There is a big difference in stretching and lengthening

In lengthening we just take the muscle to the normal length of the muscle which is the elongated state of muscle. i.e. extrafusal msucle fibers are put in the lengthen state from the lax state{ we just take the slag out} so that the intrafusal muscle fiber are elongated and the muscle spindle also under some degree of tension.
This is the optimum length of the muscle which helps in effective facilitation of muscle. this is what the Frank Starling law stated ” The length of muscle is directly proportional the strength of the muscle.” in this optimum length there are maximum number of cross bridges are available on actin and myosin filament for contraction ( walk along theory }.

where as in stretching we are not bothered about the optimum length of muscle but we ant get the normal range of the joint and length of the muscle even if the muscle has strength or not to maintain and work in that new length.

during this kind of stretching specially the spastic muscle we don’t get the change in the length of the contractile element of the muscle, instead we stretch the non contractile muscle. this will over lengthen the muscle and put under mechanical disadvantage and sometime changes the angle of pull of muscle.
all these abnormal stretching and mechanical disadvantage of muscle will reduce the strength of muscle make it permanent weak.
example over stretching of quadriceps or long flexors of hand put them in inefficient length or position. and what we call this condition as EXTENSION LAG in quadriceps. i will explain this in more details with diagram in new blog.