Pain induced by injection of hypertonic saline into the infrapatellar fat pad and effect on coordination of the quadriceps muscles

Development of a novel technique to insert intramuscular electromyography electrodes into the deep intrinsic foot muscles via the dorsum of the foot

This study aimed to develop an insertion technique for intramuscular EMG recording of the oblique head of adductor hallucis (AddH) and first dorsal interosseous (FDI) muscles in humans via the dorsum of the foot, and report feasibility of intramuscular EMG data acquisition during walking in shoes. In eight individuals without musculoskeletal pain or injury (5 males; 32 ± 8 years), intramuscular electrodes were inserted into AddH (oblique head) and FDI through the right foot's dorsum (between metatarsals I-II) with ultrasound guidance. The ultrasound transducer was positioned on the plantar surface. Intramuscular EMG was also recorded from abductor hallucis, tibialis posterior, flexor digitorum longus and peroneus longus. Participants performed six overground walking trials wearing modified shoes, and rated pain associated with the intramuscular electrodes during walking (numerical rating scale, 0-10). High-quality EMG recordings were obtained from intrinsic and extrinsic foot muscles. Analyses of power spectral densities indicated that movement artefacts commonly observed during gait were removed by filtering. Pain associated with AddH/FDI electrodes during walking was low (median[IQR] 1[2]; range 0-4) and similar to other sites. Findings demonstrate that intramuscular EMG recording from AddH (oblique head) and FDI using this insertion technique is feasible and associated with minimal pain when walking in shoes.

Effect of movement-evoked and tonic experimental pain on muscle force production

INTRODUCTION

When performing an exercise or a functional test, pain that is evoked by movement or muscle contraction could be a stronger stimulus for changing how individuals move compared to tonic pain. We investigated whether the decrease in muscle force production is larger when experimentally-induced knee pain is directly associated to the torque produced (movement-evoked) compared to a constant painful stimulation (tonic).

METHODS

Twenty-one participants performed three isometric knee extension maximal voluntary contractions without pain (baseline), during pain, and after pain. Knee pain was induced using sinusoidal electrical stimuli at 10 Hz over the infrapatellar fat pad, applied continuously or modulated proportionally to the knee extension torque. Peak torque and contraction duration were averaged across repetitions and normalized to baseline.

RESULTS

During tonic pain, participants reported lower pain intensity during the contraction than at rest (p < 0.001), whereas pain intensity increased with contraction during movement-evoked pain (p < 0.001). Knee extension torque decreased during both pain conditions (p < 0.001), but a larger reduction was observed during movement-evoked compared to tonic pain (p < 0.001). Participants produced torque for longer during tonic compared to movement-evoked pain (p = 0.005).

CONCLUSION

Our results indicate that movement-evoked pain was a more potent stimulus to reduce knee extension torque than tonic pain. The longer contraction time observed during tonic pain may be a result of a lower perceived pain intensity during muscle contraction. Overall, our results suggest different motor adaptation to tonic and movement-evoked pain and support the notion that motor adaptation to pain is a purposeful strategy to limit pain. This mechanistic evidence suggests that individuals experiencing prevalently tonic or movement-evoked pain may exhibit different motor adaptations, which may be important for exercise prescription.

Is Postural Control Affected in People with Patellofemoral Pain and Should it be Part of Rehabilitation? A Systematic Review with Meta-analysis

BACKGROUND

Growing evidence supports that exercise therapy is effective for patellofemoral pain (PFP) rehabilitation. Nevertheless, the improvements have been reported not to be sustained in the long term, suggesting that the current protocols may not comprehend all required functional factors to provide a consistent recovery. A potential neglected factor in treatment protocols for PFP is postural control. However, it is unclear whether this population presents balance impairments or the influence of postural control on pain and function during rehabilitation programmes.

OBJECTIVE

To investigate whether (Q1) balance is impaired in people with PFP compared to controls, (Q2) conservative interventions are effective to improve balance in people with PFP, and (Q3) balance exercises are effective to improve pain and function in people with PFP.

DATA SOURCES

Medline, Embase, CINAHL, SPORTDiscus, Web of Science and Cochrane Library, supplemented by hand searching of reference lists, citations and relevant systematic reviews in the field.

METHODS

A systematic review with meta-analysis was conducted according to the Cochrane recommendations and reported according to the PRISMA statement recommendations. We included cross-sectional studies comparing balance between people with and without PFP; and randomised controlled trials verifying the effect of conservative intervention on balance and the effect of balance intervention on pain and function in people with PFP. The risk of bias was assessed using the Epidemiological Appraisal Instrument for cross-sectional studies and the Physiotherapy Evidence Database scale for randomised controlled trials.

RESULTS

From 15,436 records, 57 studies (Q1 = 28, Q2 = 23, Q3 = 14) met the eligibility criteria. Meta-analyses indicated that people with PFP have worse anteroposterior (very low grade evidence, standardised mean difference [SMD] = 1.03, 95% CI 0.40-1.66) and mediolateral (moderate grade evidence, SMD = 0.87, 95% CI 0.31-1.42) balance compared to controls. Moderate grade evidence indicated that overall balance is not affected in people with PFP (SMD = 0.38, 95% CI - 0.05-0.82). Low to very low grade evidence indicates that interventions are ineffective for mediolateral (SMD = 0.01, 95% CI - 0.51-0.53) and overall (SMD = 0.49, 95% CI - 0.14-1.11) balance improvements, and low grade evidence indicates that interventions are effective to improve anteroposterior balance (SMD = 0.64, 95% CI 0.04-1.23). Moderate to low grade evidence indicated that balance interventions are effective to reduce pain (SMD = 0.82, 95% CI 0.26-1.38) and improve function (SMD = 0.44, 95% CI 0.09-0.80) when measured using questionnaires; and very low grade evidence indicated no efficacy for function measured via functional tests (SMD = 0.73, 95% CI - 0.16-1.61).

CONCLUSION

People with PFP likely present balance deficits compared to asymptomatic people. There was insufficient evidence to support the efficacy of interventions to improve or modify balance in people with PFP. Also, there was insufficient evidence to support the efficacy of balance exercises to improve pain and function in people with PFP. Trial Registration The present systematic review was registered in PROSPERO (CRD42018091717).

Impact of pathological conditions on postural reflex latency and adaptability following unpredictable perturbations: A systematic review and meta-analysis

BACKGROUND

Pathological conditions can impair responses to postural perturbations and increase risk of falls.

RESEARCH QUESTION

To what extent are postural reflexes impaired in people with pathological conditions and can exercise interventions shorten postural reflexes?

METHODS

MEDLINE, EMBASE, Scopus, SportDiscus and Web of Science were systematically searched for articles comparing muscle activation onset latency in people with pathological conditions to healthy controls following unpredictable perturbations including the effect of exercise interventions (registration: CRD42020170861).

