###### Abstract  
Falls in older adults are leading causes of fatal and non-fatal injuries, negatively impacting quality of life among those in this demographic. Most elderly falls occur due to unrecoverable limb collapse during balance control in the single-limb support (SLS) phase. To understand why older adults are more susceptible to falls than younger adults, we investigated age-related differences in lower limb kinematics, kinetics, and muscle synergy patterns during SLS, as well as their relationship to postural control strategies. Thirteen older and thirteen younger healthy adults were compared during the SLS phase of balance recovery following an unexpected surface drop perturbation. Compared to younger adults, older adults demonstrated (1) greater trunk flexion, (2) increased hip extension torque and reduced hip abduction torque of the perturbed leg, and (3) higher postural sway. Trunk flexion was correlated with a delayed latency to the start of lateral-to-medial displacement of center of mass from the perturbation onset. The group-specific muscle synergy revealed that older adults exhibited prominent activation of the hip extensors, while younger adults showed prominent activation of the hip abductors. These findings provide insights into targeted balance rehabilitation and indicate ways to improve postural stability and reduce falls in older adults. older adults; muscle synergy; movement strategy; balance control  
Prevention Rehabilitation  
W. J. A. J.  
with age, leading to reduced variability and complexity in the available muscle synergy options [6]. In addition, age-related reductions in lower limb force generation and reduced flexibility lead to altered muscle activation patterns during dynamic balance control [5; 7]. These factors collectively contribute to impaired postural stability when older adults need to respond to balance challenges. Most outdoor falls in older adults occur when they encounter challenges while walking on different types of surfaces, such as uneven and slippery terrain [8; 9]. The ability to successfully recover balance following unexpected ground perturbations depends on how well the forward and downward falling momentum is decelerated in the anterior-posterior (AP) and vertical directions while simultaneously maintaining postural stability in the medio-lateral (ML) direction [10; 11; 12; 13]. However, compared to younger adults, older adults exhibit impaired and less efficient balance control across the joints of their lower limbs. For example, older adults have been shown to demonstrate a relatively greater magnitude of electromyography (EMG) activation in ankle plantar flexor despite having no age-related differences in plantar flexion torque during ankle joint push-off [5]. At the knee joint level, older adults exhibit reduced knee extensor eccentric work and EMG burst duration during the single-limb support (SLS) phase of balance recovery, which is associated with reduced absorption of falling momentum and forward momentum control [14]. In addition, older adults display diminished hip abduction torque during protective stepping for balance recovery from lateral perturbations [15]. This reduction indicates compromised control of interlimb weight transfer at the hip joint, which is crucial for maintaining a stable relationship between the body's center of mass (CoM) and base of support (BoS). As a result, postural instability in the ML direction increases. Age-related declines in dynamic balance control in the lower extremities are associated with changes in postural control strategies in older adults while walking. The impairment in ankle joint force control which becomes more observable with age leads to an altered movement strategy for balance control, redistributing push-off propulsive force generation to more proximal knee and hip joints while walking in older adults [16].