Four Capacities, Four Different Decline Curves
Most popular training apps optimize for one thing: getting you to lift more weight over time. That is a legitimate goal, and progressive overload is well-supported training science. But strength is only one of at least four physical capacities that determine how well your body functions as you age, and it is not the one with the steepest decline curve. The other three — stability, aerobic efficiency, and peak aerobic output — decline on their own separate timelines, often earlier than most people expect, and a program that only tracks barbell numbers has no way to see any of them.
Each of these four capacities responds to a different training stimulus. Training one does not train the others as a byproduct. That is the core problem with strength-only programming: it treats one input as a stand-in for overall physical capability, when the research on age-related decline says the four capacities move independently, on independent schedules.
Stability Declines Quietly, and Most Apps Never Measure It
Postural control — the ability to hold balance during static stance and dynamic movement — degrades gradually starting in the fourth decade of life, well before it shows up in daily activities. A cross-sectional study of nearly 8,000 adults found measurable declines in postural balance beginning around age 30, with the rate accelerating after age 60 (Era et al., 2006, Gerontology). The downstream consequence is fall risk: the CDC reports that roughly one in four adults over 65 falls each year, and falls are the leading cause of both fatal and nonfatal injury in that age group (CDC, 2024, Older Adult Fall Prevention).
A strength-only app has no reason to measure any of this. A bilateral barbell squat or a machine leg press trains force production without meaningfully challenging balance, because the base of support is fixed. Stability has to be trained on purpose — typically through single-leg work, loaded carries, and movement performed on a less stable base, a category resistance-training research treats as a distinct stimulus from straight-line strength work (Behm & Anderson, 2006, Journal of Strength and Conditioning Research).
Strength Declines Too, Just More Slowly Than People Assume
Strength is the capacity strength-only apps do track, which makes its own decline curve worth stating precisely rather than assuming it is handled. Muscle strength holds relatively flat through the 30s and 40s, then declines at an accelerating rate afterward: quantitative reviews of the aging literature estimate a loss of roughly 12 to 15 percent of strength per decade after age 50, with the rate roughly doubling again after age 60 (Mitchell et al., 2012, Frontiers in Physiology).
The good news is that this is the one capacity resistance-training apps are built to address, and they generally address it well. The problem is not that these apps train strength badly. It is that strength is one-quarter of the picture they are built for, and marketing built around “get stronger” implies a completeness the research does not support.
Aerobic Efficiency Erodes Before You Notice You’re Winded
Aerobic efficiency — how much oxygen your body needs to sustain a given submaximal pace — is a function of mitochondrial density, capillary density in muscle tissue, and how readily your body draws on fat as fuel at moderate intensities. All three decline with age independent of strength, driven substantially by reduced mitochondrial content and function in skeletal muscle (Betik & Hepple, 2008, Applied Physiology, Nutrition, and Metabolism).
This decline is easy to miss, because it does not produce a number on a strength-tracking app and it rarely announces itself as a dramatic symptom. It shows up as needing to work harder to sustain the same walking pace, the same flight of stairs, or the same physically demanding afternoon, years before anyone would describe themselves as out of shape. A program built entirely around load and reps has no field for any of this.
Peak Aerobic Output Has the Steepest Curve of the Four
VO2 max — peak aerobic output, the maximum rate at which your body can consume and use oxygen — has one of the steepest age-related decline curves of any measurable physical capacity. Longitudinal data from the Baltimore Longitudinal Study of Aging found VO2 max declining by roughly 3 to 6 percent per decade before age 30, accelerating to a decline of more than 20 percent per decade after age 70, with sedentary adults declining faster than trained adults at every age (Fleg et al., 2005, Circulation).
Peak aerobic output and aerobic efficiency are related but distinct: efficiency is about the cost of submaximal work, peak output is about the ceiling of the whole system, driven by maximum heart rate, stroke volume, and how well working muscle extracts oxygen from blood. Both decline. Neither is trained by a program built entirely around sets, reps, and load.
The Strength-Only Category: What These Apps Actually Optimize For
Popular strength-only training apps are a useful example of the category, not because they are poorly built, but because they are built around a single, clearly stated goal: generate and adjust resistance-training workouts based on your logged lifts, available equipment, and recovery. That is a well-scoped product decision, and the better apps in this category execute it competently. It is also, by design, programming for one of the four capacities. Endurance work, mobility, and stability are not structural components of what the app generates each week; if you want them, you add them yourself, on top of the app, with no coordination between the two.
That gap is invisible inside the app itself, because an app only reports on what it tracks. A strength-only app will show a completed workout streak and a rising estimated one-rep max, and nothing in that interface will flag that your aerobic efficiency, peak aerobic output, or balance have gone untouched for months. The absence does not surface as an error message. It surfaces later, as the capacities described above, quietly declining on their own separate schedules.
What Multi-Modal Training Actually Requires, and What Margin Does
The exercise science guidance on this is not ambiguous. The American College of Sports Medicine’s position on exercise prescription for healthy adults calls for training cardiorespiratory, musculoskeletal, and neuromotor (balance and stability) fitness as separate, deliberate components, not as side effects of a single training mode (Garber et al., 2011, Medicine & Science in Sports & Exercise). Training one capacity does not train the others as a byproduct; each needs its own stimulus, on its own schedule.
Here is where general research ends and Margin’s actual programming begins. Margin runs a shared six-week program with three strength sessions and two conditioning sessions per week. The three strength sessions cover push, pull, hinge, carry, and squat patterns, plus lunges and jumps distributed across the week — the carry, single-leg, and jump variants are what carry the stability demand, not a separate balance session bolted on top. Of the two weekly conditioning sessions, one is bike-or-run intervals (chosen per session, not a fixed modality), and the other is an athleticism and mobility session with a dedicated mobility block; the program also expects some additional aerobic work of your own choosing each week. That combination is what addresses both the efficiency end of the aerobic spectrum and its ceiling, instead of requiring a fifth training mode you have to plan yourself. The four-capacity framework above is the general research this is built on, not a claim specific to Margin, and every program is validated before you see it.
A Proportionate Next Step
You do not need to abandon a strength app that is working for you to close this gap. You need an honest look at whether stability and aerobic work have a defined place in your week, or whether they have quietly become the thing you mean to get to eventually. If your current program has a clear answer for how it trains all four capacities, that is a good program. If it only has an answer for one of them, the gap is measurable, not hypothetical, and it does not close on its own. Margin’s six-week cycles are one structured way to close it, built the way the section above describes. Margin is currently in private testing — get on the waitlist to hear when that changes.