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## Citation

Davidson B, Madigan M, Southward S, Nussbaum M. (2010). Neural control of posture during small magnitude perturbations: Effects of aging and localized muscle fatigue. IEEE Transactions on Bio-medical Engineering. 2010. online. PUBMED

## 10 Word Summary

Feedback gains are more conservative as adults age and delay increases.

## Abstract

This study investigated the effects of aging and localized muscle fatigue on the neural control of upright stance during small postural perturbations. Sixteen young (aged 18-24 years) and 16 older (aged 55-74 years) participants were exposed to small magnitude, anteriorly-directed postural perturbations before and after fatiguing exercises (lumbar extensors and ankle plantar flexors). A single degree of freedom model of the human body was used to simulate recovery kinematics following the perturbations. Central to the model was a simulated neural controller that multiplied time-delayed kinematics by invariant feedback gains. Feedback gains and time-delay were optimized for each participant based on measured kinematics, and a novel delay margin analysis was performed to assess system robustness. A 10.9% longer effective time-delay (p=0.010) was found among the older group, who also showed a greater reliance upon velocity feedback information (31.1% higher differential gain, p=0.001) to control upright stance. Based on delay margins, older participants adopted a more robust control scheme to accommodate the small perturbations, potentially compensating for longer time-delays or degraded sensory feedback. No fatigueinduced changes in neural controller gains, time-delay, or delay margin were found in either age group, indicating that integration of this feedback information was not altered by muscle fatigue. The sensitivity of this approach to changes with fatigue may have been limited by model simplifications.

## Notes

• Linearized inverted pendulum model with delayed PD controller
• Optimize by searching for Kv, Kp, delay and initial lean angle
• Perturbations were ballistic pendulum at 10 Ns.  Whacko!
• Include passive stiffness and damping based on A. L. Hof, "In vivo measurement of the series elasticity release curve of human triceps surae muscle," J Biomech, vol. 31, pp. 793-800, Sep 1998.
• $B = 0.76 \sqrt{4 J K}$ Formula from Ian Loram.
• Differences found in derivative gain and delay between young and old subjects
• Youngsters: Delay = 188ms, Kp = 963, Kv = 98
• Oldsters: Delay = 211ms, Kp = 864, Kv = 129
• Oldsters had a greater delay margin (phase margin) than youngsters.  Appear to be selecting more stable gains?