🩺 The short version
- Stiffness is what your arm feels. A stiffer string gives a higher peak force over a shorter contact time (Tennis Warehouse University).
- String family comes first. In our catalogue, natural gut sits at 88-100 lb/in, multifilaments at 143-180, polyesters at 164.6-286.9.
- Changing string beats changing tension. In the same frame, going from Luxilon ALU Power to Wilson NXT takes the RCS index from 31 to 25; dropping ALU Power by 4 kg (about 9 lb) only takes it to 29.
- If you keep polyester, pick one of the softest: Toroline O-Toro Snap (164.6 lb/in) and Toroline O-Toro (165.7) play close to a multifilament on the index.
- Strings are not a treatment. Persistent elbow pain calls for a medical opinion.
Pain on the outside of the elbow, a weak grip after a match, a forehand you no longer dare to hit through: if that sounds familiar, the question "which string?" comes quickly. Most answers online are lists of names. This article starts from the one physical property that links a string to your arm, then puts numbers on it.
⚡ 1. Why Stiffness Is the Variable That Matters
Crawford Lindsey of Tennis Warehouse University (TWU) defines string stiffness as the force needed to stretch a string, "measured in units of how many pounds of force it takes to stretch the string one inch" — hence the lb/in unit used on every string page of our catalogue.
What it does at impact is summed up in one sentence of the same article: "Stiffer strings exhibit higher peak impact tension, higher peak force, less string deflection, and shorter dwell time." More force, delivered over less time: that is the shock that reaches your hand and forearm. And the trade-off is stated just as plainly: "A stiffer string will lead to less power, more control and less comfort."
Tension enters through the same door: "As tension goes down, so does stiffness." That is why lowering tension helps — and, as we will see, why it helps less than changing string.
🧵 2. String Families, Softest to Stiffest
Tennis Warehouse's stiffness guide gives the hierarchy in words: "Natural gut strings are generally the softest available, making them the most powerful and comfortable option"; multifilaments "tend to be quite soft as well", and "polyester strings tend to be the stiffest and lowest-powered type of string." Our catalogue puts numbers on it:
| Family | Strings in catalogue | Stiffness range (lb/in) | Median (lb/in) |
|---|---|---|---|
| Natural gut | 7 | 88 – 100 | 95 |
| Multifilament | 45 | 143 – 180 | 162.3 |
| Hybrid (pre-packed sets) | 9 | 158 – 192 | 175 |
| Synthetic gut | 16 | 165 – 185.2 | 180 |
| Polyester | 102 | 164.6 – 286.9 | 220 |
Source: Tennis String Advisor catalogue, 179 string pages, as of October 10, 2026. Values come from several sources and are not all measured with the same protocol: read them as orders of magnitude.
Two things stand out. The families overlap: the softest polyester of the catalogue (164.6 lb/in) is softer than most synthetic guts. And the spread inside polyester is huge — more than 120 lb/in between the softest and the stiffest. "Polyester" on its own tells your elbow very little; the exact string tells it a lot.
🩺 3. Seventeen Strings Through the RCS Index
The site's RCS index combines frame stiffness (RA), string stiffness and tension into one firmness score for the setup, built for a single question: does this setup put your elbow at risk? Below 20 it reads "very comfortable", 20-25 "comfortable", 25-30 "standard", 30-35 "firm", above 35 "very firm — tennis elbow risk".
To compare strings with everything else held equal, here is each one in the same frame — RA 65 — strung at 22 kg (48.5 lb):
| String | Family | Stiffness (lb/in) | RCS (RA 65, 22 kg) |
|---|---|---|---|
| Babolat Touch VS | Natural gut | 92 | 16 |
| Babolat Xcel | Multifilament | 155 | 23 |
| Tecnifibre NRG2 † | Multifilament | 164 | 24 |
| Head Velocity MLT | Multifilament | 165 | 24 |
| Toroline O-Toro † | Polyester | 165.7 | 24 |
| Tecnifibre X-One Biphase † | Multifilament | 166.9 | 24 |
| Babolat RPM Blast + VS Touch | Hybrid | 168 | 25 |
| Wilson NXT † | Multifilament | 173.7 | 25 |
| Tecnifibre Razor Code + X-One | Hybrid | 180 | 26 |
| Prince Synthetic Gut Duraflex | Synthetic gut | 185 | 26 |
| Babolat RPM Blast + Xcel | Hybrid | 192 | 27 |
| Yonex Poly Tour Air | Polyester | 195 | 28 |
| Babolat RPM Soft | Polyester | 205 | 29 |
| Luxilon Element † | Polyester | 208 | 29 |
| Luxilon ALU Power | Polyester | 230 | 31 |
| Babolat RPM Blast | Polyester | 240 | 32 |
| Luxilon 4G † | Polyester | 286.9 | 38 |
Stiffness from our string pages; RCS computed with the site's formula for one shared example. † Stiffness aligned on a measurement by the TWU lab: when the product page groups several gauges, it is the stiffest gauge measured, which can overstate another gauge on the product page, one measured softer (not always the finest), and make the RCS index look severe for it; the configurator's index is the one to trust. Hybrids are pre-packed sets: their stiffness is the set's own page value, with no lab measurement, and has not been recalculated from their components. The table compares strings with each other; it does not replace the calculation for your own frame and tension.
