Bilateral multifinger deficits in symmetric key-pressing tasks.

Area of Science:

• Neuroscience and Biomechanics
• Human Motor Control
• Physiology

Background:

• Maximal voluntary force generation is reduced when performing simultaneous bilateral or multifinger tasks compared to unilateral or single-finger exertions.
• Understanding force deficits in complex motor tasks is crucial for rehabilitation and performance optimization.

Purpose of the Study:

• To investigate the force deficit, termed bilateral deficit (BLD) and multifinger deficit (MFD), during simultaneous bilateral and multifinger isometric key-pressing tasks.
• To examine the relationship between force production deficits and electromyographic (EMG) activity during these tasks.
• To explore the underlying neural mechanisms, specifically the neural ceiling effect, contributing to these force deficits.

Main Methods:

• Eight healthy college students performed maximal isometric key-pressing tasks involving unilateral single-finger, bilateral single-finger, unilateral multifinger, and bilateral multifinger exertions.
• Recorded forces produced by individual fingers (index, middle, ring, little) and surface EMG of extrinsic finger flexors.
• Quantified BLD and MFD as percentage differences between actual and summed individual/subset finger forces and EMG signals.

Main Results:

• Significant bilateral deficits (BLD) and multifinger deficits (MFD) in both force and EMG were observed across various bilateral multifinger and some bilateral single-finger tasks.
• Both BLD and MFD were found to be dependent on the number of fingers involved in the task.
• Multifinger deficits (MFD) in force increased substantially with the number of fingers engaged, reaching over 50% for four-finger tasks. EMG deficits paralleled force deficits.
• Bilateral deficits (BLD) ranged from 3% to 22.7% for force and 8.9% to 31.0% for EMG.

Conclusions:

• The neural ceiling effect, where the central nervous system has a limited capacity to maximally activate numerous muscle groups simultaneously, is the most plausible explanation for the observed force and EMG deficits.
• Hierarchical organization of the neural ceiling effect contributes to deficits: limited neural drive shared bilaterally, then limited drive shared among fingers within each hand.
• These deficits are cumulative, leading to greater force reductions in complex bilateral multifinger tasks compared to simpler tasks.