Electrically evoked force loss of the knee extensors is equivalent for young and old females and males

Abstract

Data are scant on sex-related differences for electrically-evoked contractions, which assess intrinsic contractile properties while limiting spinal and supraspinal adaptations to mitigate fatigue. Furthermore, the few studies that exist use stimulus frequencies considerably higher than the natural motor unit discharge rate for the target force. The purpose of this study was to compare force loss to electrically-evoked contractions at a physiological stimulus frequency among young females (n=12), young males (n=12), old females (n=11) and old males (n=11). The quadriceps of the dominant leg were fatigued by 3 min of intermittent transcutaneous muscle belly stimulation (15 Hz stimulus train to initially evoke 25% of maximal voluntary force). Impairment of tetanic contractile impulse (area under the curve) did not differ between sexes for young or old adults or between age groups, with a pooled value of 55.2±12.4% control at the end of fatigue. These data contrast with previous findings at 30 Hz, when the quadriceps of females had greater fatigue resistance than males for young and old adults. The present results highlight the impact stimulus frequency has on intrinsic fatigability of muscle; the findings have implications for future fatigue paradigms and treatment approaches when utilizing electrical stimulation for rehabilitation.
 Novelty bullets:
 • Fatigue was not different between sexes with a stimulation frequency comparable to discharge rates during voluntary contractions
 • These results highlight that stimulus frequency not only influences fatigue development but also between-group differences

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

“…Although stimulation frequency has long been known to influence the contractile response of muscle, including fatigability, our data (28,29) suggest that its importance in the development of force loss during intermittent stimulation protocols has been underappreciated. The profound influence of stimulation frequency on intrinsic neural and contractile properties, and the apparent difference in results between 10, 15, and 30 Hz for the quadriceps muscle group highlights the need for further study.…”

“…Although it was not a study aim to compare fatigability at 10 Hz (28) relative to our data published at 15 Hz (29), we were intrigued by the unexpected observation of different force profiles across the 3-min NMES protocols, which led to much less force loss at 10 Hz (~20%) compared with 15 Hz (~45%) (Fig. 3).…”

“…At 15 Hz, there was a relatively rapid, near-linear decline for our measure of muscle force (contractile impulse) across the first ~75 contractions, followed by a very slow decline during the second half of the protocol. Apart from transient potentiation of the rate of force development (RFD), both RFD and rate of force relaxation (RFR) followed the same profile of contractile impulse (29). With 10-Hz stimulation, contractile impulse (as well as RFD and RFR) showed marked and prolonged potentiation for 20–30 contractions, before declining in a fashion similar to 15 Hz, albeit with a more gradual decay during the “rapid” portion (Fig.…”

“…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.…”

“…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).…”