CAUGHT LOOKING: ROSIN, PORTABLE JUMP TESTS AND PERCEPTION TRAINING

| Research
Reading Time: 8 minutes

Here are three studies the R&D department at Driveline read this week:

EFFECTS OF ROSIN POWDER APPLICATION ON THE FRICTIONAL BEHAVIOR BETWEEN A FINGER PAD AND BASEBALL

With the use of sticky substances being the talk of the town these days among MLB pitchers, this study takes an interesting and topical look at what one particular traditional and legal substance (rosin) has on the friction coefficient between the finger and the surface of a baseball.

For a long time, rosin has been used to change the feel of how the baseball releases from the pitcher’s hand. The friction coefficient is one way to quantify this characteristic by basically describing how much force it takes to slide a finger across the surface of a baseball. The idea is that with a higher friction coefficient, it’s more sticky and therefore harder for the ball to slide across the finger, leading to the ball rolling off the finger instead of sliding and causing more rotation (more spin) of the baseball. 

The illegal sticky substances that have recently been talked about, such as pine tar, are seemingly used to increase the friction coefficient, but rosin has less of a clear-cut purpose. This study looks at the effects of rosin on the fraction coefficient in wet and dry conditions.

10 young adult male subjects participated, sliding their dominant hand’s index finger across leather removed from a baseball and attached to a force measurement device.

They tested the effect when there was a large normal force (subjects pressing down really hard) and when there was a small normal force (subjects did not push down their finger very hard) Yamaguchi et al. found that:

When the subjects put a lot of pressure on the surface (large normal force)

  • In the dry condition, the friction coefficient went down when rosin was used
  • In the wet condition, the friction coefficient went up

When the subject put little pressure on the surface (small normal force)

  • In the dry condition, the friction coefficient went down
  • In the wet condition, the friction coefficient went down

Basically, when the ball is wet, rosin makes the ball “more sticky” when applying a lot of pressure, but in all other conditions, rosin makes the ball less sticky. This would suggest that if you are hoping to throw a stickier ball, then rosin is best to use when the ball or your hand is wet. Alternatively, there may be times when you want the ball to be less sticky, in which case, it looks like rosin would be a good option. 

As mentioned above, rosin has been used for a long time and likely for reasons other than trying to make the ball more sticky. From personal experience, it felt like rosin made the ball less sticky. Nonetheless, this is good information for pitchers who use rosin to know the effect it may have on the spin characteristics of their pitches. With these results, you would think that rosin will decrease the pitcher’s spin rate in most cases. 

CONCURRENT VALIDITY AND RELIABILITY OF THREE ULTRA-PORTABLE VERTICAL JUMP ASSESSMENT TECHNOLOGIES

As force plate assessments (jump testing, specifically) become more and more common in the integration of skilled and S&C training settings, it is useful to identify valid tools for measurement that are more accessible than force plates. Although force plates are considered the gold standard for many force and jump testing assessments, they are inaccessible to many due to price.

Watkins et al. at the Auckland University of Technology validated three portable and accessible vertical jump assessment technologies measuring the squat jump, counter-movement jump, and drop jump. Two-dimensional motion capture with video, G-Flight, and Push were the three technologies compared to the gold-standard force plate measurement. 

Two-dimensional motion capture was done using an iPhone with a front-view of the athlete, which was later imported to Kinovea for frame-by-frame manual analysis. G-Flight is an optical-based timing system with sensors placed on either side of the force plates, and the PUSH accelerometer-based sensor was placed on the subjects’ lower back using a strap. 

A range of statistical tests and descriptions were used to exhibit the three measurement tools’ validity with force plates in terms of jump height, contact-time, and reactive strength index. The authors concluded that reactive strength index (jump height divided by contact-time) was the most valid across the three technologies. The results for each of the three technologies include the following:

Two-dimensional motion capture:

  • Showed the most similarity to the force plate measurements overall
  • Overestimated contact-time by 12 milliseconds on average
  • Contact time error tended to be higher with shorter contact times
  • Longest processing time

G-Flight:

  • Overestimated jump height by between 1.3 and 4 centimeters
  • Greatest variability out of the three measurements
  • Reactive strength index error tended to be higher as RSI increased

PUSH:

  • Overestimated jump height by between 4.1 and 4.5 centimeters
  • Underestimated contact time by about 24 milliseconds on average
  • Reactive strength index error tended to be higher as RSI increased
  • Tough to implement with large groups without multiple units

Overall, for a coach, parent, or player who wants to implement jump testing assessments or monitoring, motion capture is the most accurate if processing time and effort is not a priority. G-Flight would be best for larger groups despite slightly larger variability because the processing time is short, and it is easy to switch between athletes (no physical attachment to the subject).

PUSH had reasonable variability, but its use becomes cumbersome for groups due to switching the unit between subjects. Overall, it is clear that accessible and portable alternatives to force plates exist. And lastly, with the appropriate considerations, including jump assessments into an athlete’s training/monitoring is a reasonable venture. 

BEHAVIORAL MEASURES IN A COGNITIVE-MOTOR BATTING TASK EXPLAIN REAL GAME PERFORMANCE OF TOP ATHLETES

There are several products on the market created to train vision and perception skills in hitting that are video-based. Most of them do not include the actual task of swinging a bat but rather identifying a pitch, working on timing, identifying a strike vs. a ball, etc. Nasu et al. in Japan did a study with softball hitters that suggests the action of swinging a bat is important for assessing their perception skill. 

This study included an assessment of identifying a “fast ball” or a “slow ball” from a live pitcher by actually swinging and trying to hit the pitch, as well as an assessment of identifying the pitch and pushing a button without having to try to hit the pitch. To compare these two assessments, a timing measurement was created to determine how accurately the hitters adjusted the initiation of their swing to the different pitch speeds. This measure was then correlated to hitting performance measured by exit velocity and whiff percentage. 

The same hitters then did the same activity, but instead of trying to hit the pitch, the subjects were instructed to push a button when it was a fastball as fast as possible. This identification performance was then compared to their exit velocity and whiff percentage to see how well it correlated to hitting performance. The authors found that the batting task explained almost twice as much of the variance in batting performance (67%) than the button-pressing task (34%). 

I’ll start this discussion by mentioning how this study is limited by the study design because I believe that for the most valid comparison of the two conditions, in the button-pressing task, the subjects should have been instructed to time the pressing of the button when the pitch crosses the strike/swing zone as opposed to pressing it as soon as they identify the pitch type. However, the significantly improved predictive performance of the batting task goes to show you that it is important to incorporate the actual act of swinging the bat in these perception tasks. 

If including an actual swing in a live test describes the batters’ ability by almost double that of a non-swinging task, it seems reasonable that training perception without actually swinging a bat will not yield optimal results. Not-to-mention, this study was done against a live pitcher. I imagine it would be even less descriptive of batter ability if the test was done while facing a virtual pitcher from a video. 

This isn’t to say that using the currently-available vision and perception training tools will not work—we have not done any training studies ourselves—but the transferability should be considered when training hitters’ vision and perception in non-live scenarios. In summary, it looks like this is more evidence to suggest training against live arms, in a live situation, is important. 

By Kyle Lindley (@kylelindley_)

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