Time Under Tension (TUT)
Also known as: TUT, Time-Under-Load, Set duration, Tempo volume
The total number of seconds a muscle is loaded during a set — the sum of the eccentric, concentric, and any pause phases across every rep. TUT is the duration component of training volume, complementary to reps and load. Two sets with identical reps and weight can have very different TUT depending on how fast you lift and how long you pause, and that difference is where most of the hypertrophy mechanism arguments live.
Formula
TUT (seconds per set) = reps × (eccentric_s + bottom_pause_s + concentric_s + top_pause_s)
Example with 3-1-2-0 tempo over 8 reps: 8 × (3 + 1 + 2 + 0) = 48 seconds of TUT per set.
Weekly TUT per muscle group = sum of per-set TUT across all working sets in the week. We don't currently aggregate this, but it's the natural extension of weekly volume tracking.Example
Two athletes both squat 100 kg × 5 reps. Athlete A uses an explosive 1-0-1-0 tempo: TUT = 5 × 2 = 10 seconds. Athlete B uses a controlled 3-1-2-0 tempo: TUT = 5 × 6 = 30 seconds. Same load, same reps, 3x the TUT for athlete B. Athlete B accumulates substantially more mechanical work and metabolic stress per set — and will likely need to drop the load 5-10% to complete the prescribed reps cleanly.
How Afitpilot Uses This
Afitpilot doesn't currently capture per-rep tempo or compute TUT directly — that's on the roadmap as a future surface. Tempo prescriptions (e.g. "3-0-1-0 tempo") flow through to the session card and the athlete is responsible for executing them. The plan generator implicitly uses TUT thinking when it picks tempo prescriptions: hypertrophy blocks favour slow eccentrics and full concentrics (high TUT per rep) at moderate loads; strength peaking blocks favour fast concentrics with minimal pause (low TUT, higher loads). Future surface: explicit per-exercise TUT capture so weekly TUT per muscle group can feed volume calculations alongside hard-set counts.
TUT ranges by training goal
| Who / Context | Value | Note |
|---|---|---|
| Max strength (1-5 reps) | ~5-20 seconds per set | Fast concentric, minimal pause — the neural stimulus dominates |
| Hypertrophy (6-12 reps) | ~30-60 seconds per set | The classic 'time-under-tension sweet spot' in bodybuilding lore |
| Muscular endurance (15-20+ reps) | ~60-90+ seconds per set | Metabolic stress and lactate accumulation dominate the adaptation |
| Slow-tempo hypertrophy protocols | 5-second eccentrics, 40-60s TUT at moderate load | Popular in coaching practice; research support is mixed at matched volume |
| Isometric hold contribution | Adds to TUT but produces different stimulus | Pause-squat or paused-bench seconds count, but aren't equivalent to lifting seconds |
| Field measurement accuracy | Self-reported TUT typically ±20% of actual | Why we treat tempo as a coaching cue, not a tracked metric |
Known Limitations
- •TUT in the field is approximate. Without a metronome or video review, prescribed tempos drift toward the athlete's natural rhythm within 2-3 sets. Self-reported TUT figures are at best ±20% of the actual lifting duration.
- •TUT alone is not a useful programming target. A 60-second set of light air squats is not equivalent to a 30-second set of heavy back squats. Load × TUT × proximity to failure together produce the stimulus; any one of them in isolation is misleading.
- •The hypertrophy literature is split on whether TUT independently drives growth or whether it's a downstream proxy for set quality. Schoenfeld and colleagues (2015) found no hypertrophic difference between 2-second and 8-second per-rep durations at matched load and proximity to failure — suggesting TUT matters less than the popular tempo-focused programming culture assumes.
- •Optimal TUT differs by training quality. Hypertrophy work is commonly programmed at 30-60 seconds per set; max-strength work lives at 5-20 seconds per set; muscular endurance work at 60-90+ seconds. The same TUT figure means different things in different contexts.
- •Pause phases (bottom or top) count toward TUT but produce a different stimulus than active lifting — isometric tension under load is more taxing on connective tissue but less metabolically demanding than continuous movement. The single TUT number papers over that distinction.
Science Context
The TUT concept entered strength training via bodybuilding coaching (Charles Poliquin formalised the four-digit tempo notation in the 1990s) and was carried into research as a candidate hypertrophy mechanism. The strongest empirical test (Schoenfeld, Ogborn & Krieger 2015 meta-analysis) found no statistically significant hypertrophy difference between fast (0.5-4 s/rep) and slow (4-8 s/rep) per-rep durations once load and proximity to failure were matched. Very slow tempos (>10 s/rep) appear to underperform because they force load reductions that erode the mechanical-tension stimulus. The practical translation: tempo prescriptions are useful for grooving technique and ensuring sets reach a meaningful proximity to failure, but TUT itself is not a programming variable that needs to be optimised — pick a tempo that lets the athlete execute the prescribed reps with control, and let volume × load × proximity to failure drive the adaptation.