Effect of Motor Task on Cortex Brainstem Modulation: Preliminary Results.

Reticulospinal Tracts (RSTs) have divergent connections to multiple spinal segments that innervate many upper extremity muscles. Therefore, increased RST engagement can often lead to muscle coactivation across multiple limb joints. The RST originates from the reticular formation (RF) and receives projections from the cortex. This provides the anatomical basis for cortex-brainstem modulation. Currently, we know little about how cortex modulates the RF to control RST engagement during motor preparation for various motor tasks, such as tasks involving proximal and distal upper limb joint coordination vs. a purely distal task. We hypothesize that since a simultaneous arm lifting and hand opening task (LIFTOPEN) requires more selective muscle recruitment than a hand opening task (OPEN), the cortex will suppress the RF to reduce the RST engagement at distal muscles during LIFTOPEN. To test this hypothesis, we investigated the startReact response in thirteen able-bodied participants performing the OPEN and LIFTOPEN tasks in response to a startling and non-startling acoustic stimulation. Our results showed that activation of distal muscles was significantly decreased, and the startle response was delayed in LIFTOPEN compared to OPEN. Both results suggest that the cortex suppressed RF and reduced the RST engagement in LIFTOPEN compared to OPEN.Clinical Relevance- Our results provide foundational knowledge of the task-specific nature of cortex-brainstem modulation. This scientific finding provides a base to compare how a unilateral brain injury may affect this cortex-brainstem modulation.

Quantification and Modeling of Ankle Stiffness During Standing Balance.

OBJECTIVE: This study investigates the factors contributing to the modulation of ankle stiffness during standing balance and evaluates the reliability of linear stiffness models. METHODS: A dual-axis robotic platform and a visual feedback display were used to quantify ankle stiffness in both the sagittal and frontal planes while subjects controlled different levels of ankle muscle co-contraction, center-of-pressure (CoP), and loading on the ankle. RESULTS: Results of 40 subjects demonstrated that ankle stiffness in the sagittal plane linearly increased with the increasing level of these three factors. The linear model relating the change in these factors from the baseline measurements during quiet standing to the change in weight normalized ankle stiffness resulted in high reliability (R2 = 0.83). Ankle stiffness in the frontal plane increased with the increasing ankle muscle co-contraction and ankle loading, but the linearity was less obvious. It also exhibited a clear nonlinear trend when CoP was shifted mediolaterally. Consequently, the reliability of the linear model was low for ankle stiffness in the frontal plane (R2 = 0.37). CONCLUSION: During standing balance, ankle stiffness in the sagittal plane could be well explained by a linear model if ankle muscle activation, CoP, and ankle loading were collectively considered. However, the linear model cannot capture highly variable and nonlinear ankle stiffness characteristics in the frontal plane. SIGNIFICANCE: The outcomes of this study could benefit the development of lower-extremity robots and their controllers. Furthermore, the ankle stiffness models could be used as a baseline in developing patient-specific ankle rehabilitation protocols.

Sex Differences in Human Ankle Stiffness During Standing Balance.

The purpose of this study is to quantify sex differences in 2-dimensional (2D) ankle stiffness during upright standing balance and investigate the mechanisms for the differences. A dual-axis robotic platform, capable of perturbing the ankle and measuring the corresponding ankle torques in both the sagittal and frontal planes, was used to reliably quantify the 2D ankle stiffness while healthy young human subjects perform a range of standing balance tasks, specifically, ankle muscle co-contraction tasks, weight-bearing tasks, and ankle torque generation tasks. In all task conditions and in both planes of ankle motion, ankle stiffness in males was consistently greater than that in females. Among all 26 experimental conditions, all but 2 conditions in the frontal plane showed statistically significant sex differences. Further investigation on the normalized ankle stiffness, scaled by weight times height, suggests that while sex differences in ankle stiffness in the sagittal plane could be explained by sex differences in anthropometric factors as well as neuromuscular factors, the differences in the frontal plane are mostly explained by anthropometric factors. This study also demonstrates that the sex differences in the sagittal plane were significantly higher as compared to those in the frontal plane. The results in this study will provide a foundation for not only characterizing sex differences in ankle stiffness during locomotion, but also investigating sex differences in lower body stability and risk of ankle injury.