Sources and References

Supplementary texts — American College of Sports Medicine. (2022). Clinical Exercise Physiology, 4th or 5th Edition. Wolters Kluwer. — Rosen, C.J. (Ed.). Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism. Wiley-Blackwell. Online resources — Osteoporosis Canada (osteoporosis.ca); Arthritis Society Canada (arthritis.ca); NIH Osteoporosis and Related Bone Diseases National Resource Center (bones.nih.gov); UpToDate clinical guidelines; ACSM Exercise Guidelines for osteoporosis and arthritis

Chapter 1: Bone Composition, Cellular Biology, and Turnover

What Is Bone Made Of?

Bone is a composite biomaterial whose remarkable combination of stiffness, strength, and toughness derives from the synergistic properties of its two principal constituents: an organic matrix dominated by type I collagen, and a mineral phase of hydroxyapatite crystals. Neither component alone possesses the full mechanical properties of bone: demineralised bone is flexible and rubbery (like a rubber band); deproteinised bone is brittle and fails catastrophically under small deformations. The composite architecture confers the fracture toughness that allows bone to absorb energy without catastrophic failure under the loading conditions of everyday life.

Bone is not an inert structural material but a living, metabolically active tissue that is continuously being renewed by the coupled processes of resorption and formation. This bone remodelling serves multiple purposes: it repairs microdamage before it accumulates to clinical fracture, it adapts the architecture of bone to changing mechanical demands, and it participates in mineral homeostasis by releasing calcium and phosphorus from the mineral phase when circulating concentrations fall.

The Cells of Bone: Osteoblasts, Osteoclasts, and Osteocytes

Three principal cell types populate bone, and their coordinated activity governs all aspects of bone biology from embryonic development through aging.

Osteoblasts are bone-forming cells derived from mesenchymal stem cells of the bone marrow stroma. They synthesise and secrete the organic matrix (principally type I collagen), and they orchestrate its mineralisation through the secretion of alkaline phosphatase and matrix vesicles that create local supersaturation with calcium phosphate, nucleating hydroxyapatite crystal formation. Osteoblast differentiation is governed by the transcription factor RUNX2 (runt-related transcription factor 2) — the master regulator of osteoblastogenesis — and its downstream targets osterix and ATF4. A proportion of osteoblasts that becomes embedded in the mineralising matrix differentiates into osteocytes; others undergo apoptosis; and the remainder remain on the bone surface as resting bone-lining cells.

Osteoclasts are multinucleated bone-resorbing cells derived from monocyte/macrophage precursors of the haematopoietic lineage. They adhere to the bone surface via integrin-rich podosomes organised into a sealing zone that isolates a resorption lacuna (Howship’s lacuna). Within this sealed space, osteoclasts acidify the lacunar environment to pH approximately 4.5 by pumping protons through a vacuolar H⁺-ATPase, dissolving the hydroxyapatite mineral. Simultaneously they secrete cathepsin K — a uniquely powerful collagenolytic lysosomal cysteine protease — that cleaves the collagen triple helix in the acidified environment, completing the degradation of the organic matrix. Osteoclast differentiation is driven by the cytokine RANK ligand (RANKL), produced by osteoblasts, osteocytes, and bone marrow stromal cells, binding to RANK on osteoclast precursors and activating the NFATc1 transcription factor.

Osteocytes are the most abundant bone cells (90–95% of all bone cells), residing within lacunae in the mineralised matrix and extending long cytoplasmic processes through a canalicular network that permeates the bone matrix in all directions. The lacunocanalicular network serves as a transport highway for nutrients, waste products, signalling molecules, and mechanical signals. Osteocytes are exquisitely sensitive to mechanical strain — they detect deformation of the lacunocanalicular fluid via primary cilia, mechanosensitive ion channels, and integrin-based adhesion complexes — and they orchestrate the adaptive remodelling response to mechanical loading by modulating RANKL/OPG expression (controlling osteoclastogenesis) and by secreting sclerostin, an inhibitor of the Wnt signalling pathway that is downregulated by mechanical loading to allow osteoblast activation.

Bone Remodelling: The Basic Multicellular Unit

Bone remodelling is executed by basic multicellular units (BMUs) — transient anatomical structures comprising osteoclasts at the cutting cone, resorbing a tunnel through cortical bone or a trench across trabecular bone surfaces, followed by osteoblasts that fill the resorption cavity with new bone matrix over a period of weeks to months. The BMU advances through the skeleton at approximately 20–40 µm per day, and at any given time the adult skeleton contains approximately a million active BMUs.

