For transradial amputees, the muscles in the residual forearm physiologically used for flexing/extending the hand fingers are the most appropriate targets for multifingered prostheses control. However, once the prosthetic socket is manufactured and fitted on the residual forearm, the electromyographic (EMG) signals recorded from the residual limb might not be originated only by the intention of performing finger movements but also by the muscular activity needed to sustain the prosthesis itself. In this work, we show that in eight healthy subjects wearing a prosthetic socket emulator, 1) variations in the weight of the prosthesis and 2) upper arm movements significantly influence the robustness of a traditional classifier based on a k-nn algorithm, causing a significant drop in performance. We demonstrate in simulated conditions that traditional pattern recognition does not allow the separation of the effects of the weight of the prosthesis because a surface recorded EMG pattern due only to the lifting or moving of the prosthesis is misclassified into a hand control movement. This suggests that a robust classifier should add to myoelectric signals, inertial transducers like multi-axes position, acceleration sensors, or sensors able to monitor the interaction forces between the socket and the end-effector. ( J Prosthet Orthot. 2012;24:86Y92.)KEY INDEXING TERMS: upper limb prosthetics, pattern recognition, myoelectric control T o myoelectrically control a multifingered dexterous prosthesis, such as the recently marketed RSL Steeper BeBionic, 1 the iLimb, 2 or research prototypes like the SmartHand, 3 VU Hand, 4 or the DARPA RP 2009 Intrinsic Hand, 5 it is necessary to map electromyographic (EMG) signals corresponding to different muscle contractions to the different existing degrees of freedom (DoFs) of the hand using a suitable algorithm. Myoelectric control techniques can be divided into two categories: nonYpattern recognition based and pattern recognition based. 6,7 NonYpattern recognition control includes proportional control, threshold control, onset analysis, and finite state machines. These schemes have a simple structure and have been mostly deployed in on/off or proportional control. In particular, in proportional control, the strength of muscle contractions controls the prosthesis speed or force. This type of control scheme has received widespread clinical acceptance but provides reduced functionality, typically limited to only one or two DoFs. In research laboratories, sophisticated algorithms implement pattern recognition. 6 This is based on the condition that amputees can voluntarily activate repeatable and distinct EMG signal patterns for each class of motion, which, in turn, can be mapped to physiologically appropriate prosthesis commands.A multitude of groups have implemented and designed controllers using different combinations of extracted features and classification methods (for a review of the EMG processing techniques, refer to the work by Oskoei and Hu 7 ) showing the feasibility of controllin...