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Torque and Discomfort During Electrically Evoked Muscle Contractions in Healthy Young Adults: Influence of Stimulation Current and Pulse Frequency

Open AccessPublished:September 23, 2022DOI:https://doi.org/10.1016/j.apmr.2022.09.004

      Abstract

      Objective

      To investigate (1) how current and pulse frequency of electrical stimulation (ES) as well as contraction mode (isometric, concentric, and eccentric) influence torque output and discomfort and (2) how familiarization by repeated ES sessions influences ratings of perceived discomfort.

      Design

      An experimental study, 3 sessions.

      Setting

      A university laboratory.

      Participants

      Eight healthy participants (5 men, 3 women; mean age 25.2 years; N=8).

      Interventions

      Participants completed 3 trial days, each including 17 electrically evoked thigh muscle contractions. On each trial day, the first 6 contractions consisted of 2 isometric, 2 concentric, and 2 eccentric muscle contractions randomly ordered with a fixed stimulation current and pulse frequency (200 mA, 20 Hz), while the remaining 11 muscle contractions were all isometric with randomly ordered combinations of current (100-250 mA) and pulse frequency (20-100 Hz).

      Main Outcome Measures

      Torque and perceived discomfort were measured for each ES-evoked contraction.

      Results

      Overall, the findings revealed that a higher stimulation frequency was associated with an increased torque without increased discomfort, while higher currents were associated with increases of both torque and discomfort. Contraction type did not influence level of discomfort, despite eccentric contractions eliciting higher torque compared with concentric and isometric contractions (P<.001). Finally, a significant familiarization to ES (P<.001) was observed after just 1 of 3 identical stimulation sessions.

      Conclusions

      The outlined data suggest that to elicit high torque levels while minimizing levels of discomfort in young subjects, eccentric muscle contractions evoked with a low stimulation current, and a high pulse frequency are preferable. Furthermore, a single familiarization session significantly lowers rating of perceived discomfort during ES.

      Keywords

      List of abbreviations:

