Altered phalanx force direction during power grip following stroke

Abstract

Many stroke survivors with severe impairment can grasp only with a power grip. Yet, little knowledge is available on altered power grip after stroke, other than reduced power grip strength. This study characterized stroke survivors' static power grip during 100 and 50 % maximum grip. Each phalanx force angular deviation from the normal direction and its contribution to total normal force was compared for 11 stroke survivors and 11 age-matched controls. Muscle activities and skin coefficient of friction were additionally compared for another 20 stroke and 13 age-matched control subjects. The main finding was that stroke survivors gripped with a 34 % greater phalanx force angular deviation of 19° ± 2° compared to controls of 14° ± 1° (p < .05). Stroke survivors' phalanx force angular deviation was closer to the 23° threshold of slippage between the phalanx and grip surface, which may explain increased likelihood of object dropping in stroke survivors. In addition, this altered phalanx force direction decreases normal grip force by tilting the force vector, indicating a partial role of phalanx force angular deviation in reduced grip strength post-stroke. Greater phalanx force angular deviation may biomechanically result from more severe underactivation of stroke survivors' first dorsal interosseous and extensor digitorum communis muscles compared to their flexor digitorum superficialis or somatosensory deficit. While stroke survivors' maximum power grip strength was approximately half of the controls, the distribution of their remaining strength over the fingers and phalanges did not differ, indicating evenly distributed grip force reduction over the entire hand.

Figure 1

Stable grip without slippage requires that each phalanx’s force not deviate from the direction normal to the object surface more than the cone of friction angle (θ), determined as arctangent of the coefficient of friction (COF) between the hand and grasped object (MacKenzie and Iberall 1994). Phalanx force deviations outside the cone of friction can lead to hand-object slippage.

Figure 2

Mean phalanx force angular deviation, shown as the spread of the fan, was significantly greater for stroke survivors compared with healthy controls. Mean grip forces were substantially reduced for stroke survivors compared with healthy controls, as seen by the shorter fan height for stroke survivors.

Figure 3

Phalanx force angular deviation was significantly greater for stroke survivors compared with controls (ANOVA subject group main effect with p<.05) (effort levels, fingers, phalanges, and subjects pooled) (a), for both 50% and maximum grip effort (b), for all three phalanges (c), and especially for the thumb, index, and little fingers (ANOVA, subject group and finger interaction with p<.05, posthoc significance marked with stars) (d). Non-transformed mean ± SE data is shown in the figure.

Figure 4

Stroke survivors’ increased phalanx force deviation was significantly correlated with lower motor function scores of the Chedoke-McMaster Assessment Hand Section (Pearson Correlation, r = −.84 with p<.05) (a) and the hand and wrist subdivision of the Fugl-Meyer Assessment (Pearson Correlation, r = −.79 with p<.05) (b).

Figure 5

The distribution of phalanx normal force across the phalanges (a and c) and fingers (b and d) for stroke and control subjects. Percent contribution (c and d) of the individual phalanges to total normal force was not significantly dependent upon the interaction of subject group and phalanx or the interaction of subject group and finger (ANOVA with p>.05) (d). Non-transformed mean ± SE data is shown in the figure.

Figure 6

Mean ± SE EMG was significantly reduced for all muscles of the stroke survivors compared with healthy controls (a). Relative to the FDS EMG, mean ± SE FDI and EDC EMG were significantly reduced for stroke survivors compared with controls (significant subject group and finger muscle interaction with p<.05, significant difference in relative FDI and EDC EMG between stroke and control with Tukey post-hoc p<.05) (b), showing an altered muscle activity pattern with a particularly underactivated intrinsic FDI muscle and the extrinsic EDC muscle for stroke survivors compared with controls. Non-transformed data is shown in the figure.