RESULTS

Fifty-three articles were included for systematic review. Significant delays in muscle activity onset following perturbations were evident in people with multiple sclerosis (n = 7, mean difference [MD]: 22 ms, 95% confidence interval [CI]: 11, 33), stroke (n = 10, MD: 34 ms, 95% CI: 19, 49), diabetes (n = 2, MD: 19 ms, 95% CI: 10, 27), HIV (n = 3, MD: 9 ms, 95% CI: 4, 14), incomplete spinal cord injury (n = 2, MD: 57 ms, 95% CI: 33, 80) and back and knee pain (n = 7, MD: 12 ms, 95% CI: 6, 18), but not in people with Parkinson's disease (n = 10) or cerebellar dysfunction (n = 4). Following exercise interventions, the paretic limb of stroke survivors (n = 3) displayed significantly faster muscle activation onset latency compared to pre-exercise (MD: -13 ms, 95% CI: -24, -4), with no significant changes in Parkinson's disease (n = 3).

CONCLUSIONS

This systematic review demonstrated that postural reflexes are significantly delayed in people with multiple sclerosis (+22 ms), stroke (+34 ms), diabetes (+19 ms), HIV (+9 ms), incomplete spinal cord injury (+57 ms), back and knee pain (+12 ms); pathological conditions characterized by impaired sensation or neural function. In contrast, timing of postural reflexes was not impaired in people with Parkinson's disease and cerebellar dysfunction, confirming the limited involvement of supraspinal structures. The meta-analysis showed exercise interventions can significantly shorten postural reflex latencies in stroke survivors (-14 ms), but more research is needed to confirm this finding and in people with other pathological conditions.

A Review of the Relationships Between Knee Pain and Movement Neuromechanics

CONTEXT

Knee injury and disease are common, debilitating, and expensive. Pain is a chief symptom of knee injury and disease and likely contributes to arthrogenic muscle inhibition. Joint pain alters isolated motor function, muscular strength, and movement biomechanics. Because knee pain influences biomechanics, it likely also influences long-term knee joint health.

OBJECTIVE

The purpose of this article is 2-fold: (1) review effects of knee pain on lower-extremity muscular activation and corresponding biomechanics and (2) consider potential implications of neuromechanical alterations associated with knee pain for long-term knee joint health. Experimental knee pain is emphasized because it has been used to mimic clinical knee pain and clarify independent effects of knee pain. Three common sources of clinical knee pain are also discussed: patellofemoral pain, anterior cruciate ligament injury and reconstruction, and knee osteoarthritis.

DATA SOURCES

The PubMed, Web of Science, and SPORTDiscus databases were searched for articles relating to the purpose of this article.

CONCLUSION

Researchers have consistently reported that knee pain alters neuromuscular activation, often in the form of inhibition that likely occurs via voluntary and involuntary neural pathways. The effects of knee pain on quadriceps activation have been studied extensively. Knee pain decreases voluntary and involuntary quadriceps activation and strength and alters the biomechanics of various movement tasks. If allowed to persist, these neuromechanical alterations might change the response of articular cartilage to joint loads during movement and detrimentally affect long-term knee joint health. Physical rehabilitation professionals should consider neuromechanical effects of knee pain when treating knee injury and disease. Resolution of joint pain can likely help to restore normal movement neuromechanics and potentially improve long-term knee joint health and should be a top priority.

Arthrogenic Muscle Inhibition: Best Evidence, Mechanisms, and Theory for Treating the Unseen in Clinical Rehabilitation

CONTEXT

Arthrogenic muscle inhibition (AMI) impedes the recovery of muscle function following joint injury, and in a broader sense, acts as a limiting factor in rehabilitation if left untreated. Despite a call to treat the underlying pathophysiology of muscle dysfunction more than three decades ago, the continued widespread observations of post-traumatic muscular impairments are concerning, and suggest that interventions for AMI are not being successfully integrated into clinical practice.

OBJECTIVES

To highlight the clinical relevance of AMI, provide updated evidence for the use of clinically accessible therapeutic adjuncts to treat AMI, and discuss the known or theoretical mechanisms for these interventions.

EVIDENCE ACQUISITION

PubMed and Web of Science electronic databases were searched for articles that investigated the effectiveness or efficacy of interventions to treat outcomes relevant to AMI.

EVIDENCE SYNTHESIS

122 articles that investigated an intervention used to treat AMI among individuals with pathology or simulated pathology were retrieved from 1986 to 2021. Additional articles among uninjured individuals were considered when discussing mechanisms of effect.

CONCLUSION

AMI contributes to the characteristic muscular impairments observed in patients recovering from joint injuries. If left unresolved, AMI impedes short-term recovery and threatens patients' long-term joint health and well-being. Growing evidence supports the use of neuromodulatory strategies to facilitate muscle recovery over the course of rehabilitation. Interventions should be individualized to meet the needs of the patient through shared clinician-patient decision-making. At a minimum, we propose to keep the treatment approach simple by attempting to resolve inflammation, pain, and effusion early following injury.

Anterior knee pain independently alters landing and jumping biomechanics

Background Biomechanical effects of anterior knee pain are difficult to distinguish from effects of other factors also related to knee injury (e.g., joint effusion). The purpose of this study was to evaluate independent effects of anterior knee pain on landing and jumping biomechanics. Methods Thirteen healthy participants performed a land and jump movement task, under three experimental conditions (pre-pain, pain, and post-pain), during one data collection session. One 1-ml injection of hypertonic saline into the infrapatellar fat pad was used to induce experimental anterior knee pain during the pain condition. Participant-perceived anterior knee pain was measured every 2 min throughout data collection. Landing and jumping biomechanics were measured and compared between the experimental conditions using a functional statistical approach. Findings The aforementioned injection increased mean participant-perceived anterior knee pain, from zero during the pre-pain condition to 2.6 ± 0.71 cm during the pain condition. Vertical ground reaction force, knee flexion angle, and internal knee extension moment decreased by approximately 0.100 body weights, 3°, and 0.010 Nm/body weight × body height, respectively, between the pre-pain and pain conditions. Conversely, hip flexion angle and internal hip extension moment increased by approximately 3° and 0.006 Nm/body weight × body height, respectively, between the pre-pain and pain conditions. Several biomechanical changes persisted after anterior knee pain abatement (the post-pain condition). Interpretation Anterior knee pain alters landing and jumping biomechanics, independent of other injury-related factors. These altered biomechanics likely change knee joint loading patterns and might increase risk for chronic knee joint injury and/or pathology.

Reduced strength, poor balance and concern about falls mediate the relationship between knee pain and fall risk in older people

BACKGROUND

Pain is an independent risk factor for falling. One in two older community-dwelling people with musculoskeletal pain fall each year. This study examined physical, psychological and medical factors as potential mediators to explain the relationship between knee pain and falls.

METHODS

Three hundred and thirty-three community-dwelling people aged 70+ years (52% women) participated in this cohort study with a 1-year follow-up for falls. Participants completed questionnaires (medical history, general health and concern about falls) and underwent physical performance tests. Participants were classified into 'pain' and 'no pain' groups based on self-reported knee pain. Poisson Regression models were computed to determine the Relative Risk (RR) of having multiple falls and potential mediators for increased fall risk.

