Do we use a priori knowledge of gravity when making elbow rotations?

Abstract

In this study, we aim to investigate whether motor commands, emanating from movement planning, are customized to movement orientation relative to gravity from the first trial on. Participants made fast point-to-point elbow flexions and extensions in the transverse plane. We compared movements that had been practiced in reclined orientation either against or with gravity with the same movement relative to the body axis made in the upright orientation (neutral compared to gravity). For each movement type, five rotations from reclined to upright orientation were made. For each rotation, we analyzed the first trial in upright orientation and the directly preceding trial in reclined orientation. Additionally, we analyzed the last five trials of a 30-trial block in upright position and compared these trials with the first trials in upright orientation. Although participants moved fast, gravitational torques were substantial. The change in body orientation affected movement planning: we found a decrease in peak angular velocity and a decrease in amplitude for the first trials made in the upright orientation, regardless of whether the previous movements in reclined orientation were made against or with gravity. We found that these decreases disappeared after participants familiarized themselves with moving in upright position in a 30-trial block. These results indicate that participants used a general strategy, corresponding to the strategy observed in situations with unreliable or limited information on external conditions. From this, we conclude that during movement planning, a priori knowledge of gravity was not used to specifically customize motor commands for the neutral gravity condition.

Fig. 1

The four movement types that were used as experimental task. Movements are defined based on their body-related movement direction and direction relative to gravity when moving in vertical plane (reclined orientation)

Fig. 2

Schematic representation of the time line of the trials. After a practice session of 70 trials in the vertical plane (reclined position), the measurement started (see vertical dashed line). Five trials of moving in the vertical plane were alternated with one trial in the horizontal plane (upright position). After the fifth alteration, the one trial in horizontal plane was followed by an additional 29 trials

Fig. 3

An example of position (0° indicating full elbow extension), velocity and net muscular torque data averaged over the five trials made either against or with gravity in de vertical plane (VP_adapt), the five first trials made in horizontal plane (HP_unadapt) and the last five trials in horizontal plane (HP_adapt). The squares indicate the point where movement came to a standstill. Data of elbow flexions made by participant 11 were used. The gravitational torque (dashed lines) encountered when moving in the vertical plane was substantial compared to the net muscular torque (solid lines)

Fig. 4

The kinematic parameters peak angular velocity (ω_peak) and overshoot (Δθ) averaged over all twelve participants. The error bars indicate the intersubject standard error of the mean. The data for elbow flexions and extensions were averaged, resulting in one set of (open gray) symbols for movements made with gravity and one set of (filled black) symbols for movements made against gravity. Significant differences between conditions as found in the planned contrasts are indicated with asterisk. Both in movements made against and in movements made with gravity, peak angular velocity and overshoot decreased for the first trial made in horizontal plane (HP_unadapt) compared to the preceding trials made in the vertical plane (VP_adapt) and the following trials in the horizontal plane (HP_adapt). The gray bar indicates the target area

Fig. 5

The trial-by-trial adaptation of the peak angular velocity (ω_peak) and the overshoot (Δθ) as a function of trial number after the switch to the horizontal plane. The mean over all twelve participants is shown, and the error bars indicate the standard error of the mean

Fig. 6

The setup used to estimate the position of the center of mass and the mass of lower arm and manipulandum together. a The setup was tilted backward, so that the plane of motion was nearly vertical. Orientation of the plane of motion relative to gravity can be described by φ _reclined. b Top view in body-related coordinate system is shown. The lower arm and manipulandum (shown in black) are attached to a force transducer. Angle definitions are shown

Fig. 7

A top view of the setup used to estimate moment of inertia of lower arm and manipulandum (shown in black) together. The participant is asked to keep the elbow angle in 90°, while a force of 25 N is applied to the manipulandum by means of a rope. A quick-release measurement was initiated by unexpectedly cutting the rope