1. Physiological Mechanisms of Hypertrophy
To understand why the "Sawtooth" model works, we must first dissect the biological imperatives of muscle growth. Hypertrophy is not merely a side effect of lifting weights; it is a specific adaptation to mechanical and metabolic stimuli.
Mechanical Tension: The Primary Driver
Current consensus identifies mechanical tension as the primary driver of hypertrophy[6]. Tension is sensed by mechanoreceptors within the muscle, triggering signaling pathways that result in protein synthesis.
Crucially, the "effective reps" theory suggests that only repetitions performed under high motor unit recruitment contribute significantly to growth. In a set of 12 reps near failure, the first 6-8 reps serve to accumulate fatigue, while the final 4 reps provide the requisite tension.
Metabolic Stress
While tension is paramount, metabolic stress (the "burn" from lactate and H+ ion accumulation) plays a synergistic role. Associated with higher rep ranges, it can amplify anabolic signaling[7], though it is secondary to tension.
The Conflict: To generate new tension (Intensity), you must increase the load. However, increasing load inherently reduces the volume you can perform. You cannot maximize both simultaneously.
2. The Sawtooth Phenomenon
The "Sawtooth" is the graphical representation of managing this trade-off. It is not a straight line, but a serrated edge of "Ramps" and "Cliffs".
The Geometry of Progression
- The Ramp (Ascent): You keep weight static and add reps (e.g., 8 → 9 → 10 → 11 → 12). Volume increases; intensity remains constant.
- The Cliff (Drop): Upon hitting the target, you increase weight. Physically, you cannot perform 12 reps at the new load. You drop back to 8. Volume plummets.
The "Runway" Concept
This "dip" is not regression; it is a strategic retreat to build a "runway" for future gains[10].
- Resets Proximity to Failure: The first session at the new weight allows acclimation without grinding to absolute failure.
- Consolidates Technique: Heavier weights demand higher stability. Lower reps allow focus on perfect execution.
- Psychological Momentum: You are not forced to hit a PR every session. You have a new, achievable baseline.
The Math Behind the Dip
Why do reps drop? The 1RM Continuum dictates it. A 5% increase in load typically results in a loss of ~2 repetitions[11].
Example: If you bench 200 lbs for 12 reps (approx. 70% 1RM) and increase to 210 lbs (approx. 74% 1RM), your max reps naturally fall to ~10. This is biological reality, not weakness.
3. Double Progression: The Engine
Double Progression (DP), coined by Alan Calvert in 1911[3], allows you to focus on one variable at a time.
Standard vs. Dynamic
Standard DP: Keep weight the same until all sets hit the top rep target (e.g., 3x12). This can be slow, as fatigue often prevents hitting the target on later sets.
Dynamic Double Progression (DDP): Treat every set individually. If you hit the top range on Set 1, increase weight for Set 1 next time, even if Set 2 and 3 are still progressing[16]. This maximizes freshness and overload.
4. Practical Weight Progression
The general guideline is the 2-for-2 Rule: if you can perform 2 extra reps over your goal in the last set for two workouts, add weight[20].
The Microloading Solution
For upper body movements, a standard 5lb jump can be a massive intensity spike (10-25% for small muscles like side delts). This leads to immediate stalling[23].
Solution: Use fractional plates (0.25 lb - 1 lb). A 1lb jump on a lateral raise keeps the intensity increase manageable (~5%), keeping you in the optimal hypertrophy zone[4].
5. Integrating Autoregulation (RPE)
Not all sets of 12 are created equal. To ensure high-quality volume, use RPE (Rate of Perceived Exertion):
- RPE 9-10: 1-0 Reps in Reserve. Target this for the top of your rep range before jumping weight.
- RPE 7-8: 2-3 Reps in Reserve. Target this for the bottom of the range after a weight jump.
This ensures "Junk Volume" is minimized and every set contributes to the overload stimulus[28].