The RANK/RANKL/OPG axis is the molecular keystone of coupling between resorption and formation. RANKL (expressed by osteoblasts, osteocytes, and T cells) drives osteoclastogenesis; osteoprotegerin (OPG), a decoy receptor for RANKL produced by osteoblasts and other cells, competes with RANK for RANKL binding and inhibits osteoclast formation. The RANKL:OPG ratio in the bone microenvironment is the critical determinant of net bone resorption; elevated RANKL:OPG drives bone loss (as in oestrogen deficiency, inflammation, and hyperparathyroidism), while suppressed RANKL:OPG reduces remodelling.

Chapter 2: Bone Growth, Development, and the Determinants of Bone Strength

Longitudinal and Appositional Growth

Bone growth proceeds by two mechanisms that must be distinguished. Endochondral ossification — the formation of long bones and most of the axial skeleton — involves the replacement of a cartilaginous template by bone, occurring at the growth plate (physis), a region of hyaline cartilage in which chondrocytes proliferate, hypertrophy, and die, leaving a calcified cartilage scaffold that is then vascularised and replaced by trabecular bone. Linear growth of long bones occurs until the growth plates close at skeletal maturity — typically late adolescence in females and early adulthood in males — an event triggered by the rising oestrogen levels of puberty (via effects on the oestrogen receptor alpha expressed in growth plate chondrocytes).

Intramembranous ossification — the formation of the flat bones of the skull, clavicle, and portions of the mandible — occurs without a cartilaginous precursor; mesenchymal condensations differentiate directly into osteoblasts that deposit bone matrix. Appositional growth — the addition of bone to the outer surface (periosteum) and the removal of bone from the inner surface (endosteum) during childhood and adolescence — increases bone diameter and adjusts cortical thickness; continued periosteal apposition in adulthood partially compensates for endocortical bone loss, maintaining bone cross-sectional area and bending resistance even as cortical thickness declines.

Factors Affecting Bone Structure and Strength

The strength of a bone — its ability to resist fracture under applied loads — is determined by a complex interplay of bone material properties (the intrinsic mechanical characteristics of the bone tissue per se), bone geometry (cross-sectional area, cortical thickness, outer diameter, trabecular architecture), and bone microstructure (microcrack density, porosity, mineralisation heterogeneity).

Hormonal Regulation of Calcium and Bone

Three calciotropic hormones — parathyroid hormone (PTH), calcitriol (1,25-dihydroxyvitamin D), and calcitonin — regulate the fluxes of calcium between intestine, blood, bone, and kidney that maintain serum calcium within the narrow range of 2.25–2.75 mmol/L. PTH is the primary acute regulator: a fall in serum calcium is sensed within seconds by the calcium-sensing receptor (CaSR) on parathyroid chief cells, which respond by increasing PTH secretion. PTH acts on the kidney to increase tubular calcium reabsorption, promotes renal 1-alpha-hydroxylase activity (increasing calcitriol production), and — through actions on osteoblasts and osteocytes that secondarily stimulate osteoclastogenesis via RANKL — increases bone resorption to release calcium from the mineral phase.

The calcium-vitamin D axis is central to bone health across the lifespan. Intestinal calcium absorption — the primary determinant of calcium balance — is almost entirely dependent on calcitriol-mediated upregulation of TRPV6 (the apical calcium entry channel) and calbindin-D9k (the cytosolic calcium binding protein that facilitates transcellular calcium transport). Without adequate vitamin D, even a diet high in calcium results in poor absorption, negative calcium balance, and compensatory secondary hyperparathyroidism that accelerates bone resorption.

Osteomalacia

Osteomalacia — literally “soft bone” — is a metabolic bone disease characterised by defective mineralisation of newly formed osteoid (the unmineralised organic matrix synthesised by osteoblasts), resulting in accumulation of hypomineralised osteoid seams and a reduction in bone stiffness and strength. In adults it is the equivalent of rickets in growing children. The most common cause is vitamin D deficiency, though osteomalacia can also result from phosphate depletion (X-linked hypophosphataemia, tumour-induced osteomalacia), renal tubular acidosis, or drug effects (prolonged anticonvulsant therapy, which accelerates hepatic catabolism of vitamin D).

