Targeted multifidus muscle activation reduces fibrosis of multifidus muscle following intervertebral disc injury.

PURPOSE: Aerobic exercise produces beneficial outcomes in patients with low back pain and partially attenuates the fibrotic changes to the multifidus in a model of intervertebral disc (IVD) degeneration. More targeted exercise might be required to fully attenuate these fibrotic alterations. This study aimed to investigate whether activation of the multifidus induced by neurostimulation could reduce fibrosis of the multifidus in a model of IVD degeneration in sheep. METHODS: IVD degeneration was induced in 18 merino sheep via a partial thickness unilateral annulus fibrosus lesion to the L1/2 and L3/4 IVDs. All sheep received an implantable neurostimulation device that provides stimulation of the L2 medial branch of the dorsal ramus. Three months after surgery, the animals were assigned to Injury or Activated groups. Activated animals received neurostimulation and the Injury group received no stimulation. Six months after surgery, the multifidus was harvested at L2 and L4. Van Gieson's, Sirius Red and immunofluorescence staining for Collagen-I and -III and quantitative PCR was used to examine fibrosis. Muscle harvested from a previous study without IVD injury was used as a control. RESULTS: Neurostimulation of the multifidus attenuated IVD degeneration dependent increases in the connective tissue, including Collagen-I but not Collagen-III, compared to the Injury group at L4. No measures of the multifidus muscle at L2, which received no stimulation, differed between the Injury and Activated groups. CONCLUSIONS: These data reveal that targeted activation of the multifidus muscle attenuates IVD degeneration dependent fibrotic alterations to the multifidus.

Uncertainty in low back pain care - insights from an ethnographic study.

PURPOSE: To explore how uncertainty plays out in low back pain (LBP) care and investigate how clinicians manage accompanying emotions/tensions. MATERIALS AND METHODS: We conducted ethnographic observations of clinical encounters in a private physiotherapy practice and a public multidisciplinary pain clinic. Our qualitative reflexive thematic analysis involved abductive thematic principles informed by Fox and Katz (medical uncertainty) and Ahmed (emotions). RESULTS: We identified three themes. (1) Sources of uncertainty: both patients and clinicians expressed uncertainty during clinical encounters (e.g., causes of LBP, mismatch between imaging findings and presentation). Such uncertainty was often accompanied by emotions - anger, tiredness, frustration. (2) Neglecting complexity: clinicians often attempted to decrease uncertainty and associated emotions by providing narrow answers to questions about LBP. At times, clinicians' denial of uncertainty also appeared to deny patients the right to make informed decisions about treatments. (3) Attending to uncertainty?: clinicians attended to uncertainty through logical reasoning, reassurance, acknowledgement, personalising care, shifting power, adjusting language and disclosing risks. CONCLUSIONS: Uncertainty pervades LBP care and is often accompanied by emotions, emphasising the need for a healthcare culture that recognises the emotional dimensions of patient-clinician interactions and prepares clinicians and patients to be more accepting of, and clearly communicate about, uncertainty.IMPLICATIONS FOR REHABILITATIONUncertainty pervades LBP care and is often accompanied by emotions.Neglecting complexity in LBP care may compromise person-centred care.Acknowledging uncertainty can enhance communication, balance patient-clinician relationships and address human aspects of care.

Deep and superficial cervical muscles respond differently to unstable motor skill tasks.

