Fatigue-related Feedback from Calf Muscles Impairs Knee Extensor Voluntary Activation

Exercise-induced pain within endurance exercise settings: Definitions, measurement, mechanisms and potential interventions

Pain can be defined as an unpleasant sensory and emotional experience associated with or resembling that associated with actual or potential tissue damage. Though consistent with this definition, different types of pain result in different behavioural and psychophysiological responses. For example, the transient, non-threatening, acute muscle pain element of exercise-induced pain (EIP) is entirely different from other pain types like delayed onset muscle soreness, muscular injury or chronic pain. However, studies often conflate the definitions or assume parity between distinct pain types. Consequently, the mechanisms through which pain might impact exercise behaviour across different pain subcategories may be incorrectly assumed, which could lead to interventions or recommendations that are inappropriate. Therefore, this review aims to distinguish EIP from other subcategories of pain according to their aetiologies and characteristics, thereby providing an updated conceptual and operational definition of EIP. Secondly, the review will discuss the experimental pain models currently used across several research domains and their relevance to EIP with a focus on the neuro-psychophysiological mechanisms of EIP and its effect on exercise behaviour and performance. Finally, the review will examine potential interventions to cope with the impact of EIP and support wider exercise benefits. HIGHLIGHTS: What is the topic of this review? Considerations for future research focusing on exercise-induced pain within endurance exercise settings. What advances does it highlight? An updated appraisal and guide of research concerning exercise-induced pain and its impact on endurance task behaviour, particularly with reference to the aetiology, measurement, and manipulation of exercise-induced pain.

The corticomuscular response to experimental pain via blood flow occlusion when applied to the ipsilateral and contralateral leg during an isometric force task

Blood flow occlusion (BFO) has been previously used to investigate physiological responses to muscle ischemia, showing increased perceptual effort (RPE) and pain along with impaired neuromuscular performance. However, at present, it is unclear how BFO alters corticomuscular activities when either applied to the exercising or nonexercising musculature. The present study therefore set out to assess the corticomuscular response to these distinct BFO paradigms during an isometric contraction precision task. In a repeated measures design, fifteen participants (age = 27.00 ± 5.77) completed 15 isometric contractions across three experimental conditions; no occlusion (CNTRL), occlusion of the contralateral (i.e., nonexercising) limb (CON-OCC), and occlusion of the ipsilateral (i.e., exercising) limb (IPS-OCC). Measures of force, electroencephalographic (EEG), and electromyographic (EMG) were recorded during contractions. We observed that IPS-OCC broadly impaired force steadiness, elevated EMG of the vastus lateralis, and heightened RPE and pain. IPSI-OCC also significantly decreased corticomuscular coherence during the early phase of contraction and decreased EEG alpha activity across the sensorimotor and temporoparietal regions during the middle and late phases of contraction compared with CNTRL. By contrast, CON-OCC increased perceived levels of pain (but not RPE) and decreased EEG alpha activity across the prefrontal cortex during the middle and late phases of contraction, with no changes observed for EMG and force steadiness. Together, these findings highlight distinctive psychophysiological responses to experimental pain via BFO showing altered cortical activities (CON-OCC) and altered cortical, corticomuscular, and neuromuscular activities (IPS-OCC) when applied to the lower limbs during an isometric force precision task.

Persistent Contralateral Pain Compromises Exercise Tolerance but Does not Alter Corticomotor Responses During Repeated, Submaximal Isometric Knee Extensions to Task Failure

Muscle pain is an important determinant of exercise tolerance, but its relationship with neurophysiological responses during a submaximal exercise trial is unclear. The purpose of this study was to determine the effect of persistent contralateral pain on neurophysiological function and perceptual responses during ipsilateral isometric knee extensions to task failure. Ten participants performed a single-leg repeated submaximal isometric knee extensions with (PAIN) or without (CTRL) constant pain induced by intermittent blood flow occlusion combined with evoked muscle contraction applied to the contralateral, resting leg. Transcranial magnetic stimulation (TMS) applied over the motor cortex was used to assess corticospinal excitability (quantified as motor evoked potentials), corticospinal inhibition (quantified as silent period duration), and short interval intracortical inhibition. Maximal voluntary contractions (MVCs), coupled with femoral nerve stimulation to the exercising leg, were performed every 12 submaximal contractions to assess neuromuscular function. Perceived leg pain and effort were also assessed throughout the exercise. The experimental pain shortened the time to task failure compared to CTRL (P = 0.019). Although time effects were present, no differences appeared between conditions for MVC force, voluntary activation, or potentiated twitch force across both tasks (all P > 0.05). Additionally, no differences between CTRL and PAIN were demonstrated for any TMS-derived measures assessing corticospinal responses. Exercising leg pain was higher in CTRL (P = 0.018), as was perceived exertion (P = 0.030). Overall, when using a persistent, submaximal experimental pain intervention, it appears that although muscle pain compromises exercise tolerance, this phenomenon occurs independently of potential alterations in corticomotor mechanisms.