Clinically, osteomalacia presents with diffuse bone pain and tenderness (particularly of the spine, ribs, and pelvis), proximal muscle weakness, and — on skeletal X-ray or DXA — reduced bone density, pseudofractures (Looser’s zones, linear zones of unmineralised osteoid visible as radiolucent bands perpendicular to the cortex), and coarsening of the trabecular pattern. The diagnosis is confirmed by elevated alkaline phosphatase, low 25(OH)D (usually below 25 nmol/L), low or normal calcium and phosphate, and elevated PTH; bone biopsy with tetracycline double-labelling remains the gold standard but is rarely necessary.

Stress Fractures and Fracture Healing

Stress fractures are incomplete fractures resulting from the accumulation of fatigue damage at rates that exceed bone’s capacity for remodelling repair. They occur at predictable sites of repetitive loading: the metatarsals (march fracture), tibial shaft, fibula, femoral neck, navicular, and sacrum. Risk factors include rapid increases in training volume, low bone density, low energy availability (particularly in female athletes with the female athlete triad or relative energy deficiency in sport), and the biomechanical characteristics of the limb (tibial curvature, leg length discrepancy).

Fracture healing follows a biologically elaborate sequence. The acute haematoma formed by bleeding from torn periosteal vessels and medullary sinusoids provides the fibrin scaffolding and growth factor reservoir (principally TGF-beta, PDGF, VEGF) that initiates the repair cascade. This is followed by formation of a soft callus of fibrocartilage, produced by periosteal and endosteal progenitors that bridge the fracture gap; hard callus formation as the fibrocartilaginous template is mineralised through endochondral ossification; and finally remodelling over months to years, restoring the original cortical architecture through BMU activity guided by mechanical loading.

Chapter 3: Physical Activity, Exercise Prescription, and Bone Health

What Makes Exercise Osteogenic?

The skeleton’s mechanostat — the theoretical regulatory system proposed by Harold Frost — maintains bone mass and architecture at a level commensurate with the prevailing mechanical loading environment. When habitual strain exceeds a modelling threshold, net bone formation occurs; when habitual strain falls below a remodelling threshold, net bone loss occurs. The osteocyte network is the primary mechanosensor of this system, detecting strain magnitudes and rates in the lacunocanalicular fluid and translating these stimuli into paracrine signals that regulate RANKL:OPG balance and Wnt signalling.

Osteogenic exercise stimuli share several features that distinguish them from non-osteogenic activities: they generate relatively high bone strains (above approximately 1500–3000 microstrain at relevant sites), they are dynamic rather than static (bone adapts poorly to constant sustained loads but responds robustly to rapidly applied and released loads), they involve relatively few loading cycles per session (the osteogenic response saturates within approximately 20–50 cycles), and they ideally load the skeleton in diverse directions that are less habitual than the predominant loading pattern.

High-impact activities — jumping, bounding, plyometrics, team sports involving sprinting and change of direction — generate the highest bone strains at the lower extremities and hip, and are the most osteogenic for these sites. Resistance training, through muscle forces transmitted to bone at tendinous insertion sites, is strongly osteogenic — particularly for the proximal femur, spine, and upper extremity. Swimming and cycling, while outstanding for cardiovascular and metabolic health, generate negligible ground reaction forces and are accordingly the least osteogenic of common aerobic exercise modalities.

Chapter 4: Osteoporosis — Epidemiology, Diagnosis, and Management

Defining and Diagnosing Osteoporosis

Osteoporosis is a skeletal disorder characterised by compromised bone strength predisposing to an increased risk of fracture. The WHO operational definition — a BMD T-score at the lumbar spine, femoral neck, or total hip of less than or equal to -2.5, where the T-score is the number of standard deviations below the young adult mean BMD — was developed for postmenopausal white women but is applied clinically across sexes and ethnicities, recognising its limitations in these contexts.

The FRAX tool (Fracture Risk Assessment Tool), developed by John Kanis and colleagues at the Sheffield University WHO Collaborating Centre, computes the 10-year probability of a major osteoporotic fracture (hip, vertebra, forearm, or humerus) and a hip fracture specifically, integrating BMD with clinical risk factors: age, sex, body weight, prior fracture history, parental hip fracture, glucocorticoid use, smoking, alcohol intake, and rheumatoid arthritis. FRAX improves upon BMD alone by capturing the substantial fracture risk attributable to clinical factors independent of BMD, and it provides the probabilistic risk estimate needed for treatment threshold decisions.

