Falls are common in older adults aged over 65 years old, and about one out of five falls causes severe fatal and non-fatal injuries 1 such as hip fractures and traumatic brain injuries. The fall rate leading to these severe injuries increases with age 2,3. Most outdoor falls in older adults occur after they experience ground balance challenges while walking over a variety of ground surfaces (e.g., uneven, slippery, or compliant surfaces such as curbs, ice, or muddy terrain) 4,5. Therefore, the ability to successfully maintain postural stability and recover balance is important for fall prevention in older adults.
A progressive decrease in balance control is part of normal aging. Previous studies have found that age-related impairment in neuromuscular control and force-generating capacity in the lower limb led to altered muscle activation patterns during walking balance recovery. For example, neuromuscular control tends to become more simplistic with age, leading to reduced variability and complexity of muscle synergy options 6. In addition, age-related reductions in lower limb force generation leads to altered muscle activation patterns during dynamic balance control. Older adults demonstrated relatively greater magnitude of ankle plantar flexor electromyography (EMG) activation compared to younger adults despite having no age-related difference in plantar flexion torque during ankle joint push-off for postural balance control 7. This age-related impairment in ankle joint force control efficiency leads to altered muscle activation patterns, which redistributes push-off force generation to more proximal muscles at the knee and hip joints 8. Regarding movement strategies for postural balance control while walking, older adults demonstrates ineffective inter-joint coordination, such as greater agonist and antagonist coactivation at the ankle and knee joints 9, and increased joint kinematic variability at the ankle, knee, and hip joints during walking with lateral balance perturbations 10. These age-related changes in neuromuscular factors and movement strategies for dynamic balance control raise the potential risk of falls in older adults.
One of the biggest challenges in aging is understanding how these age-related changes are associated with higher fall rates in older adults and, relatedly, why older adults are more likely to fall and sustain an injury from the same level of balance perturbation that their younger counterparts can successfully recover from. Falls after ground balance challenges result from what has been described as an unrecoverable limb collapse during single limb support (SLS) 11,12. Once a trailing leg protective (compensatory) step is initiated, knee extensor eccentric control during the SLS of the perturbed leg is critical as the initial defense to prevent limb collapse 11. Unless sufficient eccentric work of the quadriceps muscle in the perturbed leg adequately decelerates the falling momentum of the body, a fall will ensue. Even when limb collapse is avoided in the vertical direction, a wobbly and unstable perturbed leg during SLS due to the weak knee extensor eccentric control can lead to unrecoverable postural instability in the anterior-posterior (A-P) and mediolateral (M-L) directions 13.
Considering older adults have greater force-generating errors and larger force variability when producing knee extensor eccentric control 14, investigating age-related changes in neuromuscular eccentric control characteristics at the knee joint and differences in lower limb muscle coordination patterns to secure balance during SLS can reveal key factors underlying increased fall risk in older adults, especially after an encounter with unpredictable ground balance challenges while walking.
A compliant surface such as muddy ground, sandy soil, or soft ground challenges dynamic balance control. A compliant viscoelastic surface interrupts ankle joint proprioceptive performance in the sensorimotor system, and the unknown property of a compliant (foam, viscoelastic) surface requires partially unpredictable balance responses 15. Previous studies of balance control on a compliant surface have reported that specific movement strategies. These strategies include co-contraction of dorsiflexor and plantar flexor at the ankle joint 16 and different muscle activation patterns to compensate for relatively weaker ankle evertor activation 17 while standing on compliant (foam) surface. These findings demonstrate that when standing on a compliant surface, older adults exhibit greater postural sway and more rigid postural behavior during balance control compared to younger adults. Therefore, studying walking balance control on compliant (foam) surface during SLS will add important insight into understanding the balance recovery mechanisms during unilateral drop-landing on unstable, nonsolid ground surfaces, such as soft, muddy ground or sandy soil.
The purpose of this study was to investigate 1) age-related differences in the characteristics of knee extensor eccentric control and muscle coordination patterns of the perturbed leg, swinging (trailing) leg protective step balance control while walking on a compliant (foam) surface and 2) the relationship to postural stability (postural sway) during the SLS phase and swinging leg protective stepping response. The results of this investigation will provide insights into novel directions for lower limb strengthening exercises to improve dynamic balance control and reduce fall risk in an aging population.