      ES (electrical stimulation), MVIC (maximal voluntary isometric contraction)
      Electrical stimulation (ES) evoked contractions are widely used to assess and improve muscular function in many different populations ranging from elite athletes to persons severely impaired by spinal cord injuries.
      • Filipovic A
      • DeMarees M
      • Grau M
      • et al.
      Superimposed whole-body electrostimulation augments strength adaptations and type II myofiber growth in soccer players during a competitive season.
      • Frotzler A
      • Coupaud S
      • Perret C
      • et al.
      High-volume FES-cycling partially reverses bone loss in people with chronic spinal cord injury.
      • Hussain G
      • Wang J
      • Rasul A
      • et al.
      Current status of therapeutic approaches against peripheral nerve injuries: a detailed story from injury to recovery.
      Specifically, ES has been used in elite sport and rehabilitation to counteract suboptimal muscle activation as well as muscle atrophy.
      • Merletti R
      • Zelaschi F
      • Latella D
      • Galli M
      • Angeli S
      • Sessa MB.
      A control study of muscle force recovery in hemiparetic patients during treatment with functional electrical stimulation.
      • Levin MF
      • Huichan CWY.
      Relief of hemiparetic spasticity by tens is associated with improvement in reflex and voluntary motor functions.
      • Shariat A
      • Najafabadi MG
      • Ansari NN
      • et al.
      The effects of cycling with and without functional electrical stimulation on lower limb dysfunction in patients post-stroke: a systematic review with meta-analysis.
      • Ward NS
      • Brander F
      • Kelly K.
      Intensive upper limb neurorehabilitation in chronic stroke: outcomes from the Queen Square programme.
      In studies using ES, the electrical current amplitude used usually results only in submaximal muscle activation while being associated with some degree of discomfort.
      • Filipovic A
      • DeMarees M
      • Grau M
      • et al.
      Superimposed whole-body electrostimulation augments strength adaptations and type II myofiber growth in soccer players during a competitive season.
      ,
      • Herrero JA
      • Izquierdo M
      • Maffiuletti NA
      • García-López J.
      Electromyostimulation and plyometric training effects on jumping and sprint time.
      When using ES in an exercise or rehabilitation intervention, the degree of muscle activation and ensuing torque level is thought to be positively related to the efficacy of the ES intervention with regard to functional improvements. However, increasing ES current amplitude and/or pulse frequency to enhance torque output will most likely also be accompanied by increased discomfort. Therefore, it is important to elucidate the conditions of ES, which can elicit the most torque output with the least amount of discomfort.
      Although the effects of different ES parameters on muscle activation and torque have been investigated in several studies,
      • Collins DF.
      Central contributions to contractions evoked by tetanic neuromuscular electrical stimulation.
      • Collins DF
      • Burke D
      • Gandevia SC.
      Sustained contractions produced by plateau-like behaviour in human motoneurones.
      • Dean JC
      • Yates LM
      • Collins DF.
      Turning on the central contribution to contractions evoked by neuromuscular electrical stimulation.
      • RAdO Lima
      • Freire APCF
      • Guimarães A
      • Santos CP
      • Pacagnelli FL.
      Effectiveness of Russian current in bone regeneration process in rats.
      • Naaman SC
      • Stein RB
      • Thomas C.
      Minimizing discomfort with surface neuromuscular stimulation.
      only few present information on the optimal ES setup for muscle activation and high torque output while minimizing the discomfort.
      • Damo NLP
      • Modesto KA
      • IVdS Neto
      • Bottaro M
      • Babault N
      • Durigan JLQ.
      Effects of different electrical stimulation currents and phase durations on submaximal and maximum torque, efficiency, and discomfort: a randomized crossover trial.
      Naaman et al
      • Naaman SC
      • Stein RB
      • Thomas C.
      Minimizing discomfort with surface neuromuscular stimulation.
      investigated the discomfort, using a 0-10 verbal scale, linked to pulse width, frequency, electrode size, and shape using nerve stimulation in comparison to muscle belly stimulation. The authors found a relation between increases in frequency and pulse width and increases in discomfort, while the choice of electrode and nerve vs belly stimulation were deemed irrelevant. However, this study did not measure torque during ES. Gorgey et al
      • Gorgey AS
      • Mahoney E
      • Kendall T
      • Dudley GA.
      Effects of neuromuscular electrical stimulation parameters on specific tension.
      found a relation between ES current amplitude and degree of muscle activation using magnetic resonance imaging prior to and during ES. The linear correlation they found between torque and stimulation current at 100 Hz, together with a linear relation between torque and activated muscle cross-sectional area, indicated a need for high current to elicit high torque. However, this study did not measure discomfort during ES. Therefore, although studies have provided preliminary insight into the effects of various ES parameters on torque and discomfort separately, the combined effect of different current intensities and pulse frequencies has not been explored.
      It is well known that eccentric contractions elicit greater torque outputs compared with isometric and concentric contractions.
      • Lindstedt SL
      • LaStayo P
      • Reich T.
      When active muscles lengthen: properties and consequences of eccentric contractions.
      ,
      • Edman KA.
      Double-hyperbolic force-velocity relation in frog muscle fibres.
      If the discomfort during ES evoked contractions is solely due to the ES per se, one would assume that during ES under constant parameters a higher torque can be elicited during eccentric, than isometric or concentric contractions, without an increase in discomfort.
      Finally, it has been observed that people undergoing ES become accustomed to the procedures and feel less pain or discomfort when the stimulation is repeated. However, the extent and timing of such a familiarization effect have not, to our knowledge, previously been systematically quantified.
      The aims of the current study were, therefore, to investigate (1) how torque output and discomfort during ES-evoked contractions were influenced by variations in ES current, and frequency, and by mode of contraction and (2) how torque and discomfort during ES were influenced by familiarization over 3 identical sessions. It was hypothesized that (1) increases in both current and frequency will concomitantly increase discomfort and torque output in ES-elicited contractions; (2) ES-evoked contractions will yield higher torque when they are eccentric rather than isometric or concentric without increasing discomfort; (3) rating of perceived discomfort will decrease with familiarization without changes in torque output.

      Methods

      Design

      The study was an experimental repeated measurements trial consisting of 3 identical sessions, each assessing daily maximal voluntary isometric contraction (MVIC) followed by assessment of discomfort from ES-evoked contractions.