RESULTS

One hundred and eighteen (36%) participants were categorised as having knee pain. This group took more medications and had more medical conditions (P < 0.01) compared to the no pain group. The pain group had poorer balance, physical function and strength and reported increased concern about falls. Sixty one participants (20%) reported ≥2 falls, with the pain group twice as likely to experience multiple falls over the 12 month follow up (RR = 2.0, 95% confidence interval (CI) = 1.27-3.13). Concern about falls, knee extension torque and postural sway with eyes closed were identified as significant and independent mediators of fall risk, and when combined explained 23% of the relationship between knee pain and falls.

CONCLUSION

This study has identified several medical, medication, psychological, sensorimotor, balance and mobility factors to be associated with knee pain, and found the presence of knee pain doubles the risk of multiple falls in older community living people. Alleviating knee pain, as well as addressing associated risk factors may assist in preventing falls in older people with knee pain.

Simultaneous ice and transcutaneous electrical nerve stimulation decrease anterior knee pain during running but do not affect running kinematics or associated muscle inhibition

BACKGROUND

Runners often experience anterior knee pain and this pain is associated with altered running neuromechanics. The purpose of this study was to examine potential therapeutic benefits (reduced pain and restored running neuromechanics) of simultaneously applied ice and transcutaneous electrical nerve stimulation on experimentally-induced anterior knee pain.

METHODS

Nineteen healthy subjects completed a sham and treatment data collection session. For both sessions, hypertonic saline was infused into the infrapatellar fat pad for approximately 80 min to induce experimental anterior knee pain. Perceived pain levels were measured every two minutes and running neuromechanics were recorded at four time points: pre-pain, pain before treatment, pain immediately post-treatment, and pain 20 min post-treatment.

FINDINGS

The saline infusion significantly increased perceived knee pain from 0 to 2.8 cm. The ice/transcutaneous electrical nerve stimulation treatment significantly reduced perceived knee pain by 35%, six minutes after the treatment initiation. Perceived knee pain remained reduced until eight minutes after the treatment termination. The knee pain significantly decreased peak gluteus medius, vastus lateralis, and vastus medialis activation during running, each by an average of 17% plus/minus 6%; however, none of these decreases were resolved via the therapeutic treatment. Neither the knee pain nor the therapeutic treatment significantly affected peak gluteus maximus activation or peak hip adduction angle.

INTERPRETATION

The experimental pain model effectively produced anterior knee pain and decreased muscle activation during running. The simultaneous ice/transcutaneous electrical nerve stimulation treatment effectively decreased anterior knee pain, but did not restore running neuromechanics that were altered due to the pain.

Experimental muscle pain of the vastus medialis reduces knee joint extensor torque and alters quadriceps muscle contributions as revealed through musculoskeletal modeling

BACKGROUND

Voluntary activation deficit of the quadriceps muscle group is a common symptom in populations with knee joint injury. Musculoskeletal modeling and simulations can improve our understanding of pathological conditions; however, they are mathematically complex which can limit their clinical application. A practical subject-specific modeling framework is introduced to evaluate knee extensor inhibition and muscle force contributions to isometric knee joint torques in healthy adults with and without experimentally induced quadriceps muscle pain.

METHODS

A randomized cross-over placebo controlled study design was used. Subject-specific maximum knee joint extension torque and quadriceps electromyographic data from 13 uninjured young adults were combined in a modeling framework to determine optimal muscle strength scaling parameters and ideal torque. Strength deficit ratios (experimental torque/ideal torque) and individual muscle contribution to experimental torque was computed before and after intramuscular hypertonic (pain inducing) and isotonic (sham) saline was injected to the vastus medialis.

FINDINGS

Decreased experimental knee extension torque (-8%) and vastus medialis electromyography (-26%) amplitude pre- to post- hypertonic injection was observed. Correspondingly, significant decreases in the knee extensor strength deficit ratio (-18%) and percent contribution of vastus medialis to experimental torque (-24%) was observed pre- to post- hypertonic injection. No differences were observed with isotonic injections, confirming the validity of the model.

INTERPRETATION

Our practical method to estimate strength ratios can be easily implemented within a musculoskeletal modeling framework to improve the validity of model estimates. This, in turn, can increase our understanding of the relationship between neuromuscular deficits and functional outcomes in patient populations.

Gait mechanics contribute to exercise induced pain flares in knee osteoarthritis

Background

Exercise-induced pain flares represent a significant barrier for individuals with knee osteoarthritis to meet physical activity recommendations. There is a need to understand factors that contribute to pain flares and the potential for the motor system to adapt and reduce joint loading should a flare occur. The study aim was to examine the impact of a bout of exercise on self-reported pain, walking mechanics and muscle co-contraction for participants with knee osteoarthritis.

Methods

Thirty-six adults (17 healthy older and 19 knee osteoarthritis) participated in this study. Self-reported pain, joint mechanics and muscle co-activation during gait at two self-selected speeds were collected before and after a 20-min preferred pace treadmill walk (20MTW).

Results

Eight of nineteen osteoarthritis participants had a clinically significant pain flare response to the 20MTW. At baseline the participants that did not experience a pain flare had smaller knee flexion and total reaction moments compared to both the participants with pain flares (p = 0.02; p = 0.05) and controls (p < 0.001; p < 0.001). In addition, the 2nd peak knee adduction (p = 0.01) and internal rotation (p = 0.001) moments were smaller in the no flares as compared to controls. The pain flare participants differed from controls with smaller knee internal rotation moments (p = 0.03), but greater relative hamstrings (vs. quadriceps) and medial (vs. lateral) muscle activation (p = 0.04, p = 0.04) compared to both controls and no flare participants (p = 0.04, p = 0.007). Following the 20MTW there were greater decreases in the 1st and 2nd peak knee adduction (p = 0.03; p = 0.02), and internal rotation (p = 0.002) moments for the pain flare as compared to the no flare group. In addition, for the pain flare as compared to controls, greater decreases in the knee flexion (p = 0.03) and internal rotation (p = 0.005) moments were found.

Conclusions

Individuals who adapt their gait to reduce knee joint loads may be less susceptible to exercise-induced pain flares. This highlights a potential role of gait biomechanics in short-term osteoarthritis pain fluctuations. The results also suggest that despite the chronic nature of osteoarthritis pain, the motor system’s ability to respond to nociceptive stimuli remains intact.

Electronic supplementary material

The online version of this article (10.1186/s12891-019-2493-4) contains supplementary material, which is available to authorized users.

How does acute pain influence biomechanics and quadriceps function in individuals with patellofemoral pain?

OBJECTIVES

Beside pathophysiological factors, pain is believed to play a crucial role in the progression of patellofemoral pain (PFP). However, the isolated effect of pain on biomechanics and quadriceps function has not been investigated in PFP. Thus, this study aimed to investigate the effect of pain on quadriceps function and lower limb biomechanics in individuals with PFP.

