The sensory role of muscle for the control of balance

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Falls are a major problem for older adults, with 30% of people aged over 65 experiencing 1 or more falls per year, rising to 50% in the over 85's. The reasons for falling are multi-factorial, and include problems with both sensation and movement. One of the most important types of sensory input for balance comes from the calf muscles in the lower leg, termed 'proprioception'. There is evidence to suggest that loss of proprioception in older adults may be a key cause of postural instability. However, muscles must produce movement as well as act as sensory organs. Ongoing muscle contraction may therefore affect the ability of muscle to act as a sensor. This, in turn, is affected by individual variations in ankle stiffness, and overall muscle strength. It is well established that these factors change with age, and this may therefore degrade the ability of the muscle to detect body movement. We will test this hypothesis by measuring proprioceptive control of balance in older adults (aged 65-85). Initially, we will examine balance in younger volunteers (aged 18-40) to establish baseline performance. In particular, we will determine how natural variations in ankle stiffness and muscle strength affect the quality of sensory information emanating from the calf muscles during quiet standing. To assess the quality of proprioceptive information in each volunteer, we will use ultrasound to measure calf muscle movement to see how well muscle length tracks body sway when standing. This will be correlated with postural stability. We will then determine how mechanical factors such as ankle stiffness and ankle strength affect this relationship. Furthermore, we will test the influence of these factors upon the ability of subjects to detect very small rotations at the ankle joint. Using an innovative 'virtual' standing paradigm, we will then artificially manipulate strength and ankle stiffness to mimic the ageing phenotype in the young. Once we have established baseline performance in the young, we will study older adults to determine how the well-established musculoskeletal changes affect proprioceptive control of sway. Using the same virtual standing paradigm, we will test the effect of artificially improving function. The strength of our approach is that we can compare proprioception between groups (young vs old) as well as manipulate the above factors within subjects. This combined approach will give us a comprehensive picture of how proprioceptive information is used to control balance, both in the young and aged state. By revealing the mechanisms by which sensory information is used for balance, we will better understand how these mechanisms degrade with age leading to postural instability. This will be of benefit both in being able to better identify those at risk of falling, and being able to tailor specific therapies to improve balance function.