A neuromuscular and kinematic description of human adults with achondroplasia

Skeletal dysplasia is an umbrella term used to describe individuals with an abnormality to the skeleton. There are more than 350 reported skeletal dysplasias with the more common name for the term being ‘dwarfism’. The prevalence of skeletal dysplasia is around 1/5000 child births (Bonafe et al., 2015), the most common form of which being Achondroplasia (Krakow and Rimoin, 2010). Individuals with Achondroplasia are termed ‘rhizomelic’ (i.e. distal appendicular segments are longer than the adjoining proximal segments) and have a disproportionate limb length-to-torso ratio compared to age matched adults of average stature (controls). Average stature of an adult male with Achondroplasia is ~1.30 m while females with Achondroplasia are ~1.25 m (Horton et al., 1977; Hunter et al., 1996). The cause of Achondroplasia is well documented and is attributed to a mutation in the fibroblast growth factor receptor 3 (FGFR3) during foetal growth (Bellus et al., 1995b; Bellus et al., 1995a; Horton and Lunstrum, 2002; Horton, 2006; Horton et al., 2007). Medical and psychological complications associated with Achondroplasia are described, but no data exist on the physiological or biomechanical descriptions of the condition. The aim of this thesis was therefore to quantify the neuromuscular and biomechanical properties of adults with Achondroplasia. Specifically, this would be achieved by measuring: in vivo total-body and segment composition; maximal oxygen consumption (VO2max); submaximal oxygen consumption (VO2) and metabolic cost (C) during incremental exercise; neuromuscular and biomechanical properties of muscle and tendon during isometric maximal voluntary contraction (iMVC), and; Results showed that the adult group with Achondroplasia had disproportionate legs and arms lengths compared to torso length but were not rhizomelic. Total-body mass, bone mineral content, bone mineral density and fat free mas was lower in the group with Achondroplasia compared to controls. Fat mass however, was higher in the group with Achondroplasia than controls when relative to total-body values; differences in body composition values were lessened somewhat when relative to total-limb values. A lower absolute VO2max was observed in the group with Achondroplasia when compared to controls. This difference was removed when presented relative to total-body mass and fat free mass. The group with Achondroplasia had a higher VO2 and C than controls at all walking and running speeds, with a persistent higher C being observed when normalised to total-body mass and leg length. The group with Achondroplasia were weaker than controls when presented as absolute values and when accounting for biomechanical and physiological properties (here as specific force). Furthermore, a more compliant patella tendon during iMVC was observed in the group with Achondroplasia compared to controls. Following three-dimensional gait analysis, a number of discrete differences in joint kinematics existed between groups when walking and running, resulting in the group with Achondroplasia being more ‘flexed’ than controls throughout the walking and running stride. A global analysis of gait kinematics (here as gait profile score) showed that the gait of individuals with Achondroplasia quantifiably different to controls during walking, with a more similar gait pattern being observed between groups when running. Body morphology differences between the groups helped normalise the differences in a number of functional measures. However, the persistently higher C in individuals with Achondroplasia during walking and running is likely due to the combination of their lower force development of the knee extensors, their more compliant patella tendon and differences in their gait kinematics compared to controls.