Read from top to bottom, the table is the whole argument. Natural gut sits alone at 16. The multifilaments cluster between 23 and 25. The "soft" polyesters marketed for comfort — Element, Poly Tour Air, RPM Soft — land at 28 to 29: softer than a classic poly, still firmer than any multifilament. The classic polys climb into the "firm" band, and Luxilon 4G goes past the risk threshold (38) in a frame that is itself only medium-stiff.
One surprise: Toroline O-Toro, a polyester, scores like a multifilament (24). So does Toroline O-Toro Snap (164.6 lb/in), the softest polyester in the catalogue; on the index, these are the two least firm polys we document.
🎛️ 4. The Three Levers, Ranked by What They Change
The same formula lets you compare the three things you can change. Starting from Luxilon ALU Power at 22 kg in an RA 65 frame (RCS 31):
| Change | New RCS | Gain |
|---|---|---|
| Switch to Toroline O-Toro (soft poly), same tension | 24 | −7 |
| Switch to Wilson NXT (multifilament), same tension | 25 | −6 |
| Drop tension by 4 kg (22 → 18 kg, about 9 lb) | 29 | −2 |
| Frame 5 RA points softer (RA 65 → 60), same string and tension | 30 | −1 |
In the site's formula, 1 kg (2.2 lb) of tension is worth about half a point; 10 lb/in of string stiffness a little over one point; 10 RA points of frame stiffness about three and a half. If your arm hurts, the string is the first lever, tension the second, the frame the third. Changing frame is also by far the most expensive of the three.
💡 Expert point — what the index does not see. The RCS works from stiffness values; it does not model your technique, off-centre hits, grip pressure or how long a string has been in the racquet. Those matter to an elbow too. The index tells you how firm your setup is, not why your arm hurts.
✅ 5. A Plan, in the Order of the Levers
- Change string family. From a classic polyester to a multifilament (RCS 23-25 in the example frame), or to natural gut if budget allows (16). If you will not give up polyester, choose the softest one you are willing to play, and check its stiffness on its page in the string catalogue.
- Then lower the tension. It is the direction established by TWU: lower tension, lower stiffness. No study we know of gives a tension in pounds for an injured elbow, so we do not give one either.
- Consider a hybrid as a transition. A pre-packed hybrid with a soft component (RCS 25-27 in the example frame) sits between a multi and a poly. We have no measurement of whether the soft string works better in the mains or the crosses: the catalogue gives one stiffness for the set, without a lab measurement. Read a hybrid's index as an order of magnitude.
- Only then, the frame. A lower RA helps, but less per euro than the string.
The configurator does this arithmetic for your own frame, string and tension, and flags setups in the firm and very firm bands.
⚠️ Strings are not a treatment. A softer setup reduces the load your arm receives at impact; it does not heal a tendon. If the pain lasts, wakes you at night or does not ease with rest, see a doctor or a physiotherapist before changing more equipment.
🩺 Is your setup in the firm zone?
Enter your frame, your string and your tension. The configurator computes your RCS index and flags setups that put the elbow at risk.
Check my setup →❓ FAQ — Strings and Tennis Elbow
What is the best string type for tennis elbow?
The softest one: natural gut, then multifilament. Tennis Warehouse describes natural gut as generally the softest and most comfortable string, and polyester as generally the stiffest. In our catalogue, natural gut sits at 88 to 100 lb/in and multifilaments at 143 to 180 lb/in, against 164.6 to 286.9 lb/in for polyesters.
What tension should I use with tennis elbow?
No study we know of gives a tension in pounds for an injured arm. What is measured is the direction: per Tennis Warehouse University, as tension goes down, so does stiffness. In the site's RCS index, 2 kg (about 4.4 lb) less lowers the firmness by about one point, far less than a change of string family.
Can I keep playing polyester with tennis elbow?
If you do, choose the softest poly you are willing to play. In our catalogue, Toroline O-Toro (165.7 lb/in) reaches an RCS of 24 in a frame of RA 65 at 22 kg, close to a multifilament, while Luxilon 4G (286.9 lb/in) reaches 38. With current pain, a multifilament or natural gut generally remains a more prudent choice than a polyester.
Do strings cure tennis elbow?
No. A softer setup reduces the load your arm receives at impact; it does not treat a tendon injury. Persistent elbow pain calls for a medical opinion.