Pharmacological Blockade of Muscle Afferents and Perception of Effort: A Systematic Review with Meta-analysis

BACKGROUND

The perception of effort provides information on task difficulty and influences physical exercise regulation and human behavior. This perception differs from other-exercise related perceptions such as pain. There is no consensus on the role of group III/IV muscle afferents as a signal processed by the brain to generate the perception of effort.

OBJECTIVE

The aim of this meta-analysis was to investigate the effect of pharmacologically blocking muscle afferents on the perception of effort.

METHODS

Six databases were searched to identify studies measuring the ratings of perceived effort during physical exercise, with and without pharmacological blockade of muscle afferents. Articles were coded based on the operational measurement used to distinguish studies in which perception of effort was assessed specifically (effort dissociated) or as a composite experience including other exercise-related perceptions (effort not dissociated). Articles that did not provide enough information for coding were assigned to the unclear group.

RESULTS

The effort dissociated group (n = 6) demonstrated a slight increase in ratings of perceived effort with reduced muscle afferent feedback (standard mean change raw, 0.39; 95% confidence interval 0.13-0.64). The group effort not dissociated (n = 2) did not reveal conclusive results (standard mean change raw, - 0.29; 95% confidence interval - 2.39 to 1.8). The group unclear (n = 8) revealed a slight ratings of perceived effort decrease with reduced muscle afferent feedback (standard mean change raw, - 0.27; 95% confidence interval - 0.50 to - 0.04).

CONCLUSIONS

The heterogeneity in results between groups reveals that the inclusion of perceptions other than effort in its rating influences the ratings of perceived effort reported by the participants. The absence of decreased ratings of perceived effort in the effort dissociated group suggests that muscle afferent feedback is not a sensory signal for the perception of effort.

Motor Unit Recruitment is Altered When Acute Experimental Pain is Induced at a Site Distant to the Contracting Muscle

Acute pain alters motor unit discharge properties in muscles that are painful or influence loading of painful structures. Less is known about the changes in discharge when pain is induced in distant tissues that are unable or have limited capacity to modify the load of the contracting muscle. We aimed to determine whether acute experimental pain alters quadriceps motor unit discharge when pain is induced in; (i) a muscle that is unlikely to be mechanically influenced by modified quadriceps activity (tibialis anterior: TA), or (ii) the antagonist muscle (biceps femoris: BF). Using a within-subject design, 16 adults performed force-matched isometric knee extension during pain-free control conditions, and trials after painful hypertonic saline injections into TA or BF. Surface and intramuscular electromyography recordings were made. Despite maintained force, discharge rate of quadriceps motor units was lower during Pain than Control conditions for TA and BF trials (both P < 0.001). Redistribution of motor unit activity was observed; some units were recruited in control or pain but not both. As modified quadriceps motor unit discharge has limited/no potential to modify load in the painful tissue to protect the painful part, the findings might support an alternative hypothesis that activity is redistributed to larger motor units.

