After reading through Romans 8:1-8 one word stood out and that was the word flesh, it was used nine times in six verses. The word flesh transliterates into sarx. While it would seem simple that flesh means flesh meat on bone, I do not believe that is what Paul is referring to. So if that is the case then what is the words semantic domain (or words it could mean)? According to the Enhanced Strong’s Lexicon, sarx has “…151 occurrences; AV translates as “flesh” 147 times,….”That is not all it translates as, here are some of the other translations and definitions:
flesh, corporeal mass of human and animal (Rev 19:18; Eph 5:30 v.r.); 2. physical body (1Ti 3:16); 3. people, a physical human being (Jn 1:14; 1Pe 1:24); 4. human, physical nature (Heb 12:9); 5. nation, ethnic group (Ro 11:14); 6. human nature, the psychological human nature (1Co 1:26; Gal 5:19; 6:8); 7. physical nature, as a result of its natural development (Gal 4:23); 8. physical life (Heb 5:7); 9. σὰρξ καὶ αἷμα (sarx kai haima), human being (Mt 16:17; Gal 1:16); 10. κοινωνέω αἵματος καὶ σαρκός (koinōneō haimatos kai sarkos), be a person (Heb 2:14+); 11. have homosexual intercourse (Jude 7+), see 599; 12. trouble (2Co 12:7+), see 5022; 13. sexual desire (Jn 1:13+), see 2525
In Judaism, “Flesh is sometimes neutral, but it also denotes human creatureliness. This is bound up with sinfulness and ignorance, but it does not itself stand in contrast to spirit; instead it is the battleground of conflict between the spirit of evil and the Holy Spirit.” Paul and his uses tend to focus more contrast between the natural and the spiritual. According to the Louw-Nida, “the psychological aspect of human nature which contrasts with the spiritual nature; in other words, that aspect of human nature which is characterized by or reflects typical human reasoning and desires in contrast with those aspects of human thought and behavior which relate to God and the spiritual life—‘human nature, human aspects, natural, human.’” 
Even though we have so many different options to define the word sarx why did the authors of ESV choose to go with flesh, and the NIV chose to use sinful nature? I think it is because sinful nature gives a little more clarity on the issue and allows the contrast between the Spirit to stand out better. We can gain a clearer understanding of what Paul meant by taking a look at say Galatians and his use in chapter five. He is contrasting again the Spirit verses the flesh/Law showing our rebellion against God. “Correspondingly, on over two-thirds of the many contrasts between sarx and pneuma, sarx refers to fallen human nature. So while sarx means flesh I appreciate the way the NIV translates it making it more clear that it is not meat on bone but the battle with sinfulness.