Bilateral multifinger deficits in symmetric key-pressing tasks.

Maximal voluntary force during simultaneous bilateral and multifinger exertion has been shown to be smaller than the sum of unilateral or single-finger exertions. The goal of this study was to study the force deficit associated with bilateral multifinger tasks. Eight normal college students performed four types of maximal isometric key-pressing tasks: (1) unilateral single-finger, (2) bilateral single-finger, (3) unilateral multifinger, and (4) bilateral multifinger. Forces produced by the index (I), middle (M), ring (R), and little (L) fingers and surface electromyography (EMG) of extrinsic finger flexors were recorded. Multifinger deficit (MFD) was defined as the percentage difference between the force (or EMG) produced by a set of fingers and the sum of the forces (or EMGs) produced by the individual fingers in their unilateral single-finger tasks. Bilateral deficit (BLD) was defined as the percentage difference between the force (or EMG) produced by a set of fingers and the sum of the forces (or EMGs) produced by the finger subsets of the left and right hands. Significant BLD and MFD in force and EMG were found for all bilateral multifinger tasks and some of the bilateral single-finger tasks. Both BLD and MFD were dependent on the number of fingers involved. BLD ranged from 3% to 22.7% for force and from 8.9% to 31.0% for EMG, including bilateral single-finger and bilateral multifinger tasks. MFDs in force during bilateral I-, IM-, IMR-, and IMRL-finger tasks were 13.2%, 37.8%, 53.2%, 52.3%, respectively; and the corresponding MFDs in EMG were 11.7%, 51.3%, 67.6%, and 71.0%, respectively. BLD and MFD in EMG were found to vary in parallel with the corresponding force deficits. It was suggested that the neural ceiling effect remains the most plausible mechanism underlying the observed deficits. The central nervous system is unable to activate maximally a large number of muscle groups at the same time during tasks involving multiple body parts. During bilateral multifinger tasks, the ceiling effect may be organized hierarchically: (1) a certain limited neural drive is shared bilaterally, leading to a BLD; (2) at each hand, a certain limited neural drive is shared by multiple fingers, leading to MFD within a hand; (3) the deficits at bilateral and unilateral multifinger levels are cumulative during bilateral multifinger tasks, leading to a higher deficit associated with the tasks.