BASIC · EP 09 · AGING
Before You Listen
Episode Setup
- Topic in one line: the predictable physiologic decline that drives every geriatric rehabilitation question: sarcopenia with preferential type II fiber loss; postmenopausal trabecular-first bone loss; cardiopulmonary changes (declining maximum heart rate, arterial stiffening producing isolated systolic hypertension, baroreceptor-driven orthostatic hypotension, declining maximal oxygen uptake (VO2 max), senile emphysema, age-adjusted PaO2); renal decline that hides behind a deceptively normal serum creatinine; pharmacokinetic shifts (phase I cytochrome P450 (CYP450) decline with preserved phase II conjugation, expanded adipose Vd for lipophilic drugs, lower albumin raising free fraction of warfarin and phenytoin); and the neurologic, sensory, and sleep changes that must be distinguished from disease.
- Prerequisites: comfort with basic exercise physiology (VO2 max, cardiac output, maximum heart rate (HRmax)), bone density terminology (T-score for osteoporosis), and the cross-cutting pharmacology framework from BASIC-06 (CYP450, volume of distribution (Vd), protein binding, renal clearance).
- Runtime: approximately 30 minutes for Part 1.
- Scope boundary: Part 1 maps the physiology. Part 2 turns the physiology into prescriptions: modified exercise protocols, fall risk assessment instruments, the multifactorial fall prevention bundle, and the geriatric syndromes (frailty, delirium, polypharmacy, incontinence, pressure injury) framed within the comprehensive geriatric assessment.
Vignette. An 82-year-old retired teacher is admitted to the inpatient rehabilitation unit after a left intertrochanteric hip fracture treated with cephalomedullary nail fixation. She lives alone in a two-story home, was previously independent with ambulation using a single-point cane, and was driving locally. Her past medical history includes hypertension (lisinopril, hydrochlorothiazide), atrial fibrillation (warfarin with international normalized ratio (INR) target 2-3), osteoporosis (T-score -3.0 at the lumbar spine, on alendronate), and glaucoma (latanoprost). On admission her serum creatinine is 0.9 mg/dL with an estimated glomerular filtration rate (eGFR) of 48 mL/min by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. On day 3 of rehabilitation she becomes lightheaded when standing: blood pressure 142/88 mmHg supine and 108/68 mmHg after 1 minute standing, with heart rate 72 beats per minute supine and 86 standing. Her medication administration record shows no pharmacologic venous thromboembolism prophylaxis (the primary team deferred it because she is on warfarin for her atrial fibrillation).
Why is her serum creatinine deceptive about her actual renal function, what age-related cardiovascular mechanisms explain the orthostatic drop and the inadequate heart rate response, what does her T-score indicate and how does the postmenopausal trabecular-versus-cortical bone loss pattern relate to this intertrochanteric fracture, and what specific renal dose adjustments are mandatory for the renally cleared medications a rehabilitation team will reach for next?
(Answer at the end of this chapter)
Section 1: Musculoskeletal Aging — Sarcopenia and Bone
Bottom line: sarcopenia preferentially destroys type II (fast-twitch) fibers via anterior horn motor neuron apoptosis with collateral sprouting that produces fiber type grouping. Bone loss accelerates to 2-3 percent per year for 5-10 years after menopause when estrogen withdrawal unleashes osteoclasts, hollowing trabecular bone first and priming the hip for cortical-shell failure decades later. T-score ≤ -2.5 = osteoporosis; a fragility fracture defines osteoporosis clinically regardless of T-score.
Musculoskeletal aging runs on two parallel tracks (loss of muscle, or sarcopenia, and loss of bone), with cartilage and connective-tissue degeneration layered on top. Together they determine whether the patient falls and whether the fall produces a fracture.
Sarcopenia is the age-related loss of skeletal muscle mass, strength, and power. It begins around age 30 with losses of approximately 3-8 percent per decade and inflects sharply after age 60, when sedentary adults lose 1-2 percent per year. By age 82 the engine has been shedding mass for half a century.
The pathophysiology is selective. Sarcopenia preferentially destroys type II (fast-twitch) muscle fibers (responsible for rapid, high-force generation), while type I (slow-twitch, oxidative) fibers are relatively spared. The mechanism originates upstream in the spinal cord. Anterior horn motor neurons undergo progressive age-related apoptosis. When a motor neuron supplying a cluster of type II fibers dies, those fibers are denervated and would atrophy without rescue.
The body mounts a rescue. Surviving anterior horn cells (predominantly the slow motor units innervating type I fibers) extend collateral sprouts and adopt the orphan fibers. Because fiber type is dictated entirely by the innervating neuron, the rescued fibers are forced to convert from fast-twitch to slow-twitch. The muscle survives but loses its explosive power.
The histological signature is fiber type grouping. A young biopsy shows a mosaic of intermingled fast and slow fibers. The aging biopsy shows large consolidated clumps of a single fiber type, where one slow motor neuron has adopted a wide territory of orphan fibers. The total number of independent motor units plummets while the average size of surviving units grows, destroying fine motor precision and peak force.