Falls Risk Assessment and Prevention

Because approximately 95% of hip fractures and a large proportion of other fractures result from falls, fall prevention is a cornerstone of fracture prevention strategy in older adults. Falls risk assessment should identify: intrinsic risk factors (balance impairment, gait abnormality, lower extremity weakness, cognitive impairment, visual impairment, vestibular dysfunction, orthostatic hypotension, polypharmacy particularly with psychoactive and antihypertensive agents); extrinsic/environmental factors (poor lighting, loose rugs, inadequate footwear, bathroom hazards, icy surfaces); and behavioural factors (physical inactivity, fear of falling that paradoxically increases fall risk through altered gait biomechanics).

Validated falls risk tools include the Timed Up and Go (TUG) test (elevated risk if greater than 12 seconds to stand, walk 3 metres, turn, return, and sit), the Berg Balance Scale, the Short Physical Performance Battery (SPPB), and the STEADI (Stopping Elderly Accidents, Deaths and Injuries) initiative toolkit. Exercise-based fall prevention programmes — particularly those incorporating balance challenge and progressive resistance training — have the strongest evidence base for falls reduction, with a 23% reduction in fall rate demonstrated by the Cochrane review of exercise interventions.

Exercise Prescription for Osteoporosis

The exercise physiologist’s challenge in osteoporosis is to prescribe programmes that are osteogenic (loading the skeleton above the remodelling threshold to stimulate bone formation) and that reduce fall risk (improving balance, strength, and proprioception), while simultaneously avoiding exercises that pose unacceptable fracture risk. This requires nuanced individualization based on fracture history, spinal deformity, balance capacity, and comorbidities.

For individuals with osteoporosis without vertebral fractures, current guidelines (Osteoporosis Canada, ACSM) recommend: progressive resistance training targeting major muscle groups with free weights or machines (two to three sessions per week, working toward higher loads); weight-bearing aerobic exercise with some impact (walking with intention, hiking, tennis, dancing); and balance training (single-leg standing, tandem walking, Tai Chi). High-intensity resistance training — working at 70–85% of one-repetition maximum — has demonstrated bone-preserving or bone-forming effects at the proximal femur and spine in randomised trials in postmenopausal women, including the seminal LIFTMOR trial by Watson and colleagues.

For individuals with existing vertebral fractures, the prescription must be modified to avoid exercises that generate large compressive or flexion forces on the compromised vertebrae: deep forward flexion of the spine (sit-ups, toe-touches), high-impact activities, and heavy axial loading should be avoided or approached with extreme caution. Spinal extension exercises — which load the posterior paraspinal muscles and generate extension moments that partly counter the kyphotic forces on osteoporotic thoracic vertebrae — are generally well-tolerated and beneficial for postural alignment and pain management.

Bone Cancer and Exercise Prescription

Primary bone cancers (osteosarcoma, chondrosarcoma, Ewing sarcoma) are rare but disproportionately affect young people; more common are bone metastases from breast, prostate, lung, thyroid, and renal cell carcinoma, which preferentially establish in the trabecular-rich axial skeleton and proximal long bones. Bone metastases produce predominantly osteolytic lesions (breast, lung) that weaken bone by replacing trabecular bone with tumour tissue, mixed lesions (prostate), or osteoblastic lesions that may paradoxically increase radiological density while impairing bone quality.

Exercise prescription for individuals with bone metastases requires close interdisciplinary communication between the exercise physiologist, oncologist, and radiologist. An impending pathological fracture — indicated by a lesion greater than 50% of cortical diameter, significant endosteal scalloping, or severe localised pain — is a contraindication to weight-bearing exercise at that skeletal site. For those without impending fracture, carefully individualised exercise has demonstrated safety and efficacy for maintaining physical function and quality of life in this population.

Chapter 5: Cartilage, Joint Structure, and Synovial Joint Physiology

What Is Cartilage Made Of?

Cartilage is an avascular, aneural connective tissue that, in its hyaline articular form, covers the articulating surfaces of diarthrodial joints and provides a smooth, low-friction surface for load transmission and joint motion. Its remarkable mechanical properties — high compressive stiffness, near-frictionless surface tribology, and the ability to distribute load across the underlying subchondral bone — arise from the unique composition and architecture of its extracellular matrix.

