Limb Segment Recruitment as a Function of Movement Direction, Amplitude, and Speed

Coordination of limb segments in graphic motor behavior has been studied primarily in cyclic tasks. In the present study, limb segment recruitment patterns were investigated in a discrete line-drawing task. Subjects ( N = 11) performed pointing movements varying in direction, amplitude, and speed. The contributions of index finger, hand, and arm to the movement were analyzed by evaluating the angular displacements in 7 joint dimensions. The results showed that amplitude and direction affected limb segment involvement in the same way they have been reported to affect it in cyclic movements. Upward left(upleft) directed movements were primarily achieved by fingers and arm, whereas upward right(up-right) directed movements were accomplished with the hand and the arm. Large amplitudes elicited not only an increase of proximal but also a decrease of distal limb segment involvement, especially in the upleft direction. In the present discrete pointing task, effects of speed on limb segment involvement were different from speed effects that were observed earlier in cyclic tasks: Larger limb segments became more involved in fast than in slow discrete movements. With respect to the timing of limb segment recruitment, all joints tended to move simultaneously, but small deviations from synchronous joint movement onset and offset were present. The results are discussed in the context of recent theories of limb segment coordination. Key wordr: coordination, drawing, human arm movements, timing n drawing and writing, as well as in most other kinds of I motor behavior, we have more degrees of freedom (Bernstein, 1967) available at the level of limb segment coordination than are strictly necessary for performing the tasks successfully. For example, to move the tip of a pen to a certain position in space, a person holding that pen would need only three degrees of freedom. The human arm, however, even when the fingers and thumb are constrained to hold a pen, has many more degrees of freedom at the joint level. The redundant degrees of freedom allow for a flexible way of movement selection in response to current environmental demands (Rosenbaum, Loukopoulos, Meulenbroek, Vaughan, & Engelbrecht, 1995). But these demands are usually not strong enough to restrict the set of possible movements to a single solution. Thus, the actor is confronted with a degrees-of-freedom problem and has to choose one solution, often from many possible ones. An important question for motor control theories is how this choice is made. To explain how people select movements, investigators have proposed several approaches. First, the number of degrees of freedom that one must actually control may be effectively reduced by maintaining dependencies between limb segments (Turvey, 1990). In graphic motor behavior, coupling of elbow and shoulder has been observed in circular drawing movements; during these movements, subjects tended to realize constant phase relations between these joints (Soechting, Lacquaniti, & Tenuolo, 1986; Van Emmerik & Newell, 1990). This outcome is consistent with the hypothesis that there are dependencies between limb segments. Another approach to studying the way in which redundant degrees of freedom are dealt with is to identify efficiency constraints that may reduce the number of behavioral options. Such constraints may induce the selection of those limb segments that are most suitable to perform a task Correspondence address: J. J . Schillings, Nijmegen Institute for Cognition and Information, f? 0.Box 9104, 6500 HE Nijmegen, The Netherlands. E-mail address: SCHILLINGS@ NICI. KUN.NL 241 D ow nl oa de d by [ U ni ve rs ity o f Su ss ex L ib ra ry ] at 0 3: 39 0 3 N ov em be r 20 14 J. J. Schillings, R. G. J. Meulenbroek, & A. J. W. M. Thornassen or they may limit the range of appropriate joint angles with which the task can be performed successfully. Efficiency constraints related to optimal amplitudes and frequencies of motion and optimal suitability for movement in a certain direction have been shown to affect the involvement of limb segments in repetitive pointing and drawing tasks (Meulenbroek. Rosenbaum, Thomassen, & Schomaker. 1993; Rosenbaum, Slotta, Vaughan. & Plamondon, 1991). In these studies, the relative contributions of fingers, hand, and arm to back-and-forth movements depended on direction, amplitude, and frequency of the movements. Although efficiency constraints and dependencies that are expressed in terms of frequencies and phase relations are meaningful in describing cyclic phenomena, it is not obvious that they are also applicable to discrete, noncyclic movements. Perhaps movement speed in discrete movements is the logical equivalent of movement frequency in cyclical movements. But the question remains whether frequency-based efficiency constraints can be translated directly to speed-based ones and still be relevant. During high-frequency oscillating movements, it may be efficient to favor limb segments whose preferred frequency is high (Rosenbaum