Imagine a tightrope walker trying to maintain balance on a shaky rope. They might shift their weight or twist their body to counteract the sway. Similarly, patients with Parkinson’s disease (PD) employ compensatory mechanisms to mitigate instability during static standing. These mechanisms manifest as abnormal postures, such as stooped positions or Pisa syndrome. A recent study aimed to explore how these abnormal postures affect sway. The researchers hypothesized that increased muscle tone, which causes constant muscle activity against gravity, contributes to these postures and reduces sway. Using a computational model, they conducted simulations of various postures to determine the minimal sway for each posture. The results showed that patients with PD exhibited less sway in their abnormal postures compared to other positions. This suggests that adopting an unusual posture with increased muscle tone may serve as a valid strategy for minimizing sway in PD patients. To delve deeper into this intriguing finding and learn more about the relationship between abnormal posture and muscle tone, check out the full research article!

Patients with Parkinson’s disease (PD) exhibit distinct abnormal postures, including neck-down, stooped postures, and Pisa syndrome, collectively termed “abnormal posture” henceforth. In the previous study, when assuming an upright stance, patients with PD exhibit heightened instability in contrast to healthy individuals with disturbance, implying that abnormal postures serve as compensatory mechanisms to mitigate sway during static standing. However, limited studies have explored the relationship between abnormal posture and sway in the context of static standing. Increased muscle tone (i.e., constant muscle activity against the gravity) has been proposed as an underlying reason for abnormal postures. Therefore, this study aimed to investigate the following hypothesis: abnormal posture with increased muscle tone leads to a smaller sway compared with that in other postures, including normal upright standing, under the sway minimization criterion. To investigate the hypothesis, we assessed the sway in multiple postures, which is determined by joint angles, including cases with bended hip joints. Our approach involved conducting forward dynamics simulations using a computational model comprising a musculoskeletal model and a neural controller model. The neural controller model proposed integrates two types of control mechanisms: feedforward control (representing muscle tone as a vector) and feedback control using proprioceptive and vestibular sensory information. An optimization was performed to determine the posture of the musculoskeletal model and the accompanied parameters of the neural controller model for each of the given muscle tone vector to minimize sway. The optimized postures to minimize sway for the optimal muscle tone vector of patients with PD were compared to the actual postures observed in these patients. The results revealed that on average, the joint-angle differences between these postures was <4°, which was less than one-tenth of the typical joint range of motion. These results suggest that patients with PD exhibit less sway in the abnormal posture than in other postures. Thus, adopting an abnormal posture with increased muscle tone can potentially serve as a valid strategy for minimizing sway in patients with PD.

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