Contralateral fatigue during severe-intensity single-leg exercise: influence of acute acetaminophen ingestion

ABSTRACT


INTRODUCTION
The mechanisms of exercise-induced fatigue can be attributed to processes within the central nervous system, termed central fatigue, and within the contractile elements of the working muscle, termed peripheral fatigue.It is now recognised that peripheral and central fatigue development are interlinked, in part, via group III/IV muscle afferent feedback (25).Empirical support for a role of group III/IV muscle afferent feedback in modulating the mechanisms of neuromuscular fatigue is provided by reports that inhibiting group III/IV muscle afferent feedback, via lumbar intrathecal administration of fentanyl, lowers central fatigue development and results in increased skeletal muscle metabolic perturbation [greater and/or more rapid increases in adenosine diphosphate (ADP) and inorganic phosphate (Pi) accumulation and declines in phosphocreatine (PCr) and pH] and thus peripheral fatigue development (i.e., 1,2,8,10,11,12,38,39,40).Conversely, prior fatiguing single-limb exercise has been reported to accentuate central fatigue development and lead to lower peripheral fatigue development during subsequent fatiguing exercise in a contralateral or non-local (previously rested) muscle group, when group III/IV muscle afferent feedback would be expected to be elevated (3,22,23,26,33,40).However, the underlying mechanisms of non-local muscle fatigue, including the effect of prior fatiguing single-limb exercise on skeletal muscle metabolic perturbation during subsequent fatiguing exercise in a contralateral or non-local muscle group, have yet to be resolved (ref.23 for review).Moreover, while lumbar intrathecal administration of fentanyl and prior fatigue of a contralateral or non-local muscle group can alter group III/IV muscle afferent feedback and the physiological bases of exercise-induced neuromuscular fatigue, the effect of such interventions on exercise performance is equivocal (i.e., 1,2,3,8,10,11,12,22,23,26,28,33).
There is an emerging body of evidence to suggest that oral ingestion of acetaminophen (ACT) can blunt the development of exercise-induced neuromuscular fatigue and improve exercise capacity and/or performance (19,30,31,32).It is generally accepted that the principal mechanism of action of ACT is the inhibition of cyclooxygenase, the enzyme that catalyses the synthesis of prostaglandins from arachidonic acid (4).Since prostaglandins sensitize nociceptors (36,37), and since blocking cyclooxygenase attenuates group III/IV muscle afferent discharge during dynamic exercise (24), this might account for reports of increased work output for the same level of perceived pain and exertion (19,30), and elevated muscle activation (31,32), during exercise after ACT ingestion.Therefore, ACT administration might be ergogenic by reducing, but not abolishing, the net magnitude of group III/IV muscle afferent feedback, leading to a blunting of exercise-induced central fatigue.Since ACT appears to attenuate exercise-induced neuromuscular fatigue by abating aspects of central fatigue development (19,30,31,32), ACT might be more effective at lowering exercise-induced neuromuscular fatigue following prior exhaustive exercise in a contralateral limb.However, the effects of ACT ingestion on exercise-induced fatigue development and its underlying mechanisms following prior exercise in a contralateral limb have yet to be investigated.
The purpose of this study was to investigate the effects of ACT ingestion on exercise-induced neuromuscular fatigue and some of its underlying mechanisms during single-leg severeintensity knee extensor exercise completed with and without prior exhaustive severe-intensity knee extensor exercise in the contralateral leg.It was hypothesised that: 1) prior exhaustive exercise would impair subsequent exercise tolerance in the contralateral leg by lowering muscle activation and the degree of muscle metabolic perturbation [changes in muscle pH and

PCr ([PCr]), ADP ([ADP]) and Pi ([Pi]
) concentrations] that could be attained; 2) ACT ingestion would enhance single-leg knee extensor exercise tolerance by increasing muscle activation (higher surface EMG) and permitting the attainment of a greater degree of muscle metabolic perturbation; and 3) ACT ingestion would improve exercise tolerance to a greater extent with, compared to without, the completion of prior exercise by the contralateral leg.

