100 crunches a day, every day, with money on the line.

The 100-crunches-a-day target is one of the most-searched bodyweight goals on the internet, and one of the most consistently abandoned. The reasons are partly cultural and partly biomechanical, and worth separating before any contract gets written. Colloquially, "situps" and "crunches" are used interchangeably, but they describe different movements. A full situp drives the torso from supine to fully upright through repeated lumbar flexion against the load of the upper body. A crunch raises the shoulder blades and upper back off the floor while the lower back stays in contact with the ground — a much shorter range of motion focused on rectus abdominis contraction without grinding the lumbar spine. Stuart McGill, PhD, professor emeritus of spine biomechanics at the University of Waterloo and author of "Back Mechanic" (2015) and "Ultimate Back Fitness and Performance," has spent four decades documenting why high-volume full situps under repeated lumbar flexion accumulate disc stress in ways most novice trainees underestimate. McGill's framework is precisely why Lockin's pose-detection model is calibrated for crunches, not full situps — a controlled, short-range curl-up loads the abdominals without the cumulative spinal cost. That distinction does not settle the volume question. Brad Schoenfeld's 2016 and 2017 meta-analyses on training volume and frequency converge on a finding that applies to abdominal work as cleanly as to any other muscle group: distributing volume across multiple sessions per day or per week produces adaptation at least as efficiently as concentrating it, often more so for novice and intermediate trainees. Pavel Tsatsouline, in "The Naked Warrior" (Dragon Door, 2003), formalized this as grease-the-groove — frequent, submaximal practice that reinforces the motor pattern without ever inducing crippling fatigue. Ten sets of ten crunches across waking hours leave no single set hard enough to wreck form. Greg Lehman, DC, MSc, the Canadian biomechanist who has written extensively on spinal health for general populations, offers a useful counterweight to McGill's caution: in non-clinical, asymptomatic individuals, controlled spinal flexion is not the catastrophe internet fitness culture sometimes paints it as. The honest synthesis: crunches in moderate daily volume, with attention to form, are a defensible practice for most people. The 100-rep target is not the problem. The behavioral failure mode is the one people actually face. Eric Helms, PhD, in the MASS research review, has repeated the line that visible abs are roughly 80% kitchen — body-fat percentage, not crunch volume, controls whether the rectus abdominis shows. That research-honest framing is also a motivational poison: if abs are made in the kitchen, why grind out 100 reps? The answer is that the daily commitment is doing different work than aesthetics — it is building a baseline of trunk endurance, a daily contact point with discipline, and a habit-stack anchor for harder training to attach to. None of which the present-tense brain finds compelling at 10pm. Kahneman and Tversky's prospect theory (Econometrica, 1979) identifies why a small stake closes the gap: loss aversion registers a potential loss at roughly twice the psychological weight of an equivalent gain. A $5 stake on tonight's last set of 20 is not $5 of motivation — it functions closer to $10 of aversion pressure at the exact moment the rationalization would otherwise win.