In vivo operational fascicle lengths of vastus lateralis during sub-maximal and maximal cycling

Instantaneous contractile characteristics of skeletal muscle, during movement tasks, can be determined and related to steady state mechanical properties such as the force-length relationship with the use of ultrasound imaging. A previous investigation into the contractile characteristics of the vastus lateralis (VL) during cycling has shown that fascicles operate on the “weak” descending limb of the force-length relationship, thus not taking advantage of the “strong” plateau region. The purpose of this study was to investigate if VL fascicle lengths change from sub-maximal to maximal cycling conditions, and if maximal cycling results in VL fascicle lengths which operate across the plateau of the force-length relationship. Fifteen healthy male subjects (age 20.9±1.8 yr, wt. 67.0±6.3 kg, ht. 176.7±7.2 cm) were tested to establish the maximal force-length relationship for the VL through ten maximal isometric contractions at various knee angles. Subjects then cycled on an SRM cycle ergometer at cadences of 50 and 80 revolutions per minute at 100 W, 250 W, and maximal effort. Fascicle lengths were determined at crank angles of 0, 90, and 180°. Fascicles operated at or near the plateau of the maximal force-length relationship for maximal cycling, while operating on the descending limb during sub-maximal conditions for both cadences. However, when comparing the fascicle operating range for the sub-maximal cycling conditions to the corresponding sub-maximal force-length relationships, the VL now also operated across the plateau region. We concluded from these results that regardless of cycling effort, the VL operated through the ideal plateau region of the corresponding force-length relationship, hence always working optimally. We hypothesize that this phenomenon is due to the coupling of series elastic compliance and length dependent calcium sensitivity in the VL.