Maintaining dynamic balance during transitional movements like sit-to-walk (STW) can be challenging for older adults. Age-related neuromuscular decline can alter movement in STW, such as rising with greater trunk flexion, narrowing the feet, or using arms to push off. Initial foot and arm position can affect subsequent movement biomechanics, with different ground reaction forces (GRFs) that stabilize and advance the body center of mass (COM). The purpose of this study was to quantify whole-body biomechanics and trunk control of STW transitions. Fifteen younger adults (18–35 years) and fifteen older adults (50–79 years) performed STW from four initial foot positions and two arm positions. Three-dimensional (3D) GRFs, 3D body COM displacement, and integrated electromyography values from the lumbar paraspinals and gluteus medius were evaluated. Younger adults generated greater mediolateral GRF ranges while rising, whereas older adults generated greater mediolateral GRF ranges when stepping forward suggesting different strategies to laterally control the body COM. Initial foot position affected the STW movement, with narrow foot positions having smaller body COM displacement than wide foot positions, associated with smaller medial GRFs to move the body COM toward the stance limb. Rising with arm support required less lumbar paraspinal excitation, which was further reduced when with a posteriorly offset foot. Gluteus medius activity was greater for older adults compared to younger adults in STW. Completing STW with arm support can reduce the muscle activity required to stabilize the torso when rising, which likely has implications for balance control and low back loading.

The prevention, mitigation and treatment of movement impairments, ideally, requires early diagnosis or identification. As the human movement system has physiological and functional redundancy, movement limitations do not promptly arise at the onset of physical decline. A such, prediction of movement limitations is complex: it is unclear how much decline can be tolerated before movement limitations start. Currently, the term ‘homeostatic reserve’ or ‘physiological reserve’ is used to refer to the redundancy of the human biological system, but these terms do not describe the redundancy in the muscle architecture of the human body. The result of functional redundancy is compensation. Although compensation is an early predictor of movement limitations, clear definitions are lacking and the topic is underexposed in literature. The aim of this article is to provide a definition of compensation and emphasize its importance. Compensation is defined as an alteration in the movement trajectory and/or altering muscle recruitment to complete a movement task. Compensation for capacity is the result of a lack in neuromusculoskeletal reserve, where reserve is defined as the difference between the capacity (physiological abilities of the neuromusculoskeletal system) and the task demand. Compensation for movement objectives is a result of a shift in weighting of movement objectives, reflecting changing priorities. Studying compensation in biomechanics requires altered protocols in experimental set-ups, musculoskeletal models that are not reliant on prescribed movement, and inclusion of alternative movement objectives in optimal control theory.

Capacity is the physiological ability of the neuromusculoskeletal systems; this declines with age. This decline in capacity may result in the inability to stand up (sit-to-stand, sit-to-walk), which is an important movement for independent living. Compensation, as a result of functional redundancy, is key in understanding how much age-related decline can be tolerated before movement limitations arise. Yet, this topic has been underexposed in the biomechanics literature. The purpose of this systematic review was to approach the literature on sit-to-stand and sit-to-walk studies from the perspective of compensation and create an overview of our current understanding of compensation in standing up, identifying the limitations and providing future recommendations. A literature search was performed, using the keywords and their synonyms: strateg*(approach, technique, way) AND, sit-to-walk OR sit-to-stand OR rise (raise, arise, stand, stand-up) AND chair (seat). Inclusion criteria: full articles on biomechanics or motor control on sit-to-stand or sit-to-walk in healthy adults (<60y), healthy or frail elderly adults (>60y), and adults with osteoarthritis. The results show that the experimental set-ups and musculoskeletal models in STS and STW studies generally exclude compensation by using restricted protocols and simplifications. Moreover, factors are mostly analysed in isolation, excluding confounding causes within capacity and/or movement objectives which limits the generalization of the results. Future studies in the standing up task should consider to (1) determine the effect of varying arm push-off strategies, (2) focus on sit-to-walk, (3) determine the biomechanical implications of asymmetry, and (4) incorporate assessments of physical capacity as well as changes in psychological priorities.