The functional consequence defines the geriatric fall. A 30-year-old who catches a toe fires the type II fibers in milliseconds; the leg whips forward with explosive eccentric force and arrests the falling center of mass. The 82-year-old fires the same signal, but the type II fibers are gone or converted. The muscle contracts with the slow rhythm of a tractor. It cannot generate force fast enough to arrest the fall. The mechanical emergency brake has been disconnected.
Hormonal and inflammatory shifts compound the neurological attrition. Older adults lose anabolic drive: declining testosterone, growth hormone, insulin-like growth factor 1 (IGF-1), and abruptly at menopause estrogen. Catabolic cytokines including tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6) rise in parallel, producing the chronic low-grade inflammatory state called “inflammaging.” The net effect is anabolic resistance, where muscle protein breakdown outpaces synthesis. Resistance training plus a protein target of 1.0-1.2 g/kg/day (above the 0.8 g/kg/day RDA) attenuates the process. Prescribing intense physical therapy without adequate protein is asking a construction crew to build a house without lumber.
Bone aging follows a parallel trajectory. Peak bone mass is achieved around age 25-30. Wolff’s law governs the load contribution: bone remodels in direct proportion to mechanical load. Weight-bearing exercise stimulates osteoblasts; absence of load (microgravity, prolonged bed rest, denervation, casting) drives massive resorption, the basis for disuse osteoporosis after complete spinal cord injury.
After age 40, both sexes lose bone density at approximately 0.5-1 percent per year. In women, menopause disrupts the linear curve catastrophically. When estrogen is abruptly withdrawn, regulatory suppression on osteoclasts is removed and osteoclast activity skyrockets, driving accelerated bone loss of 2-3 percent per year for the first 5 to 10 years.
The architectural pattern is high-yield. Trabecular bone is the metabolically active spongy internal honeycomb; cortical bone is the dense solid outer shell. Trabecular bone has a vastly higher surface-to-volume ratio and is destroyed first by the postmenopausal osteoclast surge. The vertebral bodies and distal radius are trabecular-dominant, which explains the epidemiological timeline: the classic initial fragility fracture in a woman in her 60s is a Colles’ fracture of the distal radius or a silent wedge compression fracture of the thoracic spine. Cortical bone catches up over decades. The femoral neck and intertrochanteric region rely on internal trabecular struts plus a thick cortical shell to withstand the torsional loads of walking. By 82, the trabecular core was hollowed out decades ago and the cortical shell has been thinning by roughly 1 percent annually. The intertrochanteric region is structurally primed for catastrophic failure.
T-score compares current bone mineral density to a healthy 30-year-old reference. Osteopenia is a T-score between -1.0 and -2.5. Osteoporosis is ≤ -2.5 at the hip or lumbar spine. A frequently tested override: regardless of T-score, an elderly patient who sustains a fragility fracture (fall from standing height or less) has osteoporosis clinically. The dual-energy X-ray absorptiometry (DEXA) scan is still ordered to establish a baseline for pharmacologic management (bisphosphonates, denosumab, teriparatide).
Cartilage, tendons, and ligaments complete the chassis. Articular cartilage progressively desiccates (decreased water, proteoglycan, and chondrocyte synthetic capacity), driving osteoarthritis. Tendons and ligaments stiffen from collagen cross-linking and elastin loss, explaining the rise in rotator cuff tears and Achilles ruptures. The cumulative result is a 25-30 percent decrease in joint range of motion by age 70.
High Yield — Musculoskeletal aging
- Sarcopenia: ~3-8 percent per decade after age 30; 1-2 percent per year sedentary after 60.
- Type II fibers preferentially lost via anterior horn cell apoptosis with collateral sprouting from surviving slow motor units → fiber type grouping on biopsy.
- Inflammaging: declining testosterone, GH, IGF-1, estrogen + rising TNF-alpha, IL-6 = anabolic resistance.
- Protein target 1.0-1.2 g/kg/day (vs 0.8 g/kg/day RDA).
- Peak bone mass age 25-30; decline 0.5-1 percent/year after 40; postmenopausal 2-3 percent/year for 5-10 years.
- Wolff’s law: bone remodels to mechanical load (basis of disuse osteoporosis in SCI, casting, bed rest).
- Trabecular bone fails first (vertebrae, distal radius); cortical (hip, intertrochanteric) catches up.
- Osteoporosis T-score ≤ -2.5; osteopenia -1.0 to -2.5; fragility fracture defines osteoporosis clinically regardless of T-score.
- Joint ROM decreases ~25-30 percent by age 70.
Mnemonic — “Type II, trabecular, then trouble”
Type II fast-twitch fiber loss removes reactive postural control. Trabecular bone loss after menopause hollows the skeleton from the inside. The loss of type II fibers guarantees the fall; the loss of trabecular bone guarantees the fracture. That cascade (trouble) drives geriatric inpatient rehabilitation.
The muscle receives the command to fire fast, but it fires with the sluggish, sustained, low velocity rhythm of a tractor. It simply cannot generate the requisite force fast enough to arrest the downward acceleration of her center of mass.
— BASIC-09 podcast, Part 1, ~08:13