Maximal results with minimal stimuli: the fewest high-frequency pulses needed to measure or model prolonged low-frequency force depression in the dorsiflexors.

Quantifying prolonged low-frequency force depression (PLFFD) with the gold-standard 1-s trains presents challenges, so paired pulses have been used. Owing to greater impairment of high-frequency doublet than tetanic torque, paired pulses underestimate PLFFD. This study aimed to approximate the minimum number of high-frequency pulses needed to avoid such underestimation and assess the feasibility of modeling PLFFD from a limited number of experimental pulses. In 13 participants, a 1-s 10-Hz train and 100-Hz trains with 2, 4, 7, 12, 15, 25, 50, or 100 pulses were evoked before and after (15 min, 2, 4, and 7 days) eccentric exercise of the dorsiflexors. With ≤12 pulses, impairment of 100-Hz torque was greater than the 1-s train (P ≤ 0.05; e.g., 12 vs. 100 pulses at 4 days: 97.8 ± 8.5% vs. 100.5 ± 8.2% baseline). Consequently, with ≤12 pulses, PLFFD was underestimated compared with the gold-standard measure (P ≤ 0.05; e.g., 12 vs. 100 pulse 10:100-Hz torque ratio at 4 days: 86.8 ± 12.8% vs. 84.6 ± 13.5% baseline). Modeling reproduced 10:100-Hz ratios (PLFFD) with 95% limits of agreement of -13.6% to 16.7% of experimental values with ≥12 pulses. Our results indicate that a minimum of 13-25 pulses of 100 Hz are needed to accurately quantify PLFFD in the dorsiflexors. Although this may not be the minimum range for other muscles, a similar relationship with pulse number likely exists. Modeling may eventually provide an option to estimate PLFFD from experimental trains with relatively few pulses; however, further development is imperative to reduce variability.NEW & NOTEWORTHY Ideally, prolonged low-frequency force depression (PLFFD) is measured with 1-s trains of supramaximal stimuli; however, this induces considerable discomfort. We tested briefer trains to approximate the minimum number of high-frequency pulses needed to accurately determine PLFFD and the feasibility of modeling 1-s tetani with relatively few pulses. After eccentric exercise, 13-25 high-frequency pulses were needed to accurately measure PLFFD. Modeling reproduced mean experimental values but had considerable variability.