Optimal burst duration during a facilitated quadriceps femoris contraction

To determine the most efficient burst duty cycle for eliciting an involuntary quadriceps femoris contraction in healthy subjects. This information will allow clinicians to make an informed decision about the optimal burst duty cycle based upon a specific treatment goal. The logical goal for such a treatment choice is to enhance motor unit recruitment in an effort to maintain postoperative or postinjury strength, when voluntary contractions may be less effective.

Single-group and 5-measures design. All tests were performed in a university laboratory.

Forty-eight healthy subjects (27 men and 21 women; mean age, 26.4 +/- 8.5 years) performed a maximal voluntary isometric contraction (MVIC) on an isokinetic device and received neuromuscular electrical stimulation (NMES) at 5 different burst duty cycles.

Subjects first performed an MVIC for knee extension on an isokinetic dynamometer at 60 degrees of knee flexion. NMES surface electrodes were applied to the quadriceps muscle of each subject's dominant leg. The values for the NMES were 2500-Hz carrier frequency, 50-bursts-per-second treatment frequency, amplitude increased to maximum tolerance, and burst duty cycle set to each of the 5 comparison values. The peak isometric force generated by each of the 5 nonvolitional contractions was recorded, along with the maximal charge per burst tolerated by each subject. Force generated was described as percentage of MVIC. Efficiency was the amount of force per burst charge.

The mean MVIC achieved by the subjects was 553.8 newtons (N). The average force per burst charge generated at 10% burst duty cycle was 132.9 N; at 30%, 104.2 N; at 50%, 93.1 N; at 70%, 52.9 N; and at 90%, 41.3 N. The average efficiency (force per millicoulombs [mC]) at 10% burst duty cycle was the highest at 6.49 N/mC and at 90% was the lowest at 1.05 N/mC.

A burst duty cycle of 10% was the most efficient ratio of burst duration to interburst interval duration for eliciting the strongest muscle contraction. Use of this preferred duty cycle enabled only 8 of 48 subjects (16.6%) to achieve 60% MVIC within 1 trial in 1 session of NMES. Twenty-five subjects (52.0%) were able to achieve at least 10% of the MVIC in the same trial period.