The effect of hypertonic saline evoked muscle pain on neurophysiological changes and exercise performance in the contralateral limb

Non-local muscle pain may impair endurance performance through neurophysiological mechanisms, but these are relatively unknown. This study examined the effects of muscle pain on neuromuscular and neurophysiological responses in the contralateral limb. On separate visits, nine participants completed an isometric time to task failure (TTF) using the right knee extensors after intramuscular injection of isotonic saline (CTRL) or hypertonic saline (HYP) into the left vastus lateralis. Measures of neuromuscular fatigue were taken before, during and after the TTF using transcranial magnetic stimulation (TMS) and peripheral nerve stimulation. Mean pain intensity was greater in the left leg in HYP (3.3 ± 1.9) compared to CTRL (0.4 ± 0.7; P < 0.001) which was combined with a reduced TTF by 9.8% in HYP (4.54 ± 0.56 min) compared to CTRL (5.07 ± 0.77 min; P = 0.005). Maximum voluntary force was not different between conditions (all P > 0.05). Voluntary activation was lower in HYP compared to CTRL (P = 0.022). No difference was identified between conditions for doublet amplitude (P > 0.05). Furthermore, no difference in MEP·M_max⁻¹ or the TMS silent period between conditions was observed (all P > 0.05). Non-local pain impairs endurance performance of the contralateral limb. This impairment in performance is likely due to the faster attainment of the sensory tolerance limit from a greater amount of sensory feedback originating from the non-exercising, but painful, left leg.

The effects of pain induced by blood flow occlusion in one leg on exercise tolerance and corticospinal excitability and inhibition of the contralateral leg in males

Experiencing pain in one leg can alter exercise tolerance and neuromuscular fatigue (NMF) responses in the contralateral leg; however, the corticospinal modulations to non-local experimental pain induced by blood flow occlusion remain unknown. In three randomized visits, thirteen male participants performed 25% of isometric maximal voluntary contraction (25%IMVC) to task failure with one leg preceded by (i) 6-min rest (CON), (ii) cycling at 80% of peak power output until task failure with the contralateral leg (CYCL) or (iii) CYCL followed by blood flow occlusion (OCCL) during 25%IMVC. NMF assessments (IMVC, voluntary activation [VA] and potentiated twitch [Qtw]) were performed at baseline and task failure. During the 25%IMVC, transcranial magnetic stimulations were performed to obtain motor evoked potential (MEP), silent period (SP), and short intracortical inhibition (SICI). 25%IMVC was shortest in OCCL (105±50s) and shorter in CYCL (154±68s) than CON (219±105s) (P<0.05). IMVC declined less after OCCL (-24±19%) and CYCL (-27±18%) then CON (-35±11%) (P<0.05). Qtw declined less in OCCL (-40±25%) compared to CYCL (-50±22%) and CON (-50±21%) (P<0.05). VA was similar amongst conditions. MEP and SP increased and SICI decreased throughout the task while SP was longer for OCCL compared to CYC condition (P<0.05). The results suggest that pain in one leg diminishes contralateral limb exercise tolerance and NMF development and modulate corticospinal inhibition in males. Novelty: Pain in one leg diminished MVC and twitch force decline in the contralateral limb Experimental pain induced by blood flow occlusion may modulation corticospinal inhibition of the neural circuitries innervating the contralateral exercise limb.

Electrically induced quadriceps fatigue in the contralateral leg impairs ipsilateral knee extensors performance

Muscle fatigue induced by voluntary exercise, which requires central motor drive, causes central fatigue that impairs endurance performance of a different, nonfatigued muscle. This study investigated the impact of quadriceps fatigue induced by electrically induced (no central motor drive) contractions on single-leg knee-extension (KE) performance of the subsequently exercising ipsilateral quadriceps. On two separate occasions, eight males completed constant-load (85% of maximal power-output) KE exercise to exhaustion. In a counterbalanced manner, subjects performed the KE exercise with no pre-existing quadriceps fatigue in the contralateral leg on one day (No-PreF), whereas on the other day, the same KE exercise was repeated following electrically induced quadriceps fatigue in the contralateral leg (PreF). Quadriceps fatigue was assessed by evaluating pre- to postexercise changes in potentiated twitch force (ΔQtw,pot; peripheral fatigue), and voluntary muscle activation (ΔVA; central fatigue). As reflected by the 57 ± 11% reduction in electrically evoked pulse force, the electrically induced fatigue protocol caused significant knee-extensors fatigue. KE endurance time to exhaustion was shorter during PreF compared with No-PreF (4.6 ± 1.2 vs 7.7 ± 2.4 min; P < 0.01). Although ΔQtw,pot was significantly larger in No-PreF compared with PreF (-60% vs -52%, P < 0.05), ΔVA was greater in PreF (-14% vs -10%, P < 0.05). Taken together, electrically induced quadriceps fatigue in the contralateral leg limits KE endurance performance and the development of peripheral fatigue in the ipsilateral leg. These findings support the hypothesis that the crossover effect of central fatigue is mainly mediated by group III/IV muscle afferent feedback and suggest that impairments associated with central motor drive may only play a minor role in this phenomenon.

