Category Archives: Kinesiology
Accuracy Index…What is it?
For several years, Racquet Quest has been testing racquets we have strung for an “Accuracy Index.” Click here to see an example, and then we will discuss it.
For several years, Racquet Quest has been quantifying racquet string bed behavior using our internally developed Accuracy Index. This post was drafted in 2019 but not posted util now. Why now? Players, and us, are even more frustrated with the lack of quality racquet preparations than we were in 2019! We are seeing more and more really bad stuff happening so this post may be a way to address the bad stuff and do better!
The first graph presents the relative stiffness of the main and cross string arrays, as well as the average composite stiffness. On the following page, the Accuracy Index data grid provides a segmented view of the string bed, with each color-coded cell corresponding to a specific impact zone.
This grid allows us to analyze local stiffness variations, where, for example, light blue cells typically correlate with premature failure points in topspin-oriented play. The example shown references a standard 16×19 string pattern, but the analytical framework is applicable to any string configuration.
Why does the Accuracy Index matter?
In performance terms, control is effectively a function of predictability. That is, the extent to which the string bed returns the ball along an expected trajectory with minimal variability. The Accuracy Index quantifies this by comparing actual deflection characteristics across the bed to idealized stringbed behavior. Higher accuracy values imply lower angular deviation and more consistent ball exit vectors.
We also report an Efficiency Index, which assesses how effectively the stringing process preserved the frame’s intended geometric and mechanical properties. A lower efficiency score (e.g., 71) indicates that the racquet had to internally redistribute loads—compensating for distortions incurred during stringing. This can result in non-uniform string tensions and unintended stiffness profiles across the bed, reducing both playability and predictability.
At Racquet Quest, our stringing systems utilize frame stabilization techniques that eliminate distortion during tensioning. As a result, our stringbeds exhibit minimal deviation from target parameters, allowing both Accuracy and Efficiency indices to remain at consistently elevated levels.
Implications for Racquet Technicians
Regardless of the equipment used, technicians can leverage the Accuracy Index as a diagnostic tool to identify inconsistencies introduced during stringing. Adjustments to clamping sequences, pull timing, frame support, or pre-stretch protocols can measurably improve stringbed linearity and reduce compensatory distortion.
In essence, a higher Accuracy Index isn’t just a number—it’s an indicator of a racquet’s ability to perform as engineered, shot after shot.
Is this a cost effective alternative?
Everyone loves natural gut but not everyone loves the price, even though it is very much worth it!
You may think ” I will just use synthetic gut it must be about as good”. Nope, not even close. However there is string that may be close enough to consider as an alternative.
One such string is IsoSpeed Professional. Because we know “a picture is worth a thousand words” here is a picture:
IsoSpeed v VS Touch
If you are serious about playing tennis at a high level for a long time please consider an alternative to a string that does not help that effort.
This is one, however there are others that we will highlight in future posts.
Three Important Things
When dealing with tennis racquets, there are many significant performance parameters. We are going to list our three (3) most important ones:
- Swing Weight
- Overall Weight
- Effective Stiffness
These particular parameters are based on machine testing, physics, and experience.
- Swing Weight: we refer to this as a dynamic weight, meaning how heavy or light the racquet feels during the swing to the ball.
- Overall Weight: This is normally considered to be the weight found on a scale, electronic, or balance. Weight is your friend if properly associated with your capabilities and stature.
- Effective Stiffness: is a calculable and measurable stiffness of the ball hitting the string as you are holding the racquet and hitting a ball.
Why is it Important?:
- Swing Weight: contributes to power and stability, which contributes to control. A higher swing weight of, say, 320 to 340 will handle most incoming balls with ease. Swing weights of 300 to 319 will be easier to maneuver and require a little more energy from the player to handle aggressive incoming balls.
- Overall Weight: contributes to ease of movement and long play time and must be associated with the stature and style of the player. The heavier racquet will provide more power with less energy expended by the player.
