Long Interview with Tsutomu Jinji of NEXTBASE

Thank you. Yes, rather than simply providing force sensors or force plates alone,
we strive to consider the entire measurement environment, including peripheral equipment and fixtures.

【Jinji】
We also had expectations regarding the flexibility of working with a domestic manufacturer.

When building this facility (the LAB), there were no existing models to reference. We had many operational concerns. For example, when athletes pitch wearing spikes, would a turf mound be sufficient? Would we need to prepare a soil mound instead? Since we also conduct batting measurements, we had to consider the effort required to remove and reinstall the mound as well.

During the preparation of the mound, we held numerous prior meetings to discuss details such as stiffness, height alignment in relation to the motion capture system, and the angle of the force plates.

We also conducted preliminary experiments at Kokugakuin University using a partially artificial turf mound.

【Jinji】
Yes, we did. Those experiences themselves have become part of our know-how. As a result, we arrived at the current setup. Athletes can now pitch comfortably on the turf mound while wearing spikes without any discomfort. Over the past year, we have experienced no issues with the force plates, both in terms of data acquisition and noise. It is also reassuring to know that we can rely on proper after-sales service should anything arise.

Thank you very much. We will continue striving to improve!

【Jinji】
Price is another factor. As a private company, we must ensure cost recovery, so domestic manufacturers are also strong from a pricing perspective.

How Is the Back Leg Driving? This Is Where the Force Plate Becomes Essential

In what situations does the force plate contribute to feedback?

【Jinji】
We create what we call a “logic tree” to conduct structural performance analysis. First, we must define what constitutes a “mechanically sound movement.” At the LAB, we believe that increasing the energy transferred to the ball is critical.

Energy consists of potential energy, translational energy, and rotational energy. Translational energy includes velocity, and rotational energy includes angular velocity. A ball with greater total energy has higher velocity and higher spin rate. Therefore, we define such a pitch as mechanically superior.

To increase the ball’s energy, we have found that the translational velocity of the center of mass is essential. When the center of mass velocity increases, the total energy of the body increases. That energy is then efficiently transferred through the upper arm, forearm, hand, and ultimately to the ball. The fundamental source of that process is the velocity of the center of mass.

The center of mass velocity reaches its peak immediately after the lead leg makes contact with the ground. Therefore, how much the center of mass velocity can be increased before foot contact is crucial. What contributes to this increase is the “drive action of the back leg.” The force plate enables us to quantify how the back leg is pushing.

To generate greater velocity, a large force must be applied over a longer period of time. While it is relatively simple to explain differences in center of mass velocity using impulse, the specific movement patterns that produce acceleration vary from athlete to athlete. Even when performing inverse dynamics calculations, we observe several distinct patterns in how the hip and knee joints are utilized.

So the force plate allows you to quantify the fundamental source of energy generation.

【Jinji】

In response to the question of “how to push,” we evaluate force plate data to determine whether the issue lies in the direction of force, the magnitude of force, or the duration of force application. We assess these factors in a structured manner—similar to grading categories A through D—and provide feedback to the athlete.

Athletes who cannot sufficiently increase translational velocity tend to have lower ball velocity. When discussing this with them, we sometimes uncover underlying issues, such as a prior hip injury. In fact, one athlete recently returned for re-measurement after telling us his condition had improved. (laughs)

By constructing a logic tree in this way, we can mechanically extract movement errors. We then organize whether the error stems from a physical capacity issue, such as strength or power output; a conditioning issue, such as pain or limited range of motion; or a skill-related issue. Based on that classification, we prescribe training programs from the perspectives of Strength & Conditioning, physical therapy, and skill training. At this point, we can identify all movement errors! (laughs)

How have the athletes, coaches, and managers who visit reacted?

【Jinji】
Those who are not accustomed to looking at data often don’t know where to focus. Well, that’s understandable (laughs). Even if we consider only biomechanical variables—at present, we even calculate energy—there are 112 output data points.

That many?!

【Jinji】
If you ask whether all of them are important, I would say yes, they are all important—but there are priorities. Since I also played baseball, I have a general sense of the movement sensations experienced by athletes and coaches. Some data points are closer to their physical sensations, while others are more abstract. When we start by explaining the data that aligns closely with their sensations, their understanding improves much faster.

However, athletes who regularly think about their mechanics tend to grasp the data very quickly, which often surprises me. Providing feedback involves a certain level of tension, but it is also enjoyable. I sometimes feel that these feedback sessions become opportunities for athletes to study anatomy and mechanics. Even athletes who initially struggled to understand the numbers gradually begin asking proactive questions such as, “How does this compare to last time?” Because they learn using their own data, I believe the educational impact of feedback is extremely high.

Energy Evaluation Made Possible by High-Resolution AnalysisYou mentioned energy analysis as one of your unique services. Could you elaborate further?

【Jinji】
In pitching mechanics, injury prevention is a critical issue. Previous research has reported that pitchers with higher ball velocity tend to exhibit greater elbow varus torque. This varus torque is associated with the risk of injury to the medial elbow ligament, meaning that ideally, pitchers would throw with lower varus torque.