Subjects
Fourteen active males volunteered to participate in this study (mean ± SD: age 23.8 ± 4.7 y, height 1.80 ± 0.10 m, body mass 81.6 ± 14.9 kg).All procedures were approved by the Ethics Committee of the Department of Sport and Health Sciences, University of Exeter.This study conformed to the principles of the World Medical Association Declaration of Helsinki.
Subjects completed a health questionnaire, which was checked by a medical doctor, to ensure it was safe to consume ACT prior to performing exhaustive exercise.The questionnaire incorporated questions pertaining to: known allergies to medications, current intake of medication and prior use of ACT as well as any history of illnesses, cigarette and illegal drug use, alcohol consumption, and chronic illnesses (personal and family history).Prior to each visit, subjects were required to refrain from caffeine (for at least 12 h), strenuous exercise and alcohol (for at least 24 h), analgesics and any form of anti-inflammatory drug (for the duration of the experiment) and to arrive in a fully rested, hydrated state.With the exception of these restrictions, subjects were instructed to maintain their usual diet and exercise regime during the study.All tests were performed at a similar time of day (± 2 h).

Pre-experimental procedures
Subjects visited the laboratory on twelve occasions over an 8-12 week period to complete the experimental testing, with a minimum of 72 h separating all tests (figure 1).The experimental testing incorporated 4 pre-experimental trials (visits 1-4) and 8 experimental trials (visits [5][6][7][8][9][10][11][12].Visits 1-4 were completed within a replica of an MRI scanner (with no magnetic field present).Initially, subjects completed a single-limb incremental test on the left leg (visit 1, Leg1) and right leg (visit 2, Leg2) to task failure to establish the limb-specific work rates that would be applied in subsequent experimental visits (as described below).Following these preliminary tests, subjects completed a familiarisation session on visits 3 and 4 which comprised a single-leg severe-intensity constant work rate (CWR) test to task failure with the left leg (Leg1), a single-leg severe-intensity CWR test to task failure with the right leg (Leg2), and a crossover test where the Leg1 protocol was repeated and immediately followed by the Leg2 protocol to assess contralateral fatigue in Leg2.In these preliminary tests, the Leg1, Leg2 and Leg2 contralateral protocols were interspersed by 10 min of passive recovery.

Experimental procedures
During visits 5 and 6, subjects completed the Leg1 and Leg2 protocols without oral consumption of any capsules (Leg1CON and Leg2CON, respectively).On visits 7 and 8, subjects completed the crossover limb tests described above, 45 mins following the consumption of 1 g maltodextrin (placebo, PL) to determine time to task failure (Tlim) values for Leg1 (Leg1PL) and Leg2 contralateral (Leg2PL-CONTRA), and 45 mins following the consumption of 1 g ACT, to determine Tlim values for Leg1 (Leg1ACT) and Leg2 contralateral (Leg2ACT-CONTRA).PL and ACT were administered in the form of 2 identically coloured pills.The placebo was made from maltodextrin powder inserted into gelatine capsules designed to have a similar appearance to ACT without inducing any analgesic or antipyretic effects.The oral consumption of PL and ACT ~45 min prior to commencing exercise was selected to broadly coincide with attainment of the peak plasma [ACT], which occurs ~60 min post ACT ingestion (4,17), at the onset of the Leg2-CONTRA tests.The PL and ACT conditions were administered double-blind in a counterbalanced cross-over experimental design.Visits 5-8 were completed within the bore of an MRI scanner for assessment of exercise-induced changes in intramuscular phosphorouscontaining substrates and metabolites.Visits 5-8 were replicated in visits 9-12 within a replica of the MRI scanner (with no magnetic field present) to assess muscle electromyography (EMG) and ratings of perceived exertion (RPE).

Experimental set-up
Exercise tests were performed in a prone position within the bore of a 1.5 T superconducting magnet (Gyroscan Clinical Intera, Philips, The Netherlands) using a custom-built ergometer for the assessment of intra-muscular [PCr], [Pi], [ADP] and pH (visits 5-8) or within a replica of the MRI scanner for preliminary testing (visits 1-4) and the assessment of EMG and RPE responses (visits 9-12).Subjects' feet were fastened securely to padded foot braces using Velcro straps and connected to the ergometer load baskets via a rope and pulley system.The sprocket arrangement was such that when a bucket containing non-magnetic weights was attached, it provided a concentric-only resistive load, allowing for the performance of rhythmic knee-extension exercise.Single-leg knee-extensions over a distance of ∼0.22 m were performed continuously at a constant frequency which was set in unison with the magnetic pulse sequence (40 pulses min −1 ) to ensure the quadriceps muscle was in the same phase of contraction during each magnetic resonance pulse acquisition.To prevent displacement of the quadriceps relative to the magnetic resonance spectroscopy (MRS) coil, Velcro straps were also fastened over the subject's thighs, hips and lower back.