Biomechanical modelling and physiological studies suggest that various spinal muscle layers differ in their contribution to spine movement and stiffness. This study aimed to investigate the activation of deep and superficial muscles in stable and unstable task conditions. Nine healthy participants performed a task of controlling a metal ball on a plate fixed to the head in seated position. In unstable tasks, visual feedback was provided by mirrors to move the ball to the centre of the plate by small head movements and maintain the position for 3 s. Task difficulty was adjusted in a stepwise progression of difficulty using five surfaces with materials of decreasing resistance. In the stable condition, the ball was fixed to the plate's centre. EMG was recorded with surface (sternocleidomastoid, anterior scalenes, upper trapezius) and fine-wire electrodes (rectus capitis posterior major, obliquus inferior, multifidus, semispinalis cervicis, splenius capitis). The outcome variable was root mean square (RMS) EMG during the part of the task when the ball was maintained in the centre position. Results revealed greater cervical muscle activity in the unstable than stable conditions (p < 0.001, ηp2 = 0.746). Control of deep and superficial cervical muscles differed (p = 0.003, ηp2 = 0.354). Deep cervical muscle activity was greater with unstable tasks, but did not differ with task difficulty. In contrast, superficial cervical muscle activity increased in a stepwise manner with increasing challenge. These results support the notion that the central nervous system uses different strategies for control of deep versus superficial muscle layers of the cervical spine in association with instability.

Effects of a six-week exercise intervention on function, pain and lumbar multifidus muscle cross-sectional area in chronic low back pain: A proof-of-concept study.

INTRODUCTION: Exercise with the Functional Re-adaptive Exercise Device (FRED) has previously been shown to activate the lumbar multifidus (LM) and transversus abdominis (TrA) muscles in non-symptomatic volunteers. This study aimed to determine the effects of a six-week FRED exercise intervention on pain intensity, patient-reported function and LM cross sectional area (CSA) in people with chronic non-specific low back pain (LBP). METHODS: Thirteen participants undertook six weeks of FRED exercise for up to 15 min, three times per week. At six weeks pre-, immediately pre-, immediately post-, and six and 15 weeks post-intervention, participants completed the Numeric Pain Rating Scale, Patient-Specific Functional Scale, and ultrasound imaging was used to assess the size of the LM muscles at L5 level. Changes in outcomes were assessed using effect size, confidence intervals and minimum clinically important difference (MCID). RESULTS: There was no improvement in pain intensity following the intervention. Patient-reported function improved by at least twice the MCID for all follow-up assessments compared to immediately pre-intervention (d = 4.20-6.58). Lumbar multifidus CSA showed a large effect size increase from immediately pre-intervention to immediately post-intervention (d = 0.8-1.1); this was maintained at six weeks post-intervention (not measured at 15 weeks post-intervention). CONCLUSION: Six weeks of FRED exercise improved physical function in all 13 participants with chronic non-specific LBP who took part in this study and most participants' lumbar multifidus muscle CSA. On this basis, it may be an effective intervention for people with chronic LBP and should now be tested in a randomised controlled trial.

What type of exercise is most effective for people with knee osteoarthritis and co-morbid obesity?: The TARGET randomized controlled trial.

OBJECTIVE: Different exercise types may yield different outcomes in osteoarthritis (OA) subgroups. The objective was to directly compare effectiveness of two exercise programs for people with medial knee OA and co-morbid obesity. DESIGN: We performed a participant- and assessor-blinded randomized controlled trial. 128 people ≥50 years with medial knee OA and body mass index ≥30 kg/m2 were recruited from the community. Interventions were home-based non-weight bearing (NWB) quadriceps strengthening or weight bearing (WB) functional exercise for 12 weeks. Primary outcomes were change in overall knee pain (numeric rating scale, range 0-10) and difficulty with physical function (Western Ontario and McMaster Universities Osteoarthritis Index, 0-68) over 12 weeks. Secondary outcomes included other pain measures, physical function, quality-of-life, global changes, physical performance, and lower-limb muscle strength. RESULTS: 123 (96%) participants were retained. There was no evidence of a between-group difference in change in pain (mean difference 0.73 units (95% confidence intervals (0.05,1.50)) or function (2.80 units (-1.17,6.76)), with both groups reporting improvements. For secondary outcomes, the WB group had greater improvement in quality-of-life (-0.043 units (-0.085,-0.001)) and more participants reporting global improvement (overall: relative risk 1.40 (0.98,2.01); pain 1.47 (0.97,2.24); function 1.43 (1.04,1.98). Although adverse events were minor, more NWB group participants reported ≥1 adverse event (26/66 (39%) vs 14/62 (23%), p = 0.04). CONCLUSIONS: Both exercise types similarly improved primary outcomes of pain and function and can be recommended for people with knee OA and obesity. WB exercise may be preferred given fewer adverse events and potential additional benefits on some secondary outcomes. REGISTRATION: Prospectively registered (Australian New Zealand Clinical Trials Registry #12617001013358, 14/7/2017).

