Exercise & Physical Training

Aerobic capacity is the maximum rate of oxygen uptake the body can sustain to produce ATP via oxidative metabolism during prolonged exercise. Per the Fick principle (VO2 = Q × (a-v)O2), it is governed by oxygen delivery (cardiac output, hemoglobin, capillary supply) and muscle extraction (mitochondrial function), and is most often quantified as VO2max. Higher aerobic capacity supports endurance, faster recovery, and metabolic resilience, and tracks closely with healthspan and reduced cardiovascular and all-cause mortality.

The anaerobic threshold (AT) is the exercise intensity above which aerobic metabolism can no longer meet ATP demand and lactate begins to accumulate at a rate exceeding clearance; it corresponds approximately to the second lactate threshold (LT2) and lies near, but is not identical to, the maximal lactate steady state (MLSS), typically 75–85% of VO2max in fit individuals. The term is mechanistically imprecise — lactate accumulation reflects an imbalance between production and clearance rather than an onset of anaerobic metabolism per se — and some authors prefer 'lactate threshold 2' or 'respiratory compensation point'. Sustained exercise at AT drives robust mitochondrial and cardiovascular adaptations; performance at or near AT is a strong predictor of endurance capacity and correlates with cardiovascular risk reduction.

Bone mineral density is the amount of mineral — primarily hydroxyapatite — per unit area (g/cm²) or volume of bone tissue, most commonly assessed at the lumbar spine and femoral neck by DEXA. The T-score compares an individual's BMD to the young-adult mean peak reference; WHO criteria define osteopenia (T-score −1.0 to −2.5) and osteoporosis (T-score ≤ −2.5), the latter roughly doubling hip fracture risk per SD reduction. BMD declines with age, accelerating in women post-menopause; resistance and impact-loading exercise, adequate dietary calcium and vitamin D, and estrogen-related hormonal status are the principal modifiable determinants. Hip fracture in older adults carries ~20–30% one-year mortality, making BMD preservation a direct longevity target.

Cardiorespiratory fitness (CRF) is the ability of the circulatory and respiratory systems to deliver oxygen to working muscles during sustained activity, most often quantified by VO2max. It integrates lung function, cardiac output, vascular health, and muscle oxidative capacity. Some large cohort studies (e.g., Mandsager et al. 2018) suggest low CRF can carry mortality risk comparable to or greater than coronary artery disease, smoking, or diabetes, making CRF a powerful modifiable longevity predictor.

The interference effect describes the attenuation of resistance-training adaptations - strength, power, and especially hypertrophy - when endurance training is performed concurrently in the same training cycle. The proposed molecular mechanism centres on AMP-activated protein kinase (AMPK), which is activated by endurance exercise and inhibits mTORC1, the master regulator of muscle protein synthesis. Wilson and colleagues' meta-analysis (J Strength Cond Res, 2012) pooled 21 studies and 422 effect sizes: hypertrophy and power were significantly blunted by concurrent endurance, with running producing larger interference than cycling, and longer or more frequent endurance sessions worsening the effect. Strength was less affected. The magnitude is modest, typically around 10-15 percent. Practical mitigations: separate modalities by at least 6-24 hours, cap endurance volume, prefer cycling for lifters, and prioritise the goal-relevant modality first.

Critical power (CP) is a theoretically derived aerobic metabolic ceiling: the highest sustainable power output (or running speed, as critical velocity) below which a finite work capacity W' can be repeatedly reconstituted, and above which W' is depleted to exhaustion. Mathematically, CP and W' are estimated from the hyperbolic relationship between exercise power and time-to-exhaustion across several all-out efforts. CP closely approximates the maximal lactate steady state and the respiratory compensation point, demarcating the heavy-intensity from the severe-intensity exercise domain. CP declines with age and predicts endurance performance and cardiovascular risk; training interventions that shift CP upward increase the sustainable exercise ceiling.

