Prolonged Low‐frequency Force Depression is Underestimated When Assessed with Doublets Compared to Trains

Abstract

Unaccustomed eccentric exercise induces muscle damage, which leads to an impairment of force or torque that can last for several days. A common indirect measure of muscle damage is a greater depression of the force response to low‐frequency (e.g., 10Hz) compared to high‐frequency (e.g., 100Hz) stimulation; i.e., a post‐exercise reduction in the 10:100Hz ratio. This phenomenon is termed, prolonged low‐frequency force depression (PLFFD). Historically, trains of stimuli (e.g., 1s) have been used to evoke tetanic responses for the ratio. More recently, some authors have used only paired stimuli to evoke doublets. However, it is unknown if the PLFFD indicated by doublet responses is equivalent to the value derived from the traditional tetanic responses. Hence, the purpose of this study was to compare the magnitude of PLFFD determined by doublet vs. tetanic responses. We hypothesized that the doublet ratio (DR) would indicate significantly less PLFFD than the train ratio (TR). Eight participants (four females) performed 200 eccentric maximal voluntary contractions (4 sets of 50 repetitions, 1s rest between repetitions and 1 min rest between sets) of the dorsiflexors to induce muscle damage. Eccentric contractions were performed at 60°/s, started at a neutral ankle position, and finished at 30° of plantar flexion. Before the fatiguing protocol, baseline neuromuscular function was measured by a sequence of isometric contractions evoked by supramaximal electrical stimulation of the fibular nerve (paired stimuli at 10 and 100Hz, and 1s trains at 10 and 100Hz, each separated by 1s rest). This neuromuscular function test was repeated three times prior to fatigue and 2 min, 3 min, 5 min, 10 min and 48h after the fatiguing protocol. Using the peak torque of each response, 10:100Hz ratios were calculated at all time points; post‐exercise values were expressed as a percentage of the respective baseline ratio. At baseline, the DR was greater than TR (0.81 ± 0.04 vs. 0.48 ± 0.09; P < 0.01). PLFFD was observed by 2 min for TR (73 ± 12%) and 3 min for DR (83 ± 9%), continued to develop over the acute recovery phase (at 10 min, DR = 73 ± 9%; TR = 51 ± 11%), and recovered by 48h (DR = 98 ± 6%; TR = 91 ± 12%). The TR was reduced more than the DR at all acute recovery time points (P < 0.05), but not 48h. With DR and TR revealing different levels of PLFFD following damaging eccentric contractions, our results indicate that these techniques should not be used interchangeably. Whenever possible, trains of stimuli should be used to indicate the true extent of PLFFD. In cases where the reproducibility of the tetanic force response may be problematic (e.g., femoral nerve stimulation) or a train is deemed prohibitively uncomfortable, it should be acknowledged that paired stimuli will underestimate PLFFD.Support or Funding InformationSupported by NSERC, CFI and BCKDFThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.