The center of gravity of a standing human body is roughly about an inch below navel: https://www.sciencedirect.com/topics/physics-and-astronomy/center-of-gravity
For a seated person the CoG is shifted forwards and up of navel, total distance and angle will vary according to the specifics of the posture: https://www.researchgate.net/figure/Center-of-gravity-of-a-person-in-sitting-posture-Body-segmental-Lengths-are-reported-in_fig2_256203106
The antrophomorphic test dummies mimic the proportions, weight distribution and kinematics of a human body as close as possible, for obvious reasons: to accurately replicate the response of a human body to forces, the ATD must resemble it accurately, not just as a whole, but down to its individual part also.
Example information of THOR ADT: Anthropomorphic Test Devices; THOR 50th Percentile Adult Male Test Dummy; Incorporation by Reference
If you wish to build a more accurate model, here is an example of a further break-down of proportional masses of average human body parts (Source):
| Segment | Male | Female |
|---|---|---|
| Head | 8.26% | 8.20% |
| Whole Trunk | 55.1% | 53.2% |
| Thorax | 20.1% | 17.0% |
| Abdomen | 13.1% | 12.2% |
| Pelvis | 13.7% | 16.0% |
| Total Arm | 5.70% | 4.97% |
| Upper Arm | 3.25% | 2.90% |
| Forearm | 1.87% | 1.57% |
| Hand | 0.65% | 0.50% |
| Forearm & Hand | 2.52% | 2.07% |
| Total Leg | 16.7% | 18.4% |
| Thigh | 10.5% | 11.8% |
| Leg | 4.75% | 5.35% |
| Foot | 1.43% | 1.33% |
| Leg & Foot | 6.18% | 6.68% |