Effect of sustained experimental muscle pain on joint position sense.

INTRODUCTION: Joint position sense (JPS) is impaired in clinical musculoskeletal pain conditions, but when this impairment develops in the transition from initial to prolonged pain is not known. OBJECTIVES: This study assessed whether progressively developing sustained experimentally induced muscle pain impacts JPS in healthy individuals. METHODS: Twenty-eight healthy individuals received injection of nerve growth factor (NGF) into the right extensor carpi radialis brevis muscle on days 0 and 2 to induce sustained pain and hyperalgesia. Wrist JPS was assessed 2 days before day 0 (day -2), before the injection on days 0 and 2, and on days 4 and 14. Joint position sense was quantified as the ability to return the wrist to a neutral position following movements in the direction of radial and ulnar deviation. A 3-dimensional motion analysis system was used to calculate absolute, relative, and joint-angle repositioning errors. Numerical rating scale scores of pain intensity, body chart pain drawings, and pressure pain thresholds (PPTs) were recorded on each day. RESULTS: Compared with baseline, pressure pain thresholds decreased while pain intensity and area increased at day 2 (P < 0.001) and day 4 (P < 0.001) before returning to baseline on day 14 (P > 0.13). Relative to day 0, there was no change in wrist JPS at day 2, 4, and 14 following movements in either target direction (P > 0.05). CONCLUSION: Despite the presence of sustained muscle pain and hyperalgesia for 4 days at the elbow, no statistical change in wrist joint position error was observed. These findings suggest that pain and hyperalgesia lasting as long as 4 days does not impair JPS.

Impaired microvascular reactivity after eccentric muscle contractions is not restored by acute ingestion of antioxidants or dietary nitrate.

Unaccustomed eccentric exercise leads to impaired microvascular function but the underlying mechanism is unknown. In this study, we evaluated the role of oxidative stress and of nitric oxide (NO) bioavailability. Thirty young men and women performed eccentric contractions of the tibialis anterior (TA) muscle (ECC), with the contralateral leg serving as nonexercising control (CON). Participants were randomized into three groups ingesting an antioxidant cocktail (AO), beetroot juice (BR) or placebo 46 h postexercise. At baseline and 48 h postexercise, hyperemic responses to brief muscle contractions and 5 min of cuff occlusion were assessed bilaterally in the TA muscles using blood oxygen level dependent (BOLD) magnetic resonance imaging. Eccentric contractions resulted in delayed time-to-peak (~22%; P < 0.001), blunted peak (~21%; P < 0.001) and prolonged time-to-half relaxation (~12%, P < 0.001) in the BOLD response to brief contractions, with no effects of AO or BR, and no changes in CON. Postocclusive time-to-peak was also delayed (~54%; P < 0.001) in ECC, with no effects of AO or BR, and no changes in CON. Impaired microvascular reactivity after eccentric contractions is confined to the exercised tissue, and is not restored with acute ingestion of AO or BR. Impairments in microvascular reactivity after unaccustomed eccentric contractions may result from structural changes within the microvasculature that can diminish muscle blood flow regulation during intermittent activities requiring prompt adjustments in oxygen delivery.

Corticomotor excitability reduction induced by experimental pain remains unaffected by performing a working memory task as compared to staying at rest.

Experimental pain inhibits primary motor cortex (M1) excitability. Attenuating pain-related inhibition of M1 excitability may be useful during rehabilitation in individuals with pain. One strategy to attenuate M1 excitability is to influence prefrontal and premotor cortex activity. Working memory tasks, e.g. the two-back task (TBT), engage prefrontal and premotor cortices and may influence M1 excitability. We hypothesized that performing the TBT during pain would influence pain-related changes in M1 excitability. Participants (n = 28) received rigorous training in the TBT before baseline testing. Experimental pain was induced by injecting hypertonic saline into the first dorsal interosseous (FDI) muscle. Participants rated pain intensity on a 0-10 numerical rating scale (NRS) every second min until pain-resolved (PR) during the performance of the TBT (n = 14) or during REST (n = 14). In the TBT, letters were presented pseudo-randomly, and accuracy and reaction time to identified letters corresponding to letters shown two times back were recorded. M1 excitability was assessed using transcranial magnetic stimulation. Motor-evoked potentials (MEPs) were recorded at baseline, and at PR, PR + 10, PR + 20, and PR + 30 min. Four minutes after hypertonic saline injection, the pain NRS scores were higher in the TBT group than the REST group (p = 0.009). No time × group interaction was found for MEPs (p = 0.73), but a main effect of time (p < 0.0005) revealed a reduction of MEPs at PR up until PR + 30 (p < 0.008). The TBT accuracy improved at PR + 30 in both groups (p = 0.019). In conclusion, the pain-induced reduction in corticomotor excitability was unaffected by performing a working memory task, despite greater pain in the TBT group.