      Participant characteristics

      A total of 6 young (aged >18 years) healthy men and 5 young women volunteered to participate in the study. The mean age, weight, and height can be seen in table 1. Three participants withdrew or were excluded from the study after the first session. As such, 8 participants completed the 3 sessions and were included in the analysis. By also accounting for potential dropouts, a sample size of 15 was originally deemed sufficient.
      • Naaman SC
      • Stein RB
      • Thomas C.
      Minimizing discomfort with surface neuromuscular stimulation.
      However, inclusion was halted at 11 because of COVID-19-lockdown. Participants were recruited through social media and posters at Aarhus University. All participants received written and oral information regarding the project, possible nuisances, and the aim of the project prior to their signed approval. Exclusion criteria included disorders affecting the nervous and/or muscular system, joint pain, hypertension, and pregnancy. The experimental leg was randomized for each participant (50% randomized for dominant leg). All experimental procedures were approved by the ethics committee of Region Midtjylland (#1-10-72-274-19).
      Table 1Anthropometrics, MVIC, and thigh composition
      Sex5 Male/3 Female
      Age (y)25.1 (22-31)
      Dominant leg4/8
      Height (cm)178.8 (166-198)
      Weight (kg)73.2 (54.0-100.2)
      BMI22.6 (18.5-27.6)
      MVIC (Nm)
      Day 1254.6 (83.6)
      Day 2263.7 (81.3)
      Day 3271.8 (86.9)
      Average of all263.0 (80.6)
      Thigh composition (mm)

      Muscle/fat
      Proximal57.8 (9.9)/11.1 (3.8)
      Medial48.2 (9.7)/9.8 (3.3)
      Distal32.38 (7.7)/10.34 (3.5)
      Average45.7 (9.0)/10.6 (3.4)
      Abbreviation: BMI, body mass index.

      Warm-up

      Prior to each test session, each participant underwent a brief warm-up consisting of 5 minutes of low-resistance pedaling on a bike ergometer. This was followed by 2 sets of 10 repetitions of full leg extension and leg flexion against a low resistance in a dynamometer
      • Sieljacks P
      • Wang J
      • Groennebaek T
      • et al.
      Six weeks of low-load blood flow restricted and high-load resistance exercise training produce similar increases in cumulative myofibrillar protein synthesis and ribosomal biogenesis in healthy males.
      ,a

      Electrode placement

      Electrode number, configuration, and placement may affect torque and discomfort. In our experience, the least uncomfortable placement was similar to the one presented by Botter et al
      • Botter A
      • Oprandi G
      • Lanfranco F
      • Allasia S
      • Maffiuletti NA
      • Minetto MA.
      Atlas of the muscle motor points for the lower limb: implications for electrical stimulation procedures and electrode positioning.
      and was therefore chosen for the present study. Accordingly, the placement of electrodesb was standardized to the subject's femur length and 3 visual or palpable landmarks.
      • Botter A
      • Oprandi G
      • Lanfranco F
      • Allasia S
      • Maffiuletti NA
      • Minetto MA.
      Atlas of the muscle motor points for the lower limb: implications for electrical stimulation procedures and electrode positioning.
      As shown in figure 1, the most proximal electrode was placed one-third of the length from the ilium to the superior prominence of patella with the leg extended, with the middle of the electrode placed between rectus femoris and vastus lateralis. The 2 distal electrodes were placed on the muscle belly of vastus lateralis and vastus medialis, respectively. Electrode placement was measured and marked using a semi-permanent marker during the first day of trials. After the second test day, the markings were redrawn.