ISSLS Prize in Basic science 2019: Physical activity attenuates fibrotic alterations to the multifidus muscle associated with intervertebral disc degeneration.

PURPOSE: Chronic low back pain causes structural remodelling and inflammation in the multifidus muscle. Collagen expression is increased in the multifidus of humans with lumbar disc degeneration. However, the extent and mechanisms underlying the increased fibrotic activity in the multifidus are unknown. Physical activity reduces local inflammation that precedes multifidus fibrosis during intervertebral disc degeneration (IDD), but its effect on amelioration of fibrosis is unknown. This study aimed to assess the development of fibrosis and its underlying genetic network during IDD and the impact of physical activity. METHODS: Wild-type and SPARC-null mice were either sedentary or housed with a running wheel, to allow voluntary physical activity. At 12 months of age, IDD was assessed with MRI, and multifidus muscle samples were harvested from L2 to L6. In SPARC-null mice, the L1/2 and L3/4 discs had low and high levels of IDD, respectively. Thus, multifidus samples from L2 and L4 were allocated to low- and high-IDD groups compared to assess the effects of IDD and physical activity on connective tissue and fibrotic genes. RESULTS: High IDD was associated with greater connective tissue thickness and dysregulation of collagen-III, fibronectin, CTGF, substance P, TIMP1 and TIMP2 in the multifidus muscle. Physical activity attenuated the IDD-dependent increased connective tissue thickness and reduced the expression of collagen-I, fibronectin, CTGF, substance P, MMP2 and TIMP2 in SPARC-null animals and wild-type mice. Collagen-III and TIMP1 were only reduced in wild-type animals. CONCLUSIONS: These data reveal the fibrotic networks that promote fibrosis in the multifidus muscle during chronic IDD. Furthermore, physical activity is shown to reduce fibrosis and regulate the fibrotic gene network. These slides can be retrieved under Electronic Supplementary Material.

Muscle size and composition in people with articular hip pathology: a systematic review with meta-analysis.

OBJECTIVE: To synthesise and evaluate the current evidence investigating muscle size and composition in non-inflammatory articular hip pathology. METHODS: A systematic review of five electronic databases, using three concepts; articular hip pathology (e.g., osteoarthritis (OA)); hip muscles; and outcomes (e.g., muscle size and adiposity) was undertaken. Studies addressing non-inflammatory or non-traumatic articular hip pain, using measures of muscle size and adiposity were included and appraised for risk of bias. Data was extracted to calculate standardised mean differences (SMD) and pooled where possible for meta-analysis. RESULTS: Thirteen cross-sectional studies were included; all studies measured muscle size and 5/13 measured adiposity. In OA, there was low to very low quality evidence of no difference in hip muscle size, compared with matched controls. In unilateral OA, there was low to very low quality evidence of smaller size in gluteus minimus (SMD -0.38; 95% confidence interval (CI) -0.74, -0.01), gluteus medius (-0.44; 95% CI: -0.83, -0.05) and gluteus maximus (-0.39; 95% CI: -0.75, -0.02) muscles in the symptomatic limb. Individual studies demonstrated non-uniform changes in muscle size in OA. No significant difference was observed in muscle size in other pathologies or in adiposity for any group. CONCLUSION: There is some low quality evidence that specific hip muscles are smaller in unilateral hip OA. Variation in the magnitude of differences indicate changes in size are not uniform across all muscles or stage of pathology. Studies in larger cohorts investigating muscle size and composition across the spectrum of articular pathologies are required to clarify these findings.

