SHORT REPORT ABSTRACT: Force production may be influenced by the phase of ventila- tion during which force is exerted. To examine the potential influences of ventilation on variability in maximal force measurements, we recorded peak isometric forces of the finger flexors during normal breathing, forced inspi- ration, forced expiration, and the Valsalva maneuver in 14 healthy adult subjects. The peak force increased significantly from forced inspiration to forced expiration (about 10%). Both forced expiration and inspiration re- sulted in increases in the flexor/extensor cocontraction ratio, whereas the Valsalva maneuver had no significant effects on maximal force or cocon- traction ratio. Thus, this study clearly demonstrates the effects of ventilation on maximal finger force-generating capability. Ventilation needs to be con- trolled for accurate assessments of maximal force. Muscle Nerve 34: 651– 655, 2006 INFLUENCES OF VENTILATION ON MAXIMAL ISOMETRIC FORCE OF THE FINGER FLEXORS SHENG LI, MD, PhD, and JAMES J. LASKIN, PhD Motor Control Laboratory, School of Physical Therapy and Rehabilitation Science, University of Montana, Missoula, Montana 59812, USA Accepted 21 April 2006 Control of breathing has been used to enhance force production during strength exercise and train- ing. If maximal force is exerted while inhaling, ex- haling, or making a forced expiration against the closed glottis (the Valsalva maneuver), the force magnitude is expected to increase from inspiration to expiration to the Valsalva maneuver. 19 The effects, however, have not been quantified, and the under- lying mechanisms remain unknown. Although it might be considered a useful breathing technique for ultimate force production, the Valsalva maneu- ver causes increased intrathoracic pressure with con- comitant decreased venous return to the heart and alterations in arterial pressure and heart rate. 8,15,19,20 These systemic responses might be harmful and therefore contraindicated in individuals with cardiac disease. The magnitude of maximal voluntary contrac- tion (MVC) is often viewed as a reliable measure of muscle strength and is often used to assess neuro- muscular and musculoskeletal functions. Many fac- tors, however, may influence accurate measurement of this value, resulting in large variability within sub- jects 1 and among patient populations. 9 These factors include biomechanical factors (e.g., joint angle 10 and transducer position 18 ), time of day, 14 and phase of the menstrual cycle. 17 Even when these factors are controlled, large variability may still occur in MVC measurements between trials by the same subject. 3 Different phases of ventilation could influence the maximal force-generating capabilities of muscles. 19 Therefore, to provide accurate assessment of maxi- mal muscle strength, it is necessary to quantify the effects of different phases of ventilation on maximal strength. The purpose of the present study was to assess the effects of different phases of ventilation on the maximal force-generating capability of distal- limb muscles. METHODS Fourteen healthy volunteers (5 men, 9 women; age, 26.8  6.6 years; age range, 22– 42 years) took part in the experiments. All subjects gave written informed and the study was approved by our institutional re- view board. During testing, subjects sat on an adjustable chair and breathed through a facemask connected to a pneumotach system (Series 1110A; Hans Rudolph, Kansas City, Missouri). After skin preparation, differ- ential surface electrodes (DelSys, Boston, Massachu- Abbreviations: ANOVA, analysis of variance; EDC, extensor digitorum com- munis; EMG, electromyography; FDS, flexor digitorum superficialis; MVC, maximal voluntary contraction; MVC N , MVC E , MVC I , MVC V , MVC during nor- mal breathing, forced expiration, forced inspiration, and Valsalva maneuver, respectively Key words: finger flexion; isometric force; maximal voluntary contraction (MVC); respiration; Valsalva maneuver Correspondence to: S. Li; e-mail: sheng.li@umontana.edu © 2006 Wiley Periodicals, Inc. Published online 12 June 2006 in Wiley InterScience (www.interscience.wiley. com). DOI 10.1002/mus.20592 Short Reports MUSCLE & NERVE November 2006 651