Fig 1.
(A) Measurement set-up showing the PowerGlove with 18 sensor units on the fingers and hand (Kortier et al. 2014 [24], van den Noort et al. 2016 [25]). The sensors were attached to the dorsal side of the left hand (3 sensors), on the metacarpal, proximal and distal phalanges of the thumb (3 sensors) and on the proximal, middle and distal phalanges of the index, middle, ring and little fingers (12 sensors) using small Velcro straps. The arm was placed on a custom-made arm-rest during the measurements. (B) A system with small wooden bars provided restriction of the non-instructed fingers, as part of the arm-rest.
Table 1.
Flexion finger tasks: subjects were asked to perform various finger movements through the full range of motion (ROM) until the tip(s) of the finger(s) touched the palm of the hand.
Fig 2.
(A) Typical example of the movement trajectory (angle, y-axis) in time (x-axis) of the four fingers during active flexion of an instructed finger (index). (B) Corresponding typical example of enslaving in non-instructed fingers (y-axis) during active flexion of the instructed index finger (x-axis). The range of motion angle of a finger was calculated as the sum of the MCP, PIP and DIP joint angle excursions. Enslaving threshold was set at a change of more than 5 degrees of motion (black solid line) in the non-instructed finger. Based on this threshold, the range of independent movement of the instructed finger was defined, shown with the vertical thin dotted-dashed lines corresponding to the movement threshold of the non-instructed finger(s).
Table 2.
The ΣROM (in degrees) during all active finger flexion tasks averaged over all 13 subjects.
Table 3.
The enslaving effect of the non-instructed fingers over all 13 subjects (in % of ΣROM adjacent instructed finger).
Table 4.
Differences and (a)symmetry in enslaving effect of non-instructed finger(s) and in range of independent movement of instructed finger(s) in single- and multi- finger flexion tasks.
Fig 3.
ΣROM (mean and standard deviation) of the fingers during single-finger movement tasks (n = 13, ACT = active flexion (blue bar), RES = flexion with restriction in non-instructed fingers (orange bar)).
The asterisk indicates a significant difference in ACT versus RES.
Fig 4.
(A) Individuation indices of each instructed finger during single finger movement tasks in 13 young healthy subjects. The closer the value is to 1, the more independent a finger could be moved. The boxplots show the median (red line), 1st and 3rd quartiles (blue box), smallest and largest values (whisker with black lines) and outliers (red cross, >1.5 interquartile range) over the 13 subjects. The significantly highest individuation index, indicating the highest level of independence compared to the other fingers, was found for the index finger. (B) δ individuation indices of each instructed finger over the movement trajectory, per 10% ΣROM. At 70% ΣROM, the δ individuation index of the index finger decreased significantly compared to values below 70% ΣROM. From 60%-80% ΣROM, the δ individuation index of the ring finger was significantly lower to values outside that range. Also, the index finger had highest δ individuation indices compared to the little finger from 10% onwards, and to the middle finger from 20% onwards. At 40% and 60% ΣROM the middle finger showed a lower δ individuation index compared to the ring finger.
Fig 5.
Boxplots showing the range of independent movement of the instructed finger(s) of all 13 subjects.
This was defined as follows: where in the ΣROM (%) of the instructed finger(s) (vertical axes) the non-instructed finger(s) start(s) to move (horizontal axes). Data is presented for all finger movement tasks (single and multi; i = index, m = middle, r = ring, l = little). For multi-finger movement tasks, the used ΣROM of one of the adjacent instructed fingers is used. Individual results (mean over trials) per subject are presented in the blue dots. The boxplots show the median (red line), 1st and 3rd quartiles (blue box), smallest and largest values (whisker with black lines) and outliers (red cross, >1.5 interquartile range). Start of enslaving is defined as a change of more than 5deg finger movement (ΣROM) of the non-instructed finger(s) [26, 27]. A value (on the vertical axis) close to zero means enslaving early in the ΣROM of the instructed finger, whereas a high value means late or no enslaving effect, i.e. near or at the end of the ΣROM of the instructed finger. If movement of the non-instructed finger was less than 5deg, i.e. no enslaving effect, the enslaving value has been presented as being at the end ΣROM (100%) of the instructed finger because of visualisation purposes. In single finger movement tasks, it can be seen that the adjacent non-instructed finger starts to move first before the other non-instructed finger(s) start(s) to move.