Squash Lunge Asymmetry: What a Dominant-Diagonal Deceleration Bias on IMU Data Really Means

A club coach keeps a running note on his best all-court player: clean into the back-left corner every time, a half-beat slow into the back-right. Nothing looks obviously broken on video — he still gets there, still gets the ball back — but the stop reads softer, the front knee drifts inward a few extra degrees, and the sprint back to the T lands him a step behind where the back-left corner leaves him. Ask which leg feels off and the honest answer is neither. A standard bilateral leg press or single-leg jump often comes back close enough to call symmetric — the test isn't wrong, it's answering a different question than the one the coach's eye keeps asking.

A season of squash doesn't load both legs the same way. A player who volleys more off the forehand, or who keeps facing opponents who find the same side of the T, ends up lunging into one court diagonal — say, front-right and back-left — far more often than the other. That diagonal's leading leg gets genuinely good at absorbing a sprint's worth of momentum in under a quarter of a second, while the other gets the same schedule and strength program but a fraction of the real braking reps. The shortfall shows up not as weakness on a strength test but as a softer brake and a slower return every time that corner gets called — the asymmetry this piece measures, using two shin-mounted IMUs to put a number on a pattern coaches usually sense before they can prove it.

A squash court has two natural diagonals — front-left paired with back-right, front-right paired with back-left — and a rally rarely spreads evenly across both. Jones and colleagues' 2018 review of squash's physical demands describes rallies built from repeated maximal accelerations, decelerations, and direction changes over three to six meters, lasting roughly fifteen to thirty seconds — a distance that maps closely onto the T-to-corner span on a standard court. Which of those trips a player makes over and over depends on shot preference and on the patterns opponents keep exploiting: a player who volleys cross-court off the forehand more than the backhand, or who simply gets targeted into the same corner match after match, racks up lunges on one diagonal at a rate the other never sees.

The squash-specific change-of-direction test Wilkinson, Leedale-Brown, and Winter validated in 2009 — a single timed circuit through all four corners that separated competitive squash players from non-squash athletes at 10.90 versus 12.20 seconds, p under .01 — was never built to catch this. A circuit through all four corners in sequence averages the diagonal a player is strong on against the one they're not, and the two can cancel into a respectable overall time. The repeat-lunge fatigue protocol built on the same court geometry has the same blind spot: one fatigue index across a fixed sequence hides a diagonal-specific gap rather than revealing it.

Braking a lunge and pushing a leg press are different jobs for the nervous system, even using the same muscles. A leg press asks a leg to produce force concentrically, on a timeline the athlete controls. A squash lunge asks the leg to absorb a body's worth of forward momentum eccentrically, inside a ground contact window typically under 250 milliseconds, on a timeline the ball and the opponent control. Volume on one doesn't automatically transfer to the other — three times as many real braking reps a week builds an adaptation a symmetric gym program won't erase.

Bishop, Turner, and Read's 2018 systematic review of inter-limb asymmetry research is worth citing for the pattern it pulled together, not a single number: an athlete's asymmetry score on one test — a bilateral strength measure, say — frequently didn't match the direction or size of asymmetry the same athlete showed on a different test, like a jump or change-of-direction task, measured around the same time. That's a review spanning many sports and test types, not a squash-specific finding, so it doesn't prove a clean strength score and a hidden braking gap coexist in squash players specifically — but it establishes that clearing one test says almost nothing about what a mechanically different test on the same limb would show.

Once deceleration and return speed are scored separately for each diagonal rather than averaged into one number, three recurring patterns show up often enough to name.

The middle row is worth treating as a genuine flag: a leg that brakes softer and returns slower on the same diagonal, repeating across sessions, behaves like an under-conditioned decelerator, not a one-off bad rep. The third row clears the braking leg entirely and points toward push-off and reactive strength instead — a different fix a peak-deceleration number alone would never surface.

