To explain what may be happening may take a few paragraphs so bear with me.
You have let's say 5 main segments to address when talking about ergonomics; Legs, Pelvis, Spine, Arms (shoulders) and Head.
Legs
The legs are the base of your riding position. However you can have your leg comfortably fit on a bike and align your pelvis and thus the rest of your body is going to go a long way into creating comfort. If you can visualize a tribesman sitting in a squat or Malasana (Garland) Pose from yoga,
Garland Pose | Yoga Poses | Yoga Journal. Then you will get an idea of how our lower body is actually supposed to be set up for a resting posture. (or Rossi squatting to stretch before a race
View attachment 16349)We used to sit around campfires like that for hours on end. What happens in modern culture we sit with or legs at 90 degrees and eventually the hip flexors and hamstrings get tight so that when we go to stand fully there is a muscle group called the Iliopsoas
View attachment 16347 will pull tightly across the lower back and put extra pressure on your lumbar spine, especially if you have had any injuries there or degenerative disc disease * (which most of us start to develop in as early as our 20s-30s depending on our lifestyle, interestingly the Malasana pose puts a great stretch on that lumbar spine, making space for the intervertebral disc so that they can stay healthy)
* If you can imagine your spine or look at this image
View attachment 16348. The vertebrae being solid are like books and intervertebral disc are like jelly doughnuts. If you were to put the jelly doughnut between two books the doughnut would hold it up and you would be able to make it wobble a little with light controlled pressure on the corners of the book. However, if you were to press hard on one corner of the book the jelly is going to shoot out the opposite corner of the doughnut. The way the spine is set up that pushes the disc bulge or disc herniation out toward where the nerves are in the spine and generates what is called radiculopathy or radiating pain/numbness/nerve problems occur from the nerves affected by the change in the spinal support.
If you're legs are strong and well balanced then what will happen is the gluts will help support your pelvis while you use your quads to assist in change of body/seat position. Your lower legs are there for more finite control and should be used in an athletic type posture to connect you with the bike. Overall the better your legs are balanced and strong they will create the support your pelvis needs to hold up the rest of your body.
Motorcycle changes (legs). Mainly changing rear-sets will give you the opportunity to adjust your leg position to help you find the best riding position for your legs.
Pelvis
The pelvis, which is broken down to the hip bones and the sacrum/coccyx (tailbone). The hip bones, in particular the ischial tuberosity (commonly called sits bones) is the bony contact of your ass and the seat. You can see/feel how if your lower body is stronger that it can help control the position of your hip bones and how they contact the seat. The sacrum which is where the hip/legs attach to the lumbar spine and subsequently the rest of the body. So depending on how you can comfortably sit on your 'sits bones' will dictate how you will adjust your pelvis and thus your sacrum leading into your lower back. If we think back to the legs, the hamstrings attach to the ischial tuberosity (sits bones) and the gluts connect to the hip bone (pelvis) and then down the femur (leg bone). Both flexibility and strength in the gluts/hammy's will give you better control over your pelvis and seat position. The issue I've noticed with the difference in ergonomics is related to seat position/angle. Cruisers and some ADV bikes have what would be considered a neutral pelvic tilt to a backward pelvic tilt (picture the pelvis like the top of a bucket, neutral would be even on both sides, forward would be tilting the bucket into the tank and spine forward, backward tilt would be tilting the bucket toward the tail of the bike). Sport bikes and sport standards have varying degrees of neutral to forward pelvic tilt. If we can recall our book and jelly doughnut example we could consider the pelvis like the table they are sitting on, and the seat would be the equivalent of the ground. If you tilt the table forward or back and applying a down force (gravity) as long as the books and doughnut remain stacked evenly they should hold. If in a sport riding position, the forward tilt mixed with strong core muscles can keep that neutral position between the vertebrae and disc. If there is collapsing/rounding of the lumbar spine then back to the jelly doughnut example it starts to push out the jelly in this case toward the back. I also should mention also that a tilt in either direction can result in aggravated facets (intervertebral facets are the bony connection between the two adjacent vertebrae). Even in a neutral pelvic tilt sometimes (less likely) and more commonly forward or backward pelvic tilt will get what's called facet syndrome/facet arthritis will cause aggravated back pain because the facet are attempting to support the body.
Motorcycle changes for pelvis: bike itself and seats. The overall ergonomics of the bike will affect how you're able to position your pelvis. Secondary to that the seat can affect the position of the pelvis as well. With a firmer seat in an aggressive riding position for many that requires strong legs and core muscles to support the pelvis and thus the spine (rest of the body) to keep the vertebrae stacked correctly. A cruiser style seat will position the pelvis in a such a way where the strength of core muscles aren't needed but more so relying on the stacking of the spine itself which may severely aggravate a spine with weak points (bulging disc or facet arthritis). The shift in the pelvis due to how the ischial tuberosity (sits bones) contact the seat and are supported by the seat can explain why sometimes a softer seat makes the bike less comfortable because the softness is forcing the core muscles to take up and account for holding the spine in the correct position opposed to a properly supported pelvis that can thus support the spine.
Spine
to be continued (I'm tired)