Experimental protocol
To determine peak work rate (WRpeak) for each leg, subjects initially completed single-leg incremental knee-extensor exercise on visits 1 and 2 until they were unable to continue the prescribed work rate, as described previously (43).The load for the initial increment was 4 kg and this was increased by 0.5 kg•min -1 thereafter until Tlim.Tlim was recorded when subjects were unable to sustain the required contraction frequency for 3 consecutive repetitions.Following these initial tests, subjects were familiarized with the different exercise tests that comprised the experimental testing protocol.During these visits, a limb-specific, high-intensity work rate, which was expected to elicit Tlim in approximately 5-8 min, was prescribed for each subject.
The experimental exercise protocol consisted of CWR, single-leg knee-extension to Tlim.
Initially, subjects completed single-leg knee-extension exercise for each limb individually over two separate laboratory visits.Subsequently, to investigate the influence of ACT on contralateral leg fatigue, subjects completed single-leg knee-extension exercise until task failure with Leg1, followed consecutively (<3 s) by the identical task with the contralateral leg (i.e., Leg2).These crossover tests to assess contralateral fatigue in Leg2 were completed 60 min following the consumption of PL and ACT over two separate laboratory visits.For all trials, subjects received strong verbal encouragement to continue for as long as possible but no feedback was given on the elapsed time.

MRS measurements
31 P-MRS data were acquired every 1.5 s with a spectral width of 1,500 Hz and 1,000 data points.Phase cycling with four phase cycles was used, leading to a spectrum being acquired every 6 s.The subsequent spectra were quantified by peak fitting, using the AMARES fitting algorithm in the jMRUI (v3) software package.Absolute values of [PCr] and [Pi] concentrations were subsequently calculated via the ratio of PCr:adenosine triphosphate (ATP) and Pi:ATP assuming an ATP concentration of 8.2 mM.Intracellular pH was calculated using the chemical shift of the Pi spectra relative to the PCr peak.The ADP concentration was calculated as described by Kemp et al. (27).In all cases, relative amplitudes were corrected for partial saturation resulting from the short repetition time relative to T1 relaxation time, via a spectrum consisting of 24 averages that was acquired with a TR of 20 s prior to the comencement of exercise testing.

Electromyography
Throughout visits 9-12, muscle activity of the right and left m.vastus lateralis was recorded using active bipolar bar electrodes with a single differential configuration (DE2.1,DelSys Inc, Boston, MA, USA).Initially, the leg was shaved and cleaned with alcohol to minimize skin impedance.The electrodes were placed over the respective muscle bellies parallel to the longitudinal axis of each muscle (SENIAM guidelines).Double-sided adhesive tape and a hypoallergenic medical tape were used to ensure the EMG sensor stability.The position of the EMG electrodes was measured with respect to the location of the patella and the anterior superior iliac spine and marked with indelible ink to ensure placement in the same location on subsequent visits.The ground electrode was placed over the patella of the respective leg.The EMG signals were pre-amplified (1,000x), band-pass filtered (20-450 Hz, Bagnoli-8, DelSys Inc, Boston, MA, USA), and then transferred to a computer with a sampling frequency of 2 kHz.EMG data were recorded continuously and digitised synchronously with 16 bit resolution via an A/D converter (± 5 V range, CED 1401 power, Cambridge, UK) using Spike2 software (CED, Cambridge, UK).During these trials, ratings of perceived exertion (RPE) was measured at 2-min intervals from the onset of exercise using Borg's 6-20 scale (9).

Data Analysis
Baseline values for [PCr], [Pi], [ADP], and pH were defined as the mean values measured over the final 60 s of rest (i.e., prior to initiation of the severe-intensity exercise bout).Baseline values for Leg2 during the crossover protocol (for both PL and ACT) were calculated during the final 60 s of exhaustive Leg1 exercise.End-exercise values for these variables were defined as the mean values measured over the final 30 s of exercise.The changes (Δ) in [PCr], [Pi], [ADP] and pH across the protocol were then calculated as the difference between end-exercise and baseline values.[ADP] and pH, aEMG (using 30 s mean values), and RPE (using 120 s mean values).Where the ANOVA revealed a significant main or interaction effect, posthoc tests were completed using a Bonferroni correction.For calculation of effect size, partial eta squared (η 2 ) was used for omnibus tests.Cohen's d was used to calculate the effect size for paired t-tests and post-hoc comparisons.Where sphericity was violated, a greenhouse-geisser correction factor was applied.For all tests, results were considered statistically significant when P<0.05.Data are presented as means ± SD unless otherwise indicated.All statistical analyses were conducted using IBM SPSS Statistics version 24.

