Intrinsic factors influencing the post-mortem mechanical excitability of human skeletal muscle

BACKGROUND: Post-mortem, supravital muscle reaction (SMR) of human skeletal muscle is dependent on energy supply. Intrinsic variables, like peri-mortem body temperature and underlying diseases, influence the rate of cell metabolism in skeletal muscle cells post-mortem, and thus the available energy for SMR. OBJECTIVES: A field study was conducted, comprising deceased found under differing conditions. We investigated the influence of the body temperature of the deceased on the outcome of post-mortem mechanical stimulation of skeletal muscle in the first hours post-mortem. In addition, we investigated the influence of sex, age, state of rigor mortis and the length of the dying process on SMR. Furthermore, we hypothesized that the outcome of post-mortem muscle stimulation could differ between musculus biceps brachii (MBB) and the musculus brachioradialis (MBR) on the same body examined at the same time, due to a higher cooling velocity of more peripheral located skeletal muscles (MBR). METHODS: In the period of January 2017 to August 2023 four forensic physicians from two different regions in the Netherlands performed mechanical stimulation of both upper and forearms of deceased with a known or an estimated PMI of <14 h, by using a reflex hammer. The research population concerned in-hospital-deaths, out-of-hospital deaths that were transferred to the morgue of a hospital, and out-of-hospital deaths that were examined at the place where the body was found. The sex and age of the deceased, body temperature, and state of rigor mortis were registered in the Dutch national register of forensic medicine, which was used as a data resource. The length of the dying process was registered separately using SPSS 27.0 which also served as a data resource. The Welch Two Sample t-test, chi-squared test, logistic regression and multiple logistic regression were performed to investigate the influence of the PMI, body temperature, sex, age, state of rigor mortis and the length of the dying process on the outcome of SMR. Statistical analyses were performed by using R 4.4.1. Significance was accepted at P < 0.05. RESULTS: In total, 142 cases were included, in which musculus biceps brachii (MBB) and the musculus brachioradialis (MBR) on both arms were mechanically stimulated with a reflex hammer. The population consisted out of 65 males (mean age: 73.4 ± 16.8 years) and 77 females (mean age: 82.1 ± 13.0 years). SMR was present in 74 out of 142 cases (52.1 %). The mean PMI did not differ significantly between cases with a positive SMR (157.3 min) and cases showing no SMR (168.4 min). The number of male cases that showed SMR (63.1 %) was significantly higher than the number of female cases (42.9 %) that showed SMR. No relationship was detected between age, body temperature, rigor mortis and the length of the dying process, and the outcome of post-mortem muscle stimulation within the early hours post-mortem. In 28 out of 142 cases (19.7 %) a discrepancy in the outcome of post-mortem muscle stimulation was observed between the MBB and the MBR on the same body. The percentage of SMR decreased during successive stages of rigor mortis. No SMR could be observed in case of bodies with fully developed rigor mortis. CONCLUSION: The current study shows a significant difference in outcome of post-mortem muscle stimulation in the early hours after death, between male and female deceased. No influence was detected of the body temperature, age, rigor mortis and the length of the dying process, on the outcome of post-mortem muscle stimulation in the early hours post-mortem, but this outcome must be approached with caution because of the limitations of the study. In 19.7 % of cases a discrepancy in the outcome of post-mortem muscle stimulation was observed between MBB and MBR on the same body.