Associate Professor, School of Physical Therapy,
Ohio University, W277 Grover Center, Athens Ohio 45701, USA

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Reaching tasks such as ringing a doorbell, wiping a child’s face or retrieving the morning paper require the control and coordination of the trunk and limb segments in order to perform these tasks smoothly and effortlessly. These reaching tasks are so common in our everyday experience, yet we rarely contemplate the complexity of such motor tasks or the variety of movement patterns we can utilize to perform them.

Conceptually, a reaching task could be parsed into a postural control task and a movement task which the central nervous system (CNS) must plan and execute. With respect to the reaching tasks described above, the CNS must solve two problems. One, the movement must be planned such that the projection of the center of mass (COM) is within the base of support after target contact. Two, the trajectory of the trunk and limb segments must be planned such that the hand can reach the intended target. Given the number of segments involved in these whole-body reaching tasks there are an infinite number of ways in which these tasks can be completed.

How then are reaching movements planned and executed? Are there rules by which the central nervous system (CNS) apportions the rotations of the limb segments involved in the task? How are these rules altered with orthopedic impairments? The goals of the Motor Control Laboratory at Ohio University are to 1) identify sets of rules (i.e. kinematic and kinetic) by which multi-joint tasks are governed by analyzing the segmental kinematics and kinetics of individuals performing whole body reaching tasks and 2) determine how orthopedic impairments alters those rules.

Control of Standing Posture

To maintain an upright posture (under static or quasi-static conditions) the projection of the COM must remain within the base of support (the area delineated by the feet). This task can be thought of as balancing a top heavy inverted pendulum. The inverted pendulum model has been used to accurately describe the movement of the COM in a variety of postural tasks. Postural equilibrium is maintained by active muscle contractions which keep the COM within the base of support in response to external forces such as gravity or imposed accelerations, and internal forces arising from focal movements of the limbs or trunk. For example, in a trunk flexion task, the movement of the trunk may be considered the focal movement and backward movement of the pelvis, a compensatory movement which maintains the center of mass within the base of support.

Displacement of the COM caused by focal movement of the extremities or by the change in orientation of the trunk is often counteracted by muscle activity in other segments to minimize the destabilizing effect of the perturbation. A focal movement has two components. One is the static component, which is the gravitational effect of moving the position of the COM (of the moving segment). The other is the dynamic component, which is the effect of acceleration of body segments on the postural equilibrium. Therefore, the CNS must plan for both the final geometry of the limb segments and the acceleration of the limb segments in order to maintain balance. However, in a multi-joint system there is a variety of strategies that can achieve this.

Figure 2

Kinematically Redundant Systems

Starting from an upright standing posture and reaching for a target that requires some forward bending of the trunk can be accomplished in many different configur­ations of the trunk and limb segments due to the large number of joints involved in these reaching tasks. That is, there are more mechanical degrees of freedom than are strictly required to complete this task. This is the problem of kinematic redundancy articulated by Bernstein and is best illustrated by example. Consider a subject reaching for a target in the horizontal plane when motion is restricted to the shoulder and elbow (two degrees of freedom). Given that two coordinates can define a point in a plane, there is only one configuration of the limb segments that can be used to reach the target. Now consider that the wrist is free to move (a system with 3 degrees of freedom). There exists an extra degree of freedom, and therefore an infinite number of configurations of wrist, elbow, and shoulder can be used to reach the target. The question then is how does the central nervous system choose the orientation of the limb segments from an infinite number of choices?

It has been hypothesized that the CNS resolves the problem of kinematic redundancy by reducing the independent degrees of freedom required to complete a given task. By imposing some rules by which coordinated movements are performed, complex multi-joint tasks are simplified. Several relationships have been identified for reaching tasks. These could be thought of as invariant features or characteristics of a movement task. One invariant feature identified in reaching tasks is that the spatial trajectories of the wrist are largely unaffected by changes in speed and load and that the velocity profile of the end effector tends to be bell shaped. Some have observed optimization of variables such as work, joint comfort, or combinations of work and joint comfort in subjects performing reaching tasks. Other relationships such as coupling between the shoulder and elbow joints have been demonstrated for circular drawing tasks and for reaching tasks. Fixed relationships of the trunk and limb segments have also been shown for rapid trunk flexion tasks and for kinematic patterns of gait. Whether these fixed relations are neural or mechanical in nature has yet to be elucidated, but it does suggest that the CNS might reduce the complexity at the kinematic level. Alternatively, it has been proposed that the CNS may reduce the complexity of multi-joint tasks at the level of the dynamic joint torques. In this case the CNS issues a single command which leads to torques of similar time course at each of the joints. This in effect would simplify control of a multi-joint task.

Thus the evidence that the CNS plans at either the kinematic or kinetic level is murky, and at the Ohio University Motor Control Laboratory the experiments are designed to continue to tease apart these two components.

Development of Reaching Task

To examine how people apportion motion to the various segments used in multi-joint reaching tasks, we have developed a paradigm that would 1) necessitate some forward displacement of the trunk to reach targets located in a mid-sagittal plane, 2) require progressively greater amounts of forward bending to reach each target location, and 3) standardize target locations to each individual’s anthropometric characteristics (i.e., hip height, hip-to-shoulder length, and arm length). See Figure 1. Despite the fact that full body reaching tasks can be completed using an infinite number of movement patterns, our investigations of healthy subjects performing these tasks reveal certain commonalities across subjects, speeds, target positions and trial sequences. These commonalities were expressed as couplings of segmental motions. We have found that for any given standardized target location, healthy individuals choose to use either a “knee flexion/ankle dorsiflexion” strategy, or a “knee hyperextension/ankle plantar flexion” strategy. See Figure 2.

Figure 4

Regardless of the movement strategy used, subjects revealed remarkably similar movement patterns of the lumbar spine and pelvis. Furthermore, when participants performed reaches at a fast movement speed, they used larger joint excursions of the ankle and knee but minimal change in spine motion. However, when motion is restricted at one joint due to pain, or fear of pain, the movement task can usually be completed, but the movement pattern will change (i.e. a change in inter-joint coordination). For example, when an individual with low back pain needs to perform a functional task that requires some motion of the lumbar spine (e.g. ringing a doorbell) they can compensate for reduced lumbar spine motion by increased excursions at the legs and the reaching arm. While the advantage of a kinematically redundant system is that function can be retained even with impairments, changes in motor coordination may prevent full recovery, precipitate re-injury, and are most likely moderated by levels of kinesiophobia. Thus it is critical to examine movement patterns in tasks that progressively challenge patients.

Ohio University Motor Control Laboratory

The Ohio University Motor Control Lab has seven MX13 Vicon cameras for kinematic data collection, two Bertec force plates to record ground reaction forces, and a 16-Channel Delsys EMG system. Analog data are captured with a 64-channel analog to digital board. We use Vicon Workstation software for camera calibration, data collection and post processing of kinematic data. All joint torques, forces, and power calculations are performed using Matlab Simulink. We have developed a full body 3D inverse dynamics model with 45 degrees of freedom (Figure 3). Thus we use the Vicon system to capture and process the kinematic data which is then used in our inverse dynamic model. The kinematic and kinetic data, along with the muscle activation data are used to derive sets of rules for control of multi-joint movements.

The laboratory is set up to