Unusual rupture of the middle finger flexor digitorum superficialis tendon in a climber: Biomechanical analysis.

Sport climbing is increasingly popular and consultations by climbers in hand surgery departments are on the increase. The pathologies related to this sport concern essentially the pulley system, tendons being rarely affected. We report the case of a male climber who presented an atypical rupture of the flexor superficialis tendon in his left middle finger sustained when using an atypical climbing grip technique: the "hook grip". This consists in extension of the metacarpophalangeal joints and maximal flexion of the proximal interphalangeal joints with force exerted only on middle phalanx of the middle finger. A biomechanical analysis using finger musculoskeletal modeling was performed to compare the hook grip to other grips, and the patient's recovery performance was assessed. Adapted functional treatment with physiotherapy seems to have been a good option for the treatment of this atypical lesion since the patient recovered normal use of his finger in daily life. He recovered maximal force in climbing holds. The biomechanical analysis confirmed that the atypical "hook grip" was likely at the origin of the rupture, since flexor digitorum superficialis tendon force for this grip is greater than in other climbing grip techniques. The "hook grip" seems to be dangerous and should be used cautiously by climbers to prevent similar pathology. Additionally, the patient should henceforth be careful when climbing, since the biomechanical model showed that the remaining flexor digitorum profundus tendon was overused.

A scaling method to individualise muscle force capacities in musculoskeletal models of the hand and wrist using isometric strength measurements.

Because the force-generating capacities of muscles are currently estimated using anatomical data obtained from cadaver specimens, hand musculoskeletal models provide only a limited representation of the specific features of individual subjects. A scaling method is proposed to individualise muscle capacities using dynamometric measurements and electromyography. For each subject, a strength profile was first defined by measuring net moments during eight maximum isometric contractions about the wrist and metacarpophalangeal joints. The capacities of the five muscle groups were then determined by adjusting several parameters of an initial musculoskeletal model using an optimisation procedure which minimised the differences between measured moments and model estimates. Sixteen volunteers, including three particular participants (one climber, one boxer and one arthritic patient), were recruited. Compared with the initial literature-based model, the estimated subject-specific capacities were on average five times higher for the wrist muscles and twice as high for the finger muscles. The adjustments for particular subjects were consistent with their expected specific characteristics, e.g. high finger flexor capacities for the climber. Using the subject-specific capacities, the model estimates were markedly modified. The proposed protocol and scaling procedure can capture the specific characteristics of the participants and improved the representation of their capacities in the musculoskeletal model.

Comparison of three local frame definitions for the kinematic analysis of the fingers and the wrist.

Because the hand is a complex poly-articular limb, numerous methods have been proposed to investigate its kinematics therefore complicating the comparison between studies and the methodological choices. With the objective of overcoming such issues, the present study compared the effect of three local frame definitions on local axis orientations and joint angles of the fingers and the wrist. Three local frames were implemented for each segment. The "Reference" frames were aligned with global axes during a static neutral posture. The "Landmark" frames were computed using palpated bony landmarks. The "Functional" frames included a flexion-extension axis estimated during functional movements. These definitions were compared with regard to the deviations between obtained local segment axes and the evolution of joint (Cardan) angles during two test motions. Each definition resulted in specific local frame orientations with deviations of 15° in average for a given local axis. Interestingly, these deviations produced only slight differences (below 7°) regarding flexion-extension Cardan angles indicating that there is no preferred method when only interested in finger flexion-extension movements. In this case, the Reference method was the easiest to implement, but did not provide physiological results for the thumb. Using the Functional frames reduced the kinematic cross-talk on the secondary and tertiary Cardan angles by up to 20° indicating that the Functional definition is useful when investigating complex three-dimensional movements. Globally, the Landmark definition provides valuable results and, contrary to the other definitions, is applicable for finger deformities or compromised joint rotations.

Potential effects of racket grip size on lateral epicondilalgy risks.

The effects of tennis racket grip size on the forces exerted by muscles affecting lateral epicondylalgia (LE) were assessed in this study. Grip forces and joint moments applied on the wrist were quantified under three different handle size conditions, with and without induced muscle fatigue for intermediate and advanced players. The obtained experimental results were then used as input data of a biomechanical model of the hand. This simulation aimed to quantify the impact of grip strength modulation obtained in the experiment on the wrist extensor muscle forces. Our results show that there is an optimal grip diameter size defined as the handle inducing a reduced grip force during the stroke, in both fatigued and non-fatigued sessions. The results of the simulation suggested that extensor muscles were highly employed during forehand strokes, which confirms that the mechanical overuse of extensor tendons is a potential risk factor for tennis elbow occurrence. The handle grip size appeared to be a significant factor to reduce this extensor tendon loading. This suggests that grip size should be taken into account by players and designers in order to reduce the mechanical risk factors of overuse injury occurrence.