Because articular cartilage is avascular, chondrocytes depend on diffusion from the synovial fluid for oxygen and nutrients — a limitation that makes cartilage nutrition dependent on joint movement and cyclic loading (which create the convective fluid flows that facilitate diffusion). This dependence on movement for nutrition is one mechanistic rationale for the recommendation that individuals with cartilage disease maintain activity rather than rest the affected joint.

Joint Architecture and Forces During Movement

A synovial joint consists of the articular surfaces of the articulating bones (covered by hyaline cartilage), a joint capsule of dense connective tissue, a synovial membrane lining the non-cartilaginous internal surfaces of the joint and producing synovial fluid, and associated ligaments, tendons, and bursae. The synovial fluid — a viscous, thixotropic fluid containing hyaluronan, lubricin (PRG4), and phospholipid surfactants that provide boundary lubrication — reduces the coefficient of friction at the cartilage surface to values as low as 0.001, far below even the best engineered lubricating systems.

The forces experienced by the joint surfaces during common activities are substantially larger than body weight. At the knee during level walking, compressive joint contact forces approach 2–3 times body weight; during stair climbing, 3–4 times; during deep squatting, 7–8 times. At the hip during walking, peak contact forces approach 2.5–4 times body weight. These forces are generated by the combined effects of gravity, inertia, and muscle contraction — particularly the co-contraction of antagonist muscles that stabilises the joint against rotational moments but adds compressive load to the joint surface. Body weight reduction is therefore an important modifiable determinant of joint contact stress.

Effect of Exercise on Cartilage

The long-standing concern that exercise — particularly high-impact or high-load exercise — is harmful to cartilage has been substantially revised by accumulating evidence that physiological loading is essential for cartilage health and that the absence of loading is the more potent stimulus for cartilage degeneration. Chondrocytes are mechanosensitive, responding to compressive loading by increasing anabolic activity (aggrecan and collagen synthesis), modulating inflammatory gene expression, and releasing anti-catabolic mediators. In animal models of cartilage injury, controlled exercise after cartilage damage promotes repair; immobilisation accelerates cartilage loss.

In humans, cross-sectional studies show that recreational runners have thicker, better-preserved articular cartilage and lower rates of knee osteoarthritis than sedentary controls — a finding consistent with the dose-response relationship between exercise intensity and cartilage metabolism, in which moderate loading is chondroprotective while very high loading (competitive running, heavy occupational loading) may exceed cartilage’s adaptive capacity.

Chapter 6: Osteoarthritis

Pathophysiology

Osteoarthritis (OA) is the most prevalent joint disease in the world and the leading cause of pain and functional limitation in adults over 65. It is characterised neuropathologically by progressive loss of articular cartilage, subchondral bone remodelling, osteophyte formation, synovial inflammation, and changes in periarticular muscles, ligaments, and capsule. OA is not, as was once believed, a simple mechanical “wear and tear” disease; it is a complex pathological process involving aberrant mechanosignalling, low-grade synovitis, oxidative stress, and the dysregulation of anabolic and catabolic matrix metabolism in chondrocytes.

Risk factors for OA include: age (the strongest risk factor), female sex, obesity (through both mechanical joint loading and adipose-derived systemic metabolic effects — adipokines such as leptin and adiponectin directly affect chondrocyte metabolism), prior joint injury (ACL tear doubles lifetime OA risk at the knee), malalignment (varus or valgus knee alignment concentrates stress on the medial or lateral compartment respectively), and occupational or sporting high-repetition loading.

Assessment and Diagnosis

OA is primarily a clinical diagnosis, supported by imaging. The characteristic symptoms are joint pain that worsens with activity and is relieved by rest (unlike inflammatory arthritis, in which pain and stiffness are worst with inactivity and improve with movement), stiffness lasting less than 30 minutes after rest (morning stiffness lasting longer than 30 minutes suggests inflammatory arthritis), and functional limitation in activities requiring the affected joint.

Physical examination findings in OA include joint-line tenderness, bony enlargement (osteophytes palpable at the joint margins), crepitus (a coarse crackling sensation during joint movement arising from irregular cartilage surfaces), restricted range of motion, and — in advanced OA — joint effusion and deformity. Radiographic findings include joint space narrowing (reflecting cartilage loss), subchondral sclerosis, subchondral cysts, and osteophyte formation; the Kellgren-Lawrence (K-L) grading scale (0–4) is the standard for radiographic OA classification. Notably, radiographic severity correlates only modestly with symptoms — many individuals with K-L Grade 3–4 OA are asymptomatic, and many individuals with severe pain have relatively preserved joint space.

