Biomedical subjects

C J Ivens

Publications and source records attributed to C J Ivens.

4 recordsLinked to original sources

PubMed2001

Worst-case prediction strategy in force programming when visual information is obstructed.

A person's strategy for applying force while lifting an object is dependent upon visual cues. This study investigated the alteration of strategy in force programming when visual information about an object's size was obstructed at the moment of lifting. Seven subjects were instructed to use a precision grip for repeated lifts of a cube-like grip apparatus attached to a box. The grip apparatus was a special device designed to measure grip and load forces. Three different-sized plastic boxes of equal weight were pseudorandomly presented by attaching them beneath the grip apparatus to the subjects in two visual conditions. In the Full-vision condition, subjects could view the box's size prior to lifting. In the Obstructed-vision condition, a screen prevented subjects from seeing the box size prior to lifting. In the Full-vision condition, the grip force and load force used by subjects on the grip apparatus increased with box size. In contrast, the subjects in the Obstructed-vision condition used forces appropriate for the largest box regardless of box size. The present results suggest that absence of size information may cause an alteration of strategy used to determine force output in that subjects may apply a maximum force adequate for the largest box, which could be called a "worst-case" prediction strategy, i.e., when there is doubt, the most secure lift may be selected for all possible cases.

Adult↗

PubMed2000

Evidence of motor equivalence in a pointing task involving locomotion.

A pointing task was performed both while subjects stood beside and while subjects walked past targets that involved differing movement amplitudes and differing sizes. The hand kinematics were considered relative both to a fixed frame of reference in the movement environment (end effector kinematics) and to the subject's body (kinematics of the hand alone). From the former view, there were few differences between standing and walking versions of the task, indicating similarity of the kinematics of the hand. However, when the hand was considered alone, marked differences in the kinematics and spatial trajectories between standing and walking were achieved. Furthermore, kinematic analyses of the trunk showed that subjects used differing amounts of both flexion-extension and rotation movements at the waist depending on whether they were standing or walking as well as on the constraints imposed by target width and movement amplitude. The present results demonstrate the existence of motor equivalence in a combined upper and lower extremity task and that this motor equivalence is a control strategy to cope with increasing task demands. Given the complexity involved in controlling the arm, the torso, and the legs (during locomotion), the movements involved in the present tasks appear to be planned and controlled by considering the whole body as a single unit.

Attention↗

PubMed2000

Contribution of visually perceived size to the scaling of fingertip forces when lifting a 'small' object.

The effects of visually perceived size of an object on the scaling of fingertip forces during lifting tasks were investigated using a small and lightweight object. A grip apparatus was attached to the top surface of three different size boxes of equal weight. 15 healthy adults were asked to grasp the grip apparatus with the thumb and index finger, lift it to a height of 5 cm, hold it for 8 sec., and then put it down. Force transducers embedded in the grip apparatus measured grip and load forces. When subjects lifted the same size object repeatedly, there were no size effects on the grip and load forces used by each subject. When the size was pseudorandomly varied, however, the grip and load forces exerted by 7 subjects significantly increased with increased size, while there were no size effects for the remaining subjects (n = 8). These results suggest a smaller contribution of information on size to the force programming when lifting a small object than when lifting a larger and heavier object as used in previous studies.

Adult↗

PubMed1997

Increased Sensitivity to Changes in Visual Feedback With Practice.

In recent work investigating motor learning, the focus has been on the effect of modifying feedback at different levels of learning. Results suggest that learning is specific to the practiced conditions and that this specificity increases with practice. In a replication and extension of this previous work, 3 groups (N = 30 subjects) practiced a sequential positioning movement: Controls performed 300 trials with visually presented on-line kinematic feedback, whereas the other 2 groups, low practice (LP) or high practice (HP), performed, respectively, 50 or 300 trials without feedback. Pretest and posttest sessions of 10 trials each were performed with the on-line feedback. All groups improved with practice. It was apparent that the HP group exhibited more of a performance decrement in the posttest than the LP group did, suggesting that motor learning is the process of forming an increasingly specific sensorimotor representation. These results have implications for motor learning paradigms, models of motor learning, and training.

humans↗