Comparison of single- and dual-monitor approaches to differentiate sitting from lying in free-living conditions.

High levels of sedentary time have been detrimentally linked to health outcomes. Differentiating sitting from lying may help to further understand the mechanisms associated with these health impacts. This study compares the inter-method agreement between the "single-monitor" method (thigh-worn activPAL3TM ) and a more robustly validated "dual-monitor" method (trunk and thigh-worn activPAL3TM ) in their classifications of sitting and lying under free-living conditions. Thirty-five participants (20-50 years) wore two activity monitors (thigh and trunk) for 24 hours. Total time spent lying and sitting was calculated for both methods, and agreement was determined using ICC and Bland-Altman methods. As there was no gold standard, further data were collected from five participants during structured activities that were designed to challenge classification, to better understand any disagreement between the methods. ICCs were 0.81 for sitting time and 0.64 for lying time. The single-monitor method detected less lying time than the dual-monitor method, with a mean difference of -25 minutes (95% agreement limits: -172 to 221 minutes), including three cases with extreme disagreement (mostly in daytime lying classification). The additional data collection suggested a major source of disagreement was failure of the single-monitor method to identify lying that involved no rotation around the longitudinal axis. In conclusion, there was some agreement between the single- and dual-monitor estimates of lying time under free-living conditions, but measures were not interchangeable. The main disagreement was in how the methods classified daytime lying and lying tasks involving no lateral movement. Both methods yield promise for measuring time in bed.

Hip abductor muscle activity during walking in individuals with gluteal tendinopathy.

The external hip adduction moment during walking is greater in individuals with gluteal tendinopathy (GT) than pain-free controls. Although this likely represents a greater demand on the hip abductor muscles implicated in GT, no study has investigated activation of these muscles in GT. For this purpose, fine wire electrodes were inserted into the segments of the gluteus minimus and medius muscles, and surface electrodes placed on the tensor fascia lata, upper gluteus maximus, and vastus lateralis muscles of eight individuals with, and eight without, GT. Participants underwent six walking trials. Individual muscle patterns were compared between groups using a wavelet-based linear effects model and muscle synergy analysis performed using non-negative matrix factorization to evaluate muscle activation patterns, within- and between-participant variability. Compared to controls, individuals with GT exhibited a more sustained initial burst of the posterior gluteus minimus and middle gluteus medius muscle segments. Two muscle synergies were identified; Synergy-1 activated in early-mid stance and Synergy-2 in early stance. In GT participants, posterior gluteus minimus and posterior gluteus medius and tensor fascia lata contributed more to Synergy-1 active during the period of single leg support. Participants with GT exhibited reduced within-participant variability of posterior gluteus medius and reduced between-participant variability of anterior gluteus minimus and medius and upper gluteus maximus. In conclusion, individuals with GT exhibit modified muscle activation patterns of the hip abductor muscles during walking, with potential relevance for gluteal tendon loading.

Forearm muscle activity is modified bilaterally in unilateral lateral epicondylalgia: A case-control study.

Lateral epicondylalgia (LE) is associated with a reduced wrist extensor muscle activity and altered biomechanics. This study compared the coordination between forearm muscles during gripping in individuals with LE and pain-free controls. Intramuscular electrodes recorded myoelectric activity from extensor carpi radialis brevis/longus (ECRB/ECRL), extensor digitorum communis (EDC), flexor digitorum superficialis/profundus (FDS/FDP), and flexor carpi radialis (FCR), bilaterally, in 15 participants with unilateral LE and 15 pain-free controls. Participants performed a gripping task at 20% maximum force in four arm positions. The contribution of each muscle was expressed as a proportion of the summed electromyography of all muscles. In individuals with LE, ECRB contributed less to total electromyography in the symptomatic arm but not the asymptomatic arm than pain-free controls. The contribution of EDC and FDP to total electromyography was greater in both the symptomatic and asymptomatic arm of the LE group, than pain-free controls. No other differences were observed between groups. Subtle differences in muscle activation were present with differing arm positions. These findings indicate forearm muscle activity is modified in LE. It is unknown whether this is cause or effect. Changes in the asymptomatic side may imply involvement of central mechanisms.