      Maximal voluntary isometric contraction

      The settings for knee, hip, and back angle for the isokinetic dynamometer were noted during the first visit and used for each subsequent test. Settings include knee adapter length, angles for knee, hip, and upper body, and so on. Participants were seated with thighs horizontal, a 90° hip joint angle and a knee joint angle of 70° during isometric contractions. The protocol for MVIC testing was preceded by 3 submaximal attempts where participants were instructed to elicit 50%-75% of their MVIC to accustom themselves to the isokinetic dynamometer. Each participant then rested for 2 minutes prior to the first attempt. Participants were instructed to contract their muscles as rapidly and with maximal effort until they reached a plateau in the live force readings, and the assessor cued them to stop the contraction (after ̴ 5 s). The assessor motivated the participant verbally during attempts. The same assessor was responsible for all testing to ensure reliability in process and verbal motivation. Participants rested for 2 minutes between attempts. A total of 3 attempts were completed for each participant. An average of the 2 highest torque recordings were used as the measure for MVIC.
      • Sieljacks P
      • Wang J
      • Groennebaek T
      • et al.
      Six weeks of low-load blood flow restricted and high-load resistance exercise training produce similar increases in cumulative myofibrillar protein synthesis and ribosomal biogenesis in healthy males.

      Ultrasound

      Ultrasound was used to measure subcutaneous fat layer thickness and muscle thickness of the thigh.
      • Pillen S
      • van Alfen N.
      Skeletal muscle ultrasound.
      ,d These measurements were performed only on the first trial day. Each subject was scanned thrice at 25, 50 as well as 75% of thigh length, measured from the ilium to the superior prominence of the patella. The subject was placed in the dynamometer using the same settings as for testing. The leg was extended fully and held by the dynamometers lever arm to allow the subject to relax the musculature. The subject was instructed to relax and sit still during all scans. During scanning, great care was taken not to compress the musculature and to measure in the same position every time. All ultrasound pictures were analyzed offline by the same assessor to ensure reliability.
      • Franchi MV
      • Longo S
      • Mallinson J
      • et al.
      Muscle thickness correlates to muscle cross-sectional area in the assessment of strength training-induced hypertrophy.

      Electrical stimulation

      After pre-tests, including electrode placement, ultrasound, and MVIC testing, participants were introduced to ES and electrodes were tested by brief evoked contractions at 50, 100, 150, 200, and 250 mA with a square pulse width of 200 µs using a stimulator.c Participants were then instructed to relax as much as possible, and to rate discomfort of each subsequent stimulation on a 10 cm linear visual analog scale immediately after cessation of each stimulation. During each session, each participant underwent 17 stimulations of 1 second duration, separated by a 1-minute pause. The first 6 of these evoked contractions were given at 20 Hz, 200 mA, and differed only in contraction mode (isometric, eccentric, or concentric), while the following 11 were all performed isometrically, while differing in stimulation current and frequency. For eccentric and concentric contractions, angular velocity was 60°/s, between 15° and 90° knee flexion (0° corresponding to a fully extended knee). See figure 2 for an overview. Research personnel took great care to act the same prior to each contraction and keep the wording of instructions exactly the same to ensure minimal coercion regarding pain rating. The protocol was repeated on 3 different days with the order of the stimuli randomized for each visit. Peak torque was measured for each contraction. The 3 test days were conducted by the same investigator with at least 2 days between test days (range 2-6 days). The stimulation protocol was developed to assess changes in discomfort with stimulations offering high torque in a non-linear manner, to prevent participants from being able to predict the intensity of stimulations or movement.
      Fig 2
      Fig 2Overview of stimulations and order used in the study. The first 6 contractions were 2 isometric, 2 eccentric, and 2 concentric evoked ES contractions at 20 Hz and 200 mA for 1 second. The order of contraction modes (ISO, ECC, and CON) was block randomized. Eccentric and concentric contractions were performed at an angular velocity of 60°/second in a range of motion corresponding to 75° of knee flexion (between 15° and 90° knee flexion, 0° corresponding to a fully extended knee). The final 11 contractions were all isometric but were evoked by different combinations of stimulation frequency (20, 30, or 50 Hz) and current (100, 150, 200, or 250 mA) given in a randomized order.

      Data analysis

      The study yielded data on electrically evoked peak torque output and discomfort and MVIC at 3 different trial sessions. These data were analyzed with regard to change over time. Linear mixed model with repeated measurements (corresponding to 1-way analysis of variance) were used to assess the effect of familiarization. In order to test differences between various stimulation and contraction parameters, post hoc pairwise comparisons were performed using a Bonferroni corrected regression analysis. All data were analyzed using Stata softwaree. Data are presented as means and 95% confidence intervals in text and as individual values and means ± SD in figures.

