Pii: S0304-3940(99)00843-5

Trajectories of the hands and whole-body center of mass were studied during whole-body lifting tasks. The movements of different parts of the body were monitored with the ELITE system. Subjects were instructed to lift to shoulder height an object placed at one of two distances (5±45 cm) before them on the ̄oor. The lifts were performed both with and without kinematics constraints (i.e. to produce a straight hand trajectory while lifting, and to lift without any instructions, respectively). Hand trajectories were roughly straight when performed under the constrained condition, but curved when performed without instruction. Hand velocity curves showed bell-shaped pro®les. In both groups, body centers of mass (whole-body, upper and lower part) were calculated and their trajectories showed invariant sagittal displacements. These results support the idea that movement contributes to postural control and, reciprocally, that whole-body center of mass is a robust and controlled variable which plays an important role in hand trajectory formation. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Center of mass; Kinematics constraints; Trajectory formation; Whole-body Some early eighties studies have demonstrated that in pointing or reaching movements, hand trajectories are essentially straight [1,8,12] with bell-shaped velocity pro®les. In contrast, more recent experiments [2,7,14] focusing on more natural movements, reported that trajectories are curved or relatively straight, depending upon the workspace region in which the movement was performed. When comparing hand trajectory models, the perceptual process by which trajectories were evaluated must be taken into account [7,14], as well as whether both the kinematics and dynamics of movement were considered. Kinematics invariances of pointing movements in man have been studied in order to determine in what frame of references the movement was planned [12]. Invariant straight hand trajectory performance suggests that the trajectory is planned at the end effector-level [1,4,5,8]. By end-effector, we mean the distal segment (here the hand) of the effector system involved in the performance of the task. In contrast, a curved path suggests that the trajectory is planned at the joint level [2]. It has been proposed that curved path could result from an extrinsic planning of a straight trajectory altered by a perceptual distortion which makes the movement appear to be straighter than it really is [14]. In this case, the subject is unable to produce a straight trajectory. Another possibility is that trajectory formation is under postural constraints [11] (e.g. center of mass displacement inside of base of support, or joints coupling). Curved hand movements have been found in whole-body reaching tasks, suggesting that whole-body equilibrium constraints determine hand paths for a given movement speed [11]. The purpose of this study is to verify these issues by revealing the types of trajectories during lifting movements. In addition, assuming that curved trajectories are found, we will determine if subjects are able to produce a straight trajectory, when so instructed. Hand trajectory formation has been investigated during whole-body lifting tasks performed with and without kinematics constraints. The following questions were asked. Are hand trajectories curved or straight? If curved trajectories are found, are they the result of postural constraints? Can Neuroscience Letters 277 (1999) 41±44 0304-3940/99/$ see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(99)00843-5 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 133-1-4174-4470; fax: 133-14174-4535. E-mail address: [email protected] (Y. Kerlirzin) freely standing subjects performing lifts move their hands along straight trajectories when so instructed? What is really planned and controlled in trajectory formation? Eight healthy and consenting male subjects (22.5 ^ 2.5 years) were tested during this experiment. The subjects were asked to start by standing with their hands clasped together in front of the pelvis. An object which was a wooden bar (40 cm long, 7 cm in diameter and 1.8 kg in weight) mounted on two supports (15 cm high) was placed on the ̄oor in front of them at a distance of either 5 or 45 cm. They were then asked to reach with both hands for the object and to lift it as quickly as possible to shoulder height, with the upper limbs extended and near horizontal. This ®nal position was to be held for 2 s. Subjects were tested under two conditions. Under the free condition (F), the subjects were asked to lift the object without instruction in order to study hand trajectories during natural and volitional movements. Under the instructed condition (I), however, the subjects were told to produce straight hand trajectories during the lift. Before each experimental session, the subjects trained for a 2-min period to become familiarwith grasping the object from the 5 and 45 cm distances. Body segments kinematics were monitored with the `elaboratore di immagini televisive' (ELITE) system, which is a dedicated hardware system based on automatic real-time processing of TV images. The two-camera system recognizes multiple passive markers and computes their coordinates. The cameras were placed one above the other and located at heights of 1 and 2 m from the ground, respectively, on the left side of the subject. The distance between the cameras and the plane of movement was 3 m. The ®eld of view was 1:5 £ 2 m. The accuracy was 1.5 mm for linear displacement and 1.58 for angular position. Twelve hemispherical markers (5 mm in diameter) were placed on head, neck, upper limb, trunk, and lower limb. These markers were subsequently used to construct stick ®gures that consisted of eight links. The marker located at the joint between the metacarpus and the phalange was chosen to de®ne the hand trajectory. Each recording session comprised eight trials (two experimental conditions £ four repetitions). Data were sampled at a rate of 100 frames/s. The raw data were processed with a 4th order Butterworth ®lter without phase shift, and using a cut-off frequency of 6 Hz. To compare the extent of hand trajectory curvature (i.e. deviation from straightness), the maximal perpendicular distance (Dmax) was measured from the actual path to the straight line interpolated between the initial and ®nal end points of the trajectory. The distance between these two points was called (L). The ratio Dmax/L was used to quantify hand curvatures [11]. In order to compare the contribution of different segmental subdivisions of the body with hand trajectory formation, the locations of three different centers of mass were calculated, using the model of Chandler and colleagues [3]: whole-body (CoMb), lower limb (CoMl) and upper body (i.e. head plus trunk) (CoMu). The mass of the object was integrated in the model as a single-point mass located at the handle of the box. Sagittal displacements (Table 1) and the ratio Dmax/L were calculated for the three centers of mass. The c variable [9], de®ned as the ratio between the instantaneous peak velocity and the average velocity, was used to compare hand velocity pro®les under each condition (Table 2). A two-way analysis of variance (ANOVA) with repeated measures on both factors (two conditions £ distances) was performed. Differences were considered signi®cant at P , 0:05 level. A ScheffeÂtest post-hoc was used to test signi®cant differences between values (with signi®cant P , 0:05 level). Fig. 1 shows hand and centers of mass trajectories. Under the F condition (Fig. 1A,B), subjects produced hand paths that were generally curved. In contrast, under the I condition (Fig. 1C,D), subjects produced straight trajectories. As a consequence, the Dmax/L value decreased signi®cantly (F…1; 7† ˆ 25:87, P , 0:05) under the I condition compared with the F condition and under the distance 1 compared with the distance 2 (F…1; 7† ˆ 17:33, P , 05). This ratio also decreased signi®cantly, on average from 0.11 to 0.05 for F Y. Kerlirzin et al. / Neuroscience Letters 277 (1999) 41±44 42 Table 1 Centers of mass sagittal displacements for each condition (free or instructed) and each distance (D1 ˆ 5 cm, D2 ˆ 45 cm) Sagittal displacements (mm)