Differential Corticomotor Excitability Responses to Hypertonic Saline-Induced Muscle Pain in Forearm and Hand Muscles.

Experimental muscle pain inhibits corticomotor excitability (CE) of upper limb muscles. It is unknown if this inhibition affects overlapping muscle representations within the primary motor cortex to the same degree. This study explored CE changes of the first dorsal interosseus (FDI) and extensor carpi radialis (ECR) muscles in response to muscle pain. Participants (n = 13) attended two sessions (≥48 hours in-between). Hypertonic saline was injected in the ECR (session one) or the FDI (session two) muscle. CE, assessed by transcranial magnetic stimulation (TMS) motor-evoked potentials (MEPs), was recorded at baseline, during pain, and twenty minutes postinjection together with pain intensity ratings. Pain intensity ratings did not differ between the two pain sites (p = 0.19). In response to FDI muscle pain, the MEPs of the FDI muscle were reduced at 2 and 4 min postinjection (p ≤ 0.03), but not after ECR muscle pain. No significant MEP change was detected for the ECR muscle (p = 0.62). No associations between MEPs and pain intensity were found (p > 0.2). The present results indicate that the output from overlapping cortical representations of two muscles differentially adapts to acute muscle pain.

Light Touch Contact Improves Pain-Evoked Postural Instability During Quiet Standing.

Objective: To investigate if attention to additional sensory information from the fingertip can improve postural stability during pain, which is known to impair balance. Methods: In 16 healthy volunteers, experimental pain was induced by intramuscular injection of hypertonic saline in the right vastus medialis muscle (isotonic saline used as nonpainful control, intramuscular injection in the same location). Pain intensity was assessed on an 11-point numeric rating scale (NRS; 0 representing "no pain" and 10 "maximum pain"). Subjects were asked to stand as still as possible on a force plate for 40 seconds with their eyes closed. Their postural stability was quantified by the area and velocity of center of pressure (CoP) displacement. The CoP was recorded with and without pain during two different conditions: 1) no touch and 2) the subjects were asked to lightly touch a curtain with their right index finger and focus their attention on keeping it as still as possible. Results: Hypertonic injections induced higher NRS scores compared with control injections (P < 0.05). During the hypertonic injection condition, the CoP area and velocity in both directions increased during no touch compared with the light touch condition (P < 0.05). No differences were found during light touch between the hypertonic and isotonic injection conditions. Although experimental knee-related pain impaired postural stability, lightly touching a curtain with the fingertip decreased postural sway during painful conditions. Conclusions: Providing additional sensory information while pain patients are performing balance exercises may improve postural stability and increase the quality of exercise, consequent rehabilitation protocols, and clinical outcomes.

Later stages of diabetic neuropathy affect the complexity of the neuromuscular system at the knee during low-level isometric contractions.

INTRODUCTION: This study evaluates the complexity of force and surface electromyography (sEMG) during knee extension and flexion at low-level isometric contractions in individuals with different degrees of diabetic peripheral neuropathy (DPN). METHODS: Ten control and 38 diabetic participants performed isometric contractions at 10%, 20%, and 30% of maximal voluntary contraction. Knee force and multichannel sEMG from vastus lateralis (VL) and biceps femoris were acquired. The SD of force and sample entropy (SaEn) of both force and sEMG were computed. RESULTS: Participants with moderate DPN demonstrated high force-SD and low force-SaEn. Severely affected participants showed low SaEn in VL at all force levels. DISCUSSION: DPN affects the complexity of the neuromuscular system at the knee for the extension task during low-level isometric contractions, with participants in the later stages of the disease (moderate and severe) demonstrating most of the changes. Muscle Nerve 57: 112-121, 2018.

