Isotropic velocity radius (IVR) and isotropic acceleration radius (IAR) are proposed as local performance indices to quantify the dynamic responsiveness of a multi-arm robot as regards the velocity effects. These performance measures are defined on the basis of the acceleration set describing the effects of actuator torques, velocity of the manipulated object, and gravity upon the acceleration of the object. An algorithm is presented to obtain an explicit expression for calculating these measures by decomposing the torque-related non-square matrix into square matrices without using the concept of pseudoinverse for the planar multi-arm robot composed of arms each with two joints. Numerical examples for a planar robot with two 2R arms show that the proposed concepts are effective in representing the acceleration capability and velocity effects of a multi-arm robot.