et al., 1991). During a high-speed discrete movement, however, the involvement of these segments may pay off less. Instead, other constraints may become more important, so that other segments become involved. Similar differences may be expected when considering phase relations between joints. A priori, it seems to be less relevant to look for fixed phase relations between joint angles in discrete movements than to look for them in cyclic movements; this relevance is larger, of course, to the extent that even discrete movements may be assumed to be (damped) oscillations (Guiard, 1993). In either case, however, and especially with respect to timing, it is still unclear how limb segments are coordinated in discrete movements and whether or not their involvement is synchronized to the same extent as in cyclic movements. Studies focusing on limb segment coordination in the execution of discrete movements have thus far been based primarily on tasks involving pointing in a horizontal or vertical plane, during which the arm was constrained to move within that plane (e.g. Cruse, Briiwer, & Dean, 1993; Kaminski & Gentile, 1986). The joints that have been examined in these studies were shoulder, elbow, and wrist. In the present study, we investigated the contribution of the limb segments of the forearm in more detail by analyzing three-dimensional angular displacements of forearm, hand, and fingers. We paid special attention to the timing of these distal limb segments. In studies of cyclic movements, timing of limb segment actions has been considered in terms of phase relations. Little work has been dedicated to the issue of timing in discrete movements. although one study (Kaminski & Gentile, 1986) reported some effects of amplitude variation on the relative timing of shoulder and elbow movements. In the present experiment, we addressed two of the above-mentioned aspects of limb segment coordination in discrete movements. First, the findings by Meulenbroek et al. (1993) concerning the effects of direction, distance, and speed of end-effector displacement on limb segment involvement in cyclic drawing movements were compared with those obtained in a discrete drawing task. Our primary aim was to find out whether the efficiency constraints that were proposed in that study apply only to cyclical movements or to discrete ones as well. The second aspect of our study concerned the timing of limb segment movements. The question here was: What are the principles that govern the timing of the recruitment of limb segments in a discrete pointing task in which we do not expect fixed couplings of particular joints? In contrast to earlier studies of limb segment coordination, we examined a drawing task in which the end effector, that is, the pen tip, moved in a single horizontal plane but the arm could move freely in space. We analyzed joint angular velocities, from elbow to index finger, to investigate both involvement of fingers, hand, and arm and timing aspects of their recruitment. Efficiency Constraints on Limb Segment Selection Meulenbroek et al. (1993) suggested that selection of limb segment patterns during cyclic drawing depends, among other things, upon optimal frequencies and amplitudes of motion of the effector segments that can contribute to the execution of the task. In their study, subjects performed back-and-forth movements of gradually increasing or decreasing size. Four different movement directions and three different maximum amplitudes were used. Working pace, instructed to be comfortable, slow, and fast, resulted in movement frequencies that were medium, low, and high, respectively. It turned out that the contribution of the forearm and the upper arm increased as the largest amplitude of the movements increased, whereas the contributions of the fingers and the hand decreased. Small but discernible effects of movement frequency were also present: The contribution of the fingers was large in high movement-frequency conditions, the contribution of the hand was large in medium movement-frequency conditions, and the contribution of the forearm and the upper arm tended to be large in low movement-frequency conditions. These ordinal relationships are in agreement with principles of mechanics if each segment is viewed as a linear damped oscillator (French, 197 1); the observed optimal amplitude-frequency combinations can then be attributed to the length and mass of the limb segments, which determine their moment of inertia (see Rosenbaum et al., 1991). Because the moments of inertia of the limb segments are particularly important in the initial acceleration and final deceleration of limb segments, we hypothesized that length and mass of the segments may also play a role in efficiency constraints influencing limb segment coordination during discrete movements. For example, in isolated pointing movements that start and finish with zero velocity, an efficiency constraint may be the minimization of the energy 242 Journal of Motor Behavior D ow nl oa de d by [ U ni ve rs ity o f Su ss ex L ib ra ry ] at 0 3: 39 0 3 N ov em be r 20 14 Limb Segment