ELECTRICAL-STIMULATION OF HUMAN TIBIALIS ANTERIOR - (A) CONTRACTILE PROPERTIES ARE STABLE OVER A RANGE OF SUBMAXIMAL VOLTAGES - (B) HIGH-FREQUENCY AND LOW-FREQUENCY FATIGUE ARE INDUCIBLE AND RELIABLY ASSESSABLE AT SUBMAXIMAL VOLTAGES

Objectives: To investigate the validity and reliability of submaximal voltage stimulation for assessing the 'fresh' contractile properties o f human tibialis anterior muscle (TA) and the efficacy of such stimula tion in inducing and assessing high- and low-frequency fatigue. Interv entions: (A) Contractile properties of fresh TA were assessed in six n ormal volunteers using multifrequency stimulation trains (comprising 2 seconds at each of 10, 20 and 50 Hz, arranged contiguously) over a ra nge of submaximal voltages. (B) On three separate occasions, fatigue w as induced in the TA of 10 normal volunteers by means of a 3-minute un broken sequence of the described multifrequency stimulation trains, de livered at a 'standardized' submaximal voltage. This fatiguing protoco l was preceded by discrete multifrequency stimulation trains, at the s ame standardized voltage, but followed by discrete multifrequency trai ns delivered over a range of submaximal voltages (which included the s tandardized voltage). Outcome measures: In experiment A the 10:50 Hz a nd 20:50 Hz force ratios were analysed for between-voltages variabilit y using coefficients of variation (CVs), and for trends using Friedman tests and post-hoc Wilcoxon tests. In experiment B low-frequency fati gue was detected using 10:50 Hz and 20:50 Hz force ratios derived from the discrete multifrequency trains. High-frequency fatigue was calcul ated from the decline in high-frequency force which occurred during th e fatiguing protocol itself. Each parameter was assessed for between-d ays repeatability using CVs. Results: in experiment A the 'fresh' 10:5 0 Hz force ratio was clearly unreliable at voltages which generated <1 0% of maximal voluntary contractile force (MVC) (CV less than or equal to 29.7%), but was reasonably reliable af voltages which generated 20 -30% of MVC (CV less than or equal to 11.5%; p = 0.847). The 'fresh' 2 0:50 Hz force ratio was, in contrast, extremely reliable throughout th e tested voltage range (CV less than or equal to 5.8%; p= 0.636) in fr esh muscle. In experiment B paired t-tests indicated that the fatiguin g protocol induced significant high-frequency fatigue (p <0.0037) and low-frequency fatigue (p <0.0008 for 'fresh' versus 'fatigued' 10:50 H z 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 extremel y reliable in the 20-33% of MVC range (CV less than or equal to 7.3%; p = 0.847). Between-days repeatability was poor for the 10:50 Hz force ratio in both fresh and fatigued muscle CV less than or equal to 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 c onfirm the validity and reliability of submaximal voltages in assessin g contractile properties (including low-frequency fatiguability) and i nducing fatigue of human TA.