Musculoskeletal Adaptation of Young and Older Adults in Response to Environmental, Physical, and Cognitive Conditions
Authors: Holcomb Amy E. 
Published: November 2021

Accidental falls present a large functional and financial burden among people aged 65 years and older. Falls, injuries associated
with falls, and the fear of falling decrease quality of life, physical function, and independence for older adults. To prevent falls,
improve stability, and protect joints from damage or injury, the typical response to "challenging" conditions include cautious gait,
increase muscle co-contraction, and decreased range of motion. These compensatory strategies are more pronounced in the older adult
population with apprehensive "cautious" gait at slower speeds, decreased knee flexion, and increased muscle activation around the
knee and ankle. The underlying mechanisms and driving forces behind accidental falls are not well investigated. Additionally, the
effects of aging on the ability of the musculoskeletal system to adapt to changing and challenging conditions is poorly understood.
There exists a gap in knowledge regarding the relationship between accidental fall risk factors, knee joint stability, adaptation
mechanisms, and whole-body function. Establishing these relationships between stability and musculoskeletal adaptation may have far
reaching implications on improving whole-body function through targeted joint- and muscle-level interventions.

The purpose of this study was to compare neuromechanics (whole-body function) of young and older adults walking across various
external challenging conditions, quantifying adaptation strategies for both cohorts. This was accomplished through two objectives.
In the first objective, joint kinematics, ground reaction force loading and impulse, and lower-limb muscle activation strategies
for ten young and ten older adults walking on normal, slick, and uneven surfaces were compared to assess how musculoskeletal
adaptation strategies change with age. For the second objective, a pipeline to create subject-specific lower-limb finite element
models was developed to investigate joint-level behavior across cohorts. Proof-of-concept for the model development and analysis
process was demonstrated for an older and a young adult to implement a novel metric for functional stability and dynamic laxity
of the knee joint during the stance phase of gait.

Kinematic, force, and muscle activation analysis showed that an uneven surface reduced sagittal joint kinematics during the first
25% of stance, indicating a surface-specific compensatory strategy. Additionally, older adults tended to prepare for and step onto
the uneven surfaces in a more conservative manner with joints more flexed or bent. This anticipatory or cautious musculoskeletal
adaptation of older adults was also seen in reduced magnitude of initial vertical loading during the loading response of stance
(0-25% stance). Results of this research study provide insight into the differences that exist in joint stiffening and other
musculoskeletal adaption strategies for young and older adults during external challenging conditions. Specifically, understanding
the relationships between joint-level stability and whole-body musculoskeletal function has the potential to inform targeted muscle
training programs and joint-level interventions to improve whole-body musculoskeletal function and reduce risk of injuries.