R0CNitment in Drawing required to get the system going and to bring it to a full stop (Rosenbaum et al., 1995). The existence of such a constraint should result in an ordinal relationship comparable with the one mentioned above: As required movement speed gets higher, one would expect the contribution of low-inertia segments to the movement to increase, whereas high-inertia segments should be less involved, thus minimizing required energy. In the present study, we investigated whether such relations exist in the recruitment pattern of arm, hand, and fingers in fast and slow isolated graphic movements. The hypothesis of Meulenbroek et al. (1993) concerning a similar relation between the amplitude of the end-effector displacement and the involvement of limb segments should also be applicable in the case of discrete movements. In discrete movements, it is not immediately clear how optimal amplitudes are related to optimal frequencies (Kay, Kelso, Saltzman, & Schoner, 1987; Rosenbaum et al.. 1991). because movements have to be produced in isolation. However, an efficient strategy might be to favor the use of limb segments with low moments of inertia as much as possible. Only when end-effector displacements become too large to be achieved primarily by those segments should there be increased involvement of limb segments with larger moments of inertia. Such a strategy would predict for pointing movements the ordinal relationship that was also observed in cyclic movements: The contribution to pen-tip displacement by the distal (low-inertia) segments decreased, whereas the contribution by proximal (high-inertia) segments increased as the amplitude of pen-tip displacement increased. In the present study, we tested this hypothesis by using two amplitude conditions. Another factor that has been shown to affect limb segment selection in drawing is the orientation of the axis along which the pen tip is moved. Meulenbroek and Thomassen (1991) showed that right-handed subjects producing graphic movements in the horizontal plane, at table height, primarily used their lingers to produce movements along the line between upper left and lower right, as long as the amplitude of the movement was not too large. Movements along the line between upper right and lower left were accomplished primarily with the hand. It was also found that when amplitudes became too large to be achieved by the fingers or the hand, subjects respectively translated and rotated the forearm to accomplish the task in the above directions. Because these phenomena result from the orientation of the principal axes of rotation of the limb segments, given a normal writing posture, and not from the cyclic character of the task, we expected them to be replicated in the present study. Timing of L i b segment Movemeats Little is known about the timing of limb segment recruitment in discrete pointing tasks. Several control principles for the timing of limb segment movements have been suggested recently by Rosenbaum et al. (1995). One of these is that each limb segment involved in the movement has its own preferred movement time. This would imply that, in general, different limb segments have different movement times, originating from the mechanical characteristics of the segments and the required angular displacement for their joints. It is an open question how, in this case, joint movements could be coordinated (e.g., by a constraint to start at the same time or stop at the same time). One study (Kaminski & Gentile, 1986) reported that in horizontal planar pointing movements, shoulder and elbow had, in fact, different movement times. As a coordination principle, it was suggested that these joints tend to stop moving simultaneously, although they do not necessarily start at the same time. Another suggestion by Rosenbaum et al. (1995) was that, during their combined movement, all segments have the same movement times. These movement times could be externally imposed or could constitute a compromise between the individual preferred movement times of the joints. The idea of complete synchrony of limb segment motions (which would result in congruent joint velocity profiles) provides a simple timing mechanism that has the additional advantage that it ensures a straight-line movement through joint space, thus satisfying the principle of least action for motion through joint space (Rosenbaum et al., 1995). Because few empirical data exist that strongly favor either of the above-mentioned timing hypotheses, we investigated the timing of the onset and offset of angular displacements of joints in the present task to gain more insight into the timing aspects of limb segment coordination. To summarize the main goals of the present experiment, we wanted to investigate whether, in spite of the differences between discrete and cyclic movements, the efficiency constraints that have recently been established for the latter movement type can also be observed in the former. Furthermore, we tried to find out the ways in which the movement onset and offset times of different joints relate to each other.