      Results

      Familiarization

      Data from the 3 days were analyzed to determine if there was an effect of familiarization on discomfort or torque. The torque of the evoked contractions did not change significantly between trial days in either of the contraction modes (figs 3A-D).
      Fig 3
      Fig 3Effect of familiarization and contraction mode—common for all parts of the figure is that trial days 1-3 are being presented with contractions from the first 6 stimulations differing in contraction mode. P-values in panels represent differences between day 1 and the following days. Panels A-D show absolute ES evoked torque with individual and group (in bold) means. (A) Represents all movement conditions at all the used ES parameters. (B) Represents isometric contractions only. (C) Represents concentric contractions only. (D) Represents eccentric contractions only. A similar setup is presented with regard to discomfort shown in panels E-H, P values to the right of panels indicate comparisons between contraction modes.
      For each subject, an average pooled discomfort value was obtained for each of the 3 experimental days (fig 3E-H). Comparisons between days revealed a significant decrease in average discomfort from the first to the second trial day (5.6 [5.1;6.1] vs 3.5 [3.0;4.0], P=.008); however, no difference between the second and third trial day (3.5 [3.0;4.0] vs 3.5 [3.0;4.0], P=.961). Experienced discomfort differed greatly between subjects, however a familiarization-induced lowering of discomfort was noted in all 8 subjects and in all contraction modes (fig 3E-H).

      Effects on discomfort and torque of varying ES parameters

      Figure 4 presents data for the third day only, thus showing responses in fully familiarized subjects. There was a general increase in torque, with increasing current and frequency of stimulation (fig 4). Likewise, discomfort is increased with increasing current and frequency. However, increasing the frequency of stimulation appeared a less discomfortable way of increasing torque output than increasing current. Exemplifying this, a comparison of 20 Hz, 150 mA with 50 Hz, 150 mA stimulation showed that torque increased from 99.7 Nm to 151.3 Nm with only a minor non-significant increase in discomfort from 2.4 to 2.7. In comparison, when increasing the current of a 20 Hz contraction from 150 mA to 250 mA, a similar increase in torque from 99.7 Nm to 157.3 Nm was achieved at the expense of a much higher increase in discomfort level, from 2.4 to 5.7. Data from trial days 1 and 2 followed a similar pattern and are shown in Appendix 1. Accordingly, the described pattern of torque and discomfort at varying ES parameters was not materially affected by familiarization.
      Fig 4
      Fig 4Electrically induced torque and discomfort at varying stimulation settings: Torque and discomfort are presented as the mean ± SD for all subjects during the third trial day. Each dot represents 1 type of stimulation with its accompanying SD. Black dots represent stimulations at 20 Hz, blue dots at 30 Hz, green at 50 Hz, and red at 100 Hz. Squares represent 100 mA stimulation, circles 150 mA stimulation, diamonds 200 mA, and hexagons 250 mA stimulation.

      Effects on discomfort and torque of varying contraction mode

      As shown in figure 3F-H, discomfort for the third trial day was 3.66 (2.6;4.7), 3.61 (2.5;4.7), and 3.86 (2.8;4.9), for isometric, concentric, and eccentric evoked contractions, respectively. Throughout all trial days, the mode of contraction did not influence the level of perceived discomfort. In contrast to the discomfort, peak torque was significantly influenced by mode of contraction (P<.001), with post hoc tests showing differences between all modes of contraction (P≤.037) (fig 3B-D). Force during the third trial day was 148 Nm (127;169), 77 (56;99), and 189 (166;211), for isometric, concentric, and eccentric contractions, respectively. The randomized order of stimulations did not affect the rating of perceived discomfort throughout each trial day (correlation: -0.06, P=.22).

      Anthropometric variables

      When muscle thickness was correlated with relative ES-induced (50 Hz, 100 mA) isometric torque (expressed as % of MVIC), a significant negative correlation was found (r=-0.78, P<.001). However, no correlation between muscle thickness and relative torque was found when using 50 Hz and 200 mA ES (r=-0.04, P=.502). This indicates that at low current intensities, the electrical activation of muscles was less complete in thicker muscles than in thinner muscles, while this was not an issue at high currents. Fat layer depth did not correlate significantly with isometric force in any stimulation configuration. Also, no correlations were found between leg fat or muscle thickness and discomfort associated with stimulation.