Ratings of perceived exertion
RPE increased in all trials following the onset of exercise (figure 6).However, there were no differences in RPE between Leg1CON, Leg1PL and Leg1ACT at any time point (P>0.05,η 2 =0.08, figure 6).The rate of rise and the end-exercise RPE were also similar during the Leg2CON trial compared with the Leg2PL-CONTRA and Leg2ACT-CONTRA trials (P>0.05,η 2 =0.18).However, at the onset of exercise, RPE was significantly higher in Leg2PL-CONTRA and Leg2ACT-CONTRA when compared to Leg2CON (P<0.05,η 2 =0.55, figure 6).Specifically, during the first 2 min of exercise, there was a respective elevation in RPE of 14% and 13% in Leg2PL-CONTRA and Leg2ACT-CONTRA, compared to Leg2CON (P<0.05).There were no differences in RPE at any time points between Leg2PL-CONTRA and Leg2ACT-CONTRA (P>0.05,η 2 =0.21, figure 6).

DISCUSSION
The principal original finding of this study was that, while time to task failure was lower during severe-intensity single-leg knee extensor exercise after the completion of prior fatiguing exercise in the contralateral leg, this effect was not mitigated by acute ACT ingestion.We found no differences in the rates of change or end-exercise values for skeletal muscle activation (via EMG), metabolic perturbation (via 31 P-MRS) and perception of effort (via RPE) during exercise after prior contralateral leg fatigue following ACT and PL ingestion.Moreover, there were also no differences in time to task failure (i.e., Tlim), skeletal muscle activation, metabolic perturbation and RPE during single-leg exercise without the completion of prior fatiguing exercise in the contralateral leg following ACT and PL ingestion.These findings do not support our experimental hypotheses and suggest that 1 g of acute ACT ingestion does not improve time to task failure, skeletal muscle activation, metabolic perturbation or perceived exertion during single-leg severe-intensity knee extensor exercise completed with or without prior fatiguing exercise by the contralateral leg.Collectively, these results contribute to our understanding of fatigue development during exercise, performed with or without prior contralateral leg exercise, and in the presence or absence of ACT ingestion.
In the present study, Tlim in the Leg2PL-CONTRA protocol was shorter than the Leg2CON protocol, indicative of an earlier task failure after completing exhaustive exercise in the contralateral leg compared to no prior fatiguing contralateral leg exercise.This observation is consistent with some (i.e., 3,14,22,29,33,41), but not all (i.e., 16,21,34,42,45), previous studies reporting greater fatigue development after prior contralateral or non-local muscle fatigue.While the neuromuscular bases of contralateral fatigue development have yet to be fully resolved (23), there is evidence to suggest that greater central fatigue makes an important contribution to this phenomenon (3).In the current study, RPE was higher at baseline and over the initial stages of the Leg2PL-CONTRA test compared to the Leg2CON test, leading to an earlier attainment of peak RPE and Tlim, consistent with previous observations (3) and the notion that afferent feedback may contribute to increased pain and effort sensation (1,20).Amann and colleagues ( 3 (6) or by hyperoxic gas inhalation (44).
Interestingly, however, and despite a higher baseline muscle [Pi] in the Leg2PL-CONTRA condition compared to the Leg2CON condition, muscle [Pi] was lower at the point of task failure in the Leg2PL-CONTRA test.These novel observations suggest that the ergolytic effect of prior contralateral fatigue may be related, at least in part, to a limitation in the attainment of peak intramuscular [Pi].
It is unclear why prior contralateral leg fatigue limited the attainment of peak [Pi] in the Leg2PL-CONTRA condition compared to the Leg2CON condition, whereas the peak [ADP] and the nadir in pH and [PCr] were not different between these conditions.However, our observations of a limited peak perturbation of muscle [Pi], but not pH, [PCr] and [ADP], when group III/IV muscle afferent feedback would be expected to be elevated via prior contralateral fatigue (3), are in accord with studies from another group who observed greater peak perturbation of muscle [Pi], but not pH, [PCr] and [ADP], when group III/IV muscle afferent feedback was abolished via lumbar intrathecal administration of fentanyl (8,11,12).Taken together, these complementary observations suggest that intramuscular phosphorous-containing metabolites and substrates may not respond in a uniform manner to manipulations in skeletal muscle group III/IV afferent feedback and that muscle [Pi] might be a more sensitive marker of peripheral fatigue development.However, it should be acknowledged that, since inter-test variability is greater for contracting skeletal muscle [Pi] than for pH, [PCr] and [ADP] (15), further research is required to verify these observations.Although the completion of prior single-leg fatiguing exercise lowered Tlim during subsequent exercise in the contralateral leg in the current study, there were no differences between the Leg2ACT-CONTRA and Leg2PL-CONTRA conditions in Tlim, RPE, or muscle activation and phosphorous-containing metabolites and substrates.Similarly, and also in contrast to our hypothesis, acute ACT ingestion did not alter Tlim, RPE, or muscle activation, pH, [PCr], [ADP] or [Pi], during single-leg severe-intensity knee extensor exercise completed without prior fatiguing exercise in the contralateral leg, with these responses being similar between the Leg1CON, Leg1PL and Leg1ACT conditions.These findings conflict with reports that acute ACT consumption can improve exercise performance by increasing work output for the same level of pain and effort sensation (19,30) and by increasing muscle activation (31,32).

