Single-Leg Jump Asymmetry
24.6% L<R
Flag Left leg the limiter
Sean is an 11-year-old tennis player (2.5 yrs in the sport, playing 5–6 days/week) with a healthy history and no prior injuries. He presents as a mobile, "bendy" mover who is comfortable in rotation but under-developed in straight-line strength and single-leg control — a very common and very coachable profile for a young, fast-growing tennis athlete. The force-plate data and Rachel's hands-on movement screen tell the same story from two directions, which gives us high confidence in the plan.
The headline finding is a left-side single-leg deficit. On the VALD single-leg jump, Sean's left leg produced a jump height of just 7.8 cm versus 10.4 cm on the right — a 24.6% asymmetry — with the left also lagging in reactive strength (38%) and power (17%). This mirrors exactly what Rachel saw by eye: on single-leg squats and the tandem squat, the left side was "significantly worse, weaker, and needed more help." Importantly, his bilateral (two-leg) squat is near-perfectly symmetrical (0.7%) — meaning the raw strength is there, but it falls apart the moment he has to stabilize on one leg. That points squarely at glute-medius / pelvic stability, not a true strength gap.
The second finding is a tennis-classic shoulder pattern. On the DynaMo, Sean's right (hitting-side) shoulder shows strong internal rotators but lagging external rotators — an external-rotation:internal-rotation ratio of 0.74 with the right ER measuring 25% weaker than the left. In overhead athletes this posterior-cuff lag is the single most predictive set-up for shoulder overuse and impingement. His left shoulder, by contrast, is balanced (0.97). This is a build-it-now-while-it's-cheap opportunity.
The third finding — flagged by Rachel and consistent with the movement data — is low-back load. Sean defaults to lumbar extension to create power (seen in the push-up and hip-hinge) and favors the transverse (rotational) plane while showing deficits in the sagittal and frontal planes. Combined, that places moderate-to-significant overuse risk on the low back if left unaddressed during this growth window.
Bottom line for the plan: Sean does not need to "get stronger" in the gross sense — he needs control, symmetry, and joint support. The recommended block (2×/week, Wednesdays & Fridays) targets core/glute/pelvis stability, left-leg single-leg work, posterior shoulder-cuff strength, and anti-extension trunk control — directly serving his stated goals of hitting harder, faster change-of-direction, injury prevention, and a better stretch routine.
Plan agreed with family: 2×/week, Wednesdays & Fridays (tentative times TBC). Mom confirmed 2× is the target cadence.
| Screen | Result | Interpretation |
|---|---|---|
| Multi-Segmental Flexion | Reaches shins | Hamstring tightness is the limiting factor; lots of compensatory flexion through the thoracic spine. |
| Multi-Segmental Extension | Scapulae reach past heels | Large extension range — "very bendy." Hips are very loose in rotation (ER/IR) but tight in flexion/extension. |
Read: Sean is hypermobile in rotation and extension but tight in the hamstrings and in hip flexion/extension. The body finds range where it's easy (thoracic, lumbar extension, rotation) to make up for where it's stiff (hamstrings, hip hinge). That compensation pattern is the root of both the low-back load and the hinge difficulty seen below.
| Pattern | Quality | Coach Notes |
|---|---|---|
| Squat | ★★★★★ | Knee valgus and heels rising on bodyweight, weighted & overhead squats. Sumo: knee wiggle, loads into left hip on the way down, shifts to right on the way up — significant deficiency. Tandem: right improved after 3 reps (ankle/hip mobility limited); left significantly worse — foot turned out (ER), pelvic instability. |
| Hip Hinge | ★★★★★ | Weight shifts into left glute; struggles with flexion/extension through the pelvis; hamstring tightness restricts the hinge. |
| Push (Horiz/Vert) | ★★★★★ | Eccentric (lowering) was fine with cueing (flex abs, keep hips up); concentric (pressing) not yet strong enough; defaults to lumbar extension to complete the rep. |
| Pull (Horiz/Vert) | ★★★★★ | Good form overall — note "chicken-neck" cervical flexion to coach out. |
| Pattern | Left | Right | Coach Notes |
|---|---|---|---|
| Lunge | ★★★★★ | ★★★★★ | Sagittal: right forward lunge dumps into IR — lots of pelvic/core instability (bilateral issue). Cueing into ER fixed much of it → train in ER. Frontal: left good; right a little tight through hip flexor & posterior chain, heel rose, foot turned out. Transverse: really good form & adductor flexibility. |
| Step-Up | ★★★★★ | ★★★★★ | Control biased to the right; left requires more attention (consistent with single-leg findings below). |
Pattern summary (Rachel): "Sean favors the transverse plane; he has deficits in the sagittal and frontal planes, combined with excessive lumbar extension — placing moderate-to-significant injury risk on the low back."
