter force against the flexed digits 2-5.|
|4th Palmar Interossei||When the digit 4 is flexed, provides lateral rotation to increase tension|
|Abductor Digiti Minimi||When the digit 5 is flexed, provides lateral rotation to increase tension|
Other Functional Considerations
Varying the weight of the grasped object
As mentioned previously the thumb is set on a different plane to that of the palm. Napier identified a thumb placement variation in the cylindrical grip that occurs according to the weight of the object being grasped. As can be seen in figures 2-4, the heavier the object becomes the further the thumb adducts and wraps around the object. This variation confirms that the thumb significantly increases the strength of the grip.
This means there can be an element of precision in the power grip through pulp surface of the thumb pressed against the grasped object (when the object is not heavy) (Figure 2). As the object gets heavier the ability of precision decreases and the thumb becomes more abducted and eventually crosses over the object to providing maximal power as can be seen in figures 3-5 (Napier 1956).
Grip strength at different forearm and wrist orientations
Grip strength varies according to the flexion/extension of the wrist and pronation/supination of the forearm. Different orientations of the wrist and forearm result in certain muscles shortening and extending. The length-tension relationship in muscles (Gordon, Huxley & Julian 1966) means a change in length of a muscle will affect the ability of that muscle to create tension (or in our case grip strength).
Terrell and Purswell (1976) established that a supinated forearm and a neutral wrist position results in the maximum grip strength. Table 2 below shows the percentage grip strength possible at various wrist and forearm orientations.
|Table 2: Grip strength for various wrist and forearm positions|
|Neutral||Flexion||Hyper-extenion||Radial Flexion||Ulnar Flexion|
|Note. Reprinted from Terrell and Purswell (1976)|
Gordon, A, Huxley, AF & Julian, F 1966, ‘The variation in isometric tension with sarcomere length in vertebrate muscle fibres’, The Journal of physiology, vol. 184, no. 1, p. 170.
Hedge, A 2013, Cornell University, accessed 26 April 2016, <http://ergo.human.cornell.edu/studentdownloads/DEA3250pdfs/grips.pdf>.
Landsmeer, J 1962, ‘Power grip and precision handling’, Annals of the Rheumatic Diseases, vol. 21, no. 2, pp. 164-170.
Napier, JR 1956, ‘The prehensile movements of the human hand’, Bone & Joint Journal, vol. 38, no. 4, pp. 902-913.
O’Rahilly, R, Catlin, B & Lyons, J 2008, Basic Human Anatomy, Darmouth Medical School, Darmouth.
Quizlet Incorporated, 2016, accessed 26 April 2016, <https://quizlet.com/76433165/ch-11-12-13-hand-grips-flash-cards/>.
Terrell, R & Purswell, JL 1976, ‘The influence of forearm and wrist orientation on static grip strength as a design criterion for hand tools.’ paper presented at the Proceedings of the Human Factors and Ergonomics Society Annual Meeting.