SAN FRANCISCO — 49ers running back Brandon Jacobs is becoming even more frustrated with his extremely limited role for San Francisco.

Jacobs on Saturday said via Instagram that he is "on this team rotting away" while referring to his first season with the Niners as his "worst" yet during an eight-year career that included seven with the New York Giants.

The 30-year-old Jacobs has five carries for 7 yards while being active for only two games with the NFC West-leading Niners (8-3-1), who hosted the Miami Dolphins on Sunday. Jacobs — inactive again with rookie LaMichael James active for the first time — has repeatedly referred any questions about his playing time to head coach Jim Harbaugh.

On Twitter later, Jacobs said, "I don't understand why people are angry at me because I want to do what I am paid to do."

Jacobs, hampered by a knee injury earlier this season, spends approximately 20 minutes before games punching the goal-post padding.