SAN FRANCISCO (AP) - 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.

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