Knee Extensor Electromyographic Activity-to-Work Ratio is Greater with Isotonic Than Isokinetic Contractions

Objective: To determine whether isotonic or isokinetic contractions produced greater electromyographic (EMG) activity per unit of work during isotonic and isokinetic knee-extension exercise. Design and Setting: Subjects performed three 3-second maximal voluntary isometric contractions of the dominant knee extensors for EMG normalization. Exercise testing performed on the Biodex System 3 Dynamometer involved 10 isokinetic contractions at 180°·s-1 and 10 isotonic contractions with the resistance set at 50% of the previously recorded maximal voluntary isometric contraction. Subjects: Recreationally active college students (10 men and 11 women). Measurements: Surface EMG signals were collected from the vastus medialis and lateralis muscles and then integrated (IEMG) over the concentric phase of each repetition for both exercises. The IEMG was divided by the total work performed during the concentric phase for each exercise (IEMG/W). Results: We analyzed the IEMG/W data using a 1-between (sex), 2-within (exercise and muscle) repeated-measures analysis of variance. There was a significant main effect for exercise, with the isotonic IEMG/W value being significantly greater than the isokinetic IEMG/W value. Additionally, the IEMG/W relationship did not appear to be affected by sex or individual muscle tested. Conclusions: Per unit of work performed, the isotonic contractions resulted in greater motor unit recruitment or an increased rate of firing, or both. This finding may have implications for the early phase of rehabilitation, when goals include complete motor unit recruitment of injured or atrophied muscles.