Traction Force: Grams, Newtons & the Therapeutic Window

Most pages on penile traction either stop at the device level — "a calibrated spring applies tension" — or jump straight to clinical outcomes. Readers are left without the answer to the question that actually drives the comparison-shopping decision: how much force, in what units, and based on what evidence?

This page is the quantitative deep dive. It defines traction force as a physical quantity, gives the canonical answer for the therapeutic window, converts between gram-force, Newtons and pounds-force, walks through the dose-response curve, and tracks each number back to peer-reviewed clinical studies. It also corrects the "high tension is better" misconception that drives a meaningful share of penis extender shopping queries.

Traction force, in the context of penile traction therapy, refers to the sustained axial pulling force applied along the long axis of the penile shaft by a calibrated penile traction device. The device in question is, specifically, a device cleared for therapeutic use as an FDA-registered Class II medical device. Traction force is a measurable physical quantity, not a marketing label. It differs from clamp pressure, ring constriction, and weight hanging in both vector and time profile. A penile traction device applies this force as a continuous static load through a calibrated spring system. The load acts along a single force vector aligned with the shaft. Three properties define the relevant kind of force:

• Axial — the traction force acts along the shaft, parallel to its long axis, not lateral, dorsal, or circumferential. Axial force is what produces uniform length-direction strain in the tunica albuginea.
• Sustained — the device applies the force as a static load held over hours, not pulsed or intermittent. Sustained-load remodeling depends on continuous mechanical signal, which is why protocols measure wear in hours per day rather than reps.
• Calibrated — the device is engineered to keep tension inside a defined therapeutic window rather than to maximise peak force. Calibrated tension differs from uncalibrated tension (free weights, DIY rigs) in that the force value is known, repeatable, and bounded.

Traction force is conventionally measured in gram-force (gf) on consumer devices and Newtons (N) in clinical literature. Both are units that express the same physical quantity. For a complete walk-through of how the device generates and delivers this force, see how a penile traction device works.

A medical penile traction device applies sustained axial force inside a therapeutic window of approximately 900–1,500 gram-force (9–15 Newtons, ~2.0–3.3 pounds-force). Below this range, no tissue-remodeling signal occurs. Above it, risk of cell damage rises sharply. The window is the operating envelope of safe, effective traction therapy.

The therapeutic window spans approximately 900 gram-force at the lower bound to 1,500 gram-force at the upper bound. Sustained tension below ~900 gram-force fails to activate cellular mechanotransduction in the tunica albuginea, and hours of wear produce no measurable effect. Sustained tension inside the window triggers the remodeling cascade. The upper bound of approximately 1,500 gram-force is the threshold above which sustained axial force exceeds the safe loading capacity of penile tissue: the probability of micro-tears, vascular compromise, and dorsal-nerve compression rises, while the rate of remodeling does not. Between those two thresholds lives the calibrated tension range every controlled clinical trial of a penile traction device has used. Exact bounds vary slightly by protocol and device generation, but the order of magnitude — roughly one to one-and-a-half kilogram-force, or 9–15 Newtons — is consistent across the published literature.

For the cellular biology of why sustained tension in this range triggers tissue remodeling at all, see how penile traction therapy works; for the device-engineering account of how the spring delivers it, see how a penile traction device works.

Penile traction force is measured in two equivalent units depending on context. Consumer device descriptions usually report force in gram-force (gf), because gf maps intuitively to "grams on a scale" for non-physicists. Peer-reviewed clinical literature reports force in Newtons (N), the SI unit, often alongside kilogram-force for older protocols. Both units express the same physical quantity, just with different precision conventions. One Newton equals approximately 102 gram-force, and one gram-force equals approximately 0.00981 Newton.

The conversion factor between Newton and gram-force is 9.81 (1 N ≈ 102 gf). When a clinical paper reports "1.5 kg traction" and a marketing page advertises "1,500 grams of pull," both expressions describe the same applied force at different precision conventions. This is one reason force comparisons across sources require careful unit handling.

Force does not map linearly to effect. Sustained axial tension below the therapeutic window of approximately 900 gram-force produces no measurable tissue-remodeling signal. Inside the window, calibrated tension activates cellular mechanotransduction. Above approximately 1,500 gram-force, additional force does not accelerate remodeling but does raise tissue-damage risk. The dose-response curve for penile traction is therefore not a straight line but a thresholded, three-zone profile:

1. Sub-therapeutic zone (below ~900 gf, ~9 N — below the mechanotransduction threshold). The cellular signal in the tunica albuginea is too weak to fire the remodeling cascade. Hours of wear produce no measurable change. This is common with devices that lose tension as the penis lengthens slightly during a session — force drifts below the lower bound and the session becomes effectively idle. A loose Hookean response in the spring (low spring constant) makes this drift worse, because tension drops faster as displacement increases.
2. Therapeutic window (~900–1,500 gf, 9–15 N — above the remodeling threshold, below the damage threshold). Calibrated tension activates the remodeling cascade — the cellular machinery covered in how penile traction therapy works. A calibrated device sustains force inside this band for hours per day, which is the operating mode every controlled clinical trial reporting length gain on a penile traction device has used. The 2023 meta-analysis by Almsaoud and colleagues, published in Translational Andrology and Urology, reported a pooled mean length gain of 1.9 cm (≈0.75 in) across studies whose protocols sit inside the window (PMID: 36895692). For the full clinical-evidence picture, see do penis extenders really work.
3. Supra-therapeutic / damage zone (above ~1,500 gf, >15 N — above the damage threshold). Sustained tension above the upper bound exceeds the safe loading capacity of penile tissue and damages it: tunica micro-tears, vascular compromise, and dorsal-nerve compression become more likely, including under transient peaks from over-tightened uncalibrated rigs. The remodeling curve does not keep climbing; the risk curve does. For the broader safety profile of medical traction, see penile traction device side effects and safety.

