On-field low-frequency fatigue measurement after repeated drop

IntroductionAthletes, especially elite athletes, are exposed to high training and competition loads, making the monitoring of fatigue extremely relevant (Thorpe et al., 2017). As defined by Enoka and Duchateau (2016), fatigue can be conceptualized as a disabling symptom in which physical and cognitive functions are limited by interactions between performance fatigability (i.e., the decline in an objective measure of performance) and perceived fatigability (i.e., changes in the sensations that regulate the integrity of the performer). Fatigue directly impacts sport performance, and may further increase the risk of injury or induce an overtraining syndrome in case of prolonged excessive imbalance between training/competition loads and recovery periods (Thorpe et al., 2017; Cheng et al., 2020).A reduction in maximal force during an isometric maximal voluntary contraction (MVC) is thought to provide the most straightforward evidence of performance fatigability (Carroll et al., 2017). Part of the alterations within the neuromuscular system can occur at the central nervous system level, as evidenced by a decreased voluntary activation level (VA) assessed through the superimposed twitch technique (Merton, 1954). Alterations can also occur at the peripheral level through impaired muscle contractility (Allen et al., 2008), which can be assessed through peripheral nerve or muscle electrical stimulation (Millet et al., 2011). As such, performance fatigability can be ultimately defined to occur when the force output is lower than what is expected for a given voluntary or evoked stimulus (MacIntosh and Rassier, 2002). In other words, despite maximal voluntary contraction being preserved, performance fatigability may still exist if the force induced by certain types of stimulation is depreciated.Among the exercise-induced peripheral alterations, low-frequency fatigue (LFF), also known as prolonged low-frequency force depression (Allen et al., 2008), is a long-lasting form of muscle fatigability and is characterized by a larger decrease of force at low stimulation frequencies than at high stimulation frequencies (Edwards et al., 1977; Jones, 1996). LFF is suggested to reflect excitation–contraction coupling failure through decreased Ca2+ release within muscle fibers (Edwards et al., 1977; Hill et al., 2001; Keeton and Binder-Macleod, 2006; Dargeviciute et al., 2013). LFF is notably believed to be a primary source of peripheral alterations after eccentric contractions (Martin et al., 2004; Iguchi and Shields, 2010; Skurvydas et al., 2016; Kamandulis et al., 2019). But LFF and/or alteration of Ca2+ release has been observed following other types of fatiguing tasks such as intense exercise (Lattier et al., 2004; Skurvydas et al., 2016) or exercise inducing glycogen depletion (ørtenblad et al., 2011).The gold standard to assess LFF is the ratio of low- to high-frequency force responses to trains of peripheral nerve electrical stimulation at supramaximal intensity (Allen et al., 2008). Due to the discomfort induced by such tetanic nerve stimulation, evoked forces to paired stimuli at 10Hz and 100 Hz have been proposed as an alternative method (Verges et al., 2009). But because of the complexity of this kind of measurements (i.e., electrode placement, specific material, and discomfort), LFF assessments are commonly restricted to in-lab studies. Measuring LFF on-field would however provide great insight to athletes and coaches on athletes’ muscle fatigability, allowing better management of training/competition loads. This is the goal of Myocene®, a new portable device allowing quadriceps LFF assessment on the field. For that purpose, it is composed of an easy-to-transport knee extensor dynamometer integrating an electrical stimulator for muscle transcutaneous stimulation. Considering that the relative decrease in the low- to high-frequency ratio after a fatiguing task is not different whether supramaximal nerve stimulation or submaximal muscle stimulation is applied (Martin et al., 2004), the device uses trains of submaximal stimuli applied to the quadriceps muscle to reduce discomfort and simplify its acceptability by athletes. But the outcome provided by this on-field device (i.e., the so-called Powerdex) remains to be validated when compared to laboratory measurements.The aim of this study was to validate the use of a portable on-field device to measure LFF induced by a series of drop jumps, that is, exercise consisting of stretch-shortening cycles with a strong eccentric component. We therefore compared the exercise-induced decrease in Powerdex and the decrease in the ratio of 10- to 100-Hz doublets in an in-lab setting. We hypothesized that the magnitude of LFF would be correlated between both methods.MethodsParticipantsFifteen active and healthy participants (11 men and four women; age: 26 ± 5 yr; body weight: 70 ± 10 kg; height: 174 ± 9 cm) participated in the present experiment. They self-reported their main strength as either explosive (n = 9) or enduring (n = 6). They reported no history of neurological or musculoskeletal impairment. Participants were asked to refrain from strenuous and unaccustomed physical activity for 48 h before testing to minimize the risk of prior fatigue or muscle damage. The study protocol was approved by the local Ethics Committee and was in accordance with the latest update of the Helsinki Declaration (except for registration in a database). All subjects gave their written informed consent before participation.Study designParticipants visited the laboratory on two occasions. During the first session, participants were familiarized with the experimental procedures. At least seven days later, participants went back to the laboratory for the evaluation session. This first evaluation consisted of quadriceps LFF assessment (PRE) using the on-field device. Then, participants moved to the laboratory dynamometer for a neuromuscular function evaluation: maximal voluntary contraction (MVC), maximal voluntary activation level (VA), and evoked responses to 10- and 100-Hz doublet on relaxed muscle. Participants were then asked to perform an intense eccentric exercise composed of repetitive drop jumps (DJs). Participants were then retested (POST) in the same order as in PRE. A 10-min resting period was observed between the last DJ and POST measurements to avoid the neuromuscular evaluation to be affected by metabolic fatigue. All measurements were performed on the right leg. The study design is described in Figure 1.FIGURE 1. Experimental design of the study. Each participant performed the Myocene® measurement (single pulse + low-frequency train + high-frequency train) and maximal voluntary contractions (MVCs) with peripheral nerve stimulation performed before (PRE) and after (POST) series of drop jumps (DJs). Peripheral nerve stimulations are represented by double gray (paired stimulations at 100 Hz, DB100) and double black arrows (paired stimulations at 10 Hz, DB10).LFF assessment using the on-field deviceThis study was performed using the on-field Myocene® device (Figure 2). Participants sat on the seating of the device with their leg in contact with the “Myo-sensor,” that is, a dedicated force sensor recording evoked forces at a rate of 4 kHz. Muscle electrical stimulations (biphasic square wave with a pulse width of 400 µs) were applied using three electrodes (MyoPro-1-electrode, Myocene, Liège, Belgium). The cathode (5 × 10 cm) was placed transversely across the width of the proximal portion of the quadriceps femoris, and the anodes (5 × 5 cm) were, respectively, placed over the vastus lateralis and the vastus medialis muscles. Pre-programmed electrical stimuli trains were directly sent by the device that was driven by the Myocene® software. The series of stimuli consisted of sets including 1) a single pulse, 2) a train of 5 stimuli at low frequency (20 Hz), and 3) a train of 18 stimuli at high frequency (120 Hz). One second separated each stimulation (Figure 1). In total, 16 sets were performed, with 5 s in-between, and the intensity of stimulation was progressively increased by steps of 1 mA at
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