Electrical stimulation of human tibialis anterior: (A) contractile properties are stable over a range of submaximal voltages; (B) high- and low-frequency fatigue are inducible and reliably assessable at submaximal voltages

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Abstract
Objectives: To investigate the validity and reliability of submaximal voltage stimulation for assessing the ‘fresh’ contractile properties of human tibialis anterior muscle (TA) and the efficacy of such stimulation in inducing and assessing high- and low-frequency fatigue. Interventions: (A) Contractile properties of fresh TA were assessed in six normal volunteers using multifrequency stimulation trains (comprising 2 seconds at each of 10, 20 and 50 Hz, arranged contiguously) over a range of submaximal voltages. (B) On three separate occasions, fatigue was induced in the TA of 10 normal volunteers by means of a 3-minute unbroken sequence of the described multifrequency stimulation trains, delivered at a ‘standardized’ submaximal voltage. This fatiguing protocol was preceded by discrete multifrequency stimulation trains, at the same standardized voltage, but followed by discrete multifrequency trains delivered over a range of submaximal voltages (which included the standardized voltage). Outcome measures: In experiment A the 10:50 Hz and 20:50 Hz force ratios were analysed for between-voltages variability using coefficients of variation (CVs), and for trends using Friedman tests and post-hoc Wilcoxon tests. In experiment B low-frequency fatigue was detected using 10:50 Hz and 20:50 Hz force ratios derived from the discrete multifrequency trains. High-frequency fatigue was calculated from the decline in high-frequency force which occurred during the fatiguing protocol itself. Each parameter was assessed for between-days repeatability using CVs. Results: In experiment A the ‘fresh’ 10:50 Hz force ratio was clearly unreliable at voltages which generated <10% of maximal voluntary contractile force (MVC) (CV ≤29.7%), but was reasonably reliable at voltages which generated 20–30% of MVC (CV ≤11.5%; p = 0.847). The ‘fresh’ 20:50 Hz force ratio was, in contrast, extremely reliable throughout the tested voltage range (CV ≤5.8%; p = 0.636) in fresh muscle. In experiment B paired t-tests indicated that the fatiguing protocol induced significant high-frequency fatigue ( p <0.0037) and low-frequency fatigue ( p <0.0008 for ‘fresh’ versus ‘fatigued’ 10:50 Hz force ratio; p <0.0001 for ‘fresh’ versus ‘fatigued’ 20:50 Hz force ratio). In muscle thus fatigued, the 20:50 Hz force ratio was extremely reliable in the 20–33% of MVC range (CV ≤7.3%; p = 0.847). Between-days repeatability was poor for the 10:50 Hz force ratio in both fresh and fatigued muscle (CV ≤23.8 and 44.4% respectively), but was highly acceptable for both voluntary and stimulated fatigue indices and for the 20:50 Hz force ratio, the latter in both fresh and fatigued muscle. Conclusions: These results confirm the validity and reliability of submaximal voltages in assessing contractile properties (including low-frequency fatiguability) and inducing fatigue of human TA.