METHODS

Twenty-one individuals with PFP (11 males and 10 females, age: 29.76 ± 6.36 years, height: 1.74 ± 0.09 m, mass: 70.12 ± 8.56 kg) were measured at two different occasions: when not and when experiencing acute pain. Peak quadriceps torque (concentric, eccentric and isometric) and arthrogenic muscle inhibition (AMI) were assessed. Three-dimensional motion analysis and surface electromyography of the quadriceps and hamstring muscles were collected during running, a single-leg-squat and step-down task. The normality was assessed using the Shapiro-Wilk test and a MANOVA was performed at the 95% confidence interval.

RESULTS

AMI increased significantly in acute pain. The net muscle activation of the knee extensors and flexors decreased during running in acute pain. The lower limb biomechanics and the quadriceps torque did not change in acute pain.

DISCUSSION

It appears that even if individuals with PFP experience pain they can still deliver maximal quadriceps contractions and maintain their moving patterns without biomechanical changes. However, the overall reduced activation of the quadriceps and the increased AMI indicate the presence of quadriceps inhibition in acute pain.

Self-reported knee instability associated with pain, activity limitations, and poorer quality of life before and 1 year after total knee arthroplasty in patients with knee osteoarthritis

Information on the association of self-reported knee instability with clinical outcomes after Total Knee Arthroplasty (TKA) and 1 year follow-up is scarce. The aims were to determine (i) the course and prevalence of self-reported knee instability before and 1 year after TKA and (ii) the associations of preoperative, postoperative, and retained self-reported knee instability with pain, activity limitations, and quality of life (QoL) in patients with knee osteoarthritis. Patients undergoing primary TKA, selected from the Longitudinal Leiden Orthopaedics and Outcomes of OsteoArthritis Study, had their knee instability measured using a questionnaire. The Knee injury and Osteoarthritis Outcome Score pain, activity limitations, and QoL subscales were administered before and 1 year after surgery. Multivariable regression analyses were performed to examine associations between knee instability, pain, activity limitations, and QoL, adjusted for covariates (age, gender, comorbidities, and radiographic severity). Of the 908 included patients, 649 (71%) and 187 (21%) reported knee instability before and following TKA, respectively. Of the patients with preoperative knee instability, this perception was retained in 165 (25%) cases. Knee instability was preoperatively associated with pain (B -9.6; 95%CI: -12.4 to -6.7), activity limitations (B -7.5; 95%CI: -10.2 to -4.8), and QoL (B -4.7; 95%CI: -7.0 to -2.4) and postoperatively with pain (B -15.0; 95%CI: -18.5 to -11.6), activity limitations (B -15.1; 95%CI: -18.4 to -11.8), and QoL (B -18.7; 95%CI: -22.3 to -15.3). Retained knee instability was associated with postoperative pain (B -15.1; 95%CI: -18.9 to -11.2), activity limitations (B -14.1; 95%CI: -17.8 to -10.4), and QoL (B -18.0; 95%CI: -21.7 to -14.3). In conclusion, in clinical care, self-reported knee instability is retained postoperatively in 25% of the patients. Retained knee instability is associated with more pain, activity limitations, and poorer QoL postoperatively. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2671-2678, 2018.

Repeated Testing With the Hypertonic Saline Assay in Mice for Screening of Analgesic Activity

BACKGROUND

In vivo animal assays are a cornerstone of preclinical pain research. An optimal stimulus for determining the activity of potential analgesics would produce responses of a consistent magnitude on repeated testing. Intraplantar (i.pl.) injection of hypertonic saline (HS) in mice produces robust nociceptive responses to different analgesics, without evidence of tissue damage. Here, we investigated whether the nociceptive response is changed by repeating the injection at different times and sites in a mouse and whether it is attenuated by morphine.

METHODS

We conducted randomized and blinded experiments to assess responses to repeated i.pl. 10% HS in female CD-1 mice. An injection of HS was followed by a second injection into the same hind paw at 4 hours, 24 hours, or 7 days. A separate group of mice each received i.pl. injections at 5, 10, and 15 days. In 2 independent experiments, 30 minutes after initial HS injections in the ipsilateral hind paw, mice received HS injection into the contralateral hind paw or ipsilateral forepaw. The ability of morphine to block the nociceptive responses was examined by injecting morphine at 5-day intervals.

RESULTS

Repeated injection of HS did not alter the responses at 4 hours (84 vs 75 seconds; mean difference [95% CI], -9 [-40 to 23]; P = .6), 24 hours (122 vs 113 seconds; -6 [-24 to 12]; P = .5), or 7 days (112 vs 113 seconds; -0.3 [-12 to 11]; P = .95) or at multiple injections (day 0, 122 seconds vs day 5, 121 seconds; -0.3 [-28 to 27], P > .99; day 10, 118 seconds; 2.5 [-36 to 41], P = .99; day 15, 119 seconds; 2 [-36 to 38], P = .99). A previous hind paw injection did not change the responses of the contralateral hind paw (right, 93 seconds versus left, 96 seconds; -3 [-20 to 13], P = .7) or of the ipsilateral forepaw (forepaw after HS, 146 seconds versus forepaw after 0.9% saline, 149 seconds; -3 [-28 to 22], P = .8). Morphine dose-dependently attenuated HS responses (control, 94 seconds vs 4 mg/kg, 66 seconds; 29 [-7 to 64], P = .12; vs 10 mg/kg, 27 seconds; 67 [44-90], P < .0001; 4 vs 10 mg/kg, 67 [44-90], P = .03).

CONCLUSIONS

The repetition of i.pl. HS produces consistent reproducible responses without tissue damage. This results in efficient, rapid detection of analgesic activity, reducing the number of animals required.

Blood Flow Restriction induces hypoalgesia in recreationally active adult male anterior knee pain patients allowing therapeutic exercise loading

OBJECTIVE

To evaluate if a single blood flow restriction (BFR)-exercise bout would induce hypoalgaesia in patients with anterior knee pain (AKP) and allow painless application of therapeutic exercise.

DESIGN

Cross-sectional repeated measures design.

SETTING

Institutional out-patients physiotherapy clinic.

PATIENTS

Convenience sample of 30 AKP patients.

INTERVENTION

BFR was applied at 80% of complete vascular occlusion. Four sets of low-load open kinetic chain knee extensions were implemented using a pain monitoring model.

MAIN OUTCOME MEASUREMENTS

Pain (0-10) was assessed immediately after BFR application and after a physiotherapy session (45 min) during shallow and deep single-leg squat (SSLS, DSLS), and step-down test (SDT). To estimate the patient rating of clinical effectiveness, previously described thresholds for pain change (≥40%) were used, with appropriate adjustments for baseline pain levels.

RESULTS

Significant effects were found with greater pain relief immediate after BFR in SSLS (d = 0.61, p < 0.001), DSLS (d = 0.61, p < 0.001), and SDT (d = 0.60, p < 0.001). Time analysis revealed that pain reduction was sustained after the physiotherapy session for all tests (d_(SSLS) = 0.60, d_(DSLS) = 0.60, d_(SDT) = 0.58, all p < 0.001). The reduction in pain effect size was found to be clinically significant in both post-BFR assessments.

CONCLUSION

A single BFR-exercise bout immediately reduced AKP with the effect sustained for at least 45 min.

