Increased flexibility can be achieved by increasing tissue temperatures (thixotropic effects), which decrease tissue viscoelasticity. Static and proprioceptive neuromuscular facilitation (PNF) stretching can activate a number of inhibitory reflexes or disfacilitate spindle reflexes. (discharge frequency of nuclear chain and nuclear bag fibres). Golgi tendon organs (GTOs) and Renshaw cells do not play a substantial inhibitory role with static stretching. However, extensive static stretching can activate cutaneous afferent inhibition, whereas PNF stretching, which involves the contraction of the antagonist muscle, could facilitate reciprocal inhibition. Dynamic stretching tends to excite rather than inhibit these reflexes. The stretch tolerance theory can also help explain increased range of motion (ROM) as the individual accommodates the discomfort or pain associated with stretching and can push themselves past their previous ROM. Skeletal configuration, alignment, and ligaments are resistant to elongation by stretching, except with the intense flexibility training routines by gymnasts, dancers, figure skaters, and others. Excessive fat can impede joint ROM, whereas excessive elongation of nerves (>20%) can lead to injury. Very hypertrophied muscle can also provide some ROM restrictions; however, the great extensibility of muscle puts into dispute the old theory of muscle boundness. Myofibrils can elongate to double their resting length mainly due to the protein titin. Muscle is more compliant than tendons, with the latter accounting for less than half of the musculotendinous unit change. Muscle extensibility can also be altered by changes in fascicle angle (trivial), fascicle rotation (minor), and fascicle elongation (substantial). However, stretch tolerance may provide a greater impetus to enhanced ROM than decreased musculotendinous stiffness.