Dual-energy X-ray absorptiometry (DEXA) measures body composition and bone mineral density by directing two X-ray beams at different energy levels through tissue and quantifying differential attenuation; it partitions the body into lean mass, fat mass, and bone mineral content at regional and whole-body levels with high precision and low radiation (~1–5 µSv on modern scanners; up to ~10 µSv on older devices). DEXA-derived appendicular lean mass index (ALMI = lean mass in kg of arms + legs / height in m²) is used in EWGSOP2 sarcopenia criteria and visceral adipose tissue (VAT) estimates are increasingly available on modern scanners. Serial DEXA measurements quantify muscle and fat changes from training, diet, and aging interventions; the method's main limitations include hydration sensitivity for lean-mass estimates and scanner-model variability.

Dynapenia is the age-related loss of muscle strength and power that occurs independently of muscle mass loss. The term was coined by Clark and Manini (2008) to distinguish age-related strength loss from sarcopenia, which historically centred on muscle mass. It reflects neurological decline — fewer motor units, slower firing rates, reduced central drive — rather than just atrophy. Because strength predicts mortality more strongly than mass, dynapenia is now considered a distinct geriatric risk factor; power-focused training is the primary countermeasure.

Eccentric training emphasizes the lengthening phase of a muscle contraction, such as the lowering portion of a squat or curl. Muscles produce greater force eccentrically than concentrically, generating high mechanical tension with relatively low metabolic cost. This makes eccentric work effective for building strength, hypertrophy, and tendon stiffness, and it is widely used in tendinopathy rehabilitation. Older adults tolerate it well, though delayed-onset muscle soreness is common.

EPOC is the elevated oxygen uptake that persists after exercise ends, as the body restores ATP and creatine phosphate, clears lactate, refills oxygen stores, and returns hormones and temperature to baseline. The effect is largest after high-intensity or resistance work and modestly increases total energy expenditure. Although often called the afterburn, EPOC is best understood as a physiological recovery process rather than a primary fat-loss mechanism.

Grip strength is the maximal force generated when squeezing a dynamometer and serves as a low-cost proxy for whole-body muscular function. In the 17-country PURE cohort (Leong et al., Lancet 2015; ~140,000 adults), each 5 kg decrement in grip strength predicted roughly a 16% increase in all-cause mortality, outperforming systolic blood pressure as a mortality predictor. It correlates with neuromuscular health, nutritional status, and recovery capacity, making it one of the most validated biomarkers of biological aging.

Heart rate recovery (HRR) is the drop in heart rate during the first minute (HRR1) or two minutes (HRR2) after stopping peak or symptom-limited exercise testing. Mechanistically it reflects parasympathetic (vagal) reactivation early after exercise, with sympathetic withdrawal contributing later. In the landmark Cleveland Clinic cohort of 2,428 patients undergoing treadmill testing (Cole et al., NEJM 1999), an HRR1 of 12 beats per minute or fewer (with a 2-minute cool-down) was associated in unadjusted analysis with a 4-fold higher all-cause mortality over 6 years (adjusted relative risk 2.0 after accounting for exercise capacity, beta-blockers, and standard cardiovascular risk factors). Normal HRR1 in healthy adults is typically greater than 18 bpm; values under 12 bpm are clinically abnormal. HRR is reproducible, cheap, and a useful adjunct to peak VO2.

HIIT alternates short bouts of near-maximal effort with periods of low-intensity recovery, typically over 10–30 minutes total. The high-intensity intervals stress cardiac output and mitochondrial function, driving rapid gains in VO2max, insulin sensitivity, and stroke volume. Compared with steady-state cardio, HIIT delivers similar or greater cardiorespiratory adaptations in less time, making it a time-efficient longevity intervention when balanced with lower-intensity aerobic work.