      Discussion

      The primary aim of the investigation was to determine the effect of variations in ES current and frequency as well as the mode of contraction on discomfort and torque output. While increasing ES current increased both torque output and discomfort, increasing ES frequency generally elicited increases in torque with much smaller increases in discomfort. As for the mode of contraction, no differences were observed for experienced discomfort, whereas, as expected, eccentric contractions elicited higher torque than concentric and isometric contractions. Finally, 1 repeated session consistently lowered the rating of perceived discomfort related to ES, indicating a marked and early familiarization to ES.
      With regard to familiarization, the data showed a marked decrease in the rating of discomfort after the first sessions. In the future, it may be sufficient to simply use low-intensity stimulation during the first day, which could help retain research subjects, patients, or other users of ES. Future studies should investigate means of minimizing discomfort while maximizing familiarization to electrical stimuli. Further, no differences in torque from stimulations were noted, implying that subjects remained relaxed and refrained from co-contracting.
      As expected, it was observed that increases in torque during ES can be achieved by increasing either the current or frequency of stimulation. It appears from the findings that when using ES frequencies below 30 Hz, these could be substituted by higher frequency stimulation at lower currents to achieve similar levels of torque at lower discomfort. However, it must also be considered that a possible benefit from low-frequency stimulation is a lower degree of fatigue, as seen in Malesevic et al
      • Malesevic NM
      • Popović LZ
      • Schwirtlich L
      • Popović DB.
      Distributed low-frequency functional electrical stimulation delays muscle fatigue compared to conventional stimulation.
      who found lower frequencies better suited for long-term stimulation. The present study made no changes regarding voltage, pulse form, or stimulation time, as these ES parameters have been tested in previous experiments.
      • Naaman SC
      • Stein RB
      • Thomas C.
      Minimizing discomfort with surface neuromuscular stimulation.
      ,
      • Fukuda TY
      • Marcondes FB
      • NDdA Rabelo
      • RAd Vasconcelos
      • Cazarini C.
      Comparison of peak torque, intensity and discomfort generated by neuromuscular electrical stimulation of low and medium frequency.
      ,
      • Rabello R
      • Fröhlich M
      • Maffiuletti NA
      • Vaz MA.
      Influence of pulse waveform and frequency on evoked torque, stimulation efficiency and discomfort in healthy subjects.
      The current results demonstrate high activation of knee extensor musculature and torque outputs above 70% of MVIC for all participants with this setup. Therefore, it is shown that high degrees of activation and torque outputs are achievable although coupled with a certain degree of discomfort.
      The data revealed no difference in discomfort when altering the mode of contraction, while torque output was greatly increased in both isometric and eccentric contractions compared with concentric contractions. The discrepancy between torque and discomfort indicates that the muscle torque production is not the cause of discomfort and that it is rather the activation by ES of pain perceptive nerves that cause discomfort.
      • Reddy KSK
      • Naidu MUR
      • Rani PU
      • Rao TRK.
      Human experimental pain models: a review of standardized methods in drug development.
      In studies seeking high mechanical forces, one could use a combination of low stimulation intensity and high pulse frequency coupled with eccentric modes of contraction, thereby minimizing discomfort while achieving a high torque output. It should be noted, however, that the time-torque integral (ie, area under the curve) is greater for isometric contractions compared with dynamic contractions (both eccentric and concentric, data not shown). One explanation for this is that in the isometric condition the muscle is at a more optimal length for the entirety of the stimulation, compared with the eccentric and concentric contractions, where muscle length varies during the contraction.
      • Bobbert MF
      • Ettema GC
      • Huijing PA.
      The force-length relationship of a muscle-tendon complex: experimental results and model calculations.
      Interestingly, no correlation was found between subcutaneous fat layer and/or muscle thickness on relative contraction torque when comparing higher intensity stimulation. For lower intensity stimulations, the depth of the muscle layer correlated with relative torque in both 100 and 200 mA stimulations. Only 100 and 200 mA stimulations were available for the low-intensity correlation, as lower frequency stimulations with lower current did not elicit enough torque to be measured. These data suggest that low-current stimulation does not cause contractions in deeper-lying muscle fibers, while higher current activates a larger part of the muscle. Lieber and Kelly
      • Lieber RL
      • Kelly MJ.
      Factors influencing quadriceps femoris muscle torque using transcutaneous neuromuscular electrical stimulation.
      argued that differences in contractility when using the same stimulator settings between subjects could stem from the difference in nerve branching and depth of nerves in the musculature. An alternative explanation offered by Gorgey et al
      • Gorgey AS
      • Mahoney E
      • Kendall T
      • Dudley GA.
      Effects of neuromuscular electrical stimulation parameters on specific tension.
      to the difference in contractability of muscle to certain stimuli might stem from the spatial location of type IIA/X and type I muscle fibers. It would be interesting to further elucidate the cause of differences between subjects.

