Within current rehabilitation practice, a substantial focus is on applying assistive technology (AT) to help patients with neuromotor deficits regain functionality in their daily activities. Often, myosignals are used to control the AT, e.g. in myoelectric prostheses [
1‐
3], myo-powered electric wheelchairs [
4‐
6], and movement supporting devices, such as exoskeletons [
7‐
9] and orthoses [
10,
11]. Furthermore, myosignals have been applied in other human-machine interfaces which are relevant for the independence of patients with neuromotor deficits, such as in the control of a personal computer [
12] or the teleoperation of robotic arms [
13‐
16]. However, despite the technological advancement in many devices, patients often still experience problems to control myoelectric AT in daily life [
17‐
21]. For instance, movements with myoelectric AT are non-smooth and require high levels of attention [
18,
22]. One of the aspects that could cause these problems is that the control of myoelectric AT is non-intuitive, as muscles have a different function during the control of actions with myoelectric AT compared to the same action in the non-affected situation (cf. [
23,
24]). For example, in hand prosthetics, the action of closing and opening of the hand is controlled with remaining parts of wrist or elbow flexors and extensor muscles—depending on the level of amputation. This function is arguably different from the original function of these muscles, i.e. flexing and extending the wrist or elbow, respectively. The present paper was inspired by the suggestion that an intuitive interface between user and device would aid the effective control of myoelectric AT (cf. [
25‐
27]). Furthermore, the present paper was grounded on the idea that this intuitive control can be achieved if the design of myoelectric AT is based on knowledge of the neuromotor control principles underlying the production of myosignals (cf. [
28‐
31]). Therefore, we explored the extent to which the fixed muscle synergy approach [
32‐
37], a proposed control principle from the field of neurocomputational motor control, could form a basis to improve the intuitive control of myoelectric AT for upper extremities.
The basics of the fixed muscle synergy approach are that myosignals are produced by simultaneously activating a limited number of fixed muscle groups consisting of functionally related muscles across multiple joints, i.e. the muscle synergies. These fixed muscle synergies are proposed to be interneuronal networks, which do not change over time, and are organized at the level of the spinal cord (cf. [
32,
33,
35,
36]). Activation of one of these networks leads to proportional activation of the muscles within a synergy. In that way, fixed muscle synergies serve as primitives, which—if conjointly activated in a time-varying way—can produce appropriate myosignals moving the limb across different tasks and conditions [
32‐
37]. Over the last decade, findings of muscle synergies in myosignals in a variety of human behaviors—such as reaching in three dimensions [
38‐
44], upper extremity visuomotor adaptation [
45], force control [
46], planar reaching [
47‐
49], upper extremity movements in virtual reality [
50], walking [
51‐
53], and postural control [
54‐
56]—have been offered as evidence for the existence of fixed muscle synergies as used neuromotor control principle. The relevant question for the present paper is whether the proposed principle of fixed muscle synergies could aid in controlling myoelectric AT in an intuitive way.
The successful improvement of the intuitive control of myoelectric AT with fixed muscles synergies requires that muscles are indeed organized into fixed synergies and that these synergies are systematically exploited during the different use of muscles as required in the control of myoelectric AT. The aim of the present study was to gain new insights in the viability of the fixed muscle synergy approach for its application in upper extremity assistive devices. Therefore, the present study assessed whether a limited number of muscle synergies was systematically exploited when the same task had to be produced while muscles were used differently. In the present experiment, able-bodied participants made multidirectional point-to-point movements with end-effectors of different lengths—i.e. their index finger and with a rod of varying length. Due to the introduction of end-effectors of different lengths, the same point-to-point movement with the tip of the end-effector had to be produced while postural angles, and thus the use of muscles, varied over a large range [
57‐
59]. Explaining this behavior in terms of fixed muscle synergies, the same set of fixed muscle synergies had to be activated with different time-varying signals to produce the appropriate myosignals moving the tip of the end-effector both with and without the use of rods. The potential finding that a limited number of muscle synergies is systematically exploited to produce myosignals when muscles are used differently across end-effectors would encourage the use of the fixed muscle synergy approach to improve the intuitive control of myoelectric AT for upper extremities. Alternatively, the set of muscle synergies might substantially differ across end-effectors. Such a finding would warrant further examination of the idea of fixed muscle synergies as neuromotor control principle underlying the production of myosignals and places limitations on its potential to improve the intuitive control of myoelectric AT for upper extremities.