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Shoe cushioning rig
NEW! Experiment

Tennis Shoe Cushioning

For tennis players, the primary factor determining purchase of a tennis shoe is comfort. For researchers and manufacturers, injury prevention is a main area of interest, concern and design. For both groups, heel and forefoot cushioning and stability is the common denominator between comfort and injury reduction. The objective of the experiment was to examine cushioning effects at different impact energies for both heel and forefoot, determine how stiffness affects cushioning and the player's response to it, and to compare multiple shoes on all parameters.

ALSO NEW — Two new footwear comparison tools:

Shoe test section
NEW! Experiment

Tennis Shoe Traction

Traction is specific to the particular shoe-surface interface. Different surfaces will create different forces with the same shoe, and different shoes will create different forces with the same surface. These resultant traction forces are required for performance, but the open question is how much traction is required for tennis movements on a particular shoe-surface interface, how much is available, and how much is too much, contributing to many lower extremity injuries? In search of answers, a test rig was developed to measure translational and rotational traction. The initial findings are presented here.

Rod Cross explains experiment setup.
Rod Cross with test apparatus
NEW! Experiment

The Effect of Temperature on Tennis String Tension and Stiffness

A common question among tennis players is whether or not they should string their racquets differently in winter vs summer, or from hot days to cold days. It is common knowledge that most strings lose a large percentage of tension with time and play. The amount is highly material specific, with gut, nylon, and polyester losing the least tension in that order. Do variations in temperature exposure affect tension loss in these materials differently? Does it matter when the string is exposed to a temperature extreme — before, during, or after tensioning? How do variances in tension loss due to temperature changes affect the most important property of string — stiffness?

Peak impact of tennis ball on strings viewed through plexiglass window.
Peak Impact
NEW! Experiment

Spin and String Surface Roughness: Do Shaped or Textured Strings Increase Spin?

Tennis players have long held that rough-surfaced strings will create more spin. The belief is that the roughened string will more vigorously grab the backside of the ball as the racquet sweeps up and drags the ball around into topspin. Is this true? How much extra spin can a player expect?

It has also been thought that lateral string movement produces extra spin during the "snap-back" phase of the impact. This study shows that "snap-back" is only half the story and is a partial misrepresentation of actual events.

The physics of tennis ball spin is revealed as impacts with a simplified stringbed of four mains (with and without crosses) are viewed through a see-through plexiglass frame.

Impact on a single string as viewed through plexiglass window.
Single String Impact
NEW! Experiment

Impact of a Tennis Ball With a Single String

The interaction between a tennis ball and the strings of a tennis racquet is a relatively complicated process since adjacent strings tend to act independently. All of the strings tend to move in unison in a direction perpendicular to the string plane, but motion in a direction parallel to the string plane can vary from one string to the next. Some strings move tangentially more than others, or in opposite directions, depending on whether the ball lands directly on one or more of the strings or whether it lands between two strings. In order to simplify the physics of the problem, a study was made of the interaction between a tennis ball and a single string.

Digitized swing of light racuet with 200g added at 60 cm.
Experiment setup

Racquet Weighting and Swing Speed

What effect does the addition and redistribution of mass have on racquet speed, control, feel and ball speed? Players have many varied and conflicting opinions as to where and why lead tape should be added to the racquet. This in-depth study answers all your questions.


The relationships between racquet weight, balance (center of mass — CM), swingweight (moment of inertia — I), swing speed and shot speed are often topics of much confusion among tennis players. Questions arise such as, "How can a heavier racquet swing easier than a light racquet?"; "How can a head-heavy racquet have a lower swingweight than a headlight racquet?"; "Does making a racquet more headlight increase its racquet tip velocity?"; Is it true that adding weight at the axis of rotation has no affect on the swingweight and thus the racquet speed?"; "Does more mass create more plow-through?"; "Does a heavier racquet create more ball speed?"; "Is there a trade-off between racquet mass, racquet speed, and ball speed?".

String movement
String movement on spaghetti strings

Spin and String Patterns: Old, New, and Illegal

Spin is an essential part of today's game of tennis. Racquet and string advertising proclaim the spin benefits of particular products and players agonize over which racquet-string setup will generate the most spin. But how do you choose? What factors really matter to spinning a tennis ball? Why do they matter? How do you maximize the effect of those causal factors?

Ball impact with an 8 mains 0 crosses pattern on a string pattern rig.
Eight mains, zero crosses

Spin and String Pattern

The tennis world has gone topspin crazy and players want equipment that maximizes spin. Over the years there have been many theories as to what equipment configurations will increase spin. The string pattern has often been hailed as the principle spin motor. Is it? What patterns produce the most spin?