Acid-induced experimental knee pain and hyperalgesia in healthy humans

Inflammation and the related acidity in peri-articular structures may be involved in pain generation and hyperalgesia in knee osteoarthritis. This study investigated pain and associated hyperalgesia provoked by infusion of acidic saline into the infrapatellar fat pad. Twenty-eight subjects participated in two sessions in which acidic saline (AS, pH 5) or neutral saline (NS, pH 7.4) were infused into the infrapatellar fat pad for 15 min. Pain intensity, pain area, mechanical and thermal sensitivity, and maximal voluntary knee extension force were recorded. Repeated infusions were performed in 14 subjects. Infusion of AS caused significantly higher pain intensity, larger pain areas, induced hyperalgesia around the infused knee, and reduced extension force. No significant pain facilitation or spreading of hyperalgesia was found after repeated infusions as compared with single infusions. Acidic saline infused into the infrapatellar fat pad provoked pain and localized mechanical hyperalgesia. Thus, this acid-induced pain model may mimic the early-stage responses to tissue injury of knee osteoarthritis.

Acute Responses of Strength and Running Mechanics to Increasing and Decreasing Pain in Patients With Patellofemoral Pain

CONTEXT

  Patellofemoral pain (PFP) is typically exacerbated by repetitive activities that load the patellofemoral joint, such as running. Understanding the mediating effects of changes in pain in individuals with PFP might inform injury progression, rehabilitation, or both.

OBJECTIVE

  To investigate the effects of changing pain on muscular strength and running biomechanics in those with PFP.

DESIGN

  Crossover study.

SETTING

  University research laboratory.

PATIENTS OR OTHER PARTICIPANTS

  Seventeen participants (10 men, 7 women) with PFP.

INTERVENTION(S)

  Each participant completed knee pain-reducing and pain-inducing protocols in random order. The pain-reducing protocol consisted of 15 minutes of transcutaneous electric nerve stimulation (TENS) around the patella. The pain-inducing protocol was sets of 20 repeated single-legged squats (RSLS). Participants completed RSLS sets until either their pain was within at least 1 cm of their pain during an exhaustive run or they reached 10 sets.

MAIN OUTCOME MEASURE(S)

  Pain, isometric hip and trunk strength, and running mechanics were assessed before and after the protocols. Dependent variables were pain, normalized strength (abduction, extension, external rotation, lateral trunk flexion), and peak lower extremity kinematics and kinetics in all planes. Pain scores were analyzed using a Friedman test. Strength and mechanical variables were analyzed using repeated-measures analyses of variance. The α level was set at P < .05.

RESULTS

  Pain was decreased after the TENS (pretest: 3.10 ± 1.95, posttest: 1.89 ± 2.33) and increased after the RSLS (baseline: 3.10 ± 1.95, posttest: 4.38 ± 2.40) protocols (each P < .05). The RSLS protocol resulted in a decrease in hip-extension strength (baseline: 0.355 ± 0.08 kg/kg, posttest: 0.309 ± 0.09 kg/kg; P < .001). Peak plantar-flexion angle was decreased after RSLS (baseline: -13.97° ± 6.41°, posttest: -12.84° ± 6.45°; P = .003). Peak hip-extension (pretest: -2.31 ± 0.46) and hip-abduction (pretest: -2.02 ± 0.35) moments decreased after both the TENS (extension: -2.15 ± 0.48 Nm/kg, P = .015; abduction: -1.91 ± 0.33 Nm/kg, P = .015) and RSLS (extension: -2.18 ± 0.52 Nm/kg, P = .003; abduction: -1.87 ± 0.36 Nm/kg, P = .039) protocols.

CONCLUSIONS

  This study presents a novel and effective method of increasing pain in persons with PFP. Functionally increased pain after RSLS coincides with reduced hip-extensor muscle strength and decreased plantar-flexion angle during running. The TENS treatment decreased pain during running in those with PFP but failed to influence strength. Hip moments were reduced by both protocols, which may demonstrate that acute increases or decreases in pain cause runners to change their mechanics.

The Effect of Increasing Volume of Exercise on Activation Pattern of Vastus Medialis and Lateralis and its Correlation With Anterior Knee Pain in Karate Elites

Background

The effects of exercise volume on the pattern of muscle activity is one of the most important factors in training management and injury risk reduction. In the lower limb, the quadriceps muscle which plays a determining role in performing the stance and other karate techniques could be injured in intensive exercise and may induce anterior knee pain in athletes.

Objectives

The aim of this study was to determine the relationship between training volume and muscle activity of vastus medialis and vastus lateralis and its association with anterior knee pain in karate elites.

Patients and Methods

Male and female athletes from national junior and cadet karate team (14 to 18 years) were invited to participate in the study at the beginning and the end of the training camps. Studies involved measurement of electromyographic muscle activity of vastus medialis and vastus lateralis in both lower extremities with surface electromyography device and assessment of movement by electrogoniometery. Muscle activity was recorded in three tests of dachi, walking up and walking down stairs. Simultaneously, anterior knee pain was evaluated using visual analogue scale and anterior knee pain scale questionnaire.

Results

Eight athletes of a total number of 23 reported increased ratings of pain in their right knees. No differences in muscle activity were observed in tests of Dachi and stairs between the groups with and without pain. Comparing Dachi task pattern at the beginning and end of training camps, there was no significant difference in pattern of biomechanical movement; however, reducing the amount of muscle activity in early and late phases of tasks was observed in electromyographic assessment.

Conclusions

The results showed that performing the same task after a six-week training period, less muscle activity was required in all phases in two groups of tasks, including karate-specific movement (dachi) and activities of daily living (up or down stairs).

The interaction between NGF-induced hyperalgesia and acid-provoked pain in the infrapatellar fat pad and tibialis anterior muscle of healthy volunteers

BACKGROUND

Tissue pH is lowered in inflamed tissues, and the increased proton concentration activates acid-sensing ion channels (ASICs), contributing to pain and hyperalgesia. ASICs can be upregulated by nerve growth factor (NGF). The aim of this study was to investigate two new human experimental pain models combining NGF- and acid-induced pain in a randomized, controlled, double-blind study.

METHODS

In experiment 1, volunteers (N = 16) received an injection of either NGF or isotonic saline in each infrapatellar fat pad (IFP). One day after 5 mL of phosphate-buffered acidic saline was infused into each IFP at a rate of 20 mL/h. In experiment 2, the tibialis anterior (TA) muscle of additional volunteers (N = 16) was examined, following the same procedure except that the volume and infusion rate of acid were different (10 mL, 30 mL/h). Continuous pain ratings were recorded during and after acid infusions. In addition, soreness scores on a Likert scale and pressure pain thresholds (PPTs) were assessed.

RESULTS

The PPT of the IFP was significantly decreased at the NGF injection site on day 1, but acid-provoked pain ratings and the change in PPT from pre- to postinfusion between the knees were similar. In the muscle pain model, local mechanical hyperalgesia developed 3 h after the NGF injection and a significant additional decrease in PPT was found after acid infusion compared to preinfusion.