Isometric training involves contracting muscles against an immovable resistance without joint movement, as in planks, wall sits, or holding a mid-range squat. It builds tendon stiffness and joint-angle-specific strength while imposing minimal mechanical stress, making it useful in rehabilitation. A 2023 network meta-analysis (Edwards et al., Br J Sports Med) of 270 randomized trials found isometric exercise — particularly wall sits — produced the largest reductions in resting systolic (~8 mmHg) and diastolic (~4 mmHg) blood pressure among studied modalities, including aerobic and dynamic-resistance training.

Lactate threshold is used loosely for two points: LT1 (aerobic threshold, ~2 mmol/L), where blood lactate first rises above baseline, and LT2, the highest intensity sustainable without progressive accumulation. LT2 is often approximated by OBLA, a fixed ~4 mmol/L criterion, or by MLSS, the highest steady-state workload — these correlate but are not identical, and absolute values vary by protocol and individual. Training near these thresholds increases mitochondrial enzymes and lactate clearance, raising sustainable workload.

Maximum heart rate (HRmax) is the highest beats per minute the heart reaches during all-out exertion. It is largely determined by age and genetics, not fitness, and declines with age. HRmax sets training zones for Zone 2 and HIIT. The classic 220 minus age formula is rough; Tanaka (208 − 0.7 × age) outperforms it, especially in older adults, but direct measurement in a maximal test remains the gold standard.

Mitochondrial density refers to the number and volume of mitochondria per unit of muscle tissue. Higher density expands oxidative capacity, allowing more fatty acids and pyruvate to be burned aerobically and improving endurance and metabolic flexibility. Aerobic and Zone 2 training stimulate mitochondrial biogenesis via PGC-1α, while age and inactivity reduce it. Maintaining mitochondrial density is considered central to healthy aging and cardiorespiratory fitness.

Mitochondrial respiratory capacity is the maximal rate of oxygen flux through the electron transport chain (ETC) under substrate-saturating, ADP-saturating conditions, distinct from mitochondrial density which reflects organelle abundance. It is most precisely quantified ex vivo by high-resolution respirometry (HRR) in permeabilized muscle fibers: OXPHOS-coupled state-3 respiration measures ATP-linked flux, while FCCP-uncoupled (ETS) respiration reveals the theoretical ceiling of inner-membrane electron transfer capacity. Key determinants include complex I–IV catalytic activity, inner-membrane surface area, and the availability of electron donors (NADH, FADH₂). Reduced ETS capacity with aging — partly reflecting cristae remodeling and complex I dysfunction — correlates with declines in VO2max, insulin sensitivity, and physical function; aerobic training and caloric restriction upregulate ETS capacity even in older adults.

NEAT is the energy expended during all daily activity outside of structured exercise — walking, standing, fidgeting, household chores, and posture maintenance. It can vary by up to ~2,000 kilocalories per day between individuals of similar body size and often exceeds the contribution of formal workouts to total energy balance. Higher NEAT is associated with lower visceral adiposity, improved metabolic health, and reduced sedentary-time mortality risk, making it a meaningful longevity lever.

The one-repetition maximum (1RM) is the greatest load that can be lifted through a full range of motion for a given exercise in a single maximal effort with proper form, serving as the gold-standard measure of maximal dynamic strength. Percentage-based training zones (e.g., 60–70% 1RM for hypertrophy, ≥85% 1RM for strength) are typically derived from the 1RM. Direct testing carries injury risk in untrained or older individuals; validated prediction equations (e.g., Epley, Brzycki) estimate 1RM from submaximal repetition-to-failure tests, though accuracy decreases above 5–10 reps. Progressive overload is operationalized as periodic 1RM increases over a training cycle; declining 1RM with age reflects both sarcopenia and dynapenia.

Plyometrics are explosive movements — jumps, hops, bounds, throws — that exploit the stretch-shortening cycle, in which a rapid eccentric load primes a powerful concentric contraction. They train rate of force development, neuromuscular coordination, and tendon elasticity. In ageing populations, low-volume jump training improves bone mineral density, balance, and reactive strength, addressing the power deficit that drives falls. Progression and surface choice matter to manage joint load.

