INTRODUCTION: A rowing cycle involves leg flexion and subsequent leg extension. This flexion-extension cycle enhances power output by about 10% compared with leg extension only [1], which is thought to occur because rowing cycles induce active stretch-shortening cycles (SSCs) of the knee extensors at the muscle fascicle level. SSCs enable up to 50% higher muscle force, work, and power output during the shortening phase compared with isolated muscle shortening [2]. However, it remains unknown whether rowing cycles cause active muscle fascicle stretch as the knee extensors have long in-series elastic tissues that can decouple fascicle and muscle-tendon unit (MTU) length changes [3]. Thus, the present study examined muscle fascicle kinematics of the vastus lateralis (VL) during rowing cycles at two rowing intensities and at the same rowing intensity with two techniques. METHODS: Fifteen young, sub-elite, male rowers (19.5 ±1.6yr; 1.94 ± 0.06m; 91.9 ±5.4 kg; 2,000-m time-trial personal best: 456 ±33W; weekly training volume: 17.8 ±4.1h; rowing experience: 7.5 ±2.8yr) performed 60-s rowing intervals on an ergometer using a traditional style at a low (LiR) and high intensity (HiR) and a micro-pause style at a low intensity (MpR). Muscle activity amplitudes, knee joint angles, and muscle fascicle length changes from the left-sided VL were quantified using surface electromyography, inertial measurement units, and B-mode ultrasound imaging, respectively. RESULTS: All rowing conditions showed active fascicle stretch during late knee flexion (p =0.001, SMD =0.72) and subsequent active fascicle shortening throughout knee extension. Active fascicle stretch duration, amplitude and velocity (rANOVA: p =0.001, ?p2 =0.49) were not significantly different (post-hoc: p =0.17; SMD =0.26) between LiR and MpR, but were significantly increased during HiR (post hoc: p =0.001; SMD =0.70). The percentage of rowing cycles that involved active fascicle stretch (rANOVA: p =0.001, ?p2 =0.95; post-hoc: p =0.001, SMD =0.87) was also significantly higher for HiR (98.3 ±12.9%) compared with both LiR (65.0 ±48.1%) and MpR (68.3 ±46.9%). CONCLUSION: Rowing involves SSCs of the VL at both the MTU and muscle fascicle levels, but the amount of active stretch differs between rowing intensities, with the longest, largest, and fastest active stretch occurring during HiR. SSC-based mechanisms may therefore contribute more to rowing performance during HiR than LiR or MpR.
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