Exercise Prescription for Osteoarthritis

The evidence base for exercise in OA is among the most robust in musculoskeletal medicine. Multiple Cochrane systematic reviews and meta-analyses demonstrate that land-based and aquatic exercise reduce pain and improve physical function in knee and hip OA, with effect sizes comparable to those of non-steroidal anti-inflammatory drugs (NSAIDs) and with a far more favourable safety profile. Exercise is therefore recommended as a core management strategy by the OARSI (Osteoarthritis Research Society International), EULAR, and ACR guidelines.

For knee OA, effective exercise modalities include: resistance training of the quadriceps (reducing the compressive force generated by quadriceps co-contraction during walking by improving neural control and reducing the moment arm requirement), hip abductors (reducing the medial compartment loading caused by excessive hip adduction during gait), and hamstrings; aerobic exercise at moderate intensity (walking, cycling, swimming, aqua-aerobics); and neuromuscular/proprioceptive training (perturbation training, balance exercises) to reduce dynamic joint loading peaks associated with aberrant movement patterns. The GLAD (Good Life with osteoArthritis in Denmark) programme — a structured 8-week neuromuscular training and education programme — has demonstrated sustained benefits for pain and function in randomised trials.

Chapter 7: Rheumatoid Arthritis and Inflammatory Joint Disease

Rheumatoid Arthritis: A Systemic Autoimmune Disease

Rheumatoid arthritis (RA) is a chronic, systemic autoimmune disease characterised by persistent synovial inflammation — primarily of small and medium-sized peripheral joints in a characteristically symmetrical distribution — that causes progressive articular cartilage and periarticular bone destruction if inadequately controlled, along with extra-articular manifestations involving the lungs, heart, vasculature, and skin. Its global prevalence is approximately 0.5–1.0%, with a female-to-male ratio of approximately 3:1, and peak onset in the fourth and fifth decades.

The pathogenesis of RA involves a complex interaction between genetic susceptibility (the HLA-DRB1 shared epitope is the strongest genetic risk factor, present in approximately 70% of RA patients) and environmental triggers — most convincingly, cigarette smoking and periodontal disease — that initiate autoimmune responses against citrullinated proteins. Anti-citrullinated protein antibodies (ACPAs), detectable in approximately 70% of RA patients, and rheumatoid factor (RF, IgM antibodies directed against the Fc region of IgG), present in approximately 70–80% of RA patients, are serological markers with both diagnostic and prognostic utility; seropositive RA is generally more severe than seronegative RA.

Assessment and Monitoring

The Disease Activity Score 28 (DAS28) combines measures of tender and swollen joints across 28 specified joint locations, erythrocyte sedimentation rate (ESR) or CRP, and patient global assessment into a composite score that categorises disease activity as low, moderate, high, or remission. The Clinical Disease Activity Index (CDAI) provides a similar composite without the laboratory component, allowing rapid bedside assessment. Treat to target — the strategy of titrating pharmacological therapy to achieve and maintain remission or low disease activity — has transformed RA outcomes, with modern targeted synthetic disease-modifying antirheumatic drugs (tsDMARDs) including JAK inhibitors (baricitinib, tofacitinib, upadacitinib) and biological DMARDs targeting TNF (adalimumab, etanercept), IL-6 (tocilizumab), B cells (rituximab), and T cell costimulation (abatacept) enabling remission in the majority of patients.

Exercise Prescription for Rheumatoid Arthritis

Exercise prescription in RA requires adaptation to the variable activity of the disease — active flares are a relative contraindication to high-intensity exercise on acutely inflamed joints, though gentle range-of-motion exercise is generally recommended even during flares to prevent contracture. During periods of stable or low disease activity, the same principles of progressive resistance and aerobic training apply as in OA. The EULAR 2018 recommendations for physical activity in RA specifically recommend aerobic exercise (150 minutes per week of moderate intensity or 75 minutes per week of vigorous intensity) and resistance training (at least twice weekly), noting that exercise does not worsen joint inflammation and that regular physical activity reduces cardiovascular risk, which is substantially elevated in RA due to systemic inflammation and the metabolic consequences of chronic glucocorticoid use.

