Physiological and degenerative changes involving multiple systems underlying the control of human standing balance are major contributors to falls and mobility disability with ageing. During actual or anticipated imbalance, multi-segmental postural movements of the limbs such as stepping are commonly executed and powerful protective actions for preserving balance by reconfiguring the body center of mass (CoM)-base of support (BoS) relationship. This requires that the stepping limb motion be appropriately directed, timed, and scaled in magnitude to match the ongoing motion of the whole body. Although age-related deficits in balance recovery during protective stepping have generally been identified, an impaired ability to control lateral balance is particularly relevant to the problem of falling among older people. In a series of studies investigating the impairments in physiological and mechanical mechanisms underlying lateral balance instability as a risk factor for falls, we have focused on two primary types of protective stepping involving different forms of neuromotor control: 1) externally induced reactive stepping, and 2) rapid voluntary stepping. Both models involve a rapid weight transfer sequence from bipedal to single-limb to bipedal support under the different control conditions. The methods have employed biomechanical movement analysis, kinetic recordings, electromyography, and electrophysiological and acoustic stimulation. Reactive stepping was evoked mainly by a motorized position-controlled waist-pull device. A directional vulnerability to loss of balance and falls was systematically identified in community living adults by randomly delivering waist-pulls in 12 different directions at 30 deg. intervals. Younger participants mainly recovered balance with single steps in all directions while older groups predominantly took multiple steps that were least for forward-backward directions and greatest laterally with multiple interlimb collisions especially for prospectively identified fallers. Directional differences in adapting stepping parameters were also observed in relation to age and fall risk that further indicated particular difficulties with lateral balance recovery. An important determinant for successfully recovering reactive lateral balance appears to be the motor output generated by hip joint abductor (AB)-adductor (AD) torque and power production (and postural movements of the trunk) that underlie inter-limb postural weight transfer affecting the types of stepping strategies engaged. For example, when perturbed to the side, a passive increase in weight bearing load beneath the leg nearest to the side of destabilization occurs with a concomitant reduction in contralateral load. While younger adults often engage a single and more biomechanically stable loaded-limb sidestep by actively unloading and advancing the limb, older adults use less stabilizing and precarious multiple unloaded limb crossover and medial steps. These differences in first-step recovery patterns appear to involve ageing decrements in hip AB-AD joint torque and power production. Collectively, the use of multiple recovery steps in 100% of lateral perturbation trials, reduced maximum isokinetic hip AB torque and axial motion of the trunk, and shorter global first step length, significantly predict future falls among community living adults. The use of multiple steps, possibly reflecting reduced functional limits of dynamic stability involving the center of pressure position with respect to the BoS to regulate CoM momentum prior to stepping, is a comparatively robust performance variable for identifying future risk of falls. To gain further insights into why lateral challenges to balance stability linked with falls are especially challenging with ageing, we examined whether well-known sarcopenic and composition changes among lower limb muscles with different functional roles are equivalently degraded with older age. Using computed tomography (CT), the muscle cross-sectional area (CSA), intramuscular adipose tissue (IMAT) content, and muscle attenuation (density of the skeletal muscle fibers) were determined for six hip and knee muscles together with measures of muscle performance (isokinetic testing). Intermuscular comparisons indicated that gluteal muscles (gluteus maximus and gluteus medius/minimus) had the lowest muscle attenuation and highest IMAT infiltration, and that fallers were differentiated from nonfallers by lower muscle attenuation and higher IMAT infiltration despite comparable CSAs with the gluteal muscles being the most affected. The strongest associations were found between gluteus medius/minimus and hip abduction strength and power. These regionally disparate changes in muscle composition and performance may influence directional changes in lateral balance function. This possibility also has relevance for rapid voluntary stepping where anticipatory postural adjustments for lateral weight transfer normally precede step execution and choice reaction time performance is an independent discriminator between older fallers and nonfallers. These lateral balance factors have been incorporated into currently ongoing intervention trials designed to use progressive high intensity step training and resistance exercise training to enhance lateral balance function and prevent falls.