The Edit · Founder Insights
Most people walk into a gym chasing a number on the scale. Some chase a number on the barbell. Very few walk in asking the question that actually matters: how well will I age? Catalyst's entire assessment protocol is built around that question.
Four numbers predict how well you will age more reliably than your weight, your BMI, or your blood pressure. Skeletal muscle index, waist-to-height ratio, grip strength, and heart rate recovery. Each is backed by published cohort data showing it predicts mortality, frailty, and loss of independence — and each can be measured in under 60 minutes with equipment that fits on a desk.
TL;DR
- Skeletal muscle index (SMI) tells you how much functional muscle you carry. Below 7.0 kg/m² in men or 5.7 kg/m² in women meets the AWGS sarcopenia threshold.
- Waist-to-height ratio above 0.5 flags visceral adiposity — the metabolically dangerous fat — independent of total body weight.
- Grip strength predicts all-cause mortality more strongly than systolic blood pressure, with a 16% increase in mortality risk for every 5 kg drop.
- Heart rate recovery below 12 beats in the first minute after peak exercise is a clinical red flag — a 1999 NEJM study found a 4× higher mortality risk over six years.
- All four numbers can be measured in under 60 minutes. None require a hospital. All four respond to structured training.
1. Skeletal Muscle Index (SMI), how much functional muscle you carry
Skeletal muscle does more than move you around. It regulates blood glucose, anchors metabolic health, supports bone density, and is the largest endocrine organ in the body. Loss of muscle is the engine of frailty, the predictor of falls, and one of the strongest signals of approaching mortality.
The problem with using body weight or BMI as your metric is that they don't distinguish between an 80 kg person who is mostly muscle and an 80 kg person who is mostly fat. The two physiologies age very differently. Skeletal Muscle Index (SMI) corrects for this by measuring appendicular muscle mass — the lean tissue in arms and legs — divided by height squared.
The Asian Working Group for Sarcopenia 2019 consensus sets the diagnostic thresholds: below 7.0 kg/m² in men, below 5.4 kg/m² (DXA) or 5.7 kg/m² (BIA) in women is the threshold for sarcopenia. Why appendicular and not total muscle? Because leg and arm muscle is what carries you up stairs, lifts your groceries, and catches you when you trip. Trunk muscle stays relatively preserved with age while limb muscle degrades faster — appendicular SMI is the more sensitive signal.
Sarcopenia affects 32% of Singaporeans over 60, and it more than doubles mortality risk in adults over 80 in care settings. The good news: SMI responds to structured resistance training within 16 weeks, even in adults in their 80s. The intervention is unambiguous and well-evidenced.
2. Waist-to-Height Ratio, where your fat actually sits
Not all body fat carries the same risk. Subcutaneous fat — the fat directly under the skin, often where you can pinch it — is metabolically benign in moderate amounts. Visceral fat — the fat that wraps around the organs in the abdominal cavity — is the metabolically dangerous version. Visceral fat drives chronic inflammation, insulin resistance, fatty liver disease, and the cardiovascular and metabolic disease that defines unhealthy ageing.
BMI doesn't see visceral fat. A person can have a normal BMI and a high visceral fat load — a phenotype common enough in Singapore that it has its own name: "thin outside, fat inside". Waist-to-height ratio is a simple proxy that catches this. Measure your waist circumference at the navel; divide by your standing height. A ratio above 0.5 is the threshold most commonly used as a flag for visceral adiposity, validated across multiple large cohort studies and meta-analyses showing it outperforms BMI for cardiometabolic risk prediction.
Two things drive visceral fat reduction: caloric balance and resistance training. Cardio helps, but cardio without strength training tends to lose lean mass alongside fat — which compromises the SMI number. The combination of progressive resistance training, Zone 2 cardiovascular work, and a modest energy deficit (when fat loss is the goal) is the canonical prescription.
3. Grip Strength, the cheapest mortality test in medicine
Grip strength is the most under-rated number on this list because it sounds trivial. It is not. Grip strength correlates strongly with whole-body strength, requires only a handheld dynamometer, takes 30 seconds to measure, and predicts all-cause mortality more strongly than systolic blood pressure across multiple large cohort studies.
The PURE study followed 140,000 adults across 17 countries and found that every 5 kg drop in handgrip strength carried a 16% increase in all-cause mortality risk over four years. The signal held after controlling for age, sex, education, employment status, and physical activity. Grip strength outperformed systolic blood pressure as a mortality predictor in the same cohort.
AWGS 2019 thresholds: below 28 kg in men, below 18 kg in women is the diagnostic floor for sarcopenia. The Catalyst Healthspan Assessment uses a Camry digital dynamometer that takes the test from 30 seconds to about a minute including familiarisation. I've written about grip strength as a healthspan predictor in more depth elsewhere — the short version is that it's the closest thing in clinical medicine to a free mortality scan.
The four numbers measure four different things, but they all answer the same question: how robust is your body to the next two decades of normal life?
