Enhanced precision of ankle torque measure with an open-unit dynamometer mounted with a 3D force-torque sensor

Ankle joint rotation and exerted moment during plantarflexion dependents on measuring- and fixation method

We examined the effect of ankle joint fixation vs increased foot pressure (aiming to reduce dynamometer-subject elasticity (DSE)) on the exerted moment during plantarflexion contraction. We also examined the joint rotation in dependence of the measuring site (forefoot, rearfoot) and the foot condition (fixed, free). We hypothesized higher exerted moments due to reduced DSE compared to fixed condition and an effect of fixation on the joint rotation in dependence of the measuring site. Fourteen healthy individuals (28.7±6.9y) completed in randomized order maximal isometric plantarflexions in four different positions (0-3-6-9 cm) and two ankle joint conditions (fixed-free). Kinematics of the rear- and forefoot were obtained synchronously. We found higher moment in the fixed compared to the free condition at all positions. The maximum moment in the fixed condition did not differ at any position. At the fixed condition, the forefoot rotation did not differ at any position (~5°) while at free condition we observed a significant rotation reduction (form ~12 to ~5°). The rearfoot rotation did not differ between conditions at any position while a significant joint angle reduction was observed (~10 to ~6° and ~12 to ~6°; fixed-free respectively). The results indicate that with appropriate foot fixation the maximum moment can be achieved irrespective of the position. With the foot secured, the measuring site influences the rotational outcome. We suggest that for a minimization of the joint rotation a fixation and the forefoot-measuring site should be preferred. Additionally, for unconstrained foot kinematic observations both measuring sites can be obtained.

Contribution of Achilles tendon mechanical properties to torque steadiness in persons with transfemoral amputation

BACKGROUND

How Achilles tendon mechanics and plantar flexion strength and torque steadiness are altered in the intact leg of persons with trauma-related amputation is unknown. Understanding Achilles tendon mechanics following amputation will further inform rehabilitation approaches to enhance posture, balance, and force control.

OBJECTIVE

Conduct a pilot study to quantify plantar flexion maximal voluntary contraction torque, torque steadiness, and Achilles tendon mechanics in persons with unilateral trauma-related transfemoral amputation and controls without amputation.

STUDY DESIGN

Cross-sectional study.

METHODS

Isometric plantar flexion maximal voluntary contractions were performed with the intact leg of ten males with transfemoral amputation (48 ± 14 years) and the dominant leg of age-matched male controls without amputation. Torque steadiness was calculated as the coefficient of variation in torque over 6 s during submaximal tracking tasks (5%, 10%, 25%, 50%, and 75% maximal voluntary contraction). Achilles tendon elongation and cross-sectional area were recorded with ultrasound to calculate strain, stress, and stiffness.

RESULTS

Maximal voluntary contraction and torque steadiness did not differ between persons with amputation (90.6 ± 31.6 N m, 3.7 ± 2.0%) and controls (95.8 ± 26.8 N m, 2.9 ± 1.2%; p > 0.05). Tendon stiffness (21.1 ± 18.2 N/mm) and strain (5.2 ± 1.3%) did not differ between groups (p > 0.05). Tendon cross-sectional area was 10% greater in persons with amputation leading to 29% lower stress (p = 0.021). Maximal voluntary contraction was a predictor of a lower coefficient of variation in torque (R2 = 0.11, p < 0.05).

CONCLUSION

Persons with trauma-related transfemoral amputation do not differ in plantar flexion maximal voluntary contraction and torque steadiness of the intact leg compared with controls without amputation. Larger tendon cross-sectional area reduces stress and enables distribution of force across a greater area.

