You're seeking out Social Security Disability (SSD) payments because you believe you've suffered a significant loss of function. However, what you're wondering about is how the government technically defines loss of function. Does pain play into it, or do you have to physically be unable to move?
It's a valid question. What if you feel like you've lost some ability but the government says you haven't? What if they say you just have to deal with the pain, but the ability to function is still there?
As noted by the Social Security Administration (SSA), when considering the musculoskeletal system, pain can be factored into loss of function. If the pain is so severe that you can't move as you once did, as expected, that's still a loss of function.
For example, perhaps an injury has left you unable to walk properly. You feel severe pain when you try to walk or when you're on your feet for too long. It's not as if you had a leg amputated, but you still feel incredibly limited in terms of mobility. That may still count as a loss of function and the inability to walk since the pain is debilitating.
The reasoning makes sense on many levels. The pain is bad enough that you cannot work. You still need assistance for a severe, chronic condition. When your life is impacted so drastically, you may be able to apply for benefits.
If you are applying, be sure you fully understand the system, your own injury or limitation and how best to explain your case so that the real scope of the issue is clear.
Source: Social Security Administration, "1.00 Musculoskeletal System - Adult," accessed June 21, 2017