Juvenile Idiopathic Arthritis

Juvenile idiopathic arthritis (JIA) is a heterogeneous group of chronic arthritides beginning before age 16 that persist for more than 6 weeks and cannot be attributed to another cause. The major subtypes include: oligoarticular JIA (fewer than 5 joints in the first 6 months, greatest risk of chronic anterior uveitis); polyarticular JIA (five or more joints, subdivided into RF-positive and RF-negative forms); systemic JIA (associated with quotidian fever, evanescent rash, and organ involvement); enthesitis-related arthritis (associated with HLA-B27 and axial skeleton involvement); and psoriatic arthritis. JIA poses unique challenges for physical activity prescription because the disease occurs during the critical period of skeletal development, and the combination of disease activity, pain, fatigue, and systemic glucocorticoid treatment impairs bone accrual and increases the risk of poor peak bone mass.

Chapter 8: Gout, Spondyloarthropathies, and Chronic Pain Management

Gout: Crystal Arthropathy

Gout is an inflammatory arthropathy caused by the deposition of monosodium urate (MSU) monohydrate crystals in articular and periarticular tissues, resulting from chronic hyperuricaemia. It is the most common inflammatory arthritis in men over 40 in Western countries, with a global prevalence approaching 4% and rising. The characteristic presentation of gout is the acute gouty attack — typically monoarticular, most often at the first metatarsophalangeal joint (podagra), characterised by exquisitely severe pain, swelling, warmth, and erythema reaching maximum intensity within 6–24 hours and resolving spontaneously over days to weeks.

The pathogenesis of acute gout involves the phagocytosis of MSU crystals by resident macrophages and neutrophils, which activates the NLRP3 inflammasome — an intracellular multiprotein complex that processes pro-IL-1beta and pro-IL-18 to their mature, secreted forms via caspase-1 activation. The resulting IL-1beta surge drives the intense acute inflammation of the gouty attack. Urate-lowering therapy (allopurinol, a xanthine oxidase inhibitor; febuxostat) targeting a serum urate below 360 µmol/L prevents crystal formation and eventually leads to crystal dissolution, eliminating the substrate for future attacks.

Seronegative Spondyloarthropathies

The seronegative spondyloarthropathies (SpA) are a group of inter-related inflammatory arthritides sharing the genetic marker HLA-B27 (present in 80–90% of patients with ankylosing spondylitis), axial skeleton involvement, a tendency for enthesitis (inflammation at tendon and ligament insertion sites), asymmetric oligoarthritis, uveitis, and skin manifestations (psoriasis in psoriatic arthritis). The group includes ankylosing spondylitis (axial spondyloarthropathy), psoriatic arthritis, reactive arthritis, and inflammatory bowel disease-associated arthritis.

In ankylosing spondylitis, inflammation at the sacroiliac joints and vertebral end plates leads — over years to decades — to progressive bone formation (enthesophytes, syndesmophytes) that bridges adjacent vertebral bodies, producing the radiographic “bamboo spine” and severe loss of spinal mobility. Exercise — particularly extension-based exercises and swimming — is a cornerstone of management, maintaining spinal mobility and upright posture against the relentless tendency toward kyphotic ankylosis.

Physical Therapist’s Approach to Chronic Pain

Chronic musculoskeletal pain — pain persisting beyond the expected healing time, typically 3 months — is not simply prolonged acute nociception. It involves central sensitisation: a pathological amplification of pain signalling within the central nervous system characterised by reduced pain thresholds, expansion of pain receptive fields, and the decoupling of pain perception from peripheral tissue damage. The dorsal horn of the spinal cord and the medial pain processing network (insula, anterior cingulate cortex, amygdala, prefrontal cortex) undergo functional and in some cases structural changes in chronic pain, explaining why analgesics targeting peripheral inflammation often fail and why psychological and behavioural interventions are essential components of chronic pain management.

Graded exposure to activity — helping patients with chronic pain to gradually resume feared activities in a systematic way that demonstrates that activity is safe and does not cause harm — is one of the most evidence-supported behavioural interventions for chronic musculoskeletal pain. The fear-avoidance model predicts that pain catastrophising leads to avoidance of activity, which causes deconditioning, increased disability, and perpetuated pain; breaking this cycle through graduated activity under the guidance of a skilled physiotherapist or exercise physiologist can achieve meaningful improvements in function and quality of life even when pain intensity is not fully resolved.