Corticomuscular Coherence and Motor Control Adaptations after Isometric Maximal Strength Training

Strength training (ST) induces corticomuscular adaptations leading to enhanced strength. ST alters the agonist and antagonist muscle activations, which changes the motor control, i.e., force production stability and accuracy. This study evaluated the alteration of corticomuscular communication and motor control through the quantification of corticomuscular coherence (CMC) and absolute (AE) and variable error (VE) of the force production throughout a 3 week Maximal Strength Training (MST) intervention specifically designed to strengthen ankle plantarflexion (PF). Evaluation sessions with electroencephalography, electromyography, and torque recordings were conducted pre-training, 1 week after the training initiation, then post-training. Training effect was evaluated over the maximal voluntary isometric contractions (MVIC), the submaximal torque production, AE and VE, muscle activation, and CMC changes during submaximal contractions at 20% of the initial and daily MVIC. MVIC increased significantly throughout the training completion. For submaximal contractions, agonist muscle activation decreased over time only for the initial torque level while antagonist muscle activation, AE, and VE decreased over time for each torque level. CMC remained unaltered by the MST. Our results revealed that neurophysiological adaptations are noticeable as soon as 1 week post-training. However, CMC remained unaltered by MST, suggesting that central motor adaptations may take longer to be translated into CMC alteration.

Investigation of Optimal Afferent Feedback Modality for Inducing Neural Plasticity with A Self-Paced Brain-Computer Interface

Brain-computer interfaces (BCIs) can be used to induce neural plasticity in the human nervous system by pairing motor cortical activity with relevant afferent feedback, which can be used in neurorehabilitation. The aim of this study was to identify the optimal type or combination of afferent feedback modalities to increase cortical excitability in a BCI training intervention. In three experimental sessions, 12 healthy participants imagined a dorsiflexion that was decoded by a BCI which activated relevant afferent feedback: (1) electrical nerve stimulation (ES) (peroneal nerve-innervating tibialis anterior), (2) passive movement (PM) of the ankle joint, or (3) combined electrical stimulation and passive movement (Comb). The cortical excitability was assessed with transcranial magnetic stimulation determining motor evoked potentials (MEPs) in tibialis anterior before, immediately after and 30 min after the BCI training. Linear mixed regression models were used to assess the changes in MEPs. The three interventions led to a significant (p < 0.05) increase in MEP amplitudes immediately and 30 min after the training. The effect sizes of Comb paradigm were larger than ES and PM, although, these differences were not statistically significant (p > 0.05). These results indicate that the timing of movement imagery and afferent feedback is the main determinant of induced cortical plasticity whereas the specific type of feedback has a moderate impact. These findings can be important for the translation of such a BCI protocol to the clinical practice where by combining the BCI with the already available equipment cortical plasticity can be effectively induced. The findings in the current study need to be validated in stroke populations.

Differential impact of visual feedback on plantar- and dorsi-flexion maximal torque output

The effect of visual feedback on enhancing isometric maximal voluntary contractions (MVC) was evaluated. Twelve adults performed plantar-flexion and dorsi-flexion MVCs in 3 conditions (no visual feedback, visual feedback, and visual feedback with target). There was no significant effect of visual conditions on dorsi-flexion MVC but there was an effect on plantar-flexion. Irrespective of whether a target was evident, visual feedback increased plantar-flexion MVC by ∼15%. This study highlights the importance of optimal feedback to enhance MVC.

Bilateral Strength Deficit Is Not Neural in Origin; Rather Due to Dynamometer Mechanical Configuration

During maximal contractions, the sum of forces exerted by homonymous muscles unilaterally is typically higher than the sum of forces exerted by the same muscles bilaterally. However, the underlying mechanism(s) of this phenomenon, which is known as the bilateral strength deficit, remain equivocal. One potential factor that has received minimal attention is the contribution of body adjustments to bilateral and unilateral force production. The purpose of this study was to evaluate the plantar-flexors in an innovative dynamometer that permitted the influence of torque from body adjustments to be adapted. Participants were identically positioned between two setup configurations where torques generated from body adjustments were included within the net ankle torque (locked-unit) or independent of the ankle (open-unit). Twenty healthy adult males performed unilateral and bilateral maximal voluntary isometric plantar-flexion contractions using the dynamometer in the open and locked-unit mechanical configurations. While there was a significant bilateral strength deficit in the locked-unit (p = 0.01), it was not evident in the open-unit (p = 0.07). In the locked-unit, unilateral torque was greater than in the open-unit (p<0.001) and this was due to an additional torque from the body since the electromyographic activity of the agonist muscles did not differ between the two setups (p>0.05). This study revealed that the mechanical configuration of the dynamometer and then the body adjustments caused the observation of a bilateral strength deficit.