Skeletal muscles are classified as striated and voluntary. They are found in muscles that maintain low-intensity contractions for long periods without fatigue. A single skeletal muscle cell is referred to as a muscle fibre. There are three types of muscle fibres found in the human skeletal muscle system including slow-oxidative fibres, fast-oxidative fibres and fast-glycolytic fibres. These three fibres are each defined by their biochemical capacities1. The slow-oxidative muscle fibres are classified by their slow speed of contraction and oxidative phosphorylation path to form adenosine triphosphate (ATP)1. New research has suggested that slow-oxidative human skeletal muscle is functionally wider in operating length range than the sliding filament model suggests. There has been new research on the sarcomere force-length relationship in slow-oxidative human skeletal muscle fibres. The active and passive force-length relationship was measured and compared to results predicted from the sliding filament model10. At different sarcomere lengths isometric forces in chemically skinned fibres were measured and compared to the calculated prediction10. The results suggested that the experimental force-length relationship compared to the sliding filament model implies that human muscle has functionally wider operating length range10. This new found research is important for interpreting models of motor function, particularly regarding slow-oxidative fibres. 1Sherwood L. Human Physiology: From Cells to Systems. 8th ed. Brooks/Cole Cengage Learning, Belmont, California. 2013. 10Gollapudi SK, Lin DC. Experimental determination of sarcomere force-length relationship in type I human skeletal muscle fibres. Journal of Biomechanics. 2009; 42:2011-2016.