Effects of evoked pain on the electromyogram and compound muscle action potential of the brachial biceps muscle

A randomized double blinded placebo controlled study to evaluate motor unit abnormalities after experimentally induced sensitization using capsaicin

Central sensitization is a condition that represents a cascade of neurological adaptations, resulting in an amplification of nociceptive responses from noxious and non-noxious stimuli. However, whether this abnormality translates into motor output and more specifically, ventral horn abnormalities, needs to be further explored. Twenty healthy participants aged 20-70 were randomly allocated to topical capsaicin or a placebo topical cream which was applied onto their left upper back to induce a transient state of sensitization. Visual analogue scale (VAS) ratings of pain intensity and brush allodynia score (BAS) were used to determine the presence of pain and secondary allodynia. Surface electromyography (sEMG) and intramuscular electromyography (iEMG) were used to record motor unit activity from the upper trapezius and infraspinatus muscles before and twenty minutes after application of capsaicin/placebo. Motor unit recruitment and variability were analyzed in the sEMG and iEMG, respectively. An independent t-test and Kruskal-Wallis H test were performed on the data. The sEMG results demonstrated a shift in the motor unit recruitment pattern in the upper trapezius muscle, while the iEMG showed a change in motor unit variability after application of capsaicin. These results suggest that capsaicin-induced central sensitization may cause changes in ventral horn excitability outside of the targeted spinal cord segment, affecting efferent pathway outputs. This preclinical evidence may provide some explanation for the influence of central sensitization on changes in movement patterns that occur in patients who have pain encouraging of further clinical investigation.Clinical Trials registration number: NCT04361149; date of registration: 24-Apr-2020.

Modulation of the muscle and nerve compound muscle action potential by evoked pain

Background and aims To our knowledge there are no studies that have examined the effects of the experimental pain on muscle fibre excitability as measured by the amplitudes of the potentials evoked by direct muscle stimulation (DMS) in a muscle at rest. We hypothesized that evoked pain can modulate the muscle compound action potential (CMAP) obtained by DMS possibly due to changes in muscle fibre excitability. Methods Pain was evoked by intramuscular infusion of hypertonic saline in 50 men. Ten control subjects were infused with isotonic saline. The infusions were given distal to the motor end plate region of the dominant brachial biceps muscle (BBM) in a double-blind manner. The nerve CMAP was obtained by stimulating the musculocutaneous nerve and recording from the BBM using surface-electrodes. Muscle CMAPs were obtained by direct muscle stimulation with subdermal electrodes placed subcutaneously in the distal third of the muscle. A stimuli-response curve of the amplitudes from muscle CMAP was obtained by stimulating from 10 to 90 mA. Results There was a decrease of the nerve CMAP amplitudes after infusion of isotonic saline (from 13.78mV to 12.16 mV), p-value 0.0007 and of hypertonic saline (from 13.35 mV to 10.85 mV), p-value 0.0000. The percent decrease from before to after infusion was larger in the hypertonic saline group (19.37%) compared to the isotonic saline group (12.18%), p-value 0.025. There was a decrease of the amplitudes of the muscle CMAP after infusion of both isotonic (at 90 mA from 13.84mV to 10.32 mV, p value 0.001) and of hypertonic saline (at 90 mA from 14.01 mV to 8.19 mV, p value 0.000). The percent decrease was larger in the hypertonic saline group compared to the isotonic saline group for all the stimulations intensities. At 90 mA we saw a 42% decrease in the hypertonic saline group and 24.5% in the isotonic saline group, p value 0.005. There were no changes in conduction velocity. Conclusion We found a larger amplitude decrease of the muscle and nerve potentials following hypertonic saline infusion compared with that of isotonic saline. We suggest that this deferential outcome of hypertonic saline on muscle CMAP may be linked to the nociceptive effect on muscle fibre membrane excitability. Implications The study supplies with some evidence of the peripheral effect of muscle pain. However, further trials with other nociceptive substances such as capsaicin should be performed.

Penetration and spread of interferential current in cutaneous, subcutaneous and muscle tissues

OBJECTIVES

To investigate the penetration depth of interferential current (IFC) through soft tissue and the area over which it spreads during clinical application.

DESIGN

A laboratory-based study of healthy participants.

SETTING

A university research laboratory.

PARTICIPANTS

Twelve healthy subjects.

INTERVENTIONS

Premodulated IFC at 90 Hz and 'true' IFC at frequencies of 4, 40 and 90 Hz were applied via four electrodes, in a quadrant setting, to the distal medial thigh of each participant on separate occasions.

