1. The impact of upper limb activities on balance function
As age increases, the elderly will experience decline in various body functions (including loss of sensitivity in vision, hearing, etc.), degeneration of the musculoskeletal system, and disorders of balance and coordination, resulting in a significant increase in the risk of falls among the elderly. Whenever balance function is mentioned, we tend to focus on the trunk and lower limbs. Little do we know that the upper limbs and balance function are also inextricably linked. Abnormal upper limb muscle strength has become a risk factor for repeated falls in the elderly.
The human body maintains balance when its center of mass (COM) is symmetrical to its center of pressure (COP) and is located above or within the support surface. Whether it is external interference under the action of external forces, or when internal interference caused by limb activities breaks the original balance state of the human body, the projection of COM on the support surface will change or have a tendency to change, and its relationship with COP Symmetry is also affected. Yamazaki et al. found through experiments that some upper limb activities will cause the trunk to rotate. At this time, the trunk muscles and hip muscles need to contract cooperatively to offset the COM displacement caused by this rotation, thereby maintaining balance.
Illustration: The relationship between the center of mass (COM) and the center of pressure (COP) of the human body.
Supplementary explanation: The center of gravity (COG) is the projection of the center of mass in the vertical direction.
2. Mechanisms related to upper limb activities and posture adjustment
When the balance is disturbed, the human central nervous system activates the muscles of the trunk and lower limbs, and uses anticipatory postural adjustments (APAs) to offset the center of mass shift caused by small disturbances. When encountering larger, When there is interference that cannot be offset by APAs, the center of mass shift will be transmitted through the sensory feedback system, activating compensatory postural adjustments (CPAs) to return the shifted center of mass to a balanced position.
The activation of APAs muscles usually occurs before the contraction of muscles involved in active activities. That is, when the human body starts to initiate an activity, the postural adjustment muscles are already excited in advance to prepare for maintaining balance. It is also called the "feedforward mechanism" . In particular, when the upper limbs are moving voluntarily, the muscles involved in APAs show great pre-excitability. If you want to raise your arms, your brain will receive the message that your body is about to lose its balance. Afterwards, the brain will respond quickly and send instructions to control the deep core muscles (such as contracting the transversus abdominis) to stabilize the spine and maintain balance.
3. Interaction between upper limb function and trunk stability
The trunk plays an integrated role in balance and stability, and its stability and control capabilities provide necessary conditions for ensuring the normal use of upper limb function and hand function.
Stroke will affect the control of trunk muscles, disrupt the upright position of the body and the transfer of the center of gravity, and impair the static and dynamic balance adjustment and stability of postures. A large number of studies have shown that stroke patients will experience excessive trunk activity during upper limb goal-directed tasks and reaching tasks. When the movement and control of the shoulders and elbows are impaired, they will use trunk compensatory movement strategies to extend the arms. Excessive compensatory movements can lead to secondary muscle contracture, joint malalignment, pain, limb loss, and increased energy consumption that affect upper limb functional recovery.
Avoiding compensatory trunk movements in upper limb treatment can improve upper limb function. Its essence is based on the good limb position of the "integration of upper and lower limbs" theory: changing the traditional use of waist restraints and fully mobilizing the flexibility of the trunk. And stability, the trunk and pelvis are not completely fixed on the seat, which can ensure the stability of the trunk without excessively restricting the trunk activities, and can stimulate more normal upper limb movements. As shown in the figure, a high-density foam trunk support frame is used to closely fit the posterior and lateral sides of the patient's trunk (height to the level between the tenth and twelfth thoracic vertebrae), allowing the patient to move freely while the trunk is supported. Significantly improve the upper limb function of the affected side in post-stroke patients.
Research has found that compared with upper limb exercise training, trunk exercise on an unstable plane has a positive impact on improving the sitting forward range, arm lifting speed, and walking ability of stroke patients without foot support. Appropriate unstable trunk training should be selected based on the recovery of the patient's overall function.
