In the present study, 12.2% of players developed internal oblique muscle strain in a single Japanese professional baseball team, with the incidence significantly higher in fielders. Most injury sites were at the region of the muscle insertion to the lower ribs. A mean of 36.5 months after the initial injury, 21.7% of the players were injured again, which is by no means a small proportion. A mean time of 29.4 days was needed to return to play.
In the MLB, 393 cases of abdominal muscle strain were reported to occur over a period of 20 years, with the number increasing every year.4 In addition, the re-injury rate was reported to be 12%,4 which is lower than in the current study. The incidence of internal oblique muscle strain in fielders and pitchers was reported to be almost equal in the MLB, but in the present study, the incidence was significantly higher in fielders. In the present study, MRI was indicated for players who had difficulty continuing to play. It was possible that the different methods for diagnosing internal oblique muscle strain affected these results.
In the present study, most injury sites were at the region of the muscle insertion to the lower ribs. As noted by Connel et al, this was because the region is weak and vulnerable to injury.11 They also reported that most of the pain in the lateral trunk of athletes might be internal oblique muscle strain.11 Athletes who complain of pain in this region should be considered for proactive examination for suspected internal oblique muscle strain.
Regarding the injured side, it has been reported that 78% of pitchers and 70% of fielders developed abdominal oblique muscle strain on the contralateral batting and pitching sides in MLB.4 In Australian and English first-class cricket fast bowlers, the injured side of side strain of all 108 bowlers was reported to be the contralateral side.12 Furthermore, in Australian first-class cricket fast bowlers, all 10 internal oblique muscle strains diagnosed by MRI were on the contralateral side.6 A previous study that used electromyography to examine the activity of abdominal muscles during baseball pitching reported that the muscles on the contralateral side were more active than those on the dominant side.13 In the present study, although internal oblique muscle strain was more common on the side contralateral to the batting side or the pitching side, it occurred on either side, especially in second injury cases, most of which were on the side opposite to the initial injury. The details of the mechanism of abdominal oblique muscle strain are still not fully known, and further studies are needed in this area.
It has been reported that the time to return to play was approximately 4–5 weeks for athletes after a side strain.14 In the present study, fielders had a shorter time to return to play than pitchers, although the difference was not significant. Similarly, a prior study reported earlier return to play in fielders than pitchers.4 Fielders may return to play earlier because they can participate at least partially in a game through pinch batting, pinch running, or defense only, even if they have not returned to their full performance level.
The present study has some limitations. First, it included a small number of cases limited to a single Japanese professional baseball team. Second, all cases of internal oblique muscle strain could not be examined. Only the injuries that occurred during the season were examined. We intend to investigate the occurrence and recurrence of internal oblique muscle strain in order to gain a fuller understanding of this injury and to investigate preventive measures.