Flannery and I have been talking about different ways of gripping/holding a crimp that will maximize strength of the hand while pulling. We haven't really gotten anywhere, just talking about the differences. I'm not sold on any type of grip position except the open hand grip which can prevent finger injuries.

Hand positions we have spoken about but would like input on:
Closed hand with thumb wrapped over the index
closed hand with thumb wrapped over the index and pinkie over the ring finger
closed hand with thumb wrapped over the index, no pinkie on hold
open hand with all four fingers on

Below is a detailed biomechanics paper I wrote on why the open hand crimp would be better. This was for my biomechanics class. Enjoy!

Biomechanics of the Crimp Grip in Rock Climbing

Rock climbing in the United States has become a popular sport for those wanting to get fit and participate in a recreational sport, and it is now regarded as both a competitive and professional sport. Since its increase in popularity, more and more climbing-specific training and research has been developed. As a climbing and strength coach, it is within my scope of practice to understand the biomechanics of certain movements in climbing. In this paper, I discuss and summarize the biomechanics of the “crimp” grip and how these factors lead to injury and technique adjustment.
A crimp is a small “hold” (i.e., a place to temporarily hold onto during the climbing process) that is just big enough to be grasped with the tip of the fingers. The “crimp grip” is a hand position that allows a climber to hold a thin, sharp edge (i.e, crimp) on a rock’s surface or a thin plastic hold on an indoor climbing wall (Schweizer, 2001). This position must be sustained until the next move is completed, usually a combination of foot placement and adjacent hand placement. The crimp is held until all limbs are in place and the next move is completed, which usually places increased force on the grip as the limbs move into position. This may last about 3-10 seconds, inducing concentric (as the tendons initially flex) and eccentric (fighting gravitational forces and body weight) contractions within the hand tendons.
Coming into the crimp position, the climber moves the hand to the hold, so that the fingertips are placed at the back edge of the hold (this movement is done statically or dynamically, depending on the difficulty of the climb and the incline of the surface). For maximum stability, the adjacent fingers should all be touching, so that the fingertips are all held together in a row. The fingers flex (Flexor Digitorum Superficialis & Flexor Digitorum Profundus, FDS & FDP) until the proximal interphalangeal (PIP) joints of the index, middle, and ring fingers (and most times the little finger) are flexed to 90o to 100o in the sagittal plane, and the distal interphalangeal (DIP) joints are hyperextended (Schweizer, 2001. & Vigouroux et al., 2006). The thumb can be used to allow for greater stability by placing it on top of the DIP joint of the index finger, increasing the extension in the DIP joint (biomechanics of the thumb joint has not been researched in the literature). As the fingers come into flexion, the flexor tendons at the PIP joint increases the moment arm and increases the holding force, which ultimately produces the most powerful finger joint angle in climbing (Schweizer, 2001).
The force of flexion at each finger would be varied to compensate for different finger lengths (Schweizer, 2001), and the enslavement phenomenon (movement of one finger causes movement of the adjacent fingers) would also play a dominant part in varied flexion force (Wook Kim et al., 2008). According to Shim et al. (2006), during maximum voluntary force (MVF) tests of the four fingers, the index and middle fingers produce the greatest force and show that the percent distribution is 30% for the index finger and 30% for middle finger during MVF. Tendon tension of the FDP tendon at maximum force during the crimp is higher (257.5N) than the FDS tendon (147.6N). The intrinsic muscles also play an important role in sustaining maximum force in the position of the crimp. The lumbricals do not show any force development, the ulnar interosseus show a 50.9N of force, and the radial interosseus show a 29.3N of force during the crimp position (Vigouroux et al., 2006).
In addition to forces placed within the joint, high forces are applied to the fingertips, which maximize flexion of the tendon (FDP) and recruits the flexor tendon pulley system (Vigouroux et al., 2006). The three major pulley systems within the fingers involved in sustaining crimp forces are the A2, A3, and A4 pulleys (A1 and A5 have not been researched in the literature). These structures prevent bowstringing of the FDP and FDS, as well as providing a mechanical advantage during flexion (Mallo et al., 2008). Bowstring width of FDP in flexion after warming up (100 moves) increases by 0.6mm, which leads to a 3% increase of moment arm of the FDP across the PIP joint. This means that warm up of the tendons is essential before placing maximum loads on the fingers (Schweizer, 2001).
As the tension of the FDS and FDP increases at the fingertips during crimping, the force of the pulleys are often much higher to sustain the crimp position. The A2 pulley of the middle finger is the strongest of the three pulleys, and is measured to have around 375N or 407N of force at maximum strength when 118N of force is placed at the fingertip. The research of the A2 pulley in recreational climbers, aged persons, and cadaver fingers determined these values. The professional climber could in theory exhibit higher values, as their tolerance for higher forces and maximum strength is far superior (Schweizer, 2001).
In comparing the tolerance of the fingers at the A2 pulley, the values of the middle and ring finger of females are much higher than those compared to males, whereas index finger values of males are much higher than females. The A4 pulley of the index and middle finger of females exhibit higher values than their male counterparts, but these differences in values were very minimal. The overall order of A2 pulley strength of the four fingers is the middle, the index, the little, and lastly, the ring finger. This means that the ring finger tested the weakest and may be the contributing factor to the high incidence of A2 rupture of the ring finger in climbers. The overall order of A4 pulley strength of the four fingers corresponds to index, middle, little, and lastly, the ring finger. Incidence of A4 pulley rupture is relatively high, but not as predominant as the A2 pulley (Mallo et al., 2008).
According to Wright et al. (2009), overuse injuries account for over 80% of injuries at indoor climbing facilities; 75-90% of these injuries occur in the upper body. The most common injury, however, is that of the A2 pulley in the middle and ring finger. Risk of injury increases as the grade of climbing difficulty increases and as the frequency of climbing increases. Another common injury is strain of the flexor unit (pain in the entire flexor system in the hand and arm). The risk of this injury increases when holding a moderate crimp position inside a pocket hold, which often only utilizes the middle and ring finger (Rohrbough et al., 1999). From the above information, the mechanics of the crimp grip would likely lead to an A2, A3 or A4 pulley rupture or strain. These often occur from overuse (frequent crimp position) or acute episodes (occurs from forced extension, audible “pop”) (Rohrbough et al., 1999; Jones et al., 2008). A technique change in crimp grip may decrease the likelihood of these injuries, however. This grip is called the open-hand crimp, also known as the “slope” grip. The open-hand grip is often accomplished by elite rock climbers, because it requires more forearm and hand strength, and this places less stress on the pulley system (Burach, 2004). The open-hand grip is where the DIP joints are flexed between 50o to 70o and the PIP joints are extended or slightly flexed (Schweizer, 2001). The force of the FDP to reach equilibrium is much less at the DIP and PIP compared to that of the crimp grip. There is also less of a bowstring effect and a decrease in moment arm of the FDP at the DIP and PIP joints in the open-hand grip (Schweizer, 2001). The A2 pulley was shown to have much lower forces against the FDS and FDP (8.1N) during the open-hand grip than the crimp grip (254.8N). The FDP-to-FDS tendon-force ratio during the open-hand grip was equally distributed between the tendons (0.88:1), whereas the crimp grip produced higher FDP forces (1.75:1), making it the prime flexor of the finger (Vigouroux et al., 2006). This suggests that the muscle force of the FDP tendon is more effective during the open-hand grip than the crimp grip (Schweizer, 2001).
According to this information, it is clear that the A2 pulley will accrue more damage and risk of injury as a result of the crimp grip, as compared to the open-hand grip. Therefore, it is now thought that use of the open-hand grip will help to prevent finger injuries in climbers without sacrifice of performance. At times, climbers may have no choice but to use the crimp grip, if the edge is far too sharp or the hold is “incut” (i.e., angled upwards) to use the open-hand grip. Although it may take some time for individual climbers to automatically convert to using the open-hand grip, overall strength in the forearms and hand will improve the more the climber uses it.

