A methodologic approach for the comparison between dynamic contractions: influences on the neuromuscular system

Although isotonic and isokinetic exercises are commonly used in sports medicine and rehabilitation, studies comparing their effects on the neuromuscular system have provided conflicting results.

To compare responses of the neuromuscular system to isotonic and isokinetic contractions by controlling the total external amount of work performed and the mean angular movement velocity.

A familiarization session was followed by isotonic and isokinetic sessions of tests performed on an isokinetic dynamometer. Each subject participated in 3 sessions.

A sport sciences research laboratory.

Nine healthy adult males with no history of knee injury.

The isotonic session consisted of 3 sets of 8 knee extensions at 80% of each subject's maximal voluntary isotonic contraction. The isokinetic session involved 3 sets of n knee extensions at a preset velocity equivalent to the mean velocity measured during the corresponding isotonic sets; n represented the number of repetitions subjects had to achieve to equalize the total external amount of work performed during the corresponding isotonic sets.

We recorded mechanical parameters, n, and surface electromyographic signals from the vastus medialis, rectus femoris, and vastus lateralis muscles. Then root mean square, mean power frequency, and neuromuscular efficiency values were calculated for each repetition.

As expected, the total external amount of work and mean angular velocity were similar between the isotonic and isokinetic sessions. The number of repetitions performed was equivalent in both sessions. In addition, although no "shift" of mean power frequency occurred, mean neuromuscular efficiency decreased linearly with repetitions for both modes in no differentiated way.

Standardization of isotonic and isokinetic contractions based on total external amount of work and movement velocity is possible. This method can be applied by future investigators aiming to compare chronic effects of these 2 contraction modes on the neuromuscular system.