PURPOSE: To understand (1) how young and older adults adjust arm movements in response to unexpected changes of object weight and (2) how aging affects the movement adaptation to unexpected changes of object weight during and after practice

RATIONALE: In the realm of physical rehabilitation, occupational therapy (OT) puts a great emphasis on helping clients to regain arm and hand functions because they are crucial for daily occupations. For this reason, knowledge of upper-extremity movement control is essential in OT practice. In previous studies, researchers have examined kinematic and kinetic characteristics of arm-reaching and hand-grasping movement in different age groups. Few studies examined these movements while encountering unexpected perturbation of object size and location.

However, there is no study examining the control and adaptation of movement when encountering unexpected changes in object weight, which is a common phenomenon when people reach for an object (e.g., an empty soda can) but incorrectly predict its weight (e.g., think it is still full). Further, a large proportion of our clients in physical OT are seniors; therefore, seniors’ movement control strategy and adaptation while in an unexpected condition (i.e., changes in object weight) is our main focus.

DESIGN: It is a descriptive study.

PARTICIPANTS: A convenient sample was used. Sixteen younger adults (age 18 or older) and 16 older adults (age 65 or older) participated in the experiment. All participants needed to be right-hand dominant. The exclusion criteria were presence of any neurological or orthopedics deficits and cognitive deficits.

METHOD: Participants were asked to reach and lift a jar. There were two types of jar that look the same in appearance but with different weights. Three-dimensional infrared cameras were used to capture participants’ hand movements. Matlab was used to analyze the kinematic data for further statistical analysis.

ANALYSIS: Data were analyzed using two-way analysis of variance to compare the aging effect and condition effect (i.e., two types of object weights, types of changes of weight, and pretest–posttest).

RESULTS: The main results showed that under continuous changes in object weight, younger adults could adjust reaching, F(1, 30) = 10.97, p < .01, and lifting movement time, F(1, 30) = 9.50, p < .01, more steadily than seniors. At posttest, the reaching-to-lifting performances of both age groups changed. Younger adults had increasing movement time of reaching phase, F(1, 30) = 6.19, p < .025, which may indicate that they became more careful when reaching for an object. However, seniors did not have this phenomenon. Seniors had larger movement variability than the young in both movement phases, for example, reaching movement time, F(1, 30) = 8.54, p < .01, and percentage of time where peak reaching velocity occurred, F(1, 30) = 8.12, p < .01, and so forth.

DISCUSSION: The reaching-to-lifting movement was affected by age or object weight. When encountering unexpected change in object weight, both groups had similar adjustment performances. In an unpredictable environment, young adults had better adaptive performances than seniors. After practice, younger adults became more careful and seniors displayed more unstable movement. This may suggest that both age groups began to adopt different strategies in response to upcoming unexpected conditions.

IMPACT STATEMENT: The results could serve as a fundamental knowledge for physical OT while training clients’ upper-extremity motor function.