Reducing Motor Variability Enhances Myoelectric Control Robustness Across Untrained Limb Positions.

The limb position effect is a multi-faceted problem, associated with decreased upper-limb prosthesis control acuity following a change in arm position. Factors contributing to this problem can arise from distinct environmental or physiological sources. Despite their differences in origin, the effect of each factor manifests similarly as increased input data variability. This variability can cause incorrect decoding of user intent. Previous research has attempted to address this by better capturing input data variability with data abundance. In this paper, we take an alternative approach and investigate the effect of reducing trial-to-trial variability by improving the consistency of muscle activity through user training. Ten participants underwent 4 days of myoelectric training with either concurrent or delayed feedback in a single arm position. At the end of training participants experienced a zero-feedback retention test in multiple limb positions. In doing so, we tested how well the skill learned in a single limb position generalized to untrained positions. We found that delayed feedback training led to more consistent muscle activity across both the trained and untrained limb positions. Analysis of patterns of activations in the delayed feedback group suggest a structured change in muscle activity occurs across arm positions. Our results demonstrate that myoelectric user-training can lead to the retention of motor skills that bring about more robust decoding across untrained limb positions. This work highlights the importance of reducing motor variability with practice, prior to examining the underlying structure of muscle changes associated with limb position.

Delaying feedback during pre-device training facilitates the retention of novel myoelectric skills: a laboratory and home-based study.

Objective.The objective of this study was to assess the impact of delayed feedback training on the retention of novel myoelectric skills, and to demonstrate the use of this training approach in the home environment.Approach.We trained limb-intact participants to use a motor learning-based upper-limb prosthesis control scheme called abstract decoding. A delayed feedback paradigm intended to prevent within-trial adaptation and to facilitate motor learning was used. We conducted two multi-day experiments. Experiment 1 was a laboratory-based study consisting of two groups trained over a 4 day period with concurrent or delayed feedback. An additional follow-up session took place after 18 days to assess the retention of motor skills. Experiment 2 was a home-based pilot study that took place over five consecutive days to investigate delayed feedback performance when using bespoke training structures.Main Results.Approximately 35 000 trials were collected across both experiments. Experiment 1 found that the retention of motor skills for the delayed feedback group was significantly better than that of their concurrent feedback counterparts. In addition, the delayed feedback group improved their retention of motor skills across days, whereas the concurrent feedback group did not. Experiment 2 demonstrated that by using a bespoke training protocol in an environment that is more conducive to learning, it is possible for participants to become highly accurate in the absence of feedback.Significance.These results show that with delayed feedback training, it is possible to retain novel myoelectric skills. Using abstract decoding participants can activate four distinct muscle patterns without using complex algorithms. The accuracy achieved in the pilot study supports the feasibility of motor learning-based upper-limb prosthesis control after home-based myoelectric training.

Long-Term Myoelectric Training with Delayed Feedback in the Home Environment.

Myoelectric prosthesis users typically do not receive immediate feedback from their device. They must be able to consistently produce distinct muscle activations in the absence of augmented feedback. In previous experiments, abstract decoding has provided real-time visual feedback for closed loop control. It is unclear if the performance in those experiments was due to short-term adaptation or motor learning. To test if similar performance could be reached without short-term adaptation, we trained participants with a delayed feedback paradigm. Feedback was delayed until after the ~1.5 s trial was completed. Three participants trained for five days in their home environments, completing a cumulative total of 4920 trials. Participants became highly accurate while receiving no real-time feedback of their control input. They were also able to retain performance gains across days. This strongly suggests that abstract decoding with delayed feedback facilitates motor learning, enabling four class control without immediate feedback.

Intramuscular EMG For Abstract Myoelectric Control: A Proof Of Concept Study.

Myoelectric prostheses are commonly controlled by surface EMG. Many control algorithms, including the user learning-based control paradigm abstract control, benefit from independent control signals. Measuring at the surface of the skin reduces the signal independence through cross talk. To increase the number of independent signals, intramuscular EMG recordings might be a viable alternative for myoelectric control. This proof of concept study investigated if real time abstract myoelectric control is possible with intramuscular measurements. Six participants performed a 4-target and 12-target abstract control task with both surface and intramuscular EMG recordings. The results suggest that intramuscular EMG is suitable for abstract control, and that performance could be increased in the future by stabilizing the amplitude of the processed intramuscular EMG signal.

Temporal Modulation of Transcutaneous Electrical Nerve Stimulation Influences Sensory Perception.

The incorporation of sensory feedback in prosthetics can lead to a range of benefits, such as improved hand control, increased prosthesis embodiment, and the reduction of phantom limb pain. However, the creation of reliable sensory feedback is complicated by the temporal modulation of the nervous system. Sensory fibres in the hand are primed to react to changing conditions, firing when discrete mechanical events occur. In this study, we investigate the minimal possible stimulation needed to distinguish different sensory patterns that can be used to indicate events. We presented a two-alternative forced-choice task of transcutaneous electrical nerve stimuli to 10 participants. The results showed that different stimuli can be distinguished when double pulses have an inter-stimulus-interval of 10 ms. Additionally, providing a pause of at least 350 ms between stimuli increases the discrimination of the perception. These results suggest that humans can distinguish different patterns of transcutaneous electrical nerve stimulation with as little as two stimuli, illustrating the possibility of providing event-related stimulation.

Automatic Myoelectric Control Site Detection Using Candid Covariance-Free Incremental Principal Component Analysis.

The unknown composition of residual muscles surrounding the stump of an amputee makes optimal electrode placement challenging. This often causes the experimental set-up and calibration of upper-limb prostheses to be time consuming. In this work, we propose the use of existing dimensionality reduction techniques, typically used for muscle synergy analysis, to provide meaningful real-time functional information of the residual muscles during the calibration period. Two variations of principal component analysis (PCA) were applied to electromyography (EMG) data collected during a myoelectric task. Candid covariance-free incremental PCA (CCIPCA) detected task-specific muscle synergies with high accuracy using minimal amounts of data. Our findings offer a real-time solution towards optimizing calibration periods.

Distinct neural control of intrinsic and extrinsic muscles of the hand during single finger pressing.

Single finger force tasks lead to unintended activation of the non-instructed fingers, commonly referred to as enslaving. Both neural and mechanical factors have been associated with this absence of finger individuality. This study investigates the amplitude modulation of both intrinsic and extrinsic finger muscles during single finger isometric force tasks. Twelve participants performed single finger flexion presses at 20% of maximum voluntary contraction, while simultaneously the electromyographic activity of several intrinsic and extrinsic muscles associated with all four fingers was recorded using 8 electrode pairs in the hand and two 30-electrode grids on the lower arm. The forces exerted by each of the fingers, in both flexion and extension direction, were recorded with individual force sensors. This study shows distinct activation patterns in intrinsic and extrinsic hand muscles. Intrinsic muscles exhibited individuation, where the agonistic and antagonistic muscles associated with the instructed fingers showed the highest activation. This activation in both agonistic and antagonistic muscles appears to facilitate finger stabilisation during the isometric force task. Extrinsic muscles show an activation independent from instructed finger in both agonistic and antagonistic muscles, which appears to be associated with stabilisation of the wrist, with an additional finger-dependent modulation only present in the agonistic extrinsic muscles. These results indicate distinct muscle patterns in intrinsic and extrinsic hand muscles during single finger isometric force pressing. We conclude that the finger specific activation of intrinsic muscles is not sufficient to fully counteract enslaving caused by the broad activation of the extrinsic muscles.