The Impact of Shoulder Immobilization by Orthoses on Human Joint Biomechanics and Inter-joint Coordination

The human balance system is sophisticated, involving a dynamic interplay between sensory inputs, central processing, and motor outputs. Sensory inputs from the visual, vestibular, and proprioceptive systems provide continuous feedback about body position and environmental changes. The central nervous system integrates this information, coordinating responses that maintain balance by regulating muscle tone and reflexes. Motor outputs, executed through the actions of muscles in both the upper and lower limbs, adjust body position to counteract potential disturbances to stability. Balance control is a fundamental aspect of human function, playing a crucial role in daily activities ranging from simple tasks like standing and walking to more complex movements such as carrying loads or navigating uneven surfaces. Effective balance control is not just about preventing falls; it is integral to overall mobility and independence.

Lower limbs play a pivotal role in balance control. The legs and feet support body weight, adapt to surface changes, and generate the necessary forces and movements to initiate, sustain, and terminate locomotion. Muscles in the legs manage much of the corrective action required to maintain an upright stance and facilitate movement. For instance, during walking, the legs propel the body forward and adjust to varying terrain and unexpected obstacles, ensuring stability. The role of the upper limbs in balance, while sometimes less obvious, is equally vital. Arms and hands assist in counterbalancing actions during locomotion and stabilizing the body when it encounters destabilizing forces. For example, when a person trips, the instinct is to extend the arms outward or forward to regain equilibrium or brace for a fall, demonstrating the reflexive contribution of the upper limbs to balance maintenance. Furthermore, the upper limbs are essential in tasks that require holding or manipulating objects, which can alter the center of mass and require compensatory adjustments in body posture to maintain balance.

During locomotion, the arms and hands are essential in counterbalancing actions. They help stabilize the body by making fine adjustments that counteract destabilizing forces encountered during movement. For instance, when a person walks, the swinging motion of the arms helps to balance the rhythmic movement of the legs, contributing to a smooth and stable gait. Research has shown that this coordinated movement between the upper and lower limbs is critical for maintaining balance and preventing falls. When an individual encounters an unexpected trip or slip, the upper limbs instinctively extend outward or forward. This reflexive action serves multiple purposes: it helps regain equilibrium by redistributing the body's center of mass and prepares the body to brace for impact if a fall is imminent. The rapid extension of the arms increases the base of support and helps absorb the shock of the fall, reducing the likelihood of injury. This reflexive contribution of the upper limbs is crucial to the body's balance maintenance system, highlighting their importance beyond their more apparent functions. Furthermore, these actions often shift the body's center of mass, necessitating compensatory adjustments in posture to maintain stability. For example, carrying a heavy object in one hand requires the body to make lateral adjustments to avoid tipping over. Similarly, tasks that involve reaching out or moving objects can cause shifts in balance, requiring the coordination of upper limb movements with the rest of the body to prevent falls. This complex interplay ensures the body remains upright and stable even when performing dynamic or strenuous activities. Studies have also highlighted the role of the upper limbs in enhancing postural control in static and dynamic situations. During standing, minor adjustments of the arms can help maintain a steady posture, especially when the surface is unstable or when external forces act upon the body. In dynamic activities, such as sports or dancing, the arms help balance and contribute to the efficiency and fluidity of movement, further underscoring their integral role in maintaining equilibrium.

Shoulder immobilization, a standard treatment for various injuries and conditions such as fractures, dislocations, or rotator cuff tears, can significantly impact daily walking activities. The shoulder joint, one of the body's most mobile and versatile joints, plays a crucial role in the overall biomechanics of movement, including walking. The natural arm swing during walking serves several essential functions. It helps to counterbalance the movements of the legs, thereby maintaining the body's center of mass and contributing to a smooth and efficient gait cycle. When one shoulder is immobilized, the symmetry of this arm swing is disrupted, forcing the body to adapt in various ways. Typically, individuals may exhibit a reduced or asymmetrical arm swing on the immobilized side, which can lead to an uneven distribution of forces across the body. This imbalance can cause additional strain on the muscles and joints of the opposite arm, the lower back, and the hips, potentially leading to discomfort or secondary injuries over time. Research has shown that restricted upper limb movement can lead to compensatory changes in the lower limbs' kinematics and kinetics. For example, individuals may exhibit an increased trunk rotation or lateral sway to compensate for the lack of arm movement, which can affect balance and stability. These compensatory movements can also increase energy expenditure, making walking more tiring and less efficient. Postural control is another aspect significantly affected by shoulder immobilization. The shoulder girdle and upper limbs are vital in maintaining upright posture, particularly when the body encounters destabilizing forces or uneven terrain. Immobilizing the shoulder limits the ability to make fine adjustments with the arms, which are crucial for maintaining balance. As a result, individuals may become more prone to falls or adopt a more cautious and rigid gait pattern to compensate for the lack of upper limb mobility. This cautious gait can reduce walking speed and agility, impacting daily activities and overall quality of life. In the sit-to-stand (STS) transition, a movement that demands coordinated action and substantial muscle strength across various joints and body segments, the shoulders, though not primary drivers, play an essential role in maintaining balance and postural alignment. Immobilization of one or both shoulders induces significant biomechanical adaptations. The constrained shoulder mobility restricts natural arm swing and the accompanying upper body momentum, both of which are crucial in propelling the body upward from a seated position. Typically, arm swing not only facilitates momentum but also plays a pivotal role in maintaining balance. In the absence of arm use, there is a greater reliance on the strength and control of the lower body, particularly the quadriceps and hip flexors, to execute the movement. Furthermore, shoulder immobilization often necessitates compensatory strategies to offset the diminished contribution from the upper body. Individuals may adopt pronounced lateral leaning, torsional twisting of the torso, or utilize external supports, such as grabbing onto nearby objects or excessively using the non-immobilized arm. These compensatory actions can significantly alter loading patterns on the spine and lower extremities, potentially resulting in overuse injuries or increased joint stress over time.

