This answer includes some speculation in the end. Since this is also a speculative question I hope that's okay :-)
if people who suffer from paralysis, specifically those who can't walk anymore, lose strength in their skeletal structure from the waist down as well.
Yes. Bones that aren't used lose mass. In a study of 89 men who had been in a wheelchair for at least 2 months and up to 50 years:
In the femur and tibia, bone mass, total and trabecular bone mineral density (BMDtot and BMDtrab, respectively) of the epiphyses, as well as bone mass and cortical cross-sectional area of the diaphyses, showed an exponential decrease with time after injury in the spinal cord injured subjects. The decreasing bone parameters reached new steady states after 3–8 years, depending on the parameter. Bone mass loss in the epiphyses was approximately 50% in the femur and 60% in the tibia, while the shafts lost only approximately 35% in the femur and 25% in the tibia.
Relationship between the duration of paralysis and bone structure: a pQCT study of spinal cord injured individuals
Other studies agree:
In the lower extremities the BMC decreased after injury. New steady-state levels for BMC were reached at 2 years post-injury for the proximal tibia and the femoral neck at 40–50% and 60–70% respectively of normal values
Longitudinal study of bone mineral content in the lumbar spine, the forearm and the lower extremities after spinal cord injury.
if there would be an advancement that would allow them to move their bones with the aid of artificial muscles, would their bones be rigid enough to still support them? If not, would there be a type of therapy combined with supplements that would help them regain strength in their bones?
There are "standing wheelchairs", designed to give patients with paralysis the chance to stand up, which helps with household activities, etc.
The Rehabilitation Engineering & Assistive Technology Society of North America released a position paper on these devices. They cite numerous health benefits, among them benefit for bone mineral density :
Research suggests that weight bearing is superior to nutritional supplements in preventing BMD loss, and that the mechanical loading of the bones should be dynamic for full prevention of BMD loss. It also appears that with discontinuation of the weight bearing program, BMD levels will continue to decrease and/or return to pre-weight bearing values.
They cite a few studies for this benefit, among them:
However, other research does not agree with this, saying that after the first year after the injury has passed, bone mineral density does not recover even from weight-bearing exercises. From Non-pharmacological treatment and prevention of bone loss after spinal cord injury: a systematic review:
For the chronic phase (Table 2), the picture seems more uniform, with very little evidence for any gain in BMD when the first year after injury has passed.
They also looked at studies regarding other intervention, for example electrical stimulation and found no significant benefits:
The level of evidence is important and in this study area, there is no conclusive indication of any effective intervention.
However, that doesn't mean your scenario would not work:
- This is talking about losing bone mineral density compared to control groups. However, from standing assisting technology we know that the remaining bone density is enough to at least support short-term standing, bearing a significant part of the individual's body weight. A return to pre-paralysis levels might not be needed for short-term walking, for example.
- The construction of such a device would lead to more studies into this. Right now, stopping the loss of bone density is not the primary motivating factor for the use of standing wheelchairs, for example - they are to increase quality of life and also to strengthen the individual's circulation, for example.