Abstract
BACKGROUND: Grip force requirements for grasping and releasing objects when hand opening and closing is assisted by Functional Electrical Stimulation (FES) were investigated. To understand whether FES can be useful in assisting these motion primitives, the following requirements were investigated: (i) producing sufficient force, (ii) sustaining this force for an adequate duration, (iii) successfully releasing the object, and (iv) monitoring the onset of muscle fatigue during repetitive grasping. METHODS: In an observational cross-sectional design, hemiparetic patients after stroke were instructed to repetitively grasp and release a sensorized object at maximum voluntary contraction while receiving FES assistance (3 s each of finger flexor and extensor stimulation). Outcome variables included clinical grip force data of the paretic and non-paretic hand. Grip force metrics, corresponding to each investigated requirement, such as the maximum applied grip force (GFmax), the amount of time of grip force maintained in the "GFtarget ± 10% window," and the force level during the assisted release (release ratio), were extracted during the FES-assisted grasping protocol. Additionally, changes in GFmax across repetitions were analyzed to investigate muscle fatigue. RESULTS: The median GFmax of 16 analyzed patients (mean 3.4 months post stroke) was 6.9 N (2.2-56.5 N), with severely impaired patients producing significantly lower values than the mildly/moderately impaired. The maximum grip force level was maintained for a median of 1.8 s, and the median object release ratio was significantly higher in patients with spasticity. CONCLUSION: All patients were able to produce grip force during FES assistance and maintained the grip force level across repetitions, without physiologically meaningful fatigue. The grip forces produced by the patients were similar to those found in real-life object handling in healthy subjects and very close to the reference values required by stroke patients for object transportation. In addition, the repetitive grasping did not lead to a significant muscle fatigue. This grip force produced while having no relevant fatigue highlights the assistive potential of FES in activities of daily living. However, these effects need to be verified for the grasping of real-life objects. TRIAL REGISTRATION: This trial was registered on September 29, 2021 (registration number: DRKS00025889).