Movement-based subgrouping in low back pain: synergy and divergence in approaches.

BACKGROUND: Classification systems for low back pain (LBP) aim to guide treatment decisions. In physiotherapy, there are five classification schemes for LBP which consider responses to clinical movement examination. Little is known of the relationship between the schemes. OBJECTIVES: To investigate overlap between subgroups of patients with LBP when classified using different movement-based classification schemes, and to consider how participants classified according to one scheme would be classified by another. DESIGN: Cross-sectional cohort study. SETTING: University clinical laboratory. PARTICIPANTS: One hundred and two participants with LBP were recruited from university, hospital outpatient and private physiotherapy clinics, and community advertisements. INTERVENTION: Participants underwent a standardised examination including questions and movement tests to guide subgrouping. MAIN OUTCOME MEASURES: Participants were allocated to a LBP subgroup using each of the five classification schemes: Mechanical Diagnosis and Treatment (MDT), Movement System Impairment (MSI), O'Sullivan Classification (OSC), Pathoanatomic Based Classification (PBC) and Treatment Based Classification (TBC). RESULTS: There was concordance in allocation to subgroups that consider pain relief from direction-specific repeated spinal loading in the MDT, PBC and TBC schemes. There was consistency of subgrouping between the MSI and OSC schemes, which consider pain provocation to specific movement directions. Synergies between other subgroups were more variable. Participants from one subgroup could be subdivided using another scheme. CONCLUSIONS: There is overlap and discordance between LBP subgrouping schemes that consider movement. Where overlap is present, schemes recommend different treatment options. Where subgroups from one scheme can be subdivided using another scheme, there is potential to further guide treatment. An integrated assessment model may refine treatment targeting.

Sensory and motor deficits exist on the non-injured side of patients with unilateral tendon pain and disability--implications for central nervous system involvement: a systematic review with meta-analysis.

INTRODUCTION: Tendinopathy manifests as activity-related tendon pain with associated motor and sensory impairments. Tendon tissue changes in animals present in injured as well as contralateral non-injured tendon. This review investigated evidence for bilateral sensory and motor system involvement in unilateral tendinopathy in humans. METHODS: A comprehensive search of electronic databases, and reference lists using keywords relating to bilateral outcomes in unilateral tendinopathy was undertaken. Study quality was rated with the Epidemiological Appraisal Instrument and meta-analyses carried out where appropriate. Analysis focused on comparison of measures in the non-symptomatic side of patients against pain-free controls. RESULTS: The search revealed 5791 studies, of which 20 were included (117 detailed reviews, 25 met criteria). There were 17 studies of lateral epicondylalgia (LE) and one each for patellar, Achilles and rotator cuff tendinopathy. Studies of LE were available for meta-analysis revealing the following weighted pooled mean deficits: pressure pain thresholds (-144.3 kPa; 95% CI -169.2 to -119.2 p<0.001), heat pain thresholds (-1.2°C; 95% CI -2.1 to -0.2, p<0.001), cold pain thresholds (3.1°C; 95% CI 1.8 to 4.4, p<0.001) and reaction time (37.8 ms; 95% CI 24.8 to 50.7, p<0.001). DISCUSSION: Deficits in sensory and motor systems present bilaterally in unilateral tendinopathy. This implies potential central nervous system involvement. This indicates that rehabilitation should consider the contralateral side of patients. Research of unilateral tendinopathy needs to consider comparison against pain-free controls in addition to the contralateral side to gain a complete understanding of sensory and motor features.

Anticipatory postural activity of the deep trunk muscles differs between anatomical regions based on their mechanical advantage.

