Combining more than one Checboxes using string builder

public string GetMuscleGravity()

{

StringBuilder sb = new StringBuilder();

            if (chkbxLU.Checked)

{

sb.Append((int)Helper.MuscleGravityOT.chkbxLU).ToString();

sb.Append(",");

}

if (chkbxRU.Checked)

{

sb.Append((int)Helper.MuscleGravityOT.chkbxRU).ToString();

sb.Append(",");

}

if (chkLH.Checked)

{

sb.Append((int)Helper.MuscleGravityOT.chkLH).ToString();

sb.Append(",");

}

if (chkbxRH.Checked)

{

sb.Append((int)Helper.MuscleGravityOT.chkbxRH).ToString();

sb.Append(",");

}

if (sb == null)

{

sb.Append(0).ToString();

}

return sb.ToString();

}