Progressive overload is the principle of gradually increasing training demands — load, volume, density, range of motion, or proximity to failure — to keep driving adaptation. Without it, the body settles into a maintenance state and gains plateau. The progression must be small enough to be tolerated and large enough to be meaningful. It is the central mechanism behind sustained gains in strength, hypertrophy, and bone density across a training career.

Rate of force development (RFD) is the change in muscle force per unit time (N/s), quantifying how rapidly maximal force can be expressed — a key component of muscular power distinct from peak force alone. Early-phase RFD (0–50 ms) reflects neural drive, motor unit synchronization, and Type II fiber recruitment; late-phase RFD (100–200 ms) is more influenced by muscle cross-sectional area and fiber composition. RFD declines with age more rapidly than maximal strength and is closely linked to fall-prevention capacity, functional power, and reactive balance, because most daily protective movements (catching a stumble, rising from a chair) occur within 100–200 ms. Power-focused and plyometric training preferentially improve RFD.

Resting heart rate (RHR) is the number of heartbeats per minute at full rest, ideally measured supine after several minutes of quiet rest or upon waking, and is influenced by caffeine, illness, medications, and sleep. Trained individuals typically have lower RHR through a combination of intrinsic sinoatrial node remodeling (notably downregulation of the HCN4 'funny' channel, which persists after autonomic blockade) and elevated vagal/parasympathetic tone, with increased stroke volume as a parallel cardiac adaptation. Epidemiological data (e.g., Aune 2017) link elevated RHR with higher cardiovascular and all-cause mortality, making it a simple biomarker of cardiorespiratory health and recovery.

Reps in Reserve (RIR) is an autoregulation method for prescribing and grading resistance-training intensity by asking the lifter to estimate how many additional repetitions could be performed at the end of a set before reaching momentary muscular failure. An RIR of 0 corresponds to a maximal effort (RPE 10 on a 0-10 scale), RIR 1 means one rep short of failure, and RIR 3 means three reps in reserve. Validated by Zourdos and colleagues (J Strength Cond Res, 2016), the RIR-based RPE scale correlates strongly with bar-velocity and with percentage of 1-rep maximum (1RM) in both experienced and novice lifters, with stronger correlations at higher intensities. Compared with rigid %1RM prescriptions, RIR adapts to daily readiness fluctuations and is therefore favoured in modern hypertrophy and powerlifting programming.

Sarcopenia is the age-related loss of skeletal muscle mass, strength, and function; contributing factors include anabolic resistance, neuromuscular changes, chronic inflammation, and inactivity. Under the EWGSOP2 (2019) consensus, low muscle strength (grip strength or chair-stand) is the primary criterion for probable sarcopenia, confirmed by low muscle quantity or quality (DXA, BIA, CT/MRI), with poor physical performance defining severity. Since October 2016 it has its own ICD-10-CM code (M62.84), recognising it as an independent clinical condition.

Sarcopenic obesity is the concurrent presence of low skeletal muscle mass or function (sarcopenia) and excess adiposity. The combination is more adverse than either condition alone: excess fat amplifies systemic inflammation and lipotoxicity while reduced muscle mass impairs glucose uptake and energy expenditure, creating a self-reinforcing cycle. Risk for cardiometabolic disease, physical disability, and mortality is higher in sarcopenic-obese individuals than in those with only one condition, though exact thresholds vary across diagnostic frameworks. Resistance training combined with sufficient dietary protein (often ≥1.2 g/kg/day) is considered the primary intervention, targeting both muscle preservation and metabolic health.

The sit-rise test measures the ability to lower oneself to the floor and stand back up using as little support as possible, scored from zero to ten with points deducted for hand, knee, or balance assistance. It captures lower-body strength, flexibility, balance, and body composition in a single movement. In Brito and Araújo's cohort of 2,002 adults aged 51–80 (Eur J Prev Cardiol, 2014), low scores (0–3) carried roughly 5-fold higher all-cause mortality than high scores (8–10) (HR 5.44, 95% CI 3.1–9.5), with each additional point linked to ~21% better survival over a median follow-up of 6.3 years.

