Introduction: Low-Frequency Fatigue (LFF) refers to a long-lasting impairment of muscle contractility, induced by a fatiguing physical task. Its gold standard assessment, commonly restricted to laboratory studies, is based on the low- to high-frequency force ratio produced by an electrically stimulated muscle (L/HFratio); the higher the exercise-induced depression of L/HFratio, the higher the LFF. A recent study has tested a portable device (MYOCENE, Liège, Belgium) as a measurement tool of L/HFratio (called Powerdex value, expressed in percentage) in the field [1], allowing the quadriceps LFF monitoring of sport practitioners. The aim of the present study is to evaluate the repeatability and reproducibility of Powerdex measurements, under field conditions.
Methods: 16 healthy students volunteered for an initial familiarization session with MYOCENE measurements. Then, two experimental sessions (over two weeks) aimed to study 1) the repeatability of two consecutive Powerdex measurements (seven minutes apart), and 2) the effect of time of day (8:00 a.m., 12:00 p.m. and 5:00 p.m. ) on the reproducibility of Powerdex measurements. For all sessions, participants were asked to refrain from moderate to intense physical activity, the measurements were performed by the same operator, with the same device, and the position of the skin muscle electrical stimulation electrodes was exactly the same throughout each session. For each session, the agreement between the pairs of within-subjects measurements was assessed by a Bland & Altman plot [2]: the bias was estimated as the mean of the paired differences, the repeatability coefficient was estimated at 1.96 x standard deviation of paired differences, and upper and lower limits of agreement were calculated as the bias ± 1.96 x standard deviation of paired differences.
Results: The analysis of repeatability shows an anecdotal bias (-1,6%) between the second and the first Powerdex measurement, probably due to a small fatigue effect caused by electrical muscle stimulation. The repeatability coefficient is 2,97%. The largest bias between the Powerdex values measured at different times of the day (reproducibility analysis) is 1,2%, the largest upper limit of agreement is 10,9 % and the largest lower one is -11,4%.
Discussion & Conclusion: The repeatability of the paired Powerdex measurements suggests that an exercise-induced Powerdex depression of at least 3% is required to conclude to the onset of Low-Frequency Fatigue. Further studies are required to assess more accurately the reproducibility of the device, as the daily activities may have modified the muscular contractility, either by a slight potentiation or a small fatiguing effect.
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[1] Ridard J. et al. (2022). On-field low-frequency fatigue measurement after repeated drop jumps. Front Physiol. Nov 9;13:1039616.
[2] Gerke O. (2020). Reporting Standards for a Bland-Altman Agreement Analysis: A Review of Methodological Reviews. Diagnostics (Basel). May 22;10(5):334.