Protein degradation in human skeletal muscle: advancing postmortem interval estimation under natural outdoor conditions

Estimating the postmortem interval (PMI) represents a major challenge in forensic science, particularly beyond the early postmortem phase. Protein degradation has become a promising molecular approach, as many proteins disintegrate in progressive and temporally distinct degradation patterns. However, decomposition is influenced by environmental and individual factors, resulting in case-specific variability that complicates the development of generally applicable PMI estimation methods. Available human reference data are limited and often derived from autopsy samples collected at a single time point postmortem, typically without precise PMI or environmental data. Longitudinal studies under monitored natural outdoor conditions are therefore essential to establish reliable reference data for protein-based PMI estimation. This study investigated postmortem protein degradation in skeletal muscle under natural outdoor conditions at the Forensic Research Outdoor Station (FROST), a human taphonomy research facility in Marquette, Michigan. Muscle samples of the M. vastus lateralis were collected over 8-day periods from five human donors during two summer field trials (2022 and 2024). In situ sampling was complemented by an extracorporeal model, in which excised muscle tissue of each individual was stored in containers placed next to the body. This setup allowed a semi-controlled investigation of proteolytic dynamics outside the body. Additionally, morphological changes were assessed using a Total Body Score (TBS). Our findings emphasize the crucial impact of environmental conditions on postmortem protein degradation. They further support previous observations that specific proteins degrade in a regular and predictable manner, as demonstrated in this study in human tissue under natural outdoor conditions. The extracorporeal model revealed reproducible trends comparable to in situ degradation and represents a promising tool for isolated investigation of extrinsic factors under standardized conditions. This work provides valuable reference data, advancing protein-based PMI estimation in forensic research.