The functional differentiation between regions of psoas major (PM) and quadratus lumborum (QL) may underlie a mechanical basis for recruitment of motor units across the muscle. These mechanically unique fascicle regions of these complex multifascicular muscles, PM and QL, are likely to be controlled independently by the central nervous system (CNS). Fine-wire electrodes recorded the electromyographic activity of the PM fascicles arising from the transverse process (PM-t) and vertebral body (PM-v) and the anterior (QL-a) and posterior (QL-p) layers of QL on the right side during a postural perturbation associated with rapid arm movements. The findings of this study indicate that the CNS coordinates the activity of specific regions of PM and QL independently as a component of the anticipatory postural adjustments that precedes the predictable challenge to the spine associated with limb movements. The spatial and temporal features of discrete activity of different regions within PM and QL matched their differing mechanical advantage predicted from their anatomy. These findings suggest that the CNS differentially activates individual regions within complex spine muscles to control the three-dimensional forces applied to the spine. The data also point to a sophisticated control of muscle activation that appears based on mechanical advantage.

The effect of Parkinson's disease and levodopa on adaptation of anticipatory postural adjustments.

Postural support alters anticipatory postural adjustments (APAs). Efficient adaptation to changes in postural support in reactive and centrally initiated postural synergies is impaired in Parkinson's disease (PD). This study examined whether APAs are affected differently by familiar and novel supports in people with PD, ON and OFF levodopa. The effect of PD and levodopa on the ability to immediately adapt APAs to changes in support and refine with practice was also investigated. Fourteen people with PD and 14 healthy control participants performed 20 single rapid leg lift tasks in four support conditions: unsupported, bilateral handgrip (familiar), bite plate (novel) and a combined handgrip+bite plate condition. APAs, identified from force plate data, were characterized by an increase in the vertical ground reaction force under the lifted leg as a result of a shift of weight toward the stance limb. Results showed the ability to incorporate familiar and novel external supports into the postural strategy was preserved in PD. Controls and PD patients in the OFF state further refined the postural strategy with practice as evidenced by changes in amplitude of vertical ground reaction forces and forces applied to support apparatus within conditions between the initial and final trials. In the ON state, people with PD failed to refine the use of postural supports in any condition. The results suggest that immediate postural adaptation is intact in people with PD and unaffected by levodopa administration but the ability to refine postural adaptations with task experience is compromised by dopamine therapy.

Effect of airway control by glottal structures on postural stability.

Maintenance of upright posture involves complex neuromotor processes that include control of thoracic and abdominal pressures. Control of airflow by glottal structures is a primary determinant of thoracic pressure and may have a role in control of postural stability. This study aimed to investigate the effect of modulation of airway control on upright postural stability during postural perturbations. Standing balance was gently perturbed in the sagittal plane during 7 breathing/voicing tasks that ranged from completely closed (breath-hold), to partially opened (voicing) or completely open (sigh) glottal conditions in 11 healthy adults. Dependent measures were peak amplitude of displacement of the thorax and center of pressure (CoP). When the glottis was completely open during sigh, thoracic displacement in response to the perturbation was greater than in all other conditions, regardless of direction of perturbation (post hoc, all P < 0.002). The absolute amplitude of CoP displacement was greater with backward perturbation (main effect, Direction P = 0.001) and was greater at both extremes of glottal modulation (glottis closed and completely open) than when the glottis was partially opened during counting out loud (post hoc, all P < 0.04). These results show that airway modulation affects postural control during upright perturbations. The thorax was more stable when the glottis was engaged than when it was required to remain open, whereas control of CoP displacement appeared more optimal during the natural dynamic mid-range airway modulation of voicing. These data suggest that glottal control influences balance, and that glottal control strategies may be an important consideration for patients with breathing and/or balance disorders.

Temporal association between changes in primary sensory cortex and corticomotor output during muscle pain.

