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Impact of 12-s Rule on Performance and Muscle Damage of Baseball Pitchers


Medicine & Science in Sports & Exercise: December 2016 - Volume 48 - Issue 12 - p 2512–2516
doi: 10.1249/MSS.0000000000001048

Introduction A recent US Major League Baseball (MLB) rule change requires baseball pitchers to deliver pitches within 12 s.

Purposes To examine the effect of three between-pitch rest intervals on throwing performance during a simulated seven-inning game and muscle damage during postgame recovery.

Design A randomized counterbalanced study.

Methods Seven intercollegiate pitchers threw 15 pitches per inning for seven innings with rest interval trials of 8, 12, and 20 s between pitches and 5 min between innings. Pitchers threw aimed fastballs at their best effort. Trials were separated by ≥2 wk.

Results Progressive decreases in pitching speed and accuracy below baseline (first inning of 20-s trial) occurred after fourth inning during the 8-s and 12-s trials, but not the 20-s trial. Plasma creatine kinase elevated 48 h later for the 8-s and 12-s trials (+105% and +75%, P < 0.01), but not the 20-s trial (+26%, no significance). A transient interleukin (IL)-6 surges immediately after the game for the 8- and 12-s trials (+265%, +128%, P < 0.01) above baseline. IL-6 reversed below the level of 20-s trial at 48 h after game, whereas IL-10 increased significantly above the level of 20-s trial.

Conclusions Under the same pitching load, decreasing rest interval from 20 to 12 s or less results in an early-onset performance loss during a game and increases in muscle damage and inflammation for more than 2 d after a game. Our data do not favor the current rule change in concern of keeping musculoskeletal health of pitchers.

1Athletic Department, Shih Hsin University Taipei, TAIWAN; 2Graduate Institute of Athletics and Coaching Science, National Taiwan Sport University, Tao Yuan County, TAIWAN; 3Athletic Department, National Taipei University of Business, Taipei, TAIWAN; 4Department of Physical Therapy and Graduate Institute of Rehabilitation Science, China Medical University, TAIWAN; 5Department of Occupational Therapy, Asia University, Taichung, TAIWAN; 6Athletic Department, Vanung University, Taoyuan, TAIWAN; 7Department of Sports Sciences, University of Taipei, Taipei, TAIWAN; and 8Department of Kinesiology, Health, and Nutrition, The University of Texas at San Antonio, San Antonio, TX

Address for correspondence: Kuo, Chia-Hua, Ph.D., Laboratory of Exercise Biochemistry, University of Taipei, 101 Zhungcheng Road, Section 2, Shilin District, Taipei 111, Taiwan; E-mail:

Submitted for publication April 2016.

Accepted for publication July 2016.

The average length of Major League Baseball (MLB) games in the United States has increased >25 min from 2004 to 2014 (1). As a result, the MLB proposed a rule change that would put a 20-s pitch clock in games. Currently, the new rule (rule 8.04 in the MLB rule book) states a pitch must be thrown every 12 s. However, this rule has not yet been enforced by clock counting. The average pitch pace is around 22.6 s (1). The nonclocked, 12-s rule is currently being tested by the University Baseball League in Taiwan as well. The enforcement of the 12-s rule will inevitably reduce between-pitch recovery time by 40%. Although the rule does not directly affect overall pitcher throwing load during a game, throwing fatigue, defined by performance declines in velocity and accuracy of a pitcher, could result due to insufficient recovery. Data analysis of a public website comparing games with average pace (22.6 s), 12-s pace (under rule 8.04 in the MLB), and 20-s pace (2014 Arizona Fall League) conditions among 73 pitchers suggests an increased muscle fatigue as recovery time between pitches decreases (12). Muscle fatigue can weaken joint stability and alter kinematics (6,9,15), which may, in turn, increase the chance of injury in pitchers (16). No data from randomized controlled trials are currently available regarding the effect of decreasing between-pitch time intervals on pitching performance during a game and postgame muscle damage and inflammation of pitchers.

