Timing and extent of finger force enslaving during a dynamic force task cannot be explained by EMG activity patterns

Abstract

Finger enslaving is defined as the inability of the fingers to move or to produce force independently. Such finger enslaving has predominantly been investigated for isometric force tasks. The aim of this study was to assess whether the extent of force enslaving is dependent on relative finger movements. Ten right-handed subjects (22–30 years) flexed the index finger while counteracting constant resistance forces (4, 6 and 8 N) orthogonal to the fingertip. The other, non-instructed fingers were held in extension. EMG activities of the mm. flexor digitorum superficialis (FDS) and extensor digitorum (ED) in the regions corresponding to the index, middle and ring fingers were measured. Forces exerted by the non-instructed fingers increased substantially (by 0.2 to 1.4 N) with flexion of the index finger, increasing the enslaving effect with respect to the static, pre-movement phase. Such changes in force were found 260–370 ms after the initiation of index flexion. The estimated MCP joint angle of the index finger at which forces exerted by the non-instructed fingers started to increase varied between 4° and 6°. In contrast to the finger forces, no significant changes in EMG activity of the FDS regions corresponding to the non-instructed fingers upon index finger flexion were found. This mismatch between forces and EMG of the non-instructed fingers, as well as the delay in force development are in agreement with connective tissue linkages being slack when the positions of the fingers are similar, but pulled taut when one finger moves relative to the others. Although neural factors cannot be excluded, our results suggest that mechanical connections between muscle-tendon structures were (at least partly) responsible for the observed increase in force enslaving during index finger flexion.

Understanding the constraints of finger motor control

“…This was interpreted as evidence for a predominantly neural origin of enslaving. From these and other recent studies on finger enslaving (Mirakhorlo et al, 2017a;Sanei and Keir, 2013;van den Noort et al, 2016), the view emerges that both neural and musculoskeletal characteristics impose constraints on finger motor control, but that their relative importance depends on the specific conditions of the task.…”

Increased enslaving in elderly is associated with changes in neural control of the extrinsic finger muscles

“…Subjects were requested to sit on an adjustable chair resting their forearm on a horizontal platform leaving the wrist free to move, as described in detail previously (Mirakhorlo et al 2017 ). Their seating position was adjusted such that the elbow was in approximately 90° flexion.…”

“…Muscle regions of FDS and ED corresponding to the different fingers were palpated for each subject individually. Four electrodes were used for each muscle region to identify the best electrode location of each muscle region (Mirakhorlo et al 2017 ). Electrodes (KendallTM H69P Cloth Electrodes, Covidien, Zaltbommel, The Netherlands) were placed on each of the regions corresponding to the index (II), middle (III) and ring (IV) fingers for both ED and FDS muscles.…”

“…Thus, the type of task may determine which factors are the main cause of enslaving. Recently, an experiment was performed by our group, in which young subjects were instructed to flex their index finger while overcoming a constant sub-maximal resistance force (Mirakhorlo et al 2017 ). Obtained results indicated that connective tissue linkages are (at least partly) responsible in limiting finger independency.…”

“…Based on the results of Van Beek et al ( 2017 ), we expected more coactivation of the finger specific muscle regions. For this purpose, we compared older subjects to young subjects of which the data were published previously (Mirakhorlo et al 2017 ). We measured forces exerted at the finger tips and EMG activities of the different regions of flexor digitorum superficialis (FDS) and extensor digitorum (ED) muscles corresponding to the different fingers in response to isotonic flexion of the index finger.…”