Exercise-induced rib stress fractures: potential risk factors related to thoracic muscle co-contraction and movement pattern

The etiology of exercise-induced rib stress fractures (RSFs) in elite rowers is unclear. The purpose of the study was to investigate thoracic muscle activity, movement patterns and muscle strength in elite rowers. Electromyographic (EMG) and 2-D video analysis were performed during ergometer rowing, and isokinetic muscle strength was measured in seven national team rowers with a history of RSF and seven matched controls (C). RSF displayed a higher velocity of the seat in the initial drive phase (RSF: 0.25 +/- 0.03, 0.25 (0.15-0.33) m/s vs C: 0.15 +/- 0.06, 0.18 (-0.11-0.29) m/s P = 0.028) (Mean +/- SEM, median and range). Further, RSF had greater co-contraction of m. serratus anterior and m. trapezius in the mid-drive phase (RSF: 47.5 +/- 3.4, 48.5 (35.8-60.2)% EMG signal overlap vs C: 30.8 +/- 6.5, 27.0 (11.2-61.6)% P = 0.043). In addition, the RSF subjects showed a lower knee-extension to elbow-flexion strength ratio (RSF: 4.2 +/- 0.22, 4.3 (3.5-5.1) vs C: 4.8 +/- 0.16, 5.0 (4.2-5.3) P = 0.043), indicating stronger arms relative to legs compared with controls. In conclusion, increased thoracic muscle co-contraction, altered movement patterns and reduced leg/arm strength ratio were observed in the RSF subjects, which may all predispose toward an increased risk of RSF.