Forced ventilation increases variability of isometric finger forces.

The study aimed to assess the effects of forced ventilation on variability of the index finger force at the submaximal levels. Fourteen (6 men, 8 women) healthy subjects were instructed to perform self-initiated forced inspiration and forced expiration, the Valsalva maneuver and normal breathing while sustaining 15%, 30%, and 45% of maximal voluntary contraction (MVC) by the index finger. Standard deviation (S.D.) of finger forces increased significantly with the level of force. At each level of force, the mean force was not significantly changed under different breathing conditions. S.D. and coefficient variation (CV) during forced inspiration and expiration was significantly greater than S.D. and CV during normal breathing and the Valsalva maneuver at each force level. No significant differences in S.D. and CV were found between forced inspiration and expiration or between normal breathing and the Valsalva maneuver. Force variability synchronized with the initiation of forced inspiration and expiration, but not with the ventilation data during the Valsalva maneuver or normal breathing. These findings demonstrate clearly that finger force variability is affected by specific ventilation patterns at submaximal force levels. Therefore, assessment of force variability should consider the influence of ventilation.