Intrinsic hand muscle function, part 1: creating a functional grasp

What was the aim of the study?

People with a tetraplegia often report that regaining hand function would be most important when it comes to body functions that were lost due to the paralysis. For many people it is possible to regain certain finger functions with the help of a tendon transfer. The finger flexion for example can be recovered through a transfer of the tendon of the extensor carpi radialis longus (muscle that stretches the wrist) to the tendon of the flexor digitorum profundus (muscle that flexes index to little finger).
However, if only the flexion of the finger is recovered, the finger can only curl up. This means that the distance between the fingertips and the palm is very small during the grasping movement and therefore it is impossible to grasp larger objects. Therefore, it is important to increase the distance between the fingertip and the palm to obtain an optimal functional grasping movement. If the hand function is intact, the intrinsic hand muscles execute the fine coordination of the finger movements. These small muscles are situated in the palm of the hand and are attached to the sides of the four fingers (index to little finger). It was the goal of this study to find out, which role the intrinsic hand muscles play with regard to a functional grasping movement.

How did the researchers proceed?

Five hands were examined that were provided by deceased people. The tendons of the finger flexors were attached to a motor to simulate the grasping movement. The motor was adjusted so that the fingers of the open hand (stretched fingers) curled up into a fist. To simulate the different activity levels of the intrinsic hand muscles, five weights were attached to the intrinsic muscles (0 g = passive muscles, 125 g, 250 g, 375 g, and 500 g = maximal weight with which the fingers remain extended at the start of the movement).
The finger movement was video-taped. With that, the angles of each of the finger joints, the sequence of the joint movement and the distance between fingertip and palm were calculated. Then these measured values were compared between the activity levels of the intrinsic muscles.

What did the researchers discover?

In case of passive intrinsic muscles the fingers curled up (see figure 1): The fingers first flexed at the distal interphalangeal joints (exterior joints of the finger) and at the proximal interphalangeal joints (middle joints). Only in the second half of the movement the fingers also flexed in the metacarpophalangeal joints (joints that connect the fingers with the palm).

Figure 1: finger movement at three different activity levels of the intrinsic hand muscles

The movement pattern changed with strong activation of the intrinsic muscles: the fingers first flexed at the metacarpophalangeal joints and only after that they curled in (see figure 1). Through this movement pattern the distance between fingertip and palm was larger during the movement (see figure 2): Compared to the passive intrinsic muscles it increased by 2 cm with active intrinsic muscles.

Figure 2: position of the fingertip of the middle finger during the grasping movement

What do these findings mean?

The results of this study showed that the distance between the fingertips and the palm increases through the additional activation of the intrinsic finger muscles. The difference of two centimetres is quite big with regard to the grasping movement and this can be a decisive factor, if a person can grasp an object by himself or needs additional help. With active intrinsic muscles it is possible to grasp e.g. a standard soft drink can, whereas with passive muscles it is not possible. Consequently, the grasping movement was more functional with the additional intrinsic muscles.
This means, with regard to tendon transfers, that it is recommended to include also the intrinsic muscles when recovering the finger flexion to achieve an optimal functional grasping movement.

Who conducted and financed the study?

The study was funded and conducted by Swiss Paraplegic Research and the Muscle Physiology Laboratory of the University of California in San Diego (USA).