Experimental Considerations
The lack of an ergogenic effect of ACT administration in the current study might be due to differences in the ACT administration procedure compared to previous studies reporting improved performance and delayed neuromuscular fatigue development (19,30,31,32).In the present study, ACT was ingested 45 min prior to the start of the Leg1ACT test, which immediately transitioned into the Leg2ACT-CONTRA protocol that was the primary focus of the current study.Since peak plasma [ACT] is attained ~60 min post oral ACT ingestion (4,17), we elected to administer ACT such that peak plasma [ACT] was expected to coincide with the onset of the Leg2ACT-CONTRA protocol rather than the Leg1ACT protocol.This might account for the lack of an ergogenic effect of ACT during the Leg1ACT protocol compared to other studies that administered ACT 60 min prior to the performance trial (19,30,31,32).Therefore, we cannot exclude the possibility that earlier ACT ingestion (18), at the same or a greater dose (19,30), might have resulted in improved single-leg severe-intensity exercise tolerance.
However, it should also be noted that inter-study differences in participant characteristics (i.e., training status, motivation and responsiveness to analgesic medication) may have contributed to the differences in ergogenicity observed following ACT ingestion between the current study and some previous studies (19,30,31,32).
In addition to differences in the ACT dosing procedure, the lack of an ergogenic effect of ACT administration in the current study might be linked to the nature of the fatiguing exercise test administered.Our subjects completed continuous single-leg severe-intensity knee extensor exercise until task failure with no pre-determined end-point (i.e., an 'open loop' exercise test).This differs from situations in which ACT ingestion has been reported to be ergogenic, such as completion of a fixed distance (16.1 km) time trial (30), a fixed number of maximal effort repetitions (19,31), or a fixed duration of maximal effort (32), all of which have a predetermined end point (i.e., a 'closed loop' exercise task).Moreover, since exercise-induced pain sensation is positively associated with exercise intensity (5,13), and since ACT ingestion is suggested to be ergogenic by mitigating pain sensation (19,30), this might account for the lack of improvement in performance in the longer duration, continuous severe-intensity exercise test we employed compared to the improved exercise performance that has been reported during maximal-intensity exercise (19,31,32).With regard to contralateral fatigue development, we cannot exclude the possibility that ACT might have been effective at attenuating the effects of prior single-leg fatigue on Tlim during subsequent exercise if a greater degree of contralateral fatigue had been attained.For example, Tlim was lowered by 19% in Leg2PL-CONTRA compared to Leg2CON in the current study, whereas Amann et al. (3) reported a much larger (49%) reduction in Tlim following contralateral limb fatigue, which would have provided greater scope for an ergogenic effect with ACT ingestion.Moreover, since RPE is higher and Tlim is shorter at the same relative exercise intensity when a larger muscle mass is recruited (35), it is possible that ACT ingestion might have improved Tlim during exercise after prior fatigue had a larger muscle mass been recruited in either the initial or the subsequent fatiguing exercise task.Further research is required to assess the exercise settings in which ACT administration is more or less likely to be ergogenic.
In conclusion, the completion of prior single-leg fatiguing exercise compromised exercise tolerance during subsequent exercise in the contralateral leg.This ergolytic effect of prior contralateral leg fatigue was accompanied by elevated baseline RPE, muscle activation and