This needs two data streams a standard timed circuit doesn't give you: a deceleration signal from each leading leg, and a return time split by diagonal. Two IMUs strapped above the ankle, sampling at 200Hz or higher, give both — shank deceleration during braking, and, paired with photocell gates or a synced stopwatch at the T, the return time per rep.

After a warm-up plus a few submaximal lunges to each corner, run six full-effort reps to one diagonal — front-right, back-left, alternating — with fifteen seconds of recovery, then six reps on the other diagonal after a two-to-three-minute rest. Per rep, pull three numbers: peak negative acceleration during the braking window, from first ground contact to the point forward shank velocity reaches zero; braking time in milliseconds; and return time from touch back across the T line. Average across the six reps per diagonal, then compute a Deceleration Asymmetry percentage — the stronger diagonal's peak braking g-force minus the weaker's, divided by the stronger value — and a Return-Speed Asymmetry percentage the same way.

As a practical starting point, not a clinically validated cutoff, treat under roughly 8% on both measures as normal noise, 8 to 15% as a moderate gap worth a repeat test, and above 15% on a retest a week or two later as a real, repeatable pattern.

A matched pair on both metrics needs nothing beyond a note in the file — most players who split court time reasonably evenly across diagonals land here, and chasing a perfectly even number wastes training time. A moderate gap, 8 to 15% on either measure, gets a straightforward first response: route extra submaximal reps toward the weaker diagonal during warm-ups for two to three weeks, then retest rather than guessing whether it closed.

A gap above 15% that holds up on a second test a week or two later deserves more caution, mostly because of what similar-sized asymmetries have been linked to elsewhere — even though neither study measured squash lunge deceleration directly. Paterno and colleagues (2010) found female athletes with vertical ground reaction force asymmetry above 15% on a drop-landing task were roughly four times more likely to suffer an ACL injury the following season. Kyritsis and colleagues (2016) found ACL-reconstructed athletes who returned to sport with a single-leg hop symmetry index under 90% — a comparable double-digit gap — reinjured at roughly four times the rate of athletes who cleared it. The overlap in magnitude is reason to take a persistent 15%+ gap seriously, not proof the same fourfold risk applies here. Cross-referencing with an established test, such as jump asymmetry and injury prediction research or ground reaction force asymmetry research, keeps this protocol a screening signal rather than a standalone verdict.

Once a diagonal-specific gap is confirmed on a retest, the fix is rarely more general conditioning — it's targeted exposure to the task the weaker diagonal is missing. Add two to three sessions a week of lunge volume weighted toward the low-rep diagonal, kept submaximal so it doesn't compete with match load, alongside eccentric work on the leg that's braking soft — a three-to-four-second controlled lowering on a Bulgarian split squat, or a lateral bound with a stuck landing. Four to six weeks is a reasonable window to expect meaningful closure, though a player returning from a prior injury on that side may move slower.

Retest with the identical protocol every two weeks rather than adjusting the program off one session's numbers, since a single test carries the same day-to-day noise any bilateral asymmetry measure does — see separating asymmetry noise from a real difference. Pairing this with the existing squash multidirectional repeat-lunge test fills out the picture: a player who fatigues evenly across all four corners but shows a clean diagonal-specific deceleration gap is a different case than one whose whole game slows down together.

The 8% and 15% bands above are a practical starting point built from general deceleration and asymmetry research in other sports, not a squash-validated cutoff — no dataset yet reports these exact metrics for a squash population, and a program adopting them should tighten or loosen them against its own players' data. A single session isn't enough to call a pattern real, either: bilateral asymmetry measures vary day to day even in the same athlete, so a gap that shows up once needs a second test before it earns a training response, let alone a referral.

The injury-risk figures cited above describe association, not causation, and neither study measured a squash lunge — they're a magnitude reference for what double-digit asymmetries have been linked to elsewhere, not proof that closing a gap here prevents an ACL injury. A gap under this protocol says something happened during braking or the return sprint; it doesn't say why on its own. A soft brake can come from an under-trained diagonal, but also from an old ankle sprain, a hip restriction, or simple fatigue on test day, and separating those usually needs more than one field test.