The material used in tennis string can significantly affect a player’s performance, and even health. Several materials are used to make tennis strings. They vary in terms of elasticity, durability, rebound efficiency, tension holding capability, and manufacturing cost, among other considerations.
Animal intestine is the most resilient material used to make tennis strings. It has better tension retention than any other material, and also is softer than any other material used for tennis strings. It provides the most energy return, meaning it is the most efficient string. It remains soft at high tensions while other materials tend to dramatically stiffen. This allows gut string to enable players to string rather tightly to improve ball control without losing much rebound efficiency (power) and without greatly increasing impact shock, which can hurt the elbow and other joints.
Its principal drawbacks are much higher cost to manufacture and purchase than other materials, weakness to shear stresses from off-center hits (typical of beginning players), variable quality control depending upon the brand, batch, age, storage conditions, and grade, difficulty of stringing due to its delicacy, and poor durability when wetted with water. Natural gut is very sensitive to different types of weather and those players who use it normally carry several different racquets with different tensions to compensate for this. Some players, particularly those who hit flat shots and hit the sweet spot consistently, find high-quality gut to be more durable than many other types of strings due to its outstanding tension retention. This may help to offset the high initial cost. The use of a dense string pattern also generally improves the longevity of natural gut.
Natural gut is produced by drying fibers extracted from a part of the cow intestine called the serous membrane, or serosa, which contains collagen designed to withstand the stretching and contraction of the intestine. It is this elasticity that makes the fibers useful for tennis string. Sheep intestines have also been used for racquet strings in the past.
The first natural gut tennis string was rumored to be manufactured in 1875 by Pierre Babolat, who would launch the VS brand of gut fifty years later. Natural gut is usually offered in coated form, to reduce its tendency to unravel, particularly when humid or wet.
Synthetic gut is nylon, nearly always composed of a single filament. It is a very inexpensive string to manufacture and is generally the least expensive string to purchase.
Small changes from pure nylon are usually found in strings sold as “synthetic gut”. Textured coatings, colorants, and the addition of a small amount of Kevlar are the most common changes. Some manufacturers, such as Gosen, label nylon strings with words like “sheep”, as in sheep intestine, although such strings contain no gut.
Synthetic gut, as it is used for mono-filament nylon strings, is now a misnomer, as the creation of multi-filament strings has provided players with a better approximation of natural gut’s performance. Modern “synthetic gut” is actually a multi-filament string that holds its tension extremely well and which has a dynamic stiffness profile that is closest to natural gut, although the industry continues to apply the term to mono-filament nylon strings.
Multi-filament, or “Multi”
Multi-filament strings, known commonly as “multis” are strings that have more than one filament. They are most commonly made of many filaments of nylon, but can incorporate other materials such as polyurethane, Zyex, Vectran, Kevlar, and other materials. Multi-filament strings offer better elasticity than single filament strings, but usually inferior durability. No multi-filament string holds its tension as well as natural gut, and none of them is as soft.
However, in comparison to mono-filament “synthetic gut”, multi-filament strings can offer a much closer approximation of natural gut’s performance. The softest multi-filament strings can be made with Zyex and polyolefin, although some of the softest of these strings are no longer on the market.
Nylon is the most popular string material for amateur players due to its low cost and the improvement in elasticity offered by multi-filament strings. Wear-resistant coatings for nylon strings are common, especially with multi-filament strings, because the outer filaments tend to break first as the racquet is used.
Polyester is a stiff and durable string material, originally intended for use by frequent string breakers. It took the string time to become popular, primarily due to its poor tension holding capability. However, players feel they are able to apply more topspin to balls while maintaining control with polyester strings. Polyester’s support for heavy topspin in particular has made it the most popular material in the pro tour. The increased topspin due to polyester strings has been verified with controlled experiments. The exact cause for the increased spin is not known but there is strong evidence that the low friction between strings is a factor.
Kevlar is the stiffest, most durable synthetic string available,and is thus extremely hard to break. Although it is one of the best strings in terms of tension holding capability (next to natural gut), it is the most dangerous string when it comes to developing tennis elbow. Kevlar is often strung with another string, such as nylon), in order to combine both strings’ qualities, as Kevlar by itself feels too stiff for many tennis players, especially when combined with a stiff racquet. Some advocate using a very thin gauge Kevlar for increasing comfort, but even in the thinnest gauge it is a stiff string. Another strategy to increase comfort and improve rebound efficiency is to string Kevlar at a low tension.
Vectran is the penultimate string in terms of stiffness and durability. It is perhaps the least commonly used contemporary string material. It is usually added to nylon string to increase nylon’s durability and stiffness, as with Kevlar. Yonex, for instance, offers two badminton strings, made primarily from nylon, which have Vectran strands. However, the Ashaway company offers a braided Vectran tennis string.
Zyex string offers more rebound efficiency, i.e. gut-like dynamic stiffness, than other synthetic strings, particularly when strung at low tensions. This gives it playability that is more similar to natural gut than, arguably, other synthetic materials. It also has low overall stiffness. The Pro Kennex IQ Element Z string, for instance, has the lowest stiffness of any synthetic string yet tested. The drawback of Zyex is that the outer wrapping materials in Zyex tennis string tend to be much less durable than the Zyex filaments inside the string and do not bond with them. This can lead to the outer wrapping wearing away, leaving the inner Zyex filaments.
Polyolefin is one of the softest synthetic string materials, rivaling some Zyex and most nylon multi-filaments. It offers mediocre durability and tension retention, so it is generally used as the cross string in a hybrid string setup. For those who do not break strings very quickly, however, 100% polyolefin stringing may be a good alternative to natural gut and multi-filament strings. This is especially true for those who have had tennis elbow and find natural gut string to be too costly.
Metal wire, usually piano wire, was used in some historical racquets
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