Legends to figures
Protocol schematic.Visits 1-4 were completed within a replica of the MRI scanner.Subjects completed a single-leg incremental test on the left leg (visit 1, Leg1) and right leg (visit 2, Leg2).
Subjects then completed a familiarisation session on visits 3 and 4 which comprised a singleleg severe-intensity constant work rate (CWR) test to task failure with Leg1, Leg2 and a crossover test where the Leg1 protocol was repeated and immediately followed by the Leg2 protocol (interspersed by 10 min of passive recovery).During visits 5 and 6, subjects completed the Leg1 and Leg2 protocols, respectively, without oral consumption of any capsules.On visits 7 and 8, subjects commenced the crossover test, 45 mins following the consumption of 1 g maltodextrin (PL) and 45 mins following the consumption of 1 g ACT.Visits 5-8 were completed within the bore of an MRI scanner for assessment of intramuscular phosphorous substrates and metabolites and then replicated within a replica of the MRI scanner (visits [9][10][11][12] to assess muscle electromyography (EMG) and ratings of perceived exertion (RPE).The dashed vertical lines represent the limit of tolerance (i.e., Tlim) for each trial and/or leg, respectively.
[PCr], [Pi] and [ADP] were expressed as absolute concentrations and as a percentage change relative to resting baseline (i.e., 100%).The overall rate of change for [PCr], [Pi], [ADP] and pH was calculated as the difference between end-exercise and baseline values divided by Tlim.EMG was average rectified and normalised to the first 30 s of each trial (aEMG).For analysis, Tlim values obtained from visits 5-8 were used.Visits 9-12 were used to overlay EMG and RPE responses to 31 P-MRS data.Statistics Differences in Tlim, baseline and end-exercise aEMG and muscle [PCr], [Pi], [ADP], and pH between control limbs (i.e., Leg1 vs. Leg2) were assessed using paired samples t-tests.A twoway repeated measures ANOVA (time x condition) was employed to test for differences in the profiles of muscle [PCr], [Pi], ) reported a lower EMG response at task failure and reduced peripheral fatigue development after prior contralateral leg fatigue.Although the EMG amplitude was not different at task failure in the current study between the Leg2CON and Leg2PL-CONTRA tests, baseline EMG was elevated in the Leg2PL-CONTRA condition, presumably due to isometric stabilisation, leading to the earlier attainment of the same peak EMG amplitude.The greater muscle activation in the non-exercising contralateral leg during the baseline 'resting' period in the Leg2PL-CONTRA condition was accompanied by lower muscle [PCr], and higher muscle [Pi] and [ADP], compared to the Leg2CON condition.Since there were no differences in muscle [PCr] and [ADP] at Tlim, and since the rates of change in [PCr] and [ADP] were not different between the Leg2CON and Leg2PL-CONTRA tests, the muscle [PCr] nadir and [ADP] peak were attained earlier in the Leg2PL-CONTRA test.These observations cohere with reports that the end-exercise values of muscle [PCr], [ADP] and pH are consistent when several bouts of exhaustive exercise of differing duration are completed within the severe-intensity domain (7, 44), and when Tlim is altered via prior passive heating of the legs

[
ADP], and lower baseline [PCr], leading to the earlier attainment of peak (RPE, muscle activation and [ADP]) or nadir (muscle [PCr]) values in these variables, and the attainment of a submaximal end-exercise [Pi].However, acute ACT ingestion was not effective at lowering perceived exertion, increasing muscle activation or intramuscular perturbation or enhancing Tlim during single-leg severe-intensity exercise completed with or without prior fatigue in the contralateral leg.These findings do not support an ergogenic effect of analgesia, at least using the ACT administration and exercise testing procedures employed in the current study.intensity exercise in humans: a 31P magnetic resonance spectroscopy study.Exp Physiol 95: 528-540, 2010.45.Zijdewind I, Zwarts MJ and Kernell D. Influence of a voluntary fatigue test on the contralateral homologous muscle in humans?Neurosci Lett 253: 41-44, 1998.