During and after a baseball game, creatine kinase (CK) in damaged muscle of pitchers is released into blood, leading to an elevated CK level in blood (2,11). Such muscle damage is commonly known to trigger tissue inflammation (10,17). Early elevations in proinflammatory cytokine interleukin (IL)-6 and delayed elevations in anti-inflammatory IL-10 are important indications of the progression of recovery after the imposed muscle stress. Increased IL-10 shifts the inflammation process from phagocytic phase into a protracted regenerative phase until the resolution of inflammation (14). Therefore, elevated levels of muscle soreness, CK, and IL-10 collectively indicate an incomplete recovery after imposed stress.

To the best of our knowledge, the impact of the new 12-s rule on throwing performance (accuracy and speed) of pitchers during a game and on muscle damage and inflammation after a game has not yet been documented. Thus, the main purpose of the study was to compare throwing performance of pitchers during a seven-inning game between the 12-s and 20-s pace. A more applicably relevant 8-s trial was added because pitchers must release a pitch before 12 s when the rule is enforced. Participant’s plasma CK, IL-6, and IL-10 levels were measured daily throughout the 72-h postgame recovery period. A seven-inning simulated game model was used based on previous literatures (8). Average pitching length among pitchers from the 2014 season was 6.13 innings (12). We hypothesized a proportional performance loss of pitchers during a seven-inning game and delayed recovery of muscle damage after a game as rest interval between pitches shortens.

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Eleven division I intercollegiate baseball pitchers from University Baseball League in Taiwan volunteered for this study. Four participants did not complete the study protocol, and their data were not reported. Mean age, height, and weight of the remaining seven participants were 21 ± 0.3 yr, 180 ± 1.7 cm, and 82 ± 2.2 kg, respectively. Average history of playing baseball for participants was 7.2 ± 0.4 yr. The study protocol was approved by the institutional review board at National Taiwan Sports University, conducted in accordance with the guidelines in the Declaration of Helsinki. Participants provided their written informed consent to participate in this study.

The study was conducted during the off-season. Participants were instructed to maintain their normal exercise training activity although this was not recorded. Because blood CK elevation can persist for approximately 48 h after a baseball pitch performance (11), participants were instructed to refrain from pitching, and any other vigorous exercise 2 d before each trial to avoid the interference of regular training on observation. Nonthrowing bullpen and side sessions were allowed for conditioning and practice.

Three randomized, counterbalanced trials were conducted to evaluate the effect of rest interval between pitches on performance during a seven-inning simulated game and postgame muscle damage and inflammation markers. Trials were separated by at least 2 wk. Pitchers reported at 7 am after breakfast for an hour of quiet rest with the assigned trial beginning at 8 am. All participants threw 15 pitches per inning for seven innings (8) with trial between-pitch rest intervals of 8, 12, and 20 s. A 5-min rest interval was allowed between innings. Pitchers were asked to throw fastballs into the strike zone at their best effort throughout each trial. One day before each trial (Pre), muscle soreness, plasma CK, IL-6, and IL-10 of each participant were measured. The throwing performance indicators throwing speed (km·h−1) and accuracy (% strike rate) were averaged at the end of each inning. Muscle damage and inflammatory markers were measured at Pre (1 d before trial), 24, 48, and 72 h postgame.

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Pitching performance

Throwing speed was measured by a radar gun (Jugs Sports, Tualatin, OR) positioned 20 ft from the plate. Accuracy (percentage of pitches across the strike zone in flight) was judged by a professional baseball umpire.

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At the end of each inning, participants rated their perceived level of pitching effort required to produce maximal pitch velocity and accuracy using the Borg RPE scale, with light, hard, and very hard scored as 11, 15, and 17, respectively.

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Blood sampling and analyses

Venous blood samples were collected from an antecubital vein for the measurements of CK, IL-6, and IL-10. Plasma samples were obtained by centrifuging the blood for 10 min at 4°C. They were stored at −80°C until analysis. Plasma CK was measured using Fuji Dri-Chem 4000i analyzer (Fuji Photo Film Co, Tokyo, Japan). IL-6 and IL-10 were measured using with enzyme-linked immunosorbent assay kits (Quantikine ELISA Kit; R&D system, Minneapolis, USA) on an ELISA analyzer (Tecan Infinite 200 PRO, Tecan, Switzerland).