Age Interactions on Pain Sensitization in Patients With Severe Knee Osteoarthritis and Controls.

OBJECTIVES: Widespread pressure hyperalgesia, facilitated temporal summation of pain (TSP), and impaired conditioned pain modulation (CPM) have been found in knee osteoarthritis (KOA) patients compared with controls and these parameters have further been suggested to be altered in the elderly. This study investigated the influence of age on pressure hyperalgesia, TSP, and CPM in patients with KOA and controls. MATERIALS AND METHODS: One hundred thirty-three severe KOA patients and 50 age-matched and sex-matched asymptomatic controls were assessed by cuff algometry and handheld pressure algometry. Pain sensitivity was assessed around the head of the gastrocnemius muscle to identify mild pain detection threshold (MPDT) and pressure tolerance threshold (PTT). TSP was assessed by visual analogue scale scores of the pain evoked by 10 repetitive cuff stimulations. CPM was assessed as the difference in PTT before and during cuff-induced tonic arm pain. Pressure pain thresholds (PPTs) were assessed by handheld algometry at the tibialis anterior muscle. Two subgroups were analyzed in the age range below and above 65 years. Pearson correlations between age and pain parameters were applied. RESULTS: Patients demonstrated reduced MPDT, PTT, and PPT (P<0.01), facilitated TSP (P<0.02), and a trend toward impaired CPM (P=0.06) compared with controls. A negative correlation was found between MPDT, PTT, and PPT and age (P<0.05) but no age-related association was found for TSP and CPM. DISCUSSION: Pressure hyperalgesia was affected by age whereas dynamic pain mechanisms such as TSP and CPM were unaffected suggesting that these parameters are robust for a larger age range and reliable for long-term follow-up studies.

Effects of Prolonged and Acute Muscle Pain on the Force Control Strategy During Isometric Contractions.

UNLABELLED: Musculoskeletal pain is associated with multiple adaptions in movement control. This study aimed to determine whether changes in movement control acquired during acute pain are maintained over days of pain exposure. On day 0, the extensor carpi radialis brevis muscle of healthy participants was injected with nerve growth factor (NGF) to induce persistent movement-evoked pain (n = 13) or isotonic saline as a control (n = 13). On day 2, short-lasting pain was induced by injection of hypertonic saline into extensor carpi radialis brevis muscles of all participants. Three-dimensional force components were recorded during submaximal isometric wrist extensions on day 0, day 4, and before, during, and after saline-induced pain on day 2. Standard deviation (variation of task-related force) and total excursion of center of pressure (variation of force direction) were assessed. Maximal movement-evoked pain was 3.3 ± .4 (0-10 numeric scale) in the NGF-group on day 2 whereas maximum saline-induced pain was 6.8 ± .3 cm (10-cm visual analog scale). The difference in centroid position of force direction relative to day 0 was greater in the NGF group than in the control group (P < .05) on day 2 (before saline-induced pain) and day 4, reflecting changes in tangential force direction used to achieve the task. During saline-induced pain in both groups, tangential and task-related force variation was greater than before and after saline-induced pain (P < .05). PERSPECTIVE: Persistent movement-evoked pain changes force direction from the pain-free direction. Acute pain leads to increased variation in force direction irrespective of persistent movement-evoked pain preceding the acutely painful event. These differences provide novel insight into the search for and consolidation of new motor strategies in the presence of pain.

Eccentric exercise slows in vivo microvascular reactivity during brief contractions in human skeletal muscle.