Components of spin.

Physics of the Tennis Kick Serve

One of the mysteries concerning the kick serve is how the server manages to generate topspin in the first place. If the racquet needs to rise at 30 degrees to hit a good topspin forehand, how can anyone serve a ball with a significant amount of topspin when the racquet head is rising at only a few degrees?

Kettle bell drop on strings to test performance degradation.
Kettle bell drop test

How Tennis Strings "Go Dead" — Part 1

Tennis players often complain that a tennis string, especially polyester string, "goes dead." This phenomenon has various and often contradictory descriptions. Some players experience too much stringbed action and others feel complete loss of stringbed action. And these contrary opinions can be about the very same string. How can two completely opposite descriptions of a string going dead be correct? Well, as this research will show, it seems that they can.

Stretch time and return time in a 34.2 ms impact with a steel hammer.
Deflection time ratio vs tension

How Tennis Strings Go Dead — Part 2

Part 1 dealt with several ways that strings "go dead. This experiment addresses another factor not addressed there — loss of elasticity due to an internal fatigue and breakdown of the string. This is often the explanation of players who experience string death as a loss of power. But is it true? Do strings go dead because of loss of elasticity due to an internal fatigue and breakdown of the string? What does science say?

String movement during impact.
String movement during impact

Spin and String Stiffness

The optimization rules for spin appear to be "Goldilocks Principles": the stringbed must be not too soft and not too stiff, not too closed and not too open, and not too rough and not too smooth. You can have too much or too little of each.


Until recently, the prevailing theory regarding string stiffness and spin has been that the firmness and lateral rigidity of a stiffer string will create more ball-to-string friction due to more squashing and embedding, resulting in more spin. It is true that friction accounts for most of the spin produced by the stringbed, but it does not account for most of the difference in spin between stringbeds. Recent research suggests that lateral main string movement is the most important factor determining this difference. Consequently, the explanation of how string stiffness might affect the spin differential has changed. The question now becomes, "How does string stiffness affect sideways string movement and its associated storage and return of rotational energy to the ball?"

Graph of lubricated vs non-lubricated strings.
Spin with lubed and non-lubed string

Spin and String Lubrication

Lateral movement and snap-back of the main strings has been shown by TWU to increase spin? Lateral movement is facilitated by using strings with a low string-to-string stickiness. These strings are usually stiff, hard polyester strings with low friction, minimal notching, and no fraying. But what would happen if we added lubrication to any string whether polyester, gut, or nylon, used or new? Would this increase the relative string movement of each string and, with it, the spin of the ball? To find out, we added WD-40 to all strings.

Spinning ball in flight.
Spinning ball in flight

Tennis Ball Trajectories: Aerodynamic Drag and Lift in Tennis Shots

Aerodynamic forces are responsible for both the angular, diving, hopping, topspin shot and the floating, skidding, backspin slice. What makes the ball appear to suddenly change course and dive into the court or to seemingly defy gravity and float through the air?


Tennis is a game of spin, and much attention is devoted to the mechanics and equipment for producing it. Much less attention has been given to what makes spin work for the player — aerodynamics. Other than gravity, it is the flow of air around the ball that determines its trajectory when hit at any given velocity, spin, and angle. Forces created by air flow act to decrease velocity in the direction of motion (primarily the horizontal direction in tennis) and, for spinning balls, act to increase or decrease its vertical or side-to-side speed.

Experiment setup.
Experiment setup

Which Strings Generate the Most Spin?

Tennis players have claimed for many years that some tennis strings generate more spin than others. Court-side mythology had it that stiff strings that flattened and embedded the ball as well as grabbed it with high friction surfaces increased spin. This experiment was one of the first to discover that material matters, but not for the reasons most players think.

Friction device used in experiment.
Friction sled

Sliding Friction and Spin

Main strings that move sideways and snap-back can create more spin. To achieve maximum spin, you must get the string moving, keep it moving both to and fro, and do so quickly with as little loss of energy as possible. Minimizing friction between strings is essential to this process.

string snap-back model of spin production.
Snap-back model

String Snap-Back and Spin

Videos of various stringbed setups show that string movement does indeed occur and that the strings snap back for all setups and they do so, for the most part, in less than 1/1000 second.

Double pendulum in action during a serve
Double pendulum action

The Double Pendulum in Tennis

The complex interaction between the forearm and the racquet is nicely described in terms of the physics of a double pendulum. Players don't worry about that since they know from experience what happens to the racquet when they swing their arm or use their wrist to position the racquet. However, they may not be aware of some of the fine details.