OBJECTIVES:To investigate the architectural and strength adaptations of the hamstrings following 6-weeks of inertial flywheel resistance training. DESIGN:Randomised, stratified training intervention METHODS: Twenty healthy males undertook 6-weeks of a conventional (n=10) or eccentrically biased (n=10) flywheel leg-curl training intervention as well as a subsequent 4-week detraining period. Biceps femoris long head (BFlh) architecture was assessed weekly, whilst assessments of eccentric and isometric knee flexor strength and rate of force development (RFD) were conducted prior to and following the intervention and detraining periods. RESULTS:The participants who undertook the eccentrically biased flywheel intervention showed a significant 14±5% (p<0.001, d=1.98) increase in BFlh fascicle length after 6-weeks of training. These improvements in fascicle length subsequently declined by 13±4% (p<0.001. d=-2.04) following the 4-week detraining period. The conventional flywheel leg-curl training group saw no changes in BFlh fascicle length after the intervention (-0.5%±0.8%, p=0.939, d=-0.04) or detraining (-1.1%±1%, p=0.984, d=-0.03) periods. Both groups saw no changes in any of the strength or RFD variables after the intervention or the detraining period. CONCLUSIONS:Flywheel leg-curl training performed with an eccentric bias led to significant lengthening of BFlh fascicles without a change in RFD, eccentric or isometric strength. These increases in fascicle length were lost following a 4-week detraining period. Conventional flywheel leg-curl training resulted in no changes in fascicle length, strength and RFD. These findings suggest that additional eccentric bias is required during inertial flywheel resistance training to promote fascicle lengthening in the BFlh, however this may still be insufficient to cause alterations to strength and RFD.