4. Heart Rate Recovery, what your cardiovascular system is hiding
Heart rate recovery (HRR) measures how quickly your heart rate drops in the first minute after peak exercise. It's an indirect window into autonomic function — specifically the parasympathetic ("rest and digest") nervous system's ability to override the sympathetic ("fight or flight") response after stress. A vagally well-toned cardiovascular system drops the heart rate fast; a poorly conditioned one doesn't.
The clinical relevance of this number was established in 1999, when Cole et al. published a landmark study in the New England Journal of Medicine. They tracked over 2,400 adults undergoing exercise treadmill tests and found that an HRR below 12 bpm in the first minute after peak exercise was associated with a 4-fold higher risk of mortality over six years, independent of other cardiovascular risk factors.
Smartwatches don't measure HRR well. As I've written about before, the Apple Watch and most fitness trackers report resting heart rate (a different and weaker metric) and average heart rate during exercise — not the deceleration response. To get HRR you need a chest-strap monitor or a treadmill test, taking your peak heart rate during exercise and timing the decay over the next 60 seconds.
HRR responds to training. Zone 2 cardiovascular work, in particular — sustained 60-90 minute sessions at conversational pace — strengthens parasympathetic tone over a 6-12 week training block. The improvement is measurable within weeks.
How to measure these and what to do
All four numbers can be captured in under 60 minutes by a properly trained clinician or assessment service. The equipment is modest: a bioimpedance or DXA scanner for SMI, a tape measure for waist-to-height, a handheld dynamometer for grip strength, and a treadmill or bike with chest-strap monitoring for HRR.
The Catalyst Healthspan Assessment measures all four in one 60-minute session, calibrated against AWGS 2019 thresholds and published cardiovascular norms. The output is a Healthspan Score on a 0-10 scale plus per-pillar bars showing where the gaps are. We re-measure at the 16-week Checkpoint to confirm the prescription is working.
What changes the numbers, in priority order: structured resistance training (raises SMI, raises grip strength, supports body composition); Zone 2 cardiovascular work (raises HRR, raises VO2 max); a modest energy deficit when fat loss is required (lowers waist-to-height ratio); and adequate protein intake distributed across the day (supports the muscle-building response). Total time commitment for the prescription: under three hours per week of dedicated training.
If you've never had any of these measured, you are flying blind. The first measurement is the most valuable one because it converts an abstract concern about ageing into four specific numbers you can move.
Frequently asked questions
Q. What is the most important number for healthspan?
If you had to pick one, grip strength — because it's the closest single proxy for whole-body strength and the strongest single predictor of all-cause mortality among the four. SMI is a close second because it's the most directly modifiable through training. The four numbers together give a more complete picture than any single one alone.
Q. Can I measure these at home?
Partially. Waist-to-height ratio needs only a tape measure. Grip strength needs a handheld dynamometer (around SGD 100-200 for a clinical-grade model). SMI requires a bioimpedance scanner — most home scales claim to measure body fat but their muscle-mass figures are not reliable enough for SMI tracking. HRR requires either a chest-strap monitor or a clinical treadmill setup. The full battery is best done once at a qualified clinic or studio, then repeated every 4-6 months.
Q. How often should I measure these?
Twice a year is the sensible cadence. Often enough to catch a deteriorating trend before it becomes clinically meaningful, infrequent enough to avoid noise. Catalyst's 16-week Checkpoint cadence works for most adults; clinical populations or post-injury cases may benefit from more frequent measurement.
Q. What if my numbers are bad?
The trial evidence on each of the four pillars shows they all respond to training within 12-24 weeks. Strength gains begin within 4 weeks. SMI gains become measurable around 12-16 weeks. HRR improvements show up within 6-12 weeks of consistent Zone 2 work. The earlier you start, the easier the reversal — but the intervention works at every age, including in adults in their 80s.
Q. Is BMI useless?
BMI is a population screening tool, not a clinical-grade individual metric. It's useful for epidemiology and rough triage but it cannot distinguish between muscle and fat, and it cannot see visceral fat. For individual healthspan tracking, the four numbers above are far more informative. BMI as a single data point is worth knowing but should never be the only one you check.
Citations
Chen LK, Woo J, Assantachai P, et al. (2020). Asian Working Group for Sarcopenia: 2019 Consensus Update on Sarcopenia Diagnosis and Treatment. Journal of the American Medical Directors Association, 21(3), 300–307. jamda.com
Leong DP, Teo KK, Rangarajan S, et al. (2015). Prognostic value of grip strength: findings from the Prospective Urban Rural Epidemiology (PURE) study. The Lancet, 386(9990), 266–273. pubmed.ncbi.nlm.nih.gov
Cole CR, Blackstone EH, Pashkow FJ, Snader CE, Lauer MS. (1999). Heart-rate recovery immediately after exercise as a predictor of mortality. New England Journal of Medicine, 341(18), 1351–1357. nejm.org
Ashwell M, Gunn P, Gibson S. (2012). Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obesity Reviews, 13(3), 275–286. pubmed.ncbi.nlm.nih.gov
Mandsager K, Harb S, Cremer P, et al. (2018). Association of Cardiorespiratory Fitness With Long-term Mortality. JAMA Network Open, 1(6), e183605. pubmed.ncbi.nlm.nih.gov
