Experimental data suggest that movement slowness, motor variability are smaller-than-normal primary submovements in the elderly are related to a reduced capability to initiate, control and modulate force. Irregularities in force control in the elderly may be caused by the less fine-grained control of muscle force, especially in small movements due to the motoneuron reorganization that occurs with. This suggests that force variability should increase with age. Furthermore, Galganski et al. (1993) reported that the standard deviation of force amplitude increases non-proportionally with target force during isometric movements for both elderly and young control subjects, with elderly subjects showing larger mean values. Specifically, the coefficient of variation, a measure of relative variability, was significantly larger for small compared to large target force levels in the elderly. This suggests that movements requiring small force levels (e.g. handwriting) should be differentially impaired in the elderly. Thus a fine motor control task, such as handwriting, is an excellent task to study how movements are controlled and regulated in the elderly.

Cooke et al. (1989) reported that movement duration and relative deceleration time becomes increasingly variable with age, particularly for small movement amplitudes. Pratt et al. (1994), Seidler and Stelmach (1996) and Seidler et al. (1995) have shown that the young cover a greater portion of a point-to-point movement distance in the primary or ballistic portion of the movement. In contrast, the elderly shorten considerably the primary submovements, requiring them to make a large corrective movement (that may contain more than one secondary movement) to reach the target. These secondary movements result in greater spatial variability for the elderly as compared to young controls. This predicts that spatial variability should increase with age. Furthermore, Diggles-Buckles (1993) has suggested that variability and slowness in the elderly are caused by decreased coordinative structures that control many joints simultaneously. This would predict that temporal variability should also be larger in the elderly than in the young.

The aim of this study was to test whether movement variability in the elderly is due to deficits in spatial coordination, force control or both. It was hypothesized that in the elderly, impairments in spatial coordination will result in movement strokes that depart from an ideal straight line. Curved strokes would likely be caused by unequal timing of wrist and finger joints, or by improper setting of the required joint movement or force amplitudes. To measure spatial variability we developed a normalized straightness error score. It is predicted that changes in straightness scores with stroke direction in the elderly will result from a deficit in the coordination of multiple joints (e.g. wrist and fingers) during movement. Furthermore, it was hypothesized that fluctuations in on-line force modulation during movement production will result in less fluent movements in the elderly, and therefore increased movement variability and Teulings et al. (1997) have proposed the normalized jerk score as a measure of movement fluency, in which jerk is defined as the change in acceleration. At least in the frictionless case, changes (e.g. modulation) in force levels may be assumed to be proportional to the changes in acceleration or jerk. We postulate that increases in jerk scores across oblique stroke orientations, which require simultaneous coordination of finger and wrist, may reflect a deficit in force modulation. Additionally, we expect elderly subjects to write smaller and slower than young controls. This experiment reports handwriting-like performance of elderly and young subjects as quantified by normalized jerk and straightness error scores, and measurements of stroke size and duration.

Twelve elderly subjects (7 males, 4 females; age RANGE=63–78 years, MEAN=69) and eight young controls (4 males, 4 females; age RANGE=19–31 years, MEAN=25) participated in this study after having given informed consent. Subjects reported no neurological or skeletomotor dysfunctions, and were naive as to the experimental design or purpose. The methodology was approved by the Human Subjects Institutional Review Board, Arizona State University.

The subjects wrote on a digitizer-display (Wacom PL-100V) connected to a PC. The digitizer sampled the X and Y coordinates of the pen (200 Hz) with a spatial error of 0.1 mm. The digitizer-display (28W X 23H cm) was placed horizontally on top of a table and was oriented to meet each subject’s arm configuration preference. Four handwriting patterns in the horizontal plane were tested: back-and-fourth strokes in the 0–180° (right-left), 45–225° (forward slanted up and down), 90–270° (up-down), and 135–315° (backward slanted up and down) orientations. The experimenter indicated to the subject when the number of strokes was out of range so that the trial could be redone. Preferred orientation of the subject’s forearm with respect to the digitizer was kept constant during the experiment. The preferred forearm orientation was such that in all subjects the 0–180° strokes were mainly performed by the wrist joint and the 90–270° strokes by the finger joints. Oblique strokes were performed using both finger and wrist joints. Subjects were asked to write at a comfortable speed and in their normal handwriting size.