References
Burbach, M. (2004). Gym Climbing: Maximizing Your Indoor Experience. Movement Technique. The Mountaineers Books. Seattle, WA, 83-84.
Jones, G., Asghar, A., & Llewellyn, D. J. (2008). The epidemiology of rock-climbing injuries. British Journal of Sports Medicine, 42, 773-778.
Mallo, G. C., Sless, Y., Hurst, L. C., & Wilson, K. (2008). A2 and A4 pulley biomechanical analysis: comparison among gender and digit. Hand, 3, 13-16.
Rohrbough, J. T., Mudge, M. K., & Schilling, R. C. (2000). Overuse injuries in the elite rock climber. Medicine & Science in Sports & Exercise, 32, 1369-1372.
Schweizer, A. (2001). Biomechanical properties of the crimp grip position in rock climbing. Journal of Biomechanics, 34, 217-223.
Shim, J. K., Oliveira, M. A., Hsu, J., Huang, J., Park, J., & Clark, J. E. (2007). Hand digit control in children: age-related changes in hand digit force interactions during maximum flexion and extension force production tasks. Experimental Brain Research, 176, 374-386.
Vigouroux, L., Quaine, F., Labarre-Vila, A., & Moutet, F. (2006). Estimation of finger muscle tendon tensions and pulley forces during specific sport-climbing grip techniques. Journal of Biomechanics, 39, 2583-2592.
Wook Kim, S. Kun Shim, J., Zatsiorsky, V. M., & Latash, M. L. (2008). Finger interdependence: linking the kinetic and kinematic variables. Human Movement Science, 27, 408-422.
Wright, D. M., Royle, T. J., & Marshall, T. (2009). Indoor rock climbing: who get injured? British Journal of Sports Medicine, 35, 181-185.