Let’s take a look at sacroiliac syndrome (SI syndrome).
Symptoms of SI syndrome consist of pain at the base of the spine which is located usually on one side, with pain then being referred down the back of the leg, rarely going below the knee. Pain is typically a pinching or stabbing in the lower back with a background dull ache, whilst the leg pain is usually just the ache. The SI joint itself allows for movement of the pelvis as we walk, and enlarges to allow the passage of a baby during birth.
Essentially, the human body is still better evolved (in some aspects) for walking around on 4 legs, rather than 2. Our animal companions use the back of the pelvis as a suspension bridge, with the triangular sacrum hanging from big strong ligaments from the pillars of the ilia, allowing for more mobility than we humans experience here.
In humans, we then walk upright instead, with those ligaments now trying to pinch the sacrum between the ilia, with the joint line being more-or-less vertical. This means that when our foot meets the floor, and force that comes up the leg, the knee needs to flex and provide shock absorption. If that doesn’t happen (eg the ground is further away than you expected, and you land with the knee straight) then the force travels straight into the pelvis, and puts a sheering force through the SI joint – which it is poor at dealing with. The joint is therefore susceptible to spraining types of injury when dealing with forces travelling through it unevenly; whether that’s walking, running or jumping; or lifting or carrying with the weight all on one side.