The thumb during the crimp grip.

During rock-climbing, fingers grasp holds of various shapes with high force intensities. To ideally place the fingertips on the holds, the thumb is sometimes positioned on the nail of the index finger. This allows using the thumb as an additional actuator by exerting a supplementary force in the same direction as the index, middle, ring and little fingers. This study analysed how the forces exerted by the fingers are modified by the additional action of the thumb. The results showed that the thumb increases the resultant forces exerted on the hold. It was shown that the pathology risks of the middle, ring and little fingers were not modified in this condition. The finger force sharing was totally re-organized due to the support of the thumb. This led to the conclusion that the central nervous system organised the association of the 5 fingers. The results were discussed in regard to the established theories of the virtual fingers and the neutral line of the hand.

Effect of object width on precision grip force and finger posture.

This study aimed to define the effect of object width on spontaneous grasp. Participants held objects of various masses (0.75 to 2.25 kg) and widths (3.5 to 9.5 cm) between thumb and index finger. Grip force, maximal grip force and corresponding finger postures were recorded using an embedded force sensor and an optoelectronic system, respectively. Results showed that index finger joints varied to accommodate the object width, whereas thumb posture remained constant across conditions. For a given object mass, grip force increased as a function of object width, although this result is not dictated by the laws of mechanics. Because maximal grip force also increased with object width, we hypothesise that participants maintain a constant ratio between grip force and their maximal grip force at each given width. Altogether we conclude that when the task consists in manipulating objects/tools, the optimal width is different than when maximal force exertions are required.

Maximal resultant four fingertip force and fatigue of the extrinsic muscles of the hand in different sport climbing finger grips.

This study investigates the effect of simulated sport climbing finger grips on the resultant four fingertip force and the rate of fatigue of finger flexor muscles. Six elite sport climbers sat on a chair with the right forearm placed in a handgrip dynamometer modified so that only the fingertips applied direct force. They were asked to perform three maximal voluntary contractions (MVC). After ten minutes, they had to reach 80 % of the peak MVC intermittently with a 5 s contraction followed by 5 s of rest for 20 repetitions. Two common sport climbing finger grips were tested: the "slope" grip and the "crimp" grip. In the "crimp" grip, the distal interphalangeal joint (DIP) is hyper-extended and the proximal interphalangeal joint (PIP) is flexed from 90 degrees to 100 degrees . In the "slope" grip, DIP is flexed from 50 degrees to 70 degrees and PIP is flexed just slightly. The surface EMG of the hand extrinsic flexors and the maximal resultant four fingertip force were recorded. Results show that the maximal resultant four-fingertip force does not depend on the type of finger grips (on average 420 N, p > 0.05). EMG median frequency of finger flexor muscles and resultant four fingertip force rate decrease are similar between both sport climbing finger grips (p > 0.05). This shows that the fatigue rate is not dependent upon the sport climbing finger grips. In conclusion, the results suggest that the use of the "crimp" or the "slope" grip does not provide any benefit with respect to muscular fatigue in sport climbing.

Finger flexors fatigue in trained rock climbers and untrained sedentary subjects.

The present series of experiments were conducted to access the surface EMG frequency parameters during repeated fingertip isometric contractions to determine if they can be used as a fatigue index under specific grip used in rock climbing. Electromyograms of the finger flexors and extensors were characterised in ten elite climbers and ten non-climbers. The exercise consisted in reaching 80 % of maximal isometric finger force as quickly as possible intermittently with a 5-s contraction followed by 5-s of rest until exhaustion (i. e. when the subject was unable to maintain 80 - 70 % MVC force range for the 5 s). The results clearly indicate that expert climbers performed significantly greater fingertip force than sedentary subjects (420 +/- 46 N vs. 342 +/- 56 N). This force was maintained during twelve repetitions (12.88 +/- 4.96) in sedentary subjects, whereas the climbers maintained the force during nineteen repetitions (19.33 +/- 4.84). The median frequency of both the flexor and extensor EMG power spectra decreased during fatiguing isometric contractions, but at different rates in climbers and non-climbers. In non-climbers, the results replicated previous findings, whereas in climbers the results were novel.