BACKGROUND: Integration of information between multiple cortical regions is thought to underpin the experience of pain. Yet studies tend to focus on pain related changes in discrete cortical regions. Although altered processing in the primary motor (M1) and sensory cortex (S1) is implicated in pain, the temporal relationship between these regions is unknown and may provide insight into the interaction between them. METHODS: We used recordings of somatosensory-evoked potentials (SEPs) and transcranial magnetic stimulation to investigate the temporal relationship between altered excitability of the primary sensory cortex and corticomotor output during and after muscle pain induced by hypertonic saline infusion into the right first dorsal interosseous. SEPs and motor-evoked potentials (MEPs) were recorded in 12 healthy individuals. RESULTS: Participants reported an average pain intensity of 5.4 (0.5) on a 10-cm visual analogue scale. The area of the N20-P25-N33 complex of the SEP was reduced during and after pain, but MEP amplitudes were suppressed only after pain had resolved. CONCLUSIONS: Our data show that pain reduces sensory processing before motor output is altered. This temporal dispersion, coupled with the lack of correlation between pain-induced changes in S1 and M1 excitability, imply either that independent processes are involved, or that reduced excitability of S1 during acute experimental muscle pain mediates latent reductions in motor output via processes that are non-linear and potentially involve activation of a wider brain network.

New insight into motor adaptation to pain revealed by a combination of modelling and empirical approaches.

BACKGROUND: Movement changes in pain. Unlike the somewhat stereotypical response of limb muscles to pain, trunk muscle responses are highly variable when challenged by pain in that region. This has led many to question the existence of a common underlying theory to explain the adaptation. Here, we tested the hypotheses that (1) adaptation in muscle activation in acute pain leads to enhanced spine stability, despite variation in the pattern of muscle activation changes; and (2) individuals would use a similar 'signature' pattern for tasks with different mechanical demands. METHODS: In 17 healthy individuals, electromyography recordings were made from a broad array of anterior and posterior trunk muscles while participants moved slowly between trunk flexion and extension with and without experimentally induced back pain. Hypotheses were tested by estimating spine stability (Stability Index) with an electromyography-driven spine model and analysis of individual and overall (net) adaptations in muscle activation. RESULTS: The Stability Index (P < 0.017) and net muscle activity (P < 0.021) increased during pain, although no two individuals used the same pattern of adaptation in muscle activity. For most, the adaptation was similar between movement directions despite opposite movement demands. CONCLUSIONS: These data provide the first empirical confirmation that, in most individuals, acute back pain leads to increased spinal stability and that the pattern of muscle activity is not stereotypical, but instead involves an individual-specific response to pain. This adaptation is likely to provide short-term benefit to enhance spinal protection, but could have long-term consequences for spinal health.

Experimental knee joint pain during strength training and muscle strength gain in healthy subjects: a randomized controlled trial.

OBJECTIVE: Knee joint pain and reduced quadriceps strength are cardinal symptoms in many knee pathologies. In people with painful knee pathologies, quadriceps exercise reduces pain, improves physical function, and increases muscle strength. A general assumption is that pain compromises muscle function and thus may prevent effective rehabilitation. This study evaluated the effects of experimental knee joint pain during quadriceps strength training on muscle strength gain in healthy individuals. METHODS: Twenty-seven healthy untrained volunteers participated in a randomized controlled trial of quadriceps strengthening (3 times per week for 8 weeks). Participants were randomized to perform resistance training either during pain induced by injections of painful hypertonic saline (pain group, n = 13) or during a nonpainful control condition with injection of isotonic saline (control group, n = 14) into the infrapatellar fat pad. The primary outcome measure was change in maximal isokinetic muscle strength in knee extension/flexion (60, 120, and 180 degrees/second). RESULTS: The group who exercised with pain had a significantly larger improvement in isokinetic muscle strength at all angular velocities of knee extension compared to the control group. In knee flexion there were improvements in isokinetic muscle strength in both groups with no between-group differences. CONCLUSION: Experimental knee joint pain improved the training-induced gain in muscle strength following 8 weeks of quadriceps training. It remains to be studied whether knee joint pain has a positive effect on strength gain in patients with knee pathology.