I would assume that the “muscle spasm” sensation that you note is probably discomforting but you don’t interpret this sensation as pain. Muscle spasm can be reactive due to pain or instability or protective to prevent motion that could be painful. With loss of disk height and bone spur formation at L4-5 along with “moderate loss of foraminal height”, this sounds like foraminal collapse. See https://neckandback.com/conditions/foraminal-collapse-lumbar-spine/.
The foramen changes shape (volume) with flexion and extension. Flexion (bending forward or “slouching”) will “open” the canal and conversely, extension (standing/walking) will narrow this canal. This seems to fit with your symptoms even though there is “no pain”.
It is highly unlikely that the cervical spine is affecting the lumbar symptoms.
I am not a fan of interspinous spaces. The cause a forced flexion of the vertebra to “artificially” open the foramen by causing a flexion deformity (kyphosis) of this segment. Increased flexion will increase the stress of the levels above and below. In addition, these deforming devices are “wedged” between the spinous processes and I have seen many erode into the bone which returns the alignment back to normal which then allows the foramen to be recompressed.
Dr. Corenman
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.