MAIN OUTCOME MEASURE

Voltage induced by tested currents was measured at three locations (middle of the four electrodes, in line with one circuit and outside the four electrodes) and three depths (skin, subcutaneous and muscle tissues) using three Teflon-coated needle electrodes connected to a Cambridge Electronic Design data acquisition system.

RESULTS

All voltages were greater at all depths and locations compared with baseline (P<0.001): premodulated IFC [mean difference 0.112, 95% confidence interval (CI) 0.065 to 0.160], 4 Hz (mean difference 0.168, 95% CI 0.106 to 0.229), 40 Hz (mean difference 0.165, 95% CI 0.107 to 0.223) and 90 Hz (mean difference 0.162, 95% CI 0.102 to 0.221). Voltages decreased with depth. Lower voltages of all currents were recorded in the middle of the four electrodes, with the highest voltage for 'true' IFC being recorded outside the four electrodes (mean difference 0.04, 95% CI 0.01 to 0.029; P=0.011). The premodulated IFC had the highest voltage in line with one circuit.

CONCLUSIONS

IFC passed through soft tissues, with the highest voltages recorded in superficial tissue and the lowest voltages recorded in muscle. For 'true' IFC, the current spread outside the electrodes at higher voltages compared with the intersection of the four electrodes. The premodulated IFC had the highest voltage in line with one circuit. In terms of higher recorded voltages, 'true' IFCs were more efficient than the premodulated IFC when targeting deeper tissues. However, further studies with larger samples are required to confirm the results of this study.

Effects of Fatigue on Kinetic and Kinematic Variables During a 60-Second Repeated Jumps Test

Purpose:

The aim of this study was to investigate the effects of a maximal repeated-jumps task on force production, muscle activation and kinematics, and to determine if changes in performance were dependent on gender.

Methods:

Eleven male and nine female athletes performed continuous countermovement jumps for 60 s on a force platform while muscle activation was assessed using surface electromyography. Performances were videotaped and digitized (60 Hz). Data were averaged across three jumps in 10-s intervals from the initial jump to the final 10 s of the test.

Results:

No interaction between time and gender was evident for any variable; therefore, all results represent data collapsed across gender. Preactivation magnitude decreased across time periods for anterior tibialis (AT, P < .001), gastrocnemius (GAS, P < .001) and biceps femoris (BF, P = .03), but not for vastus lateralis (VL, P = .16). Muscle activation during ground contact did not change across time for BF; however, VL, G, and AT showed significant reductions (all P < .001). Peak force was reduced at 40 s compared with the initial jumps, and continued to be reduced at 50 and 60 s (all P < .05). The time from peak force to takeoff was greater at 50 and 60 s compared with the initial jumps (P < .05). Both knee fexion and ankle dorsifexion were reduced across time (both P < .001), whereas no change in relative hip angle was evident (P = .10). Absolute angle of the trunk increased with time (P < .001), whereas the absolute angle of the shank decreased (P < .001).

Conclusions:

In response to the fatiguing task, subjects reduced muscle activation and force production and altered jumping technique; however, these changes were not dependent on gender.

Deep muscle pain, tender points and recovery in acute whiplash patients: a 1-year follow-up study

Local sensitization to noxious stimuli has been previously described in acute whiplash injury and has been suggested to be a risk factor for chronic sequelae following acute whiplash injury. In this study, we prospectively examined the development of tender points and mechano-sensitivity in 157 acute whiplash injured patients, who fulfilled criteria for WAD grade 2 (n=153) or grade 3 (n=4) seen about 5 days after injury (4.8+/-2.3) and who subsequently had or had not recovered 1 year after a cervical sprain. Tender point scores and stimulus-response function for mechanical pressure were determined in injured and non-injured body regions at specific time-points after injury. Thirty-six of 157 WAD grade 2 patients (22.9%) had not recovered, defined as reduced work capacity after 1 year. Non-recovered patients had higher total tender point scores after 12 (p<0.05), 107 (p<0.05) and 384 days (p<0.05) relative to those who recovered. Tenderness was found in the neck region and in remote areas in non-recovered patients. The stimulus-response curves for recovered and non-recovered patients were similar after 12 days and 107 days after the injury, but non-recovered patients had steeper stimulus-response curves for the masseter (p<0.02) and trapezius muscles (p<0.04) after 384 days. This study shows early mechano-sensitization after an acute whiplash injury and the development of further sensitization in patients with long-term disability.

