Ankle muscle strength discriminates fallers from non-fallers

Are your therapists and doctors using this to objectively determine your falling possibilities and then creating stroke protocols to prevent those falls? And with our spasticity I'm sure the calculations would need to be reworked.
http://journal.frontiersin.org/Journal/10.3389/fnagi.2014.00336/full?
Thomas Cattagni^1,2*, Gil Scaglioni^1,2, Davy Laroche^2,3, Jacques Van Hoecke^1,2, Vincent Gremeaux^2,3,4 and Alain Martin^1,2

• ¹Faculté des sciences du sport—UFR STAPS, Université de Bourgogne, Dijon, France
• ²INSERM unité 1093, Cognition, action et plasticité sensorimotrice, Dijon, France
• ³INSERM CIC 1432, Plateforme d’Investigation Technologique, CHU de Dijon, Dijon, France
• ⁴Pôle rééducation-réadaptation, CHU de Dijon, Dijon, France

It is well known that center of pressure (CoP) displacement correlates negatively with the maximal isometric torque (MIT) of ankle muscles. This relationship has never been investigated in elderly fallers (EF). The purpose of this study was thus to analyze the relationship between the MIT of ankle muscles and CoP displacement in upright stance in a sample aged between 18 and 90 years old that included EF. The aim was to identify a threshold of torque below which balance is compromised. The MIT of Plantar flexors (PFs) and dorsal flexors (DFs) and CoP were measured in 90 volunteers: 21 healthy young adults (YA) (age: 24.1 ± 5.0), 12 healthy middle-aged adults (MAA) (age: 50.2 ± 4.5), 27 healthy elderly non-fallers (ENF) (age: 75.5 ± 7.0) and 30 EF (age: 78.8 ± 6.7). The MIT of PF and DF were summed to obtain the overall maximal ankle muscle strength. Body weight and height were used to normalize MIT (nMIT) and CoP (nCoP), respectively. nCoP correlated negatively with nMIT. 90% of EF generated an nMIT <3.1 N·m·kg⁻¹, whereas 85% of non-fallers generated an nMIT >3.1 N·m·kg⁻¹. The relationship between nMIT and nCoP implies that ankle muscle weakness contributes to increased postural instability and the risk of falling. We observed that below the threshold of 3.1 N·m·kg⁻¹, postural stability was dramatically diminished and balance was compromised. Our results suggest that measuring ankle torque could be used in routine clinical practice to identify potential fallers.

Introduction

Falls are a major concern among older adults. Indeed, approximately 30% of people over 65 years old and 50% of those over 80 years old fall each year (Tinetti et al., 1988; Campbell et al., 1989; O’Loughlin et al., 1993; Hill et al., 1999; Rubenstein and Josephson, 2002). These accidents are a common cause of injuries which can lead to hospitalization and loss of autonomy.

The increase in body sway, generally observed with aging (Sheldon, 1963; Amiridis et al., 2003; Laughton et al., 2003; Onambele et al., 2006; Cavalheiro et al., 2009; Kouzaki and Masani, 2012) has been identified as a major factor responsible for an increased risk of falling among older adults (Fernie et al., 1982; Maki et al., 1994). For instance, Maki et al. (1994) found that, in the year following the measurement of center of pressure (CoP) displacement, elderly people who fell had a significantly greater anteroposterior sway than those who did not fall.

To control body sway while standing upright, humans need to generate appropriate torques at the ankle joint (Horak and Nashner, 1986; Horak et al., 1989; Amiridis et al., 2003, 2005; Loram et al., 2004). During the aging process, it is common to observe a decline in the neuromuscular performance of plantar flexor (PF) and dorsal flexor (DF) muscles (Vandervoort and McComas, 1986; Doherty et al., 1993; Winegard et al., 1996; Simoneau et al., 2007; Billot et al., 2010), which appears to be more pronounced in elderly fallers (EF) than in the rest of the elderly population (de Rekeneire et al., 2003; Perry et al., 2007; LaRoche et al., 2010). The decrease in the maximal strength of ankle muscles may therefore be considered a main cause of postural instability.

Recently, Billot et al. (2010) reported that, in young adults (YA) and elderly non-fallers (ENF) challenged across three postural tasks (i.e., bipedal stance, unipodal stance and tandem stance), a negative linear correlation was observed between the maximal isometric torque (MIT) of ankle muscles and the length of CoP displacement. In the inverted pendulum model, which is generally used to describe upright stance in humans, the equilibrium equation for the foot shows that a variation in muscle torque developed at the ankle joint is directly and linearly translated into a variation of the CoP position (Morasso and Schieppati, 1999). In this model, the fall occurs when the maximal ankle torque capacity of an individual is lower than the torque needed to counterbalance the CoP displacement. Therefore, the analysis of the relationship between CoP displacement and maximal ankle torque in EF could provide valuable insights into balance control in frail elderly subjects. If the relationship between CoP displacement and maximal ankle torque in EF is different from that in non-fallers this would mean that their balance would be even more impaired by the decline in ankle muscle strength than would be expected if the relationship were the same. Should this be the case, it could be supposed that below a certain level of muscle strength, subjects are not able to control their balance. This level of torque could thus be considered an index of the risk of falling.

In view of these considerations, the purpose of this study was to analyze the relationship between MIT of ankle muscles and CoP displacement in a sample aged between 18 and 90 years old that included EF, in order to identify a threshold of torque below which balance is compromised.