How to use Isometric Mode for strength assessment, bilateral comparisons, return-to-sport benchmarking, and therapeutic loading.

Isometric Mode locks the cable at a fixed point and records force output against an immovable resistance. The device captures peak force, average force, time-to-peak, and force-time curve data — all exportable via the Beyond+ app.

This gives you a portable, repeatable strength assessment tool without the cost or footprint of a dedicated isokinetic dynamometer or force plate. It won't replace gold-standard lab equipment, but for clinical tracking, return-to-sport screening, and bilateral comparisons, it provides objective data that traditional equipment cannot.

For bilateral comparisons, repeat the test on the contralateral side using identical positioning. The Limb Symmetry Index (LSI) can be calculated from peak force values: (involved / uninvolved) × 100.

Mount at knee height. Patient seated with knee at 60–90° of flexion (document the angle). Ankle strap or handle attached below the knee. Patient extends against the locked cable for 3–5 seconds of maximal effort. Particularly relevant for ACL rehabilitation, patellofemoral conditions, and post-total knee replacement. Serial testing at consistent intervals (e.g., 4-week blocks) provides objective progression data.

Mount at floor level or use the Travel Platform. Patient prone or seated. Ankle strap attached. Patient flexes knee against locked resistance. Compare bilateral values for hamstring-related injury screening and return-to-sport criteria.

Mount at appropriate height for the test position. Common applications include shoulder external rotation strength (relevant for throwing athletes, rotator cuff rehab) and grip-related assessment with appropriate attachments.

Use the Travel Platform as a floor anchor. Patient stands on the platform and pulls upward against the locked cable in a standardized position. Provides a general lower-body strength metric comparable to force plate data.

Peak force — Maximum force produced during the contraction. The most commonly used metric for bilateral comparison and longitudinal tracking.

Time-to-peak — How quickly the patient reaches maximum force. May be relevant for explosive readiness or rate-of-force-development screening.

Force-time curve — Available in the CSV export. Shows the force profile across the entire contraction, which can reveal compensatory patterns or fatigue characteristics.

For return-to-sport decisions, a commonly referenced threshold is LSI ≥ 90%, though this should be interpreted alongside other clinical criteria rather than used as a standalone gate. VOLTRA I's data provides one objective input into that decision.

Isometric Mode is not only a testing tool. When used in a rehab setting, the real-time force display on VOLTRA I functions as direct biofeedback — patients can see exactly how many newtons of force they are producing and self-regulate their effort accordingly.

This is clinically valuable because it allows patients to moderate their own pain levels by grading to a force they are comfortable with. The clinician can track that quantified value over time alongside subjective pain ratings, creating an objective record of desensitisation. In contexts where muscle inhibition is common — post-immobilisation, post-surgery, or pain-avoidance patterns — the visible force target increases motor learning and motor output in ways that verbal cueing alone cannot.

1. Intensity guided by pain — The intensity of the contraction should keep pain low: within 1–4 out of 10 on a pain scale. Patients should not be pushing to a level that provokes a significant pain response. The goal is to load the tissue, not irritate it.

2. Positional holds — The muscle should initially be held in a shortened position (which approximates the musculotendinous junction and reduces strain). Progress to comfortable lengthened positions gradually over approximately 2 weeks as tissue tolerance improves.

3. Time under tension — Research supports at least 4 sets of 30-second holds, with a target of progressing toward 45–60 seconds per hold to increase the overall density of muscle and tendon fibres being challenged.

4. Analgesic neurophysiological benefit — Sustained isometric loading triggers neurophysiological adaptation that correlates with downregulation of pain fibres and desensitisation of the pain response. This is the mechanism behind isometrics being used not just to build strength but as a pain management strategy in irritable tendon conditions.

In the early acute and subacute phases of tissue healing, isometrics are often the safest and most appropriate loading strategy:

The following protocols are based on current research on tendons and muscle strains. All implement the dosing guidelines above (pain ≤ 4/10, positional progression, 4 × 30–45s holds).

Target position: 60° of knee extension isometric hold for 30–45 seconds, adjusted to patient tolerance. At 60° of knee flexion, the quadriceps muscle has its highest mechanical advantage for producing force. With the VOLTRA I cable fixed at this length, the isometric can be performed with the patient seated or in a partial squat position, ensuring the graft site (if applicable) is not compromised.

Implementing isometrics early at 90° of knee flexion while the patient is seated is a safe way to begin challenging the quadriceps while ensuring the cable length does not compromise a graft that is still remodelling and integrating into bone. Research supports an increase in relative return-to-sport timeline and a decrease in graft site pain with sustained isometrics implemented early in the rehab timeline.

Sustained isometrics at angles permitted by the surgical protocol are a useful way to introduce mechanical stimulus when the repair is still healing. Because the cable locks the joint at a fixed angle, the patient cannot inadvertently move through a range that compromises the repair. The surgical report and specific tissue repaired must always be reviewed before prescribing — early loading protocols vary between surgeons and repair types.

Isometrics have been shown to be an effective entry point for loading an irritated tendon. Non-chronic tendinitis of the rotator cuff responds well to moderately challenging isometric holds at angles that produce muscle fatigue without overloading the painful shoulder tendon. Clinically useful positions include:

Place the hamstring in a shortened position — hip in extension, knee flexed — to approximate the fibres at both attachments. The proximal hamstring fibres are shortened when the hip is in extension; the distal fibres shorten when the knee is flexed. Holding an isometric contraction in this position is safe in early rehab because the tissue is not being challenged in a lengthened position where re-injury risk is highest.

Parts of this article were contributed by Dr Kyle Olandt, a physiotherapist and long-time VOLTRA advocate. A genuine thank you for his clinical expertise and generosity in sharing it with the community. Find him on Instagram at @dr.kylejordan.