3D motion analysis system has been widely used clinically, and it provides a sensitive tool for the diagnosis of patients with neuro-musculoskeletal pathology and the subsequent planning and assessment of treatment. Generally, to perform a 3D motion analysis, at least 2 high-speed cameras and forceplates are needed to capture the motion data and the ground reaction force (GRF) during motor tasks. The use of video-based stereophotogrammetry in human movement analysis requires the determination of the poses (position and orientation) of the body segments from skin-mounted markers. The musculoskeletal system is generally modelled as a multi-link chain with each body segment as a rigid link. An array of at least three non-linear markers per segment is needed for the definition of a segment-embedded reference frame, which represents the pose of the segment. To obtain the velocities and acceleration, displacement of the segments during motor tasks will be integral once and twice, respectively. Displacement, velocity and acceleration of the body segments are so-called kinematic data. Combining the measured ground reaction force (GRF) and a mathematical model of the musculoskeletal system, the kinetic data, namely resultant forces and moments between two adjacent segments, will be calculated using the inverse dynamics technique.

Several variables describing the motion of the center of mass (COM) such as the range of motion (ROM), position, velocity and acceleration have been used to investigate the dynamic stability of the body during level walking and obstructed gait. Among these variables, the ROM of the body's COM is correlated with the overall control and energy expenditure of the whole body during functional tasks, while the acceleration of COM that is directly affected by the joint moments of the locomotor system is used to relate the COM motion with the applied force. Center of pressure (COP) and velocity of the COM have also been used to quantify an individual's balance maintenance during functional activities.

Pai and Patton used COM velocity-position to demonstrate the dynamic stability in the anterioposterior (A/P) direction during locomotion, and successfully predicted a feasible region of balance control in the A/P direction in accordance with environmental (contact force), anatomical (foot geometry) and physiological (muscle strength) constraints. They suggested that forward or backward falls would occur when exceeding the torque and state boundaries. Through the kinematic and kinetic data together with a whole-body inverted pendulum model, the possible control strategies of body's balance and posture in the frontal plane, and muscle activation synergies during sloped walking have been investigated. In addition, variables characterizing the coordination between the movement of the COM and COP including COM-COP separation distance and COM-COP inclination angles gave more useful insights into the study of balance control of the body during sloped walking. Nevertheless, the magnitudes of the COM motion and the COM-COP distance may be affected by a subject's stature. Therefore, it is suggested that the COM motion COM-COP distances should be normalized by each individual's leg length (LL) or body height (BH) to exclude the influence of inter-subject variability. COM-COP inclination angles were used to quantify the body's dynamic stability during sloped walking without the influence of stature differences among subjects. Inter-joint coordination is the relationship between the motions of two joints, including angular positions and velocities that are associated with not only the efferent motor control, but also with information from afferent joint receptors. Inter-segmental coordination patterns for the whole cycle during various other activities can be found in previous studies. This, together with previous findings, will provide useful information for clinical interventions. More recently, a few studies investigated the patterns and variability of the inter-joint coordination of the lower limbs during obstacle-crossing in various subjects using the method of relative phase plot, which combines information on joint angular positions and velocities. The variability of the relative phase plots of repeated trials was used to quantify the variability of the inter-joint coordination patterns during the movement as in other previous studies. With altered joint kinematics and kinetics, elderly subjects and subjects with shoulder immobilization are expected to have different coordination patterns with a larger variability during daily activities. Although aging and upper limbs seem to induce mechanical changes in individual joints, such changes still did not change the way the lower limb joints were coordinated. It seems that age-related changes contributed to the increased variability of the inter-joint coordination of the limbs during leading limb crossing. However, no study has investigated the effects of shoulder immobilization on the inter-joint coordination during daily activities.