However, varus torque is considered a byproduct of increased shoulder internal rotation torque, which is necessary for generating higher ball velocity. Traditionally, it was believed that increasing velocity while preventing injury was inherently contradictory. Yet, when we actually measure professional baseball players at the LAB, we find several athletes who achieve both. (laughs) They throw close to 150 km/h, yet their elbow varus torque is extremely low.

We collect data at 1,000 frames per second, apply specialized filtering techniques, and perform inverse dynamics calculations starting from the ball. We conduct motion analysis very rigorously. I believe it is precisely because of this “high-resolution analysis” that we are able to observe these findings.

We have discovered that athletes who can simultaneously improve performance and reduce injury risk exhibit distinctive energy flow characteristics. In pitching, evaluating energy and its time derivative—power—allows us to represent the generation and absorption of mechanical work. It is critical to efficiently transmit the energy flowing through the arm without excessive absorption. We are now able to prescribe training that enhances energy transfer without increasing elbow varus torque.

As we gain clearer insight into pitching forms that reduce injury risk, we hope to collaborate more closely with medical institutions in the future. Even if an athlete recovers from elbow pain through conservative treatment, continuing to throw with the same mechanics may lead to re-injury. By conducting biomechanical motion analysis, we can evaluate why pain occurs and guide athletes toward mechanics that do not provoke pain. We aim to further develop services in this area.

As a Company, at the Center of Sports Science; As an Individual, Going DeeperCould you share your future outlook for NEXTBASE and for yourself?

【Jinji】
As a company, we want to continue expanding the LAB. At the same time, we hope to increase the number of people who can directly handle data, coordinate data, and possess mechanical knowledge. From that perspective, I would like to establish something like an academy.

While expanding outward, we must also maintain depth. Personally, I need to continue digging deeper into my own research, so I plan to write more academic papers. (laughs)

This is something that can only be achieved as a team. It would be impossible within my university laboratory alone, especially since we do not have a graduate program. We have PhD-level biomechanical researchers, certified S&C specialists, individuals with master’s degrees in biomechanics, and physical therapists. I could never accomplish this alone, and unexpected discoveries emerge precisely because we work as a team. It truly demonstrates the strength of teamwork.

That is why role allocation is important. My role is to deepen the research. Others operate the LAB. Our CEO and other team members promote and expand the business. If we can continue functioning as a cohesive team, I believe we can achieve our goals.

Thank you very much for sharing such valuable insights.
This turned into a much longer interview than originally planned (laughs).

【Jinji】
Thank you very much. I look forward to working with you again.

Likewise, thank you very much!

Bonus Section: A Deeper Look into Pitching Biomechanics

【Jinji】
During the translational phase of motion, the force vector of the back leg is directed toward the catcher. Once the lead leg makes foot contact, the vector shifts toward the second-base side.
(*See attached figure*)

Athletes who struggle to generate velocity often have an unfavorable force direction (for example, standing too upright). They are unable to fully direct force toward the intended direction of motion, and as a result, ball velocity does not increase.

Posture is also extremely important. If the pelvis is excessively closed at foot contact, it takes longer to rotate forward, resulting in energy loss. Both temporal factors and force-related factors must be understood.

Regarding energy flow, the following graph shows the magnitude of energy itself, measured in joules.

Energy peaks sequentially from
[Pelvis (LOWER TORSO)] → with a slight time lag → [Thorax (UPPER TORSO)] → [Upper Arm (UPPER ARM)] → [Forearm (FOREARM)] → [Hand (HAND)] → and almost simultaneously to the [Ball (BALL)].

(*See attached figure*)

Pelvis ➡ Thorax
Immediately after foot contact, pelvic energy reaches its maximum, followed by peak energy in the thorax. Thoracic energy becomes approximately three times greater than pelvic energy. If the lead leg collapses at this stage, force rapidly decreases. Ideally, the lead knee should extend firmly to ensure proper force transmission. If this does not occur, energy cannot be effectively transferred to the thorax (and instead returns toward the lower body). No matter how much energy is generated earlier in the sequence, it cannot be transmitted to the upper body. Athletes with poor transmission need to use the lead leg more rigidly and develop spring-like qualities.

Thorax ➡ Upper Arm
Energy decreases to roughly one-third at this stage. Athletes with poor transmission may have underlying shoulder issues, in which case a physical therapist should intervene. If no structural problem exists, the issue may lie in improper shoulder joint mechanics or insufficient strength. In cases of insufficient strength, pull-up exercises—targeting shared muscle groups—can be an effective training method.

Upper Arm ➡ Forearm
Forearm energy reaches approximately 90% of upper arm energy. In athletes with poor transmission, excessive eccentric contraction of the biceps brachii may be occurring. Some of these athletes may also be overly concerned with control. When discussing transmission inefficiencies in depth, we have sometimes found that their coach strongly disliked walks, influencing their mechanics.

Hand ➡ Ball
Athletes with poor transmission at this stage often exhibit excessive finger extension. Increasing finger stiffness is important. Improving ball grip technique is also one possible solution.

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