Poor posture can reduce the optimum length and tension relationship of skeletal muscle leading to stiffness in the joints as they are prevented from moving through their full ROM.

How does poor posture exacerbates diminished muscle strength and endurance?

Over time, poor posture that demands support from phasic fibres causes the deeper supporting muscles to waste away from lack of use. Weak, unused muscles tend to tighten, and this shortening of muscle length can compact the bones of the spine (vertebrae) and worsen posture.

What are the effects of having a poor level of flexibility?

The range of motion will be influenced by the mobility of the soft tissues that surround the joint. These soft tissues include: muscles, ligaments, tendons, joint capsules, and skin. A lack of stretching, especially when combined with activity can lead to a fatigue induced soft tissue shortening over time.