Group III and IV muscle afferents differentially affect the motor cortex and motoneurones in humans

The influence of group III and IV muscle afferents on human motor pathways is poorly understood. We used experimental muscle pain to investigate their effects at cortical and spinal levels. In two studies, electromyographic (EMG) responses in elbow flexors and extensors to stimulation of the motor cortex (MEPs) and corticospinal tract (CMEPs) were evoked before, during, and after infusion of hypertonic saline into biceps brachii to evoke deep pain. In study 1, MEPs and CMEPs were evoked in relaxed muscles and during contractions to a constant elbow flexion force. In study 2, responses were evoked during elbow flexion and extension to a constant level of biceps or triceps brachii EMG, respectively. During pain, the size of CMEPs in relaxed biceps and triceps increased (by approximately 47% and approximately 56%, respectively; P < 0.05). MEPs did not change with pain, but relative to CMEPs, they decreased in biceps (by approximately 34%) and triceps (by approximately 43%; P < 0.05). During flexion with constant force, ongoing background EMG and MEPs decreased for biceps during pain (by approximately 14% and 15%; P < 0.05). During flexion with a constant EMG level, CMEPs in biceps and triceps increased during pain (by approximately 30% and approximately 26%, respectively; P < 0.05) and relative to CMEPs, MEPs decreased for both muscles (by approximately 20% and approximately 17%; P < 0.05). For extension, CMEPs in triceps increased during pain (by approximately 22%) whereas MEPs decreased (by approximately 15%; P < 0.05). Activity in group III and IV muscle afferents produced by hypertonic saline facilitates motoneurones innervating elbow flexor and extensor muscles but depresses motor cortical cells projecting to these muscles.

The Stretch-Shortening Cycle

Neuromuscular fatigue has traditionally been examined using isolated forms of either isometric, concentric or eccentric actions. However, none of these actions are naturally occurring in human (or animal) ground locomotion. The basic muscle function is defined as the stretch-shortening cycle (SSC), where the preactivated muscle is first stretched (eccentric action) and then followed by the shortening (concentric) action. As the SSC taxes the skeletal muscles very strongly mechanically, its influence on the reflex activation becomes apparent and very different from the isolated forms of muscle actions mentioned above. The ground contact phases of running, jumping and hopping etc. are examples of the SSC for leg extensor muscles; similar phases can also be found for the upper-body activities. Consequently, it is normal and expected that the fatigue phenomena should be explored during SSC activities. The fatigue responses of repeated SSC actions are very versatile and complex because the fatigue does not depend only on the metabolic loading, which is reportedly different among muscle actions. The complexity of SSC fatigue is well reflected by the recovery patterns of many neuromechanical parameters. The basic pattern of SSC fatigue response (e.g. when using the complete exhaustion model of hopping or jumping) is the bimodality showing an immediate reduction in performance during exercise, quick recovery within 1-2 hours, followed by a secondary reduction, which may often show the lowest values on the second day post-exercise when the symptoms of muscle soreness/damage are also greatest. The full recovery may take 4-8 days depending on the parameter and on the severity of exercise. Each subject may have their own time-dependent bimodality curve. Based on the reviewed literature, it is recommended that the fatigue protocol is 'completely' exhaustive to reduce the important influence of inter-subject variability in the fatigue responses. The bimodality concept is especially apparent for stretch reflex responses, measured either in passive or active conditions. Interestingly, the reflex responses follow parallel changes with some of the pure mechanical parameters, such as yielding of the braking force during an initial ground contact of running or hopping. The mechanism of SSC fatigue and especially the bimodal response of performance deterioration and its recovery are often difficult to explain. The immediate post-exercise reduction in most of the measured parameters and their partial recovery 1-2 hours post-exercise can be explained primarily to be due to metabolic fatigue induced by exercise. The secondary reduction in these parameters takes place when the muscle soreness is highest. The literature gives several suggestions including the possible structural damage of not only the extrafusal muscle fibres, but also the intrafusal ones. Temporary changes in structural proteins and muscle-tendon interaction may be related to the fatigue-induced force reduction. Neural adjustments in the supraspinal level could naturally be operative, although many studies quoted in this article emphasise more the influences of exhaustive SSC fatigue on the fusimotor-muscle spindle system. It is, however, still puzzling why the functional recovery lasts several days after the disappearance of muscle soreness. Unfortunately, this and many other possible mechanisms need more thorough testing in animal models provided that the SSC actions can be truly performed as they appear in normal human locomotion.