What your force curve is telling you about your stroke
Split and rate tell you how fast you're going. The force curve tells you why.
Split tells you how fast. Rate tells you how often. Neither tells you how well you're applying power during the drive. The force curve does.
Every stroke on a PM5 erg produces a force curve: a graph of how much force the handle sees across the drive, from catch to finish. Most rowers have never looked at theirs. I ignored mine for two years. It's the single most informative piece of data the erg captures — and it sits there unused.
What a force curve shows
The curve plots force against drive position. A stroke lasts roughly 0.8 to 1.2 seconds. In that window, force rises from zero at the catch, hits a peak somewhere in the first third of the drive, sustains through the body swing, and drops back to zero at the finish.
The shape tells you how efficiently you're converting effort into boat speed. Two rowers pulling the same split can have radically different curves. One has a smooth, sustained arc. The other has a spike and drop-off, or a double hump with a valley in the middle. Same split. Very different efficiency. Very different ceiling for improvement.
From catch to finish
Catch engagement. How quickly force rises after you reverse direction is one of the strongest technique markers in the data. A sharp rise means the legs connect immediately. A slow, gradual rise means you're compressing into the catch without loading. I spent months with a slow catch before I saw it in my own data. It felt normal because I'd never had a reference point.
Where the peak lands. Peak force should fall in the first third of the drive, around 30-35% of the way through. That means the legs are doing the heavy lifting. They're the biggest muscle group. When peak force lands past 45%, you're relying on your back and arms to generate power, which works at low intensity but falls apart in a race.
How long you hold it. A good drive sustains force above 80% of peak for a meaningful portion of the stroke. A bad one spikes and drops. Think of it as the difference between pushing a car and punching a wall. The erg rewards sustained force, not impulse.
The finish. Force should decrease smoothly at the end of the drive as the handle accelerates into the body. A sharp cliff in the last 20% of the curve means you're letting go early, losing centimeters of effective drive length. That's free speed you're leaving on the erg.
The double hump
This gets its own section because it's the most common technique flaw I see in erg data, and most rowers who have it don't know.
The curve shows two peaks with a dip in the middle. The first peak is the legs. The dip is where the legs finish pushing and the back hasn't engaged yet. The second peak is the back swing. The gap between them means force drops mid-stroke, exactly when you should be maintaining it.
In a connected stroke, there's no dip. The back starts opening while the legs are still pushing. Force stays high through the handoff. The cue that fixes this for most rowers: "Back opens as legs push."
How technique breaks under fatigue
Your force curve at stroke 10 looks different from your curve at stroke 450. That's expected. What matters is the pattern of degradation, because it tells you where your conditioning has gaps.
Three things happen reliably as rowers fatigue.
The catch gets sluggish. Early in a 2k, leg drive is sharp and immediate. By the third 500, rise rate drops. The legs are still pushing, but the initial snap is gone. Force builds gradually instead of explosively. This is often the first sign of fatigue, and it's invisible in split and rate data. You can only see it in the curve.
Power migrates to the back. As the legs tire, peak force shifts later in the drive. The back and shoulders compensate. This isn't a conscious decision. Your body routes around the exhausted muscles. But the backup power sources are smaller and fatigue faster, which accelerates the decline.
Strokes stop looking alike. A fresh rower's force curves are remarkably consistent stroke to stroke. Under fatigue, the curves diverge: some clean, some ragged, the shape unpredictable. Elite rowers maintain consistent curves even under heavy fatigue. Developing rowers' curves break down two to three times as much over the same piece. That gap is trainable.
What to do with this
Look at your curves. If you've been rowing on a PM5, the data exists. Most rowers have hundreds of workouts with force curve data they've never examined.
Pick one flaw. If you have a double hump, work on the leg-back transition. If your catch is slow, focus on immediate leg engagement at the reversal. Trying to fix everything at once doesn't work. Your body can only attend to one cue at a time.
Compare across sessions, not within them. Your curve degrades during a hard piece. That's normal. The useful comparison is stroke 10 today versus stroke 10 three months ago. Is the catch sharper? Is the peak earlier? Is the curve more sustained? That trend tells you whether your technique work is landing.
Pin one of these cues to your monitor for four weeks, then check the curve again:
- Double hump: "Back opens as legs push."
- Slow catch: "Legs the instant you reverse."
- Early dropout: "Accelerate the handle to the body."
ErgToday captures your force curve on every PM5 stroke and tells you exactly what to fix. Try it free for 14 days at erg.today.