- Effective Stiffness: this is a metric we use to ensure the racquet is within the stiffness range needed by the player. For example, if you come into the World Headquarters of Racquet Quest for the first time with your current racquet and the strings are intact, we can determine the effective stiffness and, based on your satisfaction with the current playability, arrive at a corresponding effective stiffness…not on a reference tension. A range of 24 to 30 pounds is acceptable. Anything over 30 becomes quite stiff and should be avoided by most players.
If you look at our racquet reviews, you will see a lot more than three (3) performance parameters! We believe they are all important, but players should understand the three (3) described here.
Yonex V-Core 100
The Yonex V-Core is a great opportunity to review the racquet with IsoSpeed Professional 17 gauge string. If you know anything about the racquet you will know it is a great platform for many player types.
But what about IsoSpeed strings? These strings, and there are a series of them, are unusual in the material and construction, therefore not very well considered!
The string is polypropylene fibers bonded together then stretched and packaged on a stiff molded reel to retain some of the stretching.
If you have seen our previous review on our Questron data you will notice some similarity to natural gut.
| Manufacturer | Yonex |
|---|---|
| Racquet Model | Yonex V-Core 100 |
| Reference Tension | 55 |
| String | IsoSpeed Professional 17 |
| Machine Used | True Tension Pro |
| String Bed Stiffness - RDC | 41 |
| String Bed Stiffness -FlexFour | 53 |
| String Bed Stiffness SBS | 48.6 |
| String Bed Stiffness - ERT | 31 Kg/Cm^ |
| Racquet Flex, RDC | 66 |
| Racquet Flex, FlexFour | 48.6 |
| Racquet - In Plane Stiffness | 423.7 lbs/In |
| Weight, Grams | 321 |
| Weight, Ounces | 11.32 |
| Balance, mm | 324 |
| Balance, Inch | 12.76 |
| Length, Cm | 68.8 |
| Length, Inch | 27.08 |
| Head Width | 9.87 |
| Head Length | 12.82 |
| Head Area, cm2 | 641.3 |
| Head Area, Sq. Inch | 99.4 |
| Beam Height @ Grip, mm | 22 |
| Beam Height @ Mid, mm | 25.6 |
| Beam Height @ Tip | 25.5 |
| Beam Width @ Grip, mm | 16.3 |
| Beam Width @ Throat, mm | 15.2 |
| Beam Width @ Mid, mm | 10.3 |
| Beam Width @ Tip, mm | 11.6 |
| Number of Main Strings | 16 |
| Number of Cross Strings | 19 |
| Ratio | .649 |
| Main String Grid Inch/mm | 7.50 / 190.5 |
| Cross String Grid Inch/mm | 10.15 / 257.8 |
| Density (% of head filled with string) | .747 |
| Average Cross String Space | .529 |
| Average Main String Space | .461 |
| Dynamic Tension, Kp, ERT | 31 |
| Dynamic Tension, Lbs/in | 173.4 |
| First Moment, Nm | .800 |
| Polar Moment | 328 |
| Torsional Stability | 15 |
| Swing Weight, Kg/cm2 | 313.0 Kg/Cm^ |
| Swing Weight, Ounces | 11.094 |
| Swing Weight Calculated | 337.0 |
| Power, RDC | 58 |
| Control, RDC | 41 |
| Manueverability, RDC | 78 |
| Power, Calculated | 2073.9 |
| Head Points | 5.98 |
| Head Weight, % | 47.2% |
| Center of Percussion | 21.1 |
| Dwell Time, ms | 7.31 |
| Efective Stiffness - lbs | 25.3 |
| K, Lb/In | 131.4 |
| Recoil Weight | 154.2 |
| Twist Weight | 233.8 |
| End Weight | 130.7 |
| Tip Weight | 188.4 |
| 9 O'Clock | 90.5 |
| 3 O'Clock | 89.4 |
| Butt End | 140.2 |
| COF, Main | .392 |
| COF, Cross | .321 |
Racquet Quest, LLC
