Falls are a major problem for older adults, with 30% of people aged over 65 experiencing one or more falls per year (Campbell et.al,1981; Prudham and Evans, 1981) , rising to 50% in the over 85 year olds (Lord et.al., 2007). Half of over 70 year olds suffering from falls face hospital admissions, costing the NHS close to £1.7bn as estimated in 2010. Falls are therefore not only associated with physiological and psychological costs but also considerate financial implications. Deficits in sensation and motor outputs are just a couple of factors that contribute to the physiological reasons for falling (Lord et.al., 2007). The current literature has reported a wealth of information linking age-related musculoskeletal changes to postural instability and falls (Onambele et.al., 2006 & 2007; Karamanidis et.al., 2008; Butler et.al., 2008). The evidence also suggests that age-related postural instability is linked to a loss of proprioception.
Proprioception refers to the sensory signals which providing a sense of limb position. These are essential in maintaining postural stability (balance). Loss of proprioception due to peripheral neuropathy is very strongly associated with falls (Richardson et.al., 1992). During quiet stance most movement occurs about the ankle joint, the majority of proprioceptive input is therefore derived from the ankle joint, in particular the signals derived from the calf muscles are of most importance. This is because the calf muscles (& Achilles tendon) are responsible for maintaining adequate levels of ankle stiffness to overcome the tendency of the body to topple forwards. Muscles, however, perform a dual function, both as sensors and force producers. The ability of calf muscles to act as accurate sensors could therefore be compromised during active contractions. It is precisely this relationship between motor and sensory function that has left unanswered questions.
An inverted pendulum with a dampened spring mass system about the ankle joint has long been used to model and describe balanced standing and its associated body sway in man (Winter et.al., 1998). Balance was initially thought to come about due to a series of mechanoreflex responses eliciting calf muscle activation due to muscle lengthening, as a result of leaning forwards. Posture would therefore be regulated via a simple stretch reflex. If this were to be true, the calf muscles should lengthen as we sway forwards, but we now know the situation to be more complex. Novel techniques using ultrasound tracking have shown that as the body sways forward, muscles actually shorten (Loram et.al., 2004, 2005 & 2009). This ‘paradoxical’ movement, a negative correlation between muscle length and sway, has been attributed to the combination of ongoing changes in motor output and a relatively low stiffness of the ankle joint during quiet stance.
We are therefore left with an ambiguous relationship between calf muscle length and body sway as the low ankle stiffness is insufficient in maintaining balance solely through passive mechanical stabilisation (Loram and Lakie, 2002; Casadio et.al., 2005). To add to the complexity ankle stiffness itself varies considerably between people (Loram and Lakie, 2002) and is limited by its most compliant component, the Achilles tendon, which in turn has been shown to alter in stiffness with age (Onambele et.al., 2006 & 2007).
This study therefore aims to investigate changes in ankle joint proprioception with age, the mechanisms underlying these changes, and the consequences these have on postural stability. In summary we will study:
- Variations in ankle stiffness and how this affects coupling between ankle rotation and muscle length during stance.
- The influence of muscle-sway coupling on proprioceptive acuity
- The effects of age upon proprioceptive control of balance
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