      Study limitations

      Some methodological considerations deserve to be mentioned. First, only 8 of the 11 included subjects in the present study completed all 3 sessions. Because of unforeseen circumstances (the Covid-19 lockdown) the present investigation was halted for a prolonged period, causing the exclusion of 1 research subject. Another subject withdrew consent without explanation. Lastly, 1 subject withdrew from the research because of articular pain in the knee. As such, care should be taken when using ES in unaccustomed populations, a more gradual approach to delivering ES than the one presented might be beneficial to retention. Second, while we did not observe any apparent sex differences in the experience of ES or the degree of familiarization experienced, this was based on very small participant count of each sex. Future studies should enroll more men and women to investigate potential sex-related differences in pain perception during ES and try to develop specific ES parameters based on sex.
      • Maffiuletti NA
      • Herrero SJ
      • Jubeau M
      • Impellizzeri FM
      • Bizzini M.
      Differences in electrical stimulation thresholds between men and women.
      ,
      • Sherman JJ
      • LeResche L.
      Does experimental pain response vary across the menstrual cycle? A methodological review.
      The study only investigated discomfort in young healthy adults, as such, care should be taken when extrapolating data to patient populations, or older populations as their pain perception might be altered. It could be interesting to investigate how different populations experience pain during ES as culture,
      • Pillay T
      • van Zyl HA
      • Blackbeard D.
      Chronic pain perception and cultural experience.
      age,
      • Daoust R
      • Paquet J
      • Piette É
      • Sanogo K
      • Bailey B
      • Chauny J-M.
      Impact of age on pain perception for typical painful diagnoses in the emergency department.
      and disease might be important factors in the experience of ES.

      Conclusions

      In conclusion, the current investigation suggests that in ES increasing the frequency of stimulation leads to increases in torque with minor increases in discomfort while it appears that increases in current lead to increases in torque coupled with larger increases in discomfort. Furthermore, when performing ES interventions requiring high peak torques, eccentric contractions could be considered as an option, as these yield higher peak force with similar discomfort as isometric or concentric contractions. Finally, a solid and rapid familiarization-related decrease in the discomfort ratings during ES on repeated occasions in young men and women was observed. Future studies aiming to maximize ES torque output and minimize discomfort should consider using high stimulation frequencies, moderate currents, and 1 extensive familiarization session prior to ES intervention.

      Suppliers

      • a.
        Humac NORM; CSMi, Stoughton.
      • b.
        Biofex CF5090 5 × 9 cm; Biofina A/S.
      • c.
        Model DS7A; Digitimer Electronic.
      • d.
        HS-2100; Honda Electronics.
      • e.
        Stata 16; StataCorp LLC.

      Appendix 1. Graphs Showing Mean Relative Torque and Mean Discomfort for Days 1, 2, and 3

      A side-by-side comparison shows similar force levels on the days with a decrease in perceived discomfort on days 2 and 3. Torque and discomfort are presented as the mean ± SD for all subjects during the respective day. Each dot represents 1 type of stimulation with its accompanying SD. Black dots represent stimulations at 20 Hz, blue dots at 30 Hz, green at 50 Hz, and red at 100 Hz. Squares represent 100 mA stimulation, circles 150 mA stimulation, diamonds 200 mA, and hexagons 250 mA stimulation. The right panel, day 3, is identical to the figure in the manuscript.

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