Effects of induced local ischemia during a 4-km cycling time trial on neuromuscular fatigue development

The present study analyzed the effects of local ischemia during endurance exercise on neuromuscular fatigue (NMF). Nine cyclists performed, in a counterbalanced order, two separate 4-km cycling time trials (TT) with (ISCH) or without (CONTR) induced local ischemia. NMF was characterized by using isometric maximal voluntary contractions (IMVC), whereas central [voluntary activation (VA)] and peripheral fatigue [peak torque of potentiated twitch (TwPt)] of knee extensors were evaluated using electrically evoked contractions performed before (PRE) and 1 min after (POST) the TT. Electromyographic activity (EMG), power output (PO), oxygen uptake (V̇o₂), and rating of perceived exertion (RPE) were also recorded. The decrease in IMVC (-15 ± 9% vs. -10 ± 8%, P = 0.66), VA (-4 ± 3% vs. -3 ± 3%, P = 0.46), and TwPt (-16 ± 7% vs. -19 ± 14%, P = 0.67) was similar in ISCH and CONTR. Endurance performance was drastically reduced in ISCH condition (512 ± 29 s) compared with CONTR (386 ± 17 s) (P < 0.001), which was accompanied by lower EMG, PO, and V̇o₂ responses (all P < 0.05). RPE was greater in ISCH compared with CONTR (P < 0.05), but the rate of change was similar throughout the TT (8.19 ± 2.59 vs. 7.81 ± 2.01 RPE.% of total time^-1, P > 0.05). These results indicate that similar end-exercise NMF levels were accompanied by impaired endurance performance in ISCH compared with CONTR. These novel findings suggest that the local reduced oxygen availability affected the afferent feedback signals to the central nervous system, ultimately increasing perceived effort and reducing muscle activity and exercise intensity to avoid surpassing a sensory tolerance limit before the finish line.

Unilateral Quadriceps Fatigue Induces Greater Impairments of Ipsilateral versus Contralateral Elbow Flexors and Plantar Flexors Performance in Physically Active Young Adults

Non-local muscle fatigue (NLMF) studies have examined crossover impairments of maximal voluntary force output in non-exercised, contralateral muscles as well as comparing upper and lower limb muscles. Since prior studies primarily investigated contralateral muscles, the purpose of this study was to compare NLMF effects on elbow flexors (EF) and plantar flexors (PF) force and activation (electromyography: EMG). Secondly, possible differences when testing ipsilateral or contralateral muscles with a single or repeated isometric maximum voluntary contractions (MVC) were also investigated. Twelve participants (six males: (27.3 ± 2.5 years, 186.0 ± 2.2 cm, 91.0 ± 4.1 kg; six females: 23.0 ± 1.6 years, 168.2 ± 6.7 cm, 60.0 ± 4.3 kg) attended six randomized sessions where ipsilateral or contralateral PF or EF MVC force and EMG activity (root mean square) were tested following a dominant knee extensors (KE) fatigue intervention (2×100s MVC) or equivalent rest (control). Testing involving a single MVC (5s) was completed by the ipsilateral or contralateral PF or EF prior to and immediately post-interventions. One minute after the post-intervention single MVC, a 12×5s MVCs fatigue test was completed. Two-way repeated measures ANOVAs revealed that ipsilateral EF post-fatigue force was lower (-6.6%, p = 0.04, d = 0.18) than pre-fatigue with no significant changes in the contralateral or control conditions. EF demonstrated greater fatigue indexes for the ipsilateral (9.5%, p = 0.04, d = 0.75) and contralateral (20.3%, p < 0.01, d = 1.50) EF over the PF, respectively. There were no significant differences in PF force, EMG or EF EMG post-test or during the MVCs fatigue test. The results suggest that NLMF effects are side and muscle specific where prior KE fatigue could hinder subsequent ipsilateral upper body performance and thus is an important consideration for rehabilitation, recreation and athletic programs.