The force curve answers a narrow question well: how much force a muscle produces, how fast it gets there, and how steadily it holds under a controlled isometric effort. That number anchors a lot of rehab, and it should. But a clinician deciding whether an athlete is ready to return to sport is answering a wider question than any single contraction can. Custom metrics, recorded in the Clinical Outcomes panel alongside the force data, are where the rest of that question gets answered: hop distance and symmetry, range of motion, girth, and patient-reported outcomes. They are easy to skip when the force trace is the headline of the session, and that is exactly why they are worth a deliberate look.
Force is necessary, not sufficient
Return-to-sport guidance has converged on a biopsychosocial model: strength and rate of force development, functional performance such as hopping, movement quality, and the athlete's own sense of readiness all carry independent information (Randsborg 2022, Piussi 2022). Each captures something the others miss. Isometric force quantifies the capacity of the muscle. Hop tests load that capacity through a stretch-shortening cycle the isometric test never sees. Patient-reported measures capture fear, confidence, and perceived function, which routinely diverge from objective scores. A patient can clear a strength threshold and still be a long way from playing.
The practical consequence is that an assessment built only on force looks more complete than it is. Recording outcome measures next to the curve is how the gaps stay visible.
Hop tests, and which ones earn their place
Hop tests are reliable, sensitive to change after ACL reconstruction, and they relate to function and to the odds of returning to sport (Davies 2019, West 2023, Crinion 2024). They are not a clearance test on their own. Passing a hop-and-strength battery is associated with a higher chance of returning, but it does not clearly lower the risk of a second ACL injury, and reinjury rates are similar between athletes who pass and athletes who fail (Girdwood 2024, Weber 2024). Some cohorts even hop better in the limbs that are later injured, which should temper any reading of "more is better" (Senorski 2026).
Given that, the value is in choosing tests that add information rather than repeating it:
- Single forward hop for distance. Simple, reliable, prognostic for return and for self-reported function. Often enough on its own when paired with one repeated hop (Davies 2019, Weber 2024).
- Triple or repeated forward hop. Adds a load, power, and endurance element, but correlates strongly with the single hop and becomes redundant once several forward hops are in the battery (Davies 2019, Crinion 2024).
- Vertical, lateral, and side hops. These expose residual asymmetry that forward hops hide, and they track more closely with strength deficits (Fältström 2023, Ebert 2026).
- Timed 6 m hop and speed hops. Common in soccer and return batteries, but they tend to add limited unique information beyond the distance hops (Sharp 2025, Greenberg 2020).
A pragmatic battery is roughly two horizontal hops (single and triple for distance) plus hops in other planes, which covers more than the classic four-hop horizontal set on its own (Davies 2019, Ebert 2026).
One limit is worth stating plainly: hop distance and symmetry say nothing about how the hop looked. Knee valgus, hip control, and landing mechanics are not in the symmetry score, and dedicated movement-quality tools do not correlate with hop symmetry, which means technique and performance are separate constructs that both need assessing (Welling 2020, Mengis 2025). If you score movement quality, record it as its own custom metric rather than assuming a good distance implies a good landing.
Patient-reported outcomes, especially psychological readiness
Patient-reported outcome measures (PROMs) carry the part of recovery the athlete feels and the clinician cannot read off a dynamometer. Psychological readiness and self-efficacy, captured by scales such as the ACL-RSI and kinesiophobia measures, are consistently associated with actually returning to sport (Pauw 2023, Webster 2023, Lo 2025). Short-term PROM scores have genuine prognostic value, predicting later return well enough to be clinically useful (Xiao 2022, Raisch 2024). In nonoperative ACL injury, better self-reported knee function and higher psychological readiness raised the odds of returning to preinjury sport (Cronström 2023).
The same caution applies as with hop tests. Common psychological cut-offs could not separate athletes who went on to a second ACL injury from those who did not, so a single PROM or threshold is not a risk-stratification tool (Slater 2022, Valier 2022). What PROMs do well is surface fear, low confidence, and perceived deficits that warrant targeted work even when the physical tests look clean (Webster 2023, Lo 2025). Many athletes are cleared and return while still carrying substantial PROM deficits, which suggests these measures are underused in the actual decision rather than overused (Petushek 2024).
What the numbers can and cannot do
The throughline across both literatures is that outcome measures inform a multifactorial decision and do not replace it. There is no symmetry percentage and no PROM threshold that reliably clears an athlete or predicts the next injury, and the pediatric evidence is thinner still (Randsborg 2022, Slater 2022). The strength of custom metrics is not that any one of them is decisive. It is that recording several of them, on the same patient, over time, makes the shape of the recovery legible in a way a single force number cannot.
Putting it on one timeline
This is where the Clinical Outcomes panel matters. Each custom metric you record sits beside the force history for the same patient, on the same dates, so the strength curve, the hop symmetry, and the psychological score move together as the case progresses.
A few things are worth using deliberately:
- Record the trials you actually perform. Hop tests are conventionally three efforts per side. Entering each trial keeps the spread visible and reports the mean rather than a single lucky or unlucky hop. A tight cluster and a wide one with the same average are different clinical pictures.
- Bilateral measures get a limb symmetry index. For hop distance, range of motion, girth, and any other left-versus-right measure, the same symmetry logic used for force applies, so a hop LSI sits next to a peak-force LSI and reads the same way. See the LSI article for how that number behaves and what it hides.
- Surveys and single scores track longitudinally too. An ACL-RSI or a pain score recorded at each visit becomes a trend, which is more informative than any single timepoint and is exactly the view the return decision needs.
- Define your own when the built-in set doesn't fit. Movement-quality scores, sport-specific tests, and clinic-specific measures can be added as custom definitions and tracked the same way.
Force data earns its place at the center of the assessment. Custom metrics are how the assessment stops being only about force. These measures exist for the athletes whose strength looks ready while their hop symmetry, landing quality, or confidence is not, and a force-only view is exactly the one that misses them.
Evidence base
The studies below inform the points above. They are drawn from the return-to-sport literature on hop testing and patient-reported outcomes after ACL and other lower-limb injuries.
- Hop testing reliability, prognosis, and battery selection: Davies (2019), West (2023), Crinion (2024), Fältström (2023), Ebert (2026), Sharp (2025), Greenberg (2020).
- Hop testing and reinjury risk: Girdwood (2024), Weber (2024), Farraye (2023), Senorski (2026).
- Movement quality versus symmetry: Welling (2020), Mengis (2025), Zarro (2023).
- PROMs, psychological readiness, and return: Pauw (2023), Webster (2023), Lo (2025), Xiao (2022), Raisch (2024), Cronström (2023), Read (2020).
- PROMs, reinjury thresholds, and gaps in standardization: Slater (2022), Valier (2022), Randsborg (2022), Piussi (2022), Petushek (2024), Bley (2022).