CONCLUSIONS

NGF sensitization in the IFP was not facilitated by acid, whereas an acid-provoked enhancement of muscle hyperalgesia was found. NGF sensitization of adipose tissue responds differently to acid provocation compared to muscle tissue.

SIGNIFICANCE

Quantification of two novel pain models combining NGF and acid. Hyperalgesia developed after NGF injection in the infrapatellar fat pad, but it was not facilitated by acid provocation. Contrary, NGF-induced hyperalgesia in muscle tissue was enhanced by acid.

Running Injuries: The Infrapatellar Fat Pad and Plica Injuries

When considering knee pain in runners, clinicians differentiate sources of symptoms and determine their cause. Knee problems arise when a runner increases the amount/frequency of the loading through the lower limb. The way the loading is distributed through the knee determines which tissues are abnormally loaded. Knee problems cannot be considered in isolation, requiring a thorough investigation of static and dynamic lower limb mechanics, and footwear and surfaces. This article examines potential sources of knee pain and explores the role of the infrapatellar fat pad and synovial plica in the mechanics of the knee and its involvement in knee symptoms.

Center of Pressure Displacement of Standing Posture during Rapid Movements Is Reorganised Due to Experimental Lower Extremity Muscle Pain

Background

Postural control during rapid movements may be impaired due to musculoskeletal pain. The purpose of this study was to investigate the effect of experimental knee-related muscle pain on the center of pressure (CoP) displacement in a reaction time task condition.

Methods

Nine healthy males performed two reaction time tasks (dominant side shoulder flexion and bilateral heel lift) before, during, and after experimental pain induced in the dominant side vastus medialis or the tibialis anterior muscles by hypertonic saline injections. The CoP displacement was extracted from the ipsilateral and contralateral side by two force plates and the net CoP displacement was calculated.

Results

Compared with non-painful sessions, tibialis anterior muscle pain during the peak and peak-to-peak displacement for the CoP during anticipatory postural adjustments (APAs) of the shoulder task reduced the peak-to-peak displacement of the net CoP in the medial-lateral direction (P<0.05). Tibialis anterior and vastus medialis muscle pain during shoulder flexion task reduced the anterior-posterior peak-to-peak displacement in the ipsilateral side (P<0.05).

Conclusions

The central nervous system in healthy individuals was sufficiently robust in maintaining the APA characteristics during pain, although the displacement of net and ipsilateral CoP in the medial-lateral and anterior-posterior directions during unilateral fast shoulder movement was altered.

Effects of transcutaneous electrical nerve stimulation on quadriceps function in individuals with experimental knee pain

Knee joint pain (KJP) is a cardinal symptom in knee pathologies, and quadriceps inhibition is commonly observed among KJP patients. Previously, KJP independently reduced quadriceps strength and activation. However, it remains unknown how disinhibitory transcutaneous electrical nerve stimulation (TENS) will affect inhibited quadriceps motor function. This study aimed at examining changes in quadriceps maximum voluntary contraction (MVC) and central activation ratio (CAR) before and after sensory TENS following experimental knee pain. Thirty healthy participants were assigned to either the TENS or placebo groups. All participants underwent three separate data collection sessions consisting of two saline infusions and one no infusion control in a crossover design. TENS or placebo treatment was administered to each group for 20 min. Quadriceps MVC and CAR were measured at baseline, infusion, treatment, and post-treatment. Perceived knee pain intensity was measured on a 100-mm visual analogue scale. Post-hoc analysis revealed that hypertonic saline infusion significantly reduced the quadriceps MVC and CAR compared with control sessions (P < 0.05). Sensory TENS, however, significantly restored inhibited quadriceps motor function compared with placebo treatment (P < 0.05). There was a negative correlation between changes in MVC and knee pain (r = 0.33, P < 0.001), and CAR and knee pain (r = 0.62, P < 0.001), respectively.

Longitudinal (4 year) change of thigh muscle and adipose tissue distribution in chronically painful vs painless knees--data from the Osteoarthritis Initiative

OBJECTIVE

To evaluate 4-year longitudinal change in thigh muscle and adipose tissue content in chronically painful vs painless knees.

METHODS

Knees from Osteoarthritis Initiative (OAI) participants with non-acceptable symptom status (numerical rating scale (NRS) ≥4) and frequent pain (≥6 months at baseline, year 2 and year 4 follow-up) were studied. These were matched with painless controls (bilateral NRS pain intensity ≤1 and ≤infrequent pain at all 3 timepoints). 4-year longitudinal changes in thigh muscle anatomical cross-sectional areas (CSAs), isometric muscle strength, and in subcutaneous (SCF) and intermuscular fat (IMF) CSAs were obtained from magnetic resonance images (MRI) and were compared between groups (paired t-tests).

RESULTS

43 participants fulfilled the inclusion criteria of chronic pain, had complete thigh muscle MRI acquisitions and strength measurements, and a matched control. Quadriceps CSAs, but not extensor strength, showed a significant longitudinal decrease in chronically painful knees (-3.9%; 95% confidence interval [95 CI] -6.3%, -1.5%) and in painless controls (-2.4%; 95% CI -4.1%, -0.7%); the difference in change was not statistically significant (P = 0.33). There was a significant 4-year gain in SCF in painful knees (8.1%; 95% CI 3.1%, 13%) but not in controls (0.0%; 95% CI -4.4%, +4.4%) with the difference in change being significant (P = 0.03). The gain in IMF (∼5.2%) was similar between painful and painless knees.

CONCLUSION

This is the first paper to show a significant impact of (chronic) knee pain on longitudinal change in local subcutaneous adipose tissue. The effect of pain on subcutaneous fat appeared stronger than that on intermuscular adipose tissue and on muscle status.

Holistic approach to understanding anterior knee pain. Clinical implications

Anterior knee pain is one of the most frequent reasons for consultation within knee conditions. The aetiology is not well known, which explains the sometimes unpredictable results of its treatment. Normally, when we see a patient in the office with anterior knee pain, we only study and focus on the knee. If we do this, we are making a big mistake. We must not forget to evaluate the pelvis and proximal femur, as well as the psychological factors that modulate the course of the illness. Both the pelvifemoral dysfunction as well as the psychological factors (anxiety, depression, catastrophization and kinesiophobia) must be included in our therapeutic targets of the multidisciplinary treatment of anterior knee pain. We must not only focus on the knee, we must remember to "look up" to fully understand what is happening and be able to solve this difficult problem. The aetiology of anterior knee pain is multifactorial. Therefore, diagnosis and treatment of patellofemoral disorders must be individualized. Our findings stress the importance of tailoring physiotherapy, surgery and psycho-educational interventions to each patient.

Potential interaction of experimental knee pain and laterally wedged insoles for knee off-loading during walking

BACKGROUND

Laterally wedged insoles are one of the gait modifications potentially slowing down progression of medial knee osteoarthritis. Clinical studies have, however, found large individual differences in the biomechanical effect and an insufficient pain reduction. To clarify if and how pain mediates mechanical changes during gait the current study investigated how acute experimental knee pain changes the mechanical effect of laterally wedged insoles in healthy subjects during walking.

