A returning ACL patient hits the gym again. Six months out from surgery. Knee feels strong. The brace comes off in physical therapy. They run a knee-flexion drill: cable, single-leg, 3×10 each side.
The right (operated) side and the left both show “60 lbs × 10 reps × 3 sets” in the log. They look identical.
tensr’s symmetry index says the right side is producing 22% less impulse per rep than the left.
The patient is not recovered. The log can’t see that. The two sensors can.
What symmetry actually is
When two sensors record the same exercise — one on each side — every metric in the catalog gains a left value and a right value. The relationship between them is the symmetry index:
symmetry index = (L_impulse − R_impulse) / (L_impulse + R_impulse) × 100A symmetry index of 0% means the two sides did equal work. ±5% is normal handedness for most populations. ±15% or more on a movement that should be symmetric is a finding.
The full bilateral cluster:
- Symmetry index at three scopes (rep / set / session).
- Bilateral deficit — the gap between bilateral peak force and the sum of single-side peaks. Usually 5–15% even in healthy athletes; large deficits suggest coordination issues.
- Timing offset — milliseconds between left peak and right peak per rep. Concentration and coordination signal.
- Coordination index — how synchronized the two force–time curves are throughout the rep.
- Side dominance — which side is consistently producing more total impulse over a session or block.
- Per-side fatigue — does one side decay faster than the other across the set?
Why it matters
Asymmetry is invisible in a normal set log because bilateral movements get recorded as a single number. Two sensors expose what one sensor averages over.
For rehab and post-injury lifters, the value is direct: was the work symmetric, yes or no. A patient who feels recovered after ACL reconstruction can be 20%+ asymmetric on the operated side without realizing it — because they’ve reorganized their movement to compensate. The symmetry index catches that compensation in the first set.
For everyone else, the value is trend-watching. Most athletes have a small dominance — 5% on a “good” day, 8% on a tired one. The interesting question is whether the gap is growing over months. A widening asymmetry usually flags an undiagnosed niggle: a side that’s avoiding loading because something hurts, even subtly.
What to track together
Treat the cluster as four lenses on the same signal: where, when, how, and how-much.
| Metric | What it tells you |
|---|---|
| Symmetry index (rep) | Was this single rep asymmetric? Useful for live coaching cues. |
| Symmetry index (set) | Did the asymmetry compound across the set? |
| Symmetry index (session) | Aggregate today. Smooths out individual-rep noise. |
| Symmetry trend (over weeks) | Is the gap growing or shrinking? The compliance signal. |
The reading worth attending to is the trend. A point-in-time 12% gap on one set tells you almost nothing — sets vary. A 12% gap that’s been climbing 1% per week for two months tells you something is wrong.
What gear it needs
Two sensors. Symmetry is a comparative metric; one sensor produces no signal.
Once you have two:
- Impulse-based symmetry is honest at any sample rate. Both sides at 8 Hz still gives you a clean per-set symmetry index because impulses smooth out under-sampling.
- Timing offset (the millisecond gap between left peak and right peak) wants ≥ 80 Hz on both sides. At 8 Hz the resolution is 125 ms, so a reported “100 ms offset” is below the noise floor.
- Coordination index (cross-correlation of curves) wants 80 Hz too.
For rehab/PT applications, set TUT and impulse symmetry at 8 Hz are usually plenty. For elite-athlete coordination work, the high-rate sensors earn their cost.
What to do tomorrow
The fastest way to baseline asymmetry: run the same band exercise unilaterally, one side at a time. Same band, same handle position, same rep count. Compare set total impulse.
If the gap is under 10%, that’s normal. If it’s 15%+, it’s worth investigating: a movement-specific weakness, a healing pattern, or an old injury you’ve worked around without noticing.
Don’t treat symmetry as a target to perfect. Most strong athletes have a dominant side and shouldn’t fight the small gap. Treat it as a trend to watch — the value isn’t in any single number, it’s in whether the number is moving the wrong direction.
Two sensors. One picture. The side you train less is bigger than you think.