Strength training is structured exercise that loads muscles against resistance — free weights, machines, bands, or bodyweight — to drive neural adaptation and muscle protein synthesis. Beyond building muscle and bone, it improves insulin sensitivity, mitochondrial function, and metabolic health. In longevity research, regular resistance training is consistently linked to lower all-cause mortality, preserved independence in later life, and reduced risk of frailty and falls.

Tendon stiffness is the mechanical property describing how much force a tendon transmits per unit of elongation (ΔForce/ΔLength, typically reported in N/mm); the related material property Young's modulus normalises stiffness for cross-sectional area and resting length. In vivo, stiffness is measured by combining ultrasound imaging of the tendon under isometric muscle contraction with dynamometry. The systematic review and meta-analysis by Bohm, Mersmann and Arampatzis (Sports Medicine - Open, 2015) found that high-magnitude loading (around 80–90 % of maximal voluntary contraction) and longer durations (around 3 seconds per repetition) - as in heavy-slow or isometric protocols - produce the largest gains in stiffness, modulus, and cross-sectional area. Aging, disuse, and tendinopathy reduce stiffness; well-trained tendons store and return elastic energy more efficiently, improving running economy and reducing injury risk.

Skeletal muscle fibers are broadly classified into Type I (slow-oxidative) and Type II (fast-glycolytic and fast-oxidative-glycolytic) on the basis of myosin heavy chain isoform expression, metabolic profile, and contractile speed. Type I fibers are fatigue-resistant, mitochondria-dense, and reliant on oxidative metabolism; they dominate endurance activity and Zone 2 training stimulus. Type II fibers — subdivided into IIa (intermediate) and IIx (fast-glycolytic; humans lack the rodent IIb fiber type) — generate higher force and power but fatigue more rapidly and are preferentially recruited during heavy resistance exercise and sprinting. With aging, Type II fibers show selective atrophy and denervation before Type I, contributing to dynapenia and fall risk; resistance and power training selectively preserve and hypertrophy these fast fibers.

Visceral adipose tissue (VAT) is the metabolically active fat depot surrounding the intra-abdominal organs, distinct from subcutaneous adipose tissue. VAT adipocytes drain into the portal circulation and secrete pro-inflammatory adipokines including TNF-α, IL-6, and resistin while producing less adiponectin than subcutaneous fat, creating a systemic inflammatory and insulin-resistant milieu. High VAT is associated with increased risk of type 2 diabetes, cardiovascular disease, non-alcoholic fatty liver disease, and all-cause mortality independently of total body fat or BMI. Gold-standard quantification uses abdominal CT or MRI; DEXA and waist circumference are practical surrogates. Aerobic exercise and weight loss preferentially reduce VAT relative to subcutaneous depots.

VO2max is the maximum rate of oxygen consumption during intense exercise, typically expressed in mL/kg/min. Per the Fick principle, it reflects oxygen delivery (cardiac output, hemoglobin) multiplied by muscle extraction at the mitochondria. VO2max is among the strongest predictors of all-cause mortality: higher VO2max is robustly associated with lower long-term risk across cohort studies (e.g., Mandsager 2018), making it a central marker of cardiorespiratory fitness in longevity research.

Zone 2 training is sustained aerobic exercise at or just below the first lactate threshold (LT1, ~1.5–2.0 mmol/L), often roughly 60–70% of max heart rate, though the precise percentage varies; lactate testing or the talk-test is more accurate. At this intensity, fat oxidation supplies most energy in slow-twitch fibers, with rising carbohydrate use near the upper end. Regular Zone 2 work increases mitochondrial density, capillarization, and metabolic flexibility.