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Muscle soreness

A visual analogue scale (VAS) was used to rate the soreness of pitchers experienced after game. Muscle soreness was assessed using a 100-mm VAS continuum with 0 mm represented “no pain at all” and 100 mm represented “unbearable pain.”

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Statistical analysis

Shapiro–Wilk test was used to test whether the data were normally distributed. Homogeneity of variances was confirmed by Levene test. Comparisons between paired data were analyzed using the nonparametric Wilcoxon signed rank test. To analyze changes over time, a Friedman test was used and when a significant F ratio was found, Mann–Whitney U test was used for post hoc analysis. A level of significance was set at P < 0.05, and all values are expressed as means ± SE.

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A proportional decrease in throwing performance below baseline (mean value of first inning during the 20-s trial) was observed as throwing pace of pitchers increased from 20 to 8 s in the simulated seven-inning game (Fig. 1). During the 8-s and 12-s trials, significant declines in average throwing speed (Fig. 1A) below baseline began at the fourth inning (8-s: P = 0.05; 12-s: P = 0.05, respectively) and persisting until the seventh inning (8-s: P < 0.01; 12-s: P < 0.01, respectively), whereas no significant change was observed during the 20-s trial at inning 7 (P = 0.26). Similarly, significant declines in pitching accuracy (% strike rate) below baseline began at the fifth inning (8 s: P = 0.03; 12 s: P = 0.04, respectively) throughout the seventh inning (8 s: P < 0.01; 12 s: P < 0.01, respectively) during the 8-s and 12-s trials. No significant change was observed during the 20-s trial at inning 7 (P = 0.71). The accuracy for the 8-s and 12-s trials at the end of the seventh inning declined 43% and 9% below baseline (Fig. 1B). Reported RPE during the game increased for all trials (P < 0.01; P < 0.01; P < 0.01, respectively) over the seven innings (Fig. 2).





Subjective soreness after the pitching trial, assessed by VAS (Fig. 3A), peaked at 24 h after game. Time effects of VAS soreness for all trials reached statistical significance (8 s: P < 0.01; 12 s: P < 0.01; 20 s: P < 0.01), with minimal difference detected among the three trials at the seventh inning. No significant differences in plasma CK at Pre of all participants was found among the three trials (first trial: 138 ± 26, second trial: 145 ± 35, third trial: 150 ± 30 U·L−1). Plasma CK rise was highest during the 8-s trial, followed by the 12-s and 20-s trials (48 h: 12 s vs 20 s, P < 0.01; 8 s vs 20 s, P < 0.01; 8 s vs 12 s, P < 0.25, respectively) (Fig. 3B). Time effects of CK for all trials during the same recovery period reached statistical significance (8 s: P < 0.01; 12 s: P < 0.01; 20 s: P < 0.01).



Inflammatory cytokine data are shown in Figure 4. Plasma IL-6 peaked immediately after the simulated game after the 8-s trial and, to a lesser extent, the 12-s trial; but the rise in IL-6 after the 20-s trial did not reach statistical significance (Fig. 4A) (Post: 12 s vs 20 s, P < 0.01; 8 s vs 20 s, P < 0.01; 8 s vs 12 s, P = 0.25, respectively). Time effects of IL-6 response were significant for the 8-s and 12-s trials (P < 0.01; P < 0.01, respectively), but not for the 20-s trial (P = 0.39). Two days after the simulated game, IL-6 values for the 8-s and 12-s trials diminished below the mean value of the 20-s trial. During the same period, plasma IL-10 gradually increased after the 8-s and 12-s trials, but not the 20-s trial (24 h: 12 s vs 20 s, P < 0.01; 8 s vs 20 s, P < 0.01; 8 s vs 12 s, P = 0.71, respectively) (Fig. 4B). Time effects on IL-10 were significant for the 8-s and 12-s trials (P < 0.01; P < 0.01, respectively), but not for the 20-s trial (P = 0.15).



