The thesis examined in vivo contractile properties of human skeletal muscle during cycling. Force-length and force-velocity relationships were measured for the vastus lateralis muscle. Muscle fascicle length and contraction velocity were measured during sub-maximal and maximal cycling conditions and were compared with the developed relationships. Sub-maximal and maximal cycling conditions resulted in the vastus lateralis to be contracting at fascicle lengths similar to those considered optimal, as determined via comparison with corresponding sub-maximal and maximal force-length curves. This novel result provides new insight into the functional contractile capacity of human skeletal muscle at sub maximal and maximal efforts due to the coupling of bio-chemical and mechanical influences on the force-length relationship and contractile length respectively. Comparisons of vastus lateralis contractile velocity, during the sub-maximal and maximal cycling, with the measured force-velocity relationship confirmed results of previous published work pertaining to optimal cadence for maximal power production. Furthermore, the importance of measuring subject specific contractile properties was emphasized as muscle architecture can change between subject populations. Interpretation of functional contractile lengths and velocities may be inappropriate if force-length and force-velocity properties are not subject specific.