METHODS

3D gait analysis was carried out for twelve healthy individuals. The study followed a cross-over design and data were collected with both a neutral and a 10-degree laterally wedged insole with experimental pain induced by hypertonic and isotonic saline injections into the infrapatellar fat pad. Peak knee adduction moment was the primary outcome. A repeated ANOVA (analysis of variance) was used to evaluate the relationship between the factors wedge, condition and test number.

FINDINGS

Wedges significantly reduced peak knee adduction moment but experimental knee pain did only marginally affect its magnitude in either condition. While frontal plane mechanics were relatively unaffected by pain, the sagittal plane knee extension moment increased with laterally wedging (P=0.008), whereas late knee flexion moment was reduced by experimental knee pain (P=0.04).

INTERPRETATION

The effect of laterally wedged insoles in attenuating knee adduction moment during walking is independent of experimental knee pain. The present study provides evidence that subjects with experimental knee pain reduce knee loading by reducing extension moment, whereas lateral wedges have the opposite effect and increase the extension moment.

Task dependency of motor adaptations to an acute noxious stimulation

This study explored motor adaptations in response to an acute noxious stimulation during three tasks that differed in the number of available degrees of freedom. Fifteen participants performed three isometric force-matched tasks (single leg knee extension, single leg squat, and bilateral leg squat) in three conditions (Control, Pain, and Washout). Pain was induced by injection of hypertonic saline into the vastus medialis muscle (VM; left leg). Supersonic shear imaging was used to measure muscle shear elastic modulus as this is considered to be an index of muscle stress. Surface electromyography (EMG) was recorded bilaterally from six muscles to assess changes in neural strategies. During tasks with fewer degrees of freedom (knee extension and single leg squat task), there was no change in VM EMG amplitude or VM shear elastic modulus. In contrast, during the bilateral leg squat, VM (−32.9 ± 15.8%; P < 0.001) and vastus lateralis (−28.7 ± 14.8%; P < 0.001) EMG amplitude decreased during Pain. This decrease in activation was associated with reduced VM shear elastic modulus (−17.6 ± 23.3%; P = 0.029) and reduced force produced by the painful leg (−10.0 ± 10.2%; P = 0.046). This work provides evidence that when an obvious solution is available to decrease stress on painful tissue, this option is selected. It confirms the fundamental assumption that motor adaptations to pain aim to alter load on painful tissue to protect for further pain and/or injury. The lack of adaptation observed during force-matched tasks with fewer degrees of freedom might be explained by the limited potential to redistribute stress or a high cost induced by such a compensation.

Pain severity is associated with muscle strength and peak oxygen uptake in adults with fibromyalgia

Background

The associations between pain, lower extremity strength, and aerobic conditioning have not been widely investigated in adults with fibromyalgia (FM). The principle objective of this study was to investigate the relationship between pain severity and knee strength in patients seeking treatment for FM. A secondary objective was to investigate the relationship between pain scores and aerobic conditioning.

Methods

Three measures of knee strength (isokinetic extensor, isokinetic flexor, isometric extensor) were quantified in the dominant leg of 69 adults with FM using a dynamometer at speeds of 60 degrees per second (60°/s) and 180°/s. Peak oxygen uptake (VO₂) was assessed using a cycle ergometer, and pain was assessed using the pain severity subscale of the Multidimensional Pain Inventory.

Results

In univariable linear regression analyses using pain severity as the dependent variable, lesser values of isokinetic knee extensor strength at 60°/s (P=0.041) and 180°/s (P=0.010), isokinetic knee flexor strength at 60°/s (P=0.028) and 180°/s (P=0.003), and peak VO₂ uptake (P=0.031) were predictive of greater pain severity scores. In multiple variable linear regression models adjusted for age, sex, body mass index, and opioid use, the following associations retained statistical significance; isokinetic knee extensor strength at 60°/s (P=0.020) and 180°/s (P=0.003), isokinetic knee flexor strength at 60°/s (P=0.015) and 180°/s (P=0.001), and peak VO₂ uptake (P=0.014). However, no significant associations were found between pain severity and isometric knee extensor strength.

Conclusion

The main findings from this study were that lesser values of isokinetic knee strength and peak VO₂ uptake were predictive of greater pain severity scores. These results build on the findings of previous investigations, but ongoing research is needed to further characterize the effects of strength and peak VO₂ uptake on the pathophysiology of FM.

Knee extensor strength is associated with pressure pain thresholds in adults with fibromyalgia

OBJECTIVE

Individuals with fibromyalgia (FM) have lower muscle strength and lower pressure pain thresholds (PPT). The primary aim of this study was to determine the associations between muscle strength and PPT in adults with FM to test the hypothesis that greater measures of muscle strength would be associated with greater values of PPT. Secondary aims included determining the effects of pain severity and the peak uptake of oxygen (VO2) on the associations between muscle strength and PPT.

METHODS

Knee extensor and flexor strength (N = 69) was measured in the dominant leg using a dynamometer, and PPT was assessed using an electronic algometer. Pain severity was determined using the Multidimensional Pain Inventory, and peak VO2 uptake was quantified using an electronically braked cycle ergometer.

RESULTS

Univariable linear regression analysis demonstrated a significant association between PPT (dependent variable) and isometric knee extensor (P<.001), isokinetic (60°/s) knee extensor (P = .002), and isokinetic (60°/s) knee flexor strength (P = .043). In a multiple variable linear regression analysis adjusted for age, sex, pain severity, body mass index and peak VO2 uptake, a significant association was found between PPT and isometric knee extensor strength (P = .008). In a similar multiple variable analysis, a significant association was found between PPT and isokinetic knee extensor strength (P = .044).

CONCLUSION

Greater measures of isometric and isokinetic knee extensor strength were significantly associated with greater values of PPT in both univariable and multiple variable linear regression models.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01253395.

Experimental pain inhibits infraspinatus activation during isometric external rotation

BACKGROUND

The effect of pain on muscle activation is poorly understood. This study examined the effects of acute experimental pain on rotator cuff muscle force and voluntary activation (VA). We hypothesized that acute subacromial pain would cause inhibition of infraspinatus VA with a corresponding decrease in external rotation force.

MATERIALS AND METHODS

Seventeen healthy adults with no known shoulder pathology were tested. Isolated external rotation force was tested on a dynamometer. Participants performed 2 baseline maximum voluntary isometric contractions of external rotation, during which maximal electrical stimulation was used to assess VA. To elicit pain, 1.5 mL 5% hypertonic saline was injected into the subacromial space, and testing of maximum voluntary isometric contractions force and VA was repeated 3 times at 5-minute intervals.

RESULTS

Mean ± standard deviation initial pain from the injection was 6.6 ± 1.3 points of 10 possible and produced a 32.8% decline in force and a 22.7% decline in VA (P < .05). Pain diminished over a 10-minute period. As pain resolved, force and VA improved (P < .0125). There was a strong relationship between force and VA (r(2) = 0.78, P < .05) and a moderate relationship between pain and VA (r(2) = 0.31, P < .05).