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The recent MLB rule change shortens the time of recovery between pitches during a baseball inning. No existing literature is currently available regarding the impact of this rule change on throwing performance during a game and muscle damage and inflammation after a game for a pitcher performing the same throwing load with decreased interpitch rest. In this randomized controlled study, we found that decreasing rest interval between pitches results in 1) an early onset decline in throwing performance of pitchers, including both pitching speed and accuracy; and 2) increased muscle damage and inflammation of pitchers after the game.

In this study, pitching performance is defined by an interaction of two major components: velocity and accuracy. Increasing pitching pace appears to impair accuracy (−43% and −9% for the 8-s and 12-s rest trials, respectively) more notably than speed (−3% and −2% for the 8-s and 12-s rest trials, respectively). The performance loss may be associated with increased muscle fatigue, suggested by previous observations on estimated muscle fatigue of pitchers comparing conditions under MLB and Arizona Fall League rules (12). To avoid loss in pitching performance due to the rule change, an early pitcher replacement after the fourth inning may be required to ensure optimal game outcomes of a team while preserving the musculoskeletal health of a team’s pitching cadre.

Inflammatory response after a pitching performance among pitchers with a different throwing pace has not been measured previously. Most previous reports on pitching related inflammation have been based on clinical examination without measuring the actual changes in blood inflammatory mediators and muscle damage markers (3). In this study, elevated inflammatory mediators and CK levels in plasma confirm that reduced recovery time between pitches to less than 12 s mandates greater recovery time of a pitcher after a game.

It is not surprising that IL-6 was suppressed during 8- and 12-s rest trials when IL-10 is elevated, because IL-10 has been shown to inhibit IL-6 release from human monocytes and T cells (7). Inflammation is a wound healing process consisting phagocytic (destructive) and nonphagocytic (regenerative) phases (13). During the first phase, unhealthy muscle fibers are removed and stem cells are recruited to the damaged site, characterized by a dramatic increase of circulating IL-6 released from injured muscle (5,13). During the second phase, new muscle fibers regenerate and differentiate into mature muscle tissue, characterized by a protracted IL-10 level in circulation (5,7). IL-10 is essential to switch off the phagocytic phase and maintain the regenerative process of recovering skeletal muscle (4,5). The sequential responses of IL-6 and IL-10 are in agreement with the findings from monocyte cell culture experiments (4,7), where production of IL-10 by human monocytes occurs relatively late compared with the production of many early inflammatory mediators, such as IL-6, which were secreted at high levels during the first day after damage (4). Sustained soreness and elevated CK and IL-10 levels in the current study suggest that the damaged tissue of pitchers has not yet fully recovered at 48 h after game, when rest interval between pitch is 12 s or less. Our findings suggest that coaches should promote >2 d rest for sufficient recovery after a seven-inning game under the current 12-s rule.

The major limitation of the study is the small number of participants. Elite pitchers represent a relatively small population in Taiwan. Therefore, the increased CK and IL-6 after the 20-s trial did not reach statistical significance. It would be more valuable if such study can be conducted in greater scale to precisely estimate the appropriate pace for pitcher health at high school, college and professional levels. Furthermore, an important unanswered question is whether increasing pitching pace will compromise dynamic joint stability and kinematics of pitching arm due to accumulated muscle damage and fatigue (12). It is possible that fatigue, accumulated muscle damage, and inflammation with high pitching pace can weaken joint stability, proprioception, and kinematics of pitchers (6,9,15), and thus increase the chance of more severe overuse-type injury throughout a pitcher’s career. A long-term prospective investigation of relative risk of musculoskeletal injury and professional longevity of pitchers is warranted before the 12-s rule is globally adopted.

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The results of the study demonstrate that decreasing the rest interval between pitches results in an early-onset performance loss during a game and increased muscle damage and inflammation after a game. To ensure musculoskeletal health of pitchers as well as preserving their performance merit and team winning potential, early replacement by relief pitchers during a game or providing a longer recovery time for pitchers after a game may be needed.

This work was supported by Shih Hsin University, grant number P9907 and P10012. There is no conflict of interest in any aspect of this study. The authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by ACSM.

S. C. Y., C. C. W., and S. C. L. equally contributed to this work.

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