| Trial 1 | 7.49 s |
| Trial 2 (best) | 6.81 s |
| Trial 3 | 6.95 s |
Improved 0.68 s from trial 1→2 (learning effect). 6.81 s is the baseline to beat at re-test. Faster left-side plant/decel should drop this further once the L-leg asymmetry closes.
| Mode | Result | Avg RT |
|---|---|---|
| 15 hits | 12:03 | 653 ms |
| 15 hits | 10:21 | 531 ms |
| 30 s window | 40 hits | 571 ms |
| 30 s window | 41 hits | 573 ms |
Reaction time improved 122 ms across the 15-hit sets — strong trainability. Baselines set for tracking.
Eccentric controlled with cueing; concentric not yet strong enough to drive the rep — currently substitutes lumbar extension. Priority: build pressing strength with a braced trunk (anti-extension), removing the low-back compensation.
| Left | Right | |
|---|---|---|
| Calf Raise (SL) | 21* | 22 |
| SL Squat | 13 | 13 |
*Left calf-raise eccentric circled into supination. SL squat: knee valgus & frontal-plane hip drift (glute-med instability); needed the opposite leg to assist on the way up — left worse, weaker, needed more help.
Captured on VALD ForceDecks (dual-plate), 8 Jun 2026, at 35.1 kg bodyweight. For an 11-year-old pre-pubertal athlete there is no clean elite normative band, so values below are reported as a baseline to track rather than scored against adult references. The asymmetry numbers are the most actionable output at this age.
| Metric | Value | What it tells us |
|---|---|---|
| CMJ Jump Height | 19.8 cm (7.8″) | Counter-movement vertical — baseline lower-body power. |
| CMJ Peak Power | 1,222 W · 34.8 W/kg | Relative power baseline for an athlete his size. |
| CMJ RSI-Modified | 0.27 | Jump-strategy efficiency — room to make his jump "springier" with less ground time. |
| Countermovement Depth | −21.5 cm | Uses a moderate dip — appropriate. |
| CMRJ Rebound Retention | 89% (17.8 / 20.0 cm) | Keeps ~89% of his height on the rebound jump — good elastic quality already. |
| Drop-Jump RSI | 0.78 · 258 ms contact | Reactive strength baseline; short contact time is a positive sign for footwork. |
| CMJ Concentric-Impulse Asymmetry | 4.5% | Within acceptable range bilaterally (<10%). |
| CMJ Peak Landing-Force Asymmetry | 10.2% | Just over threshold — landing absorption slightly uneven, worth watching. |
Peak force 486 N split almost exactly evenly — Left 248 N / Right 250 N, a 0.7% asymmetry. When Sean has both feet down he distributes load beautifully. The deficit is purely a single-leg / stability problem, not a raw-strength one — which is exactly why the plan emphasizes unilateral and stability work over heavy bilateral loading.
Single-Leg Jump (SLJ), best trial per leg. Left leg is the limiter across every output metric — yet raw peak force is symmetrical, confirming the gap is in rate, power and control (i.e. glute-med / pelvic stability), not maximal strength.
| SLJ Metric | Left | Right | Asym | Flag |
|---|---|---|---|---|
| Jump Height (cm) | 7.8 | 10.4 | 24.6% L | Flag |
| RSI-Modified | 0.09 | 0.15 | 38.0% L | Flag |
| Take-off Power (W/kg) | 19.7 | 23.8 | 17.3% L | Watch |
| Peak Landing Force (N) | 1,545 | 1,880 | 17.8% R | Watch |
| Peak Concentric Force (N) | 678 | 666 | 1.8% | OK |
Read: The left leg jumps 25% lower and is 38% less reactive, despite producing essentially the same maximal force as the right. That signature — strong but slow and unstable on one leg — is a rate-of-force / stabilization problem driven by glute-medius and pelvic control. The right leg meanwhile absorbs ~18% more landing force, the over-worked "trusted" side. This is the through-line that connects the SL-squat findings, the tandem-squat instability, the hip-hinge weight-shift into the left glute, and the lunge pelvic instability — all four point to the same fix.