Unaccustomed exercise involving eccentric contractions results in muscle soreness and an overall decline in muscle function, however, little is known about the effects of eccentric exercise on microvascular reactivity in human skeletal muscle. Fourteen healthy men and women performed eccentric contractions of the dorsiflexor muscles in one leg, while the contralateral leg served as a control. At baseline, and 24 and 48 h after eccentric exercise, the following were acquired bilaterally in the tibialis anterior muscle: 1) transverse relaxation time (T2)-weighted magnetic resonance images to determine muscle cross-sectional area (mCSA) and T2; 2) blood oxygen level-dependent (BOLD) images during and following brief, maximal voluntary contractions (MVC) to monitor the hyperemic responses with participants positioned supine in a 3T magnet; 3) muscle strength; and 4) pain pressure threshold. Compared with the control leg, eccentric exercise resulted in soreness, decline in strength (∼20%), increased mCSA (∼7%), and prolonged T2 (∼7%) at 24 and 48 h (P < 0.05). The BOLD response to a brief MVC was altered 24 and 48 h after eccentric exercise, such that time-to-peak (∼35%, P < 0.05) and time-to-half-recovery (∼23%, P < 0.05) were prolonged. The altered contraction-induced hyperemic response suggests slowed microvascular reactivity and altered matching of O2 delivery to O2 utilization within muscle tissue showing signs of muscle damage. These changes in microvascular regulation after eccentric exercise may impede rapid adjustments in muscle blood flow at exercise onset and during activities involving brief bursts of muscle activation, which may impair O2 delivery and contribute to reduced muscle function after eccentric exercise.

The effect of experimental neck pain on pressure pain sensitivity and axioscapular motor control.

UNLABELLED: Clinical neck pain affects pain sensitivity and coordination of neck muscles, but the impact on the shoulder muscles is unclear. This study investigated the effect of experimental neck pain on the activity of the axioscapular muscles during arm movements and changes in pain sensitivity. Experimental neck pain was induced in 24 healthy volunteers by injecting hypertonic saline into the splenius capitis. Isotonic saline was injected as control. Before, during, and after injections, electromyography was recorded bilaterally from 8 muscles during standardized arm movements (140° scapular plane elevation), and the root mean square amplitude was extracted. Likewise, pressure pain thresholds were assessed bilaterally on 3 sites. The root mean square electromyography was decreased for the ipsilateral upper trapezius (P < .01) and increased for the ipsilateral middle deltoid (P < .03) during upward movements. The root mean square electromyography was reduced for the ipsilateral upper trapezius (P < .01) during downward movement, whereas an increase was recorded in the contralateral external oblique (P < .02). At the injection site, the pressure pain threshold increased during pain compared with the post condition (5 minutes after potential pain had subsided; P < .03). In this study, trunk and axioscapular muscle activities were reorganized in response to localized and referred pain evoked by hypertonic saline injection into an intrinsic neck muscle with no direct attachments to the trunk or shoulder girdle. PERSPECTIVE: Reorganized activity of the axioscapular muscles has been shown previously in neck pain patients and is believed to happen during the transition from acute to chronic pain. The present study demonstrates for the first time that such reorganization may happen acutely, adding to our understanding of the effects of acute neck pain.

Experimental Pelvic Pain Impairs the Performance During the Active Straight Leg Raise Test and Causes Excessive Muscle Stabilization.

OBJECTIVES: The active straight leg raise (ASLR) test is widely used clinically to assess severity of lumbopelvic pain due to decreased stability of the sacroiliac joint (SIJ). This study aimed to bypass the influence of decreased SIJ stability on the ASLR test by investigating the effect of experimental pelvic pain and hyperalgesia on the outcome of the ASLR test. METHODS: Thirty-four healthy participants took part in this randomized crossover study. Pelvic pain was induced by injecting hypertonic saline into the long posterior sacroiliac ligament. Isotonic saline was injected on the contralateral side as control. Pain intensity was assessed on an electronic visual analogue scale. The Likert scores of difficulty performing the ASLR test and simultaneous electromyography of trunk and thigh muscles were recorded before, during, and postpain. Pressure pain thresholds were assessed bilaterally in the pelvic area and lower limb. RESULTS: Compared with the control condition and baseline, hypertonic saline injections caused (P<0.05): (1) higher visual analogue scale scores of the pain intensity; (2) reduced pressure pain thresholds at the injection site and lateral to S2; (3) increased difficulty in performing the ASLR rated on the Likert scale; and (4) bilateral increase in the electromyography activity of stabilizing trunk and thigh muscles during pain. DISCUSSION: These data demonstrate that pain and hyperalgesia in conditions unaffected by biomechanical SIJ impairments change the outcome of the ASLR test toward what is seen in clinical lumbopelvic pain. This may implicate pain-related changes in motor control strategies potentially relevant for the transition from acute into chronic pain.