CONCLUSIONS

Experimental subacromial pain elicits a decline in force and VA of the infraspinatus. Although this study only examines acute experimental pain, it supports the concept that pain affects rotator cuff muscle recruitment and function, which may contribute to abnormal shoulder mechanics in patients with rotator cuff pathology.

A novel experimental knee-pain model affects perceived pain and movement biomechanics

CONTEXT

Knee injuries are prevalent, and the associated knee pain is linked to disability. The influence of knee pain on movement biomechanics, independent of other factors related to knee injuries, is difficult to study and unclear.

OBJECTIVE

(1) To evaluate a novel experimental knee-pain model and (2) better understand the independent effects of knee pain on walking and running biomechanics.

DESIGN

Crossover study.

SETTING

Biomechanics laboratory.

PATIENTS OR OTHER PARTICIPANTS

Twelve able-bodied volunteers (age = 23 ± 3 years, height = 1.73 ± 0.09 m, mass = 75 ± 14 kg).

INTERVENTION(S)

Participants walked and ran at 3 time intervals (preinfusion, infusion, and postinfusion) for 3 experimental conditions (control, sham, and pain). During the infusion time interval for the pain and sham conditions, hypertonic or isotonic saline, respectively, was continuously infused into the right infrapatellar fat pad for 22 minutes.

MAIN OUTCOME MEASURE(S)

We used repeated-measures analyses of variance to evaluate the effects of time and condition on (1) perceived knee pain and (2) key biomechanical characteristics (ground reaction forces, and joint kinematics and kinetics) of walking and running (P < .05).

RESULTS

The hypertonic saline infusion (1) increased perceived knee pain throughout the infusion and (2) reduced discrete characteristics of each component of the walking ground reaction force, walking peak plantar-flexion angle (range = 62°-67°), walking peak plantar-flexion moment (range = 95-104 N·m), walking peak knee-extension moment (range = 36-49 N·m), walking peak hip-abduction moment (range = 62-73 N·m), walking peak support moment (range = 178-207 N·m), running peak plantar-flexion angle (range = 38°-77°), and running peak hip-adduction angle (range = 5-21°).

CONCLUSIONS

This novel experimental knee pain model consistently increased perceived pain during various human movements and produced altered running and walking biomechanics that may cause abnormal knee joint-loading patterns.

Induced anterior knee pain immediately reduces involuntary and voluntary quadriceps activation

OBJECTIVE

To examine the immediate effects of experimentally induced anterior knee pain (AKP) on involuntary and voluntary quadriceps strength and activation.

DESIGN

Crossover 3 × 3 randomized controlled laboratory study with repeated measures.

SETTING

Human Performance Research Center, Brigham Young University.

PARTICIPANTS

Thirteen neurologically sound volunteers (age, 21.9 ± 3.2 years).

INTERVENTIONS

Subjects underwent 3 different conditions (pain, sham, and control). To induce AKP and sham condition, 5% sodium chloride and 0.9% sodium chloride (total volume of 1.0 mL for each condition), respectively, were injected into the infrapatellar fat pad on the dominant leg. No injection was performed for the control condition.

MAIN OUTCOME MEASURES

The vastus medialis peak Hoffmann reflex normalized by the peak motor response (H:M ratio) was used to measure involuntary quadriceps activation. Quadriceps central activation ratio (CAR) using maximal isometric knee extension torque (N·m) was calculated to assess voluntary quadriceps activation. The visual analog scale was used to measure pain perception.

RESULTS

Our pain model increased perceived pain immediately after the 5% hypertonic saline injection and pain lasted for 12 minutes on average (F40,743 = 16.85, P < 0.001). During the pain condition, subjects showed a 12% decrease in H:M ratio (F2,59 = 8.64, P < 0.001), a 34% decrease in maximal isometric knee extension torque (F2,59 = 5.89, P < 0.01), and a 5% decrease in CAR (F2,59 = 3.83, P = 0.03).

CONCLUSIONS

Our data showed that joint pain may be an independent factor to alter function of the muscles surrounding the painful joint. Both involuntary and voluntary inhibitory pathways may play a role in an immediate reduction of muscle activation.

A theoretical framework for understanding neuromuscular response to lower extremity joint injury

Background:

Neuromuscular alterations are common following lower extremity joint injury and often lead to decreased function and disability. These neuromuscular alterations manifest in inhibition or abnormal facilitation of the uninjured musculature surrounding an injured joint. Unfortunately, these neural alterations are poorly understood, which may affect clinical recognition and treatment of these injuries. Understanding how these neural alterations affect physical function may be important for proper clinical management of lower extremity joint injuries.

Methods:

Pertinent articles focusing on neuromuscular consequences and treatment of knee and ankle injuries were collected from peer-reviewed sources available on the Web of Science and Medline databases from 1975 through 2010. A theoretical model to illustrate potential relationships between neural alterations and clinical impairments was constructed from the current literature.

Results:

Lower extremity joint injury affects upstream cortical and spinal reflexive excitability pathways as well as downstream muscle function and overall physical performance. Treatment targeting the central nervous system provides an alternate means of treating joint injury that may be effective for patients with neuromuscular alterations.

Conclusions:

Disability is common following joint injury. There is mounting evidence that alterations in the central nervous system may relate to clinical changes in biomechanics that may predispose patients to further injury, and novel clinical interventions that target neural alterations may improve therapeutic outcomes.

Immediate Effects of Acupuncture and Cryotherapy on Quadriceps Motoneuron Pool Excitability: Randomised Trial Using Anterior Knee Infusion Model

Objective

The authors asked the following research questions: will an anterior knee infusion model induce constant pain? will perceived pain alter motoneuron pool (MNP) excitability? and will treatments alter perceived pain and/or MNP excitability?

Methods

Thirty-six neurologically healthy volunteers participated in this randomised controlled laboratory study. To induce anterior knee pain (AKP), 5% hypertonic saline (0.12 ml/min with a total volume of 8.5 ml over 70 min) was injected into the infrapatellar fat pad of the dominant leg. One of four 30-min treatments was randomly assigned to each subject after pain was induced (acupuncture, cryotherapy, sham cryotherapy and no treatment). Five acupuncture needles (SP9, SP10, ST36, GB34 and an ah shi point) were inserted to a depth of 1 cm. Vastus medialis (VM) maximum Hoffmann reflexes normalised by maximum motor response were recorded from each subject at baseline, 20 min post-injection, 50 min post-injection and 70 min post-injection. To record pain perception, a visual analogue scale was used every 5 min after injection.

Results

An anterior knee infusion pain model increased perceived pain (p<0.0001). No change was found in VM MNP excitability among the four treatments (p<0.19) or at any of the time intervals (p<0.52). Cryotherapy reduced perceived pain compared with acupuncture (p=0.0003) and sham treatment (p=0.0002).

Conclusions

A pain model may be used in other neurophysiological intervention studies related to AKP. AKP alone may not directly alter quadriceps activation. None of the treatments altered VM MNP excitability. Cryotherapy reduced pain while a single session of acupuncture and sham treatments did not.