Isometric internal/external rotation tested at 90°/90° (DynaMo handheld dynamometer). The external-rotators (posterior cuff) are the decelerators of the serve and groundstroke follow-through; a healthy ER:IR ratio (≈0.66–0.75+) protects the shoulder. Sean's right (hitting-side) shoulder shows the classic overhead-athlete lag; his left is balanced.
| Shoulder | Internal Rot. (IR) | External Rot. (ER) | ER:IR Ratio | Status |
|---|---|---|---|---|
| Right (hitting side) | 48.1 N | 35.7 N | 0.74 | Watch |
| Left | 49.2 N | 47.6 N | 0.97 | Balanced |
| Side-to-side | −2.2% (even) | −25% (R weaker) | — | Watch |
Read: Internal rotation (the "hitting" muscles) is strong and even side-to-side. But the right external rotators are 25% weaker than the left and sit at a low ER:IR ratio of 0.74. In a young athlete hitting 1.5–2 hrs/day, this posterior-cuff lag is the most common precursor to shoulder overuse / impingement. It is cheap to fix now with light, high-quality cuff work — and doing so also tends to add racquet-head speed because the shoulder can decelerate (and therefore accelerate) with more confidence. (Dominant hand assumed right-handed based on the rotation pattern — please confirm; interpretation flips if Sean is left-handed.)
Cadence: 2×/week — Wednesdays & Fridays (confirmed with family; session times to finalize). Rachel recommended 2–3×/week; 2× is the agreed starting point. Format: 8–12 week foundational block, then re-test.
| Priority | Focus | Sample Work | Re-Test KPI |
|---|---|---|---|
| 1 | Left-leg single-leg strength & stability | Tempo split squats & step-downs (left-biased volume), single-leg RDL, lateral band walks, controlled SL calf raises — quality over load | SLJ height asym < 12% |
| 2 | Glute-med / pelvic & core control (anti-extension) | Dead-bugs, Pallof press, side-plank progressions, hip-airplane, banded squats cueing knees-out — kill the valgus | SL-squat: no valgus, no opposite-leg assist |
| 3 | Posterior shoulder cuff (right-biased) | Light ER work (side-lying, banded ER at 90/90), prone Y/T, rhythmic stabilizations — low load, high quality | Right ER:IR ratio > 0.85 |
| 4 | Hip hinge & hamstring length | Hip-hinge patterning to dowel, RDL progressions, 90/90 hip mobility, hamstring loading through range | Multi-segmental flexion past shins; clean hinge |
| 5 | Pressing strength with braced trunk | Incline / box push-ups with ab-brace cue, then progress; eliminate lumbar-extension cheat | Full push-up, neutral spine |
| 6 | Train the lunge in ER; agility/footwork | ER-cued lunges (per screen), ladder & reactive Blaze-Pod drills, decel-to-stick both directions | T-Test < 6.7 s; balanced left-side plant |
Programming note: Sean is pre-pubertal and hypermobile — the emphasis is motor control, symmetry and tissue capacity, not heavy external load. Keep the work crisp, well-cued, and rotation-friendly (he loves the transverse plane — use it as a reward and a teaching tool while we shore up the sagittal/frontal planes). This plan directly serves his goals: hitting harder (cuff + power transfer), faster COD (left-leg + footwork), injury prevention (shoulder + low back + symmetry), and a better stretch routine (hamstring/hip mobility).
| Window | Battery | Decision Threshold |
|---|---|---|
| 6 weeks | SLJ + SL squat + DynaMo shoulder (focused check) | SLJ asym trending < 15%; right ER improving |
| 12 weeks | Full battery (CMJ, CMRJ, SLJ, SQT, DynaMo) + T-Test & Blaze Pod | SLJ asym < 12%; ER:IR > 0.85; T-Test < 6.7 s; clean squat & hinge |