Effects of quadriceps strength asymmetry, ageing and external loading on stair negotiation

Ageing is associated with reduced stability and increased risk of falls. One risk factor for falls is sarcopenia: age-related muscle loss. Exaggeration of age-related muscle loss in one limb over the other results in strength asymmetry. This asymmetry typically increases with age and previous research has identified negative impact on gait and stability. This thesis develops to, firstly, identify a reliable method to assess strength asymmetry levels (chapter 5), then investigate the effects of quadriceps strength asymmetry on the biomechanics of stair negotiation in a young population (chapter 6) and “symmetrical” and “asymmetrical” older adults (chapter 7), before, finally, studying the effects of external loading (chapter 8).
In the first study, within- and between-day testing of isometric maximal voluntary contractions of the quadriceps identified this method as a reliable method to assess strength levels in both legs, and the percentage asymmetry between legs. Consequent studies used this method to analyse the effect of age (young adults versus older adults), asymmetry (<15% versus >15%) and load carriage (unloaded versus 5% body weight bilaterally versus 10% body weight unilaterally) on a range of kinetic and kinematics variables during stair ascent and descent. The young healthy population (8.8±8.5% asymmetry) demonstrated only two significant differences in biomechanical variables between the stronger and weaker leg out of 60 variables measured. This was possibly as a result of type I error as absolute differences in mean values were negligible. Older adults (16.6±14.2% asymmetry) demonstrated detrimental effects of ageing regardless of asymmetry level, suggesting the overall loss of quadriceps strength due to age seems to play a key role. Results demonstrated greater effects on centre of mass and centre of pressure inclination angles and knees angles, rather than contact time and ground reaction force, suggesting the loss of strength can be controlled for to a greater extent in contact time and force. Under additional demands of external loading, effects of group (i.e., age and asymmetry) were typically not found and significant effects were instead a result of loading, primarily during unilateral load carriage. Greater ground reaction force and lateral inclination towards the load was demonstrated when stepping on the leg ipsilateral to the load. Overall, while the impact of strength asymmetry was minimal, this thesis highlights the important impact of age-related loss of muscle strength on stability during stair negotiation. Furthermore, the findings suggest splitting loads bilaterally when under the additional demands of loading.

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