研究目的：視覺動作任務(visuomotor task)包含兩種主要的形式，代償性追蹤(compensatory tracking)及覓隨追蹤(pursuit tracking)。前者利用封閉式迴路控制(close-loop control)的方式以抗衡一外來虛擬阻力(virtual perturbation)並維持穩定力量輸出；後者則針對一可被視覺預測的動態軌跡並耦合(couple)其目標軌跡。視覺回饋的中樞訊息處理(central processing)已被證實會隨年齡老化而逐漸退化，並導致動作的穩定性產生改變。此外，因年齡(age-related differences)所產生的力量行為(force behaviors)表現差異可以反映出力量控制(force control)在中樞系統的回饋(feedback)及前饋(feedforward)處理會隨著老化而有不同程度的退化。因此，本研究目的為探討年齡效應於代償性追蹤及覓隨追蹤下各力量變異因子(force fluctuation properties)的差異。
研究方法：本研究共收取17位23 ± 2.3歲健康年輕人及17位68.4 ± 7.3歲健康老年人。所有受試者均執行兩種不同追蹤作業。於覓隨追蹤作業下，受試者在9%至11%的最大自主用力(maximal voluntary contraction)範圍內，以右手的第一背側骨間肌(first dorsal interosseus)執行外展(abduction)等長收（isometric contraction）以耦合螢幕所出現之0.2Hz正弦波(sinusoidal wave)；代償性追蹤作業則需維持10%的最大自主用力以抗衡一0.2Hz的虛擬正弦波。年輕組與老年組於兩種不同追蹤作業之力量行為表現與力量變異因子的標準化後的改變(normalized change)均以獨立t檢測是否有明顯不同，統計顯著水準為p < .05。
實驗結果：結果顯示老年組的力量變異程度(size of force fluctuations)改變量較年輕組大且有統計學顯著的差異。此外，老年組標準化動作複雜程度(SampEn)改變量顯著小於年輕組。然而，其它力量因子參數變化量則未達到統計學上的顯著性。
結論：年齡效應在兩種追蹤作業之力量特徵的差異顯示老年組執行視覺動作任務時的變動性較大。標準化力量變異程度於老年組有較大的改變，顯示老年人由代償性追蹤轉換至覓隨追蹤過程中有較大幅度的力量變動。此外，老年組標準化力量複雜度改變較年輕組小，顯示其覓隨追蹤轉換至代償性追蹤的過程中有較規律的力量表現。總結以上研究發現，老年人的中樞回饋控制系統會隨年齡而退化，因此，當執行須以回饋機制控制(feedback-based)的視覺動作任務時，老年人的動作表現會因對於視覺動作任務的修正策略(corrective strategy)能力退化而變得較為不穩定。

Objective: There are two major forms of visuomotor tracking, compensatory and pursuit tracking. The former tracking is to counter virtual perturbation in need of closed-loop control, while the later entails to couple movement output to a moving target with a predictable trajectory. Central processing of visual feedback have been proved to degenerate with aging that causes changes in structure of movement variability. The age-related changes in force behaviors reflect a selective decline in the feedback and feedforward processes for force control with aging. The purpose of the present study was to investigate the age-related differences in force fluctuation properties between the conditions of compensatory tracking and pursuit tracking.
Methods: 17 young health adults (23.0 ± 2.3) and 17 older health adults (68.4 ± 7.3) participated in the study. All participants were required to exert a load-varying isometric force to couple 0.2 Hz sinusoidal target wave in the range of 9%–11% MVC of index abduction of right first dorsal interosseus in pursuit tracking. The participants stabilized the target movement on the monitor at a constant level of 10% MVC, by resisting sinusoidal virtual perturbation during compensatory tracking. Normalized changes in force behaviors and force fluctuation properties between compensatory tracking and pursuit tracking for the two populations were contrasted.
Results: The results of the study indicated that normalized difference in force fluctuations of the older adults was significantly greater than that of the young adults. In addition, the normalized difference in SampEn of the older adults was significantly greater than that of the young adults. Other force fluctuation properties in ratio of residual power, task error, correlation between the target and force trajectory and peak amplitude did not significantly differ with the older and the young adults.
Conclusions: In summary, older adults had more force variability in visuomotor tasks by virtue of the differential age effect on force characteristics for the two tracking patterns. The greater normalized change in the size of force fluctuations for the elderly indicated the elderly were more variable in force control than young adults, when tracking task shifts from pursuit modes to compensatory mode. In addition, the less normalized change in the complexity of force fluctuations for the aged symbolized the aged exhibited a more regular force fluctuations than the young, when tracking task shifts from compensatory modes to pursuit modes. Thus, the provision of feedback-based visuomotor practice to the elderly should be aware of the fact that degenerative changes in feedback control could destabilize force output with less strategic richness for a visuomotor task.

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