Let us assume that you have a degenerative L5-S1 segment and that all your other segments are normal. The L5-1 segment is now painful and a fusion will be beneficial.
More likely than not- your range of motion now is limited by pain at endpoints of motion (flexion and extension). In addition, this degenerative segment is also very limited in motion by the significant loss of disc height and wear of the facets.
What is surprising is that your motion may improve with a fusion of this segment. The loss of motion from fusion is normally minimal because of the preexisting limited motion of the degenerative segment. In addition, the reduction of pain with fusion may allow greater motion of the segments above than previously existed without the fusion.
Dr. Corenman