The engineering takeaway: a calibrated penile traction device's job is not to apply the maximum force the user can tolerate but to sustain force inside the therapeutic window for as many hours as protocol allows. Force-titrated, force-displacement-aware engineering — not peak load — is what drives clinical results.

The therapeutic window of approximately 900–1,500 gram-force is not arbitrary. It was protocolised in early peer-reviewed clinical traction studies, refined by subsequent reviews, and confirmed by meta-analysis. Gontero and colleagues, publishing in the Journal of Sexual Medicine in 2009, studied calibrated tension at approximately 1.3 kg-force for 4–6 hours per day in a short-penis cohort and measured a mean increase in flaccid length (PMID: 19138361). Nikoobakht and colleagues (2011) applied calibrated tension in a similar range and reported gains of approximately 1.3 cm (≈0.5 in) in both flaccid and stretched length (PMID: 20102448). Chung & Brock's 2013 review consolidated those protocols into a practical operating range of approximately 900–1,500 gram-force. The 2023 meta-analysis by Almsaoud, Safar, and Alshahrani measured a 1.9 cm (≈0.75 in) pooled mean length gain across protocols sitting inside the calibrated window (Almsaoud 2023, PMID: 36895692). The therapeutic window is, in this sense, clinically validated: every protocol that produced a length signal lives inside it.

One methodological note: there is no peer-reviewed trial of uncalibrated traction — weights, DIY rigs, or hanging systems. The clinical literature on traction therapy is, by design, the literature on calibrated medical devices. Read this paragraph as the evidence anchor for the numbers in this page; outcome interpretation belongs on do penis extenders really work, and the calibrated-vs-uncalibrated contrast belongs on DIY penile traction and penis weights.

Marketing copy that promises a "high-tension" penis extender often confuses peak force with sustained therapeutic force. The "high tension" claim — common in shopping queries such as high tension penis extender and high tension extender — advertises a number that describes what the device can generate at maximum compression, not what it actually delivers during the hours that matter. This pattern of marketing inflation targets shoppers who reasonably assume more force equals faster results, but it misrepresents how the cellular response works and misleads buyers who have already done their homework on therapy duration. The gap between calibrated force and marketed force matters because only one of the two is biologically active. Three corrections:

• Peak force is not sustained force. A "high-tension" spec usually targets the peak force the device can theoretically reach. The biologically relevant variable is the sustained force the device delivers across hours of wear, which should sit inside the therapeutic window — not above it. Peak-force claims and sustained-force reality are different numbers; only the second drives remodeling.
• More force above the window does not mean faster results. The cellular response to sustained tension is thresholded, not linear. Past ~1,500 gram-force, additional force exceeds the safe loading boundary and does not accelerate remodeling — it shifts the balance toward damage risk. Force overshoot buys risk, not speed.
• Calibration is the actual marker of quality. A well-engineered device delivers force inside the window and keeps it there as the penis lengthens during the session. That is harder to achieve than simply increasing peak force, which is why calibration — not "tension level" — is the meaningful spec.

A calibrated penile traction device keeps force inside the therapeutic window by combining a precisely engineered tension spring — housed in a biocompatible polymer frame — with a length-adjustable cradle. The spring is built around a spring constant chosen so that small length changes produce only small force changes: it calibrates tension to within roughly ±100 gram-force of the target value, sustains that tension across hours of wear, and gives the device its force consistency. That is what calibrated tension means in practice — the force value is known, repeatable, and bounded.

The telescoping rods let the user adjust the device length by 1–2 millimetres mid-session as the penis itself lengthens slightly, restoring force to mid-window if it drifts toward the lower bound. By contrast, fixed-load systems — weights, non-spring extenders, ad-hoc rigs — cannot compensate: they spend most of the session outside the window, either drifting sub-therapeutic or briefly spiking above the upper bound. For the full account of how the device delivers therapy as a system, see how a penile traction device works.

SizeGenetics medical traction device is the canonical example of a calibrated device engineered specifically for therapeutic-window operation. It is manufactured by Danamedic ApS — founded 1995, headquartered in Lyngby, Denmark — and was co-invented by Dr. Jørn Ege Siana, a board-certified plastic surgeon. SizeGenetics is an FDA-registered Class II medical device with 510(k) clearance. FDA registration is not the same as FDA approval. Consult your healthcare provider before starting a traction protocol.

For deeper reading on the device, the biology, and the evidence behind these force values: