Analysis of a fatigue protocol for knee extensor and flexor muscle groups

Complementary to the muscle strength measurement, the isokinetic dynamometry offers the opportunity to investigate another essential variable: the fatigue which can be defined as a deterioration in maximum torque generating capacity. Many articles in the literature refer to isokinetic fatigue protocols, yet we may consider that there is a lack of consensus about testing modalities and methodological aspects. The overall aim of that preliminary study was to investigate concentric fatigue protocols in terms of contraction number, measurement reproducibility and appropriateness of parameters classically analysed.

Twelve male subjects (23  2 years old; 71  3 kg) without history of lower limb injury were included in the study. After standardized warm-up and familiarization with the isokinetic exercise (Biodex 3 dynamometer), each subject performed a unilateral concentric testing on their dominant side knee flexors and extensors. The protocol consisted in 50 maximal contractions at 180°/s angular velocity along a constant 100° range of motion. The fatigue protocol was repeated at 3 different sessions, separated by one week, in the same standardized conditions.

Measured and calculated parameters were analysed: maximal work (Wmax); total work (Wtot); cumulated work on 10-20-30-40-50 repetitions (W10-20-30-40-50); different fatigue indexes: mean on the 3 last reps/Wmax (W3L/Wmax), mean on the 5 last reps/Wmax (W5L/Wmax), W3L/mean on the 3 first reps (W3L/W3F), W5L / mean on the 5 first reps (W5L/W5F). These fatigue indexes were established from the 50 repetitions protocol but also calculated on the 30 and 40 first repetitions of the whole test. The heart rate (HR) throughout the fatigue protocol was recorded using a Polar cardiofrequencemeter.

A generalized linear mixed model (GLMM) was used for the statistical analysis of reproducibility.

The cumulated work calculated on 20, 30, 40 or 50 repetitions was found to be reproducible for the Q muscle group. The same parameters calculated on the Fl muscle group showed a less satisfactory reproducibility. Among indexes of fatigue, the most reproducible were those using Wmax as denominator (by contrast with W3F or W5F as denominator), in particular the W5L/Wmax index for the Q. From 20 repetitions to the end of exercise, the fatigue indexes calculated on the Fl were significantly (p < 0.05) inferior to the indexes established for the Q, suggesting a more marked effect of fatigue on the Fl decrease of maximal strength production. With respect to the HR expressed in percentage of the theoretical maximal HR, the value averaged 53 % before exercise, 83 % after 20 reps and peaked at 87 % after 50 reps. After 2 minutes of recovery, the HR reached 58 % of the HR max.

These findings must be taken into account when designing a fatigue isokinetic protocol, in terms of exercise duration and parameters to be analysed.