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Sagittal-Plane Thoracic and Lumbar Curvature Angles in Male High-Level Baseball Players: A Comparison Between Pitchers and Position Players

This cross-sectional study utilizing the Spinal Mouse reveals that male high-level baseball pitchers exhibit significantly smaller standing thoracic kyphosis and lumbar lordosis angles compared to position players, indicating distinct sagittal spinal profiles associated with playing position.

Original authors: Kyosuke Numaguchi, Daisuke Momma, Masaru Munemori, Yuki Matsui, Eiji Kondo, Norimasa Iwasaki

Published 2026-09-11

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CC BY 4.0

Original authors: Kyosuke Numaguchi, Daisuke Momma, Masaru Munemori, Yuki Matsui, Eiji Kondo, Norimasa Iwasaki

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The human spine is not a rigid pillar but a flexible column of bones that allows us to stand, twist, and bend. In the world of sports medicine, researchers often look at how the spine curves in its natural resting position. Two of these curves are particularly important: the upper back, which naturally bows outward, and the lower back, which naturally curves inward. These shapes are not just about posture; they are part of a larger system that transfers power from the legs to the arms. When an athlete throws a ball at high speed, the force travels up through the legs, spins through the hips and torso, and finally snaps out through the shoulder. Because this motion is so intense and repetitive, scientists have long wondered if the spine itself changes shape over time to accommodate the specific demands of a sport.

Baseball offers a unique laboratory for this question because the game splits players into two distinct groups with very different physical jobs. Pitchers spend years perfecting a single, explosive motion that requires extreme twisting and bending of the trunk. Position players, such as infielders and outfielders, also throw and run, but their movements are more varied and less repetitive in their specific mechanics. While previous studies have shown that pitchers often develop different shoulder and hip characteristics than their teammates, it remained unclear whether their spines also adapt in a unique way. Specifically, researchers wanted to know if the natural curves of the upper and lower back differ between these two groups of elite athletes, and if those differences hold true when the players bend forward or backward as far as they can.

A team of researchers from Hokkaido University set out to answer this question by examining ninety-one male baseball players, ranging from top-tier college athletes to professional players. The group included fifty-four pitchers and thirty-seven position players. To understand the shape of their spines, the scientists used a handheld device that slides along the skin of the back to measure the angles of the curves without the need for X-rays. They asked each player to stand still, then bend forward as far as possible, and finally arch backward as far as possible. To ensure their measurements were accurate, they also compared the device readings against actual X-ray images for a smaller subset of the players, confirming that the handheld tool provided a reliable picture of the spine's alignment.

The results revealed a clear and consistent difference between the two groups. When standing upright, the pitchers had a noticeably straighter spine than the position players. Their upper backs were less curved outward, and their lower backs were less curved inward. In fact, the difference was significant enough that the researchers could distinguish the two groups based on these measurements alone. This pattern of a straighter spine was not just a resting trait; it persisted even when the players bent forward. The pitchers maintained a straighter profile in their upper back and a different curve in their lower back compared to the position players during this movement. However, when the players arched backward, the differences between the two groups disappeared, and their spinal curves looked much more similar.

These findings suggest that the years of repetitive throwing do more than just strengthen muscles; they appear to be associated with a distinct, flatter spinal alignment in pitchers. The researchers emphasize that this straighter spine should not be viewed as a defect or a sign of injury. Instead, it seems to be a natural adaptation to the specific demands of pitching, where the body may benefit from a different spinal shape to handle the forces of the throw. The study highlights that when doctors or trainers evaluate a baseball player's spine, they must consider the player's position on the field. A spine that looks straighter than average might be perfectly normal for a pitcher, whereas the same alignment might be unusual for a position player. By recognizing these position-specific profiles, experts can better understand the unique physical makeup of elite athletes without mistakenly labeling their natural adaptations as problems.

Technical Summary: Sagittal-Plane Thoracic and Lumbar Curvature in High-Level Baseball Players

Problem Statement
Baseball involves high-velocity, whole-body movements where force transfer from the lower extremities to the upper extremities relies heavily on the spine as a central component of the kinetic chain. While previous research has established position-specific musculoskeletal adaptations in the shoulder, elbow, and hip (e.g., humeral retrotorsion in pitchers), and has examined trunk motion or thoracolumbar rotation ranges, it remains unclear whether distinct sagittal spinal profiles exist between pitchers and position players. Specifically, no prior study has separately evaluated regional sagittal alignment (thoracic vs. lumbar) in standing, maximal flexion, and maximal extension to determine if playing position correlates with specific curvature angles.

Methodology
This cross-sectional study analyzed 91 male high-level baseball players (54 pitchers, 37 position players) comprising professional and collegiate athletes. Participants were required to have at least three years of experience in their specific position and be free of low back pain affecting participation or history of spinal surgery.

Measurement Protocol: Sagittal spinal curvature angles were measured using the Spinal Mouse® (Idiag, Switzerland), a non-invasive device that tracks intersegmental angles along the skin surface. Measurements were taken in three postures: standing, maximal flexion, and maximal extension.
- Standing: Comfortable stance, knees extended, arms relaxed.
- Maximal Flexion: Trunk flexion with knees extended.
- Maximal Extension: Trunk extension with arms crossed, heels and knees on the floor.
Definitions: The thoracic curvature angle was defined as the cumulative angle from T1/2 to T12/L1, and the lumbar curvature angle from L1/2 to L5/S1. Positive values indicated kyphosis; negative values indicated lordosis.
Validation: Intra-examiner reliability was assessed via two repeated measurements on all 91 players. Additionally, a subset of 39 players underwent same-day lateral whole-spine radiographs to validate the Spinal Mouse's correlation with radiographic sagittal alignment.
Statistical Analysis: Comparisons between groups utilized Welch 2-sample t-tests. Effect sizes were calculated using Hedges g. P-values were adjusted for multiple comparisons using the Holm method. Stratified analyses were performed by competitive level (professional vs. collegiate).

Key Results

Reliability and Validity: The Spinal Mouse demonstrated acceptable intra-examiner reliability (Intraclass Correlation Coefficients ranging from 0.78 to 0.88). Standing measurements correlated significantly with radiographic thoracic kyphosis ( $r=0.704$ ) and showed a weaker but significant correlation with radiographic lumbar lordosis ( $r=0.418$ ).
Standing Alignment: Pitchers exhibited significantly smaller standing thoracic kyphosis ( $31.0^\circ \pm 7.6^\circ$ ) compared to position players ( $36.8^\circ \pm 5.9^\circ$ ; $P=0.0005$ ). Similarly, pitchers had significantly smaller standing lumbar lordosis (mean angle $-22.3^\circ \pm 7.2^\circ$ ) compared to position players ( $-28.6^\circ \pm 5.6^\circ$ ; $P<0.0001$ ). This indicates a straighter sagittal spinal alignment in pitchers.
Maximal Flexion: Position-specific differences persisted in maximal flexion. Pitchers showed smaller thoracic curvature angles ( $48.2^\circ$ vs. $52.8^\circ$ ; $P=0.0266$ ) and larger lumbar curvature angles (indicating less kyphosis or more lordosis in the flexed state, $34.5^\circ$ vs. $29.5^\circ$ ; $P=0.0266$ ) compared to position players.
Maximal Extension: No statistically significant differences were observed between pitchers and position players in either thoracic or lumbar curvature angles during maximal extension.
Stratification: The direction of these findings (smaller thoracic kyphosis and lumbar lordosis in pitchers) remained consistent across both professional and collegiate competitive levels.

Significance and Claims
The study concludes that male high-level baseball pitchers possess a distinct sagittal spinal profile characterized by smaller standing thoracic kyphosis and smaller standing lumbar lordosis compared to position players. The authors assert that these findings should not be immediately interpreted as pathological alignment or injury risk indicators, but rather as position-specific characteristics resulting from repeated baseball movements and trunk-use patterns.

The paper highlights that evaluating the thoracic and lumbar regions separately provides a more detailed description of regional alignment than treating the spine as a single trunk segment. These position-specific profiles offer reference information for spinal assessment in high-level baseball players. The authors note that while the differences are clear in standing and flexion, the lack of difference in maximal extension suggests the divergence is primarily in static alignment rather than end-range extension capability.

Limitations
The authors acknowledge that the cross-sectional design precludes determining causality (whether the alignment is acquired through pitching or pre-existing). Furthermore, the findings are limited to asymptomatic male professional and collegiate players and may not generalize to female, younger, recreational, or symptomatic populations. The study did not evaluate dynamic pitching motion, adjacent-joint characteristics, or injury incidence.

Technical Summary: Sagittal-Plane Thoracic and Lumbar Curvature in High-Level Baseball Players

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