Presented at NACOB 98: North American Congress on Biomechanics Canadian Society for Biomechanics - American Society of Biomechanics University of Waterloo Waterloo, Ontario, Canada August 14-18, 1998

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THE CONTRIBUTION OF FINGER FLEXOR MUSCLES
TO THE FINGER JOINT MOMENTS IN ISOMETRIC TASKS

Z. M. Li, M. L. Latash, V. M. Zatsiorsky
Department of Kinesiology, The Pennsylvania State University
University Park, PA 16802

INTRODUCTION

Several biomechanical models of the hand and fingers have been developed (Chao et al. 1976; Ketchum et al. 1978; An et al. 1979; Buchner et al. 1988; Darling et al. 1994; Brook et al. 1995). The purposes of the current study are (1) to develop a method for evaluating forces of the finger flexor muscles -- flexor digitorum profundus (FDP), flexor digitorum superficialis (FDS), and intrinsic muscles (INT), and (2) to determine the contribution of the flexor muscles to the finger joint moments.

METHODS

Experimental procedures

Four force sensors (model 208A03, Pizeotronic) were hung in an inverted U-shape steel frame for the recording of MVC forces of four fingers. The hand was configured with 20 degrees of extension of the wrist joint, 20 degrees of flexion of the metacarpophalangeal (MCP) joint, and fully extended position of the proximal and distal interphalangeal (PIP and DIP) joints. The wire-sensor units were vertically aligned during the force production. The locations of the force application were at the middle of the distal phalanges. Three consecutive trials were performed by ten subjects. See Figure 1.

Biomechanical analysis

A simplified biomechanical model of a finger was proposed (Figure 2). Several assumptions were made in the model. (1) The FDP has insertion on the distal phalanx. It contributes to the joint moments at the DIP, PIP, and MCP joints. (2) The FDS has insertion on the intermediate phalanx, and contributes to the joint moments at the PIP and MCP joints. (3) The intrinsic muscles were lumped and viewed as a single equivalent muscle (INT) producing moment solely at the MCP joint. (4) The tendon tension by the extensors and the ligament action around the joints was ignored. (5) The DIP, PIP, and MCP joints were assumed to be ideal pivot joints.

Figure 1. Experimental setup. (a) device; (b) hand configuration.

Figure 2. Biomechanical model with tendon tensions of the FDP, FDS, and intrinsic muscle group (INT) that acting across the DIP, PIP, and MCP joints.

Based on the biomechanical model, the muscle forces of a finger can be calculated as follows:

where P is the recorded external force; L_DP, L_IP , and L_PP are segment lengths of the distal, intermediate, and proximal phalanx (Buchholz et al. 1992); b_i,j is the moment arm (An et al. 1979) of muscle i (i = FDP, FDS, and INT) at joint j (j = DIP, PIP, and MCP).

RESULTS

The average data, including hand length and registered external forces, were used for the calculation of the internal muscle forces. Figure 3 shows the forces of the individual compartments of the extrinsic muscles (FDP and FDS) and intrinsic muscle groups that serve individual fingers. The middle finger had relatively large FDS forces. The force of the intrinsic muscles of the ring finger was very small. The intrinsic muscles of the index and little finger produced relatively large forces as compared to the middle and ring fingers.

Figure 3. Forces of flexor muscles of individual fingers.

The moment contributions of the extrinsic and intrinsic muscles to the PIP and MCP joints are shown in Figure 4, and Figure 5. The relative contribution of the FDP and FDS to the moment at the PIP joint was different for individual fingers. At the MCP joint, the contribution of the intrinsic muscles to the moment equilibrium was relative small for all fingers. Note that for the ring finger the extrinsic muscles contributed more than 97% of the MCP joint moment.

Figure 4. Contribution, %, of the FDP, FDS, and INT (= 0) to the moment at the PIP joint.

Figure 5. Contribution, %, of the FDP, FDS, and INT to the moment at the MCP joint.

SUMMARY

The method for calculating finger joint moments and internal muscle forces is proposed. The contribution of FDP, FDS, and INT to the joint moments was calculated. The relative contribution of individual muscles to the finger joint moment was different for different fingers. The extrinsic muscles are the major contributors to the finger joint moments. The contribution of the intrinsic muscles to the moments at the MCP joint was relatively small (<16%).

REFERENCES

An KN et al. (1979) J Biomech 12:775-788

Brook N et al. (1995) Med Eng Phys 17(1):54-63

Buchholz B et al. (1992) Ergonomics 35:261-273

Buchner HJ et al. (1988) J Biomech 6:459-468

Chao EY et al. (1976) J Biomech 9:387-396

Darling WG et al. (1994) J Biomech 27(4):479- 491

Ketchum LD et al. (1978) J Hand surg 3(6):571- 578