Divergent muscle fatigue during unilateral isometric contractions of dominant and non-dominant quadriceps

Objectives: We examined mechanical and electromyographic responses of unilateral dominant and non-dominant m. quadriceps femoris during fatiguing submaximal isometric contractions and early recovery. Design: Within subjects randomized. Methods: Healthy males (n = 18, age: 20 +/- 2 yr, height: 181 +/- 7 cm, and body mass: 79.4 +/- 10.5 kg) attended two sessions. Leg dominance was based on the preferred kicking leg. Maximal voluntary isometric force, endurance time and force fluctuations during a 20%MVIF until exhaustion were measured simultaneously with surface electromyography (EMG) of m.vastus lateralis and m.vastus medialis at a knee angle of 900 as well as the MVIF 20 s after exhaustion (early recovery). Results: The maximal voluntary isometric force of dominant m. quadriceps femoris was 4.6% higher (D: 749 +/- 178 N, ND: 716 +/- 184 N, and P = 0.01). The m. quadriceps femoris of both legs had similar endurance times during the 20%MVIF (D: 367 +/- 157 s, ND: 381 +/- 153 s, and P = 0.40). Force fluctuations during the 20%MVIF increased over time (two-way ANOVA, P < 0.05) with no differences between legs at comparable time points. Changes in median frequency and root mean square of m.vastus lateralis and m.vastus medialis during the 20%MVIF were similar for both legs. However, after the 20%MVIF, early recovery, quantified by the fatigue index, showed larger force loss for dominant m. quadriceps femoris (D: 39.9 +/- 15.7%, ND: 34.8 +/- 16.0%, and P < 0.05). Conclusions: Muscle fatigue that develops during submaximal sustained isometric contractions may not be accurately quantified by force loss soon after exhaustion. The present study has implications for unilateral studies to examine mechanisms of muscle fatigue. (C) 2012 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.