The inclusion of interstimulus interval variability does not mitigate electrically-evoked fatigue of the knee extensors

Intrinsic Neuromuscular Fatigability in Humans: The Critical Role of Stimulus Frequency

“…Although we anticipated that our protocol (eight unique VFTs repeated 18 times for a total of 144 contractions) would not engage the catchlike property of the muscle to the same extent as an initial doublet, it was theorized that introducing variability among all ISIs might mitigate force loss when compared with a 10-Hz CFT. Despite our hypothesis, our VFTs failed to slow the decline of force seen with CFTs (28).…”

“…Although some stimulation sites are not affected by this issue (e.g., stimulation of the ulnar nerve at the wrist or elbow to activate adductor pollicis), the limitation is particularly problematic for nerves such as the femoral and musculocutaneous because the electrode is next to the targeted muscle fibers and easily displaced when the muscle contracts (e.g., (41)). For example, in our two recent studies (28,29), it was our intention to use femoral nerve stimulation to activate the quadriceps muscle group. However, despite extensive pilot testing with many different electrode configurations, it was not possible to consistently elicit what we deemed a valid force response (Fig.…”

“…Nonetheless, before our recent work (28,29), the only three studies to investigate whether sex-related differences exist for intrinsic fatigability all used repeated trains of NMES at a stimulation frequency well above the mean MUDR expected for the target force (26,27,52). Establishing the presence or absence of a sex-related difference for intrinsic fatigability would not only enhance our understanding of muscle physiology but, depending on the results, could lead to sex-specific training or rehabilitation programs (53).…”

“…, (41)). For example, in our two recent studies (28,29), it was our intention to use femoral nerve stimulation to activate the quadriceps muscle group. However, despite extensive pilot testing with many different electrode configurations, it was not possible to consistently elicit what we deemed a valid force response (Fig.…”

“…As depicted in Figure 1, the benefits of using a stimulus frequency that matches the MUDR of the target muscle outweigh those benefits associated with higher frequency stimulation. To support our hypothesis, we will primarily focus on the quadriceps muscle group because it is used in most studies investigating intrinsic fatigability in humans (22)(23)(24)(25)(26)(27), including our own recent work (28,29).…”