Tightness vs weakness
If you have difficulty straightening your leg, if it feels very tight behind the knee/hamstring, if it's difficult to lift your leg to wear pants, then it's a high possibility its to do with the muscles in the lower limb (toe, foot, calf muscle etc) being too tight.
When the muscles are very tight, it pulls on the fascia and skin, causing the total "usable" area of the fascia to reduce, hence restricting the range of movement, so patients will for example experience "heaviness" or difficulties in lifting their legs.
Such a condition is often confused with muscle weakness, because when the muscles tighten to an extent, the limbs will feel weak, but the primary reason in this case is the "space" for muscle contraction has decreased rather than atrophy (shrinkage of muscle).
Muscle atrophy is often associated with lack of muscle use, for example, in the case of post surgery where the person has to be bedridden for a few weeks. Muscle tightness has to do with over use of muscles. An easy way to differentiate is that muscle atrophy causes muscles to reduce in size, and the person does not necessarily feel tightness.
Treatment wise, muscle tightness needs treatments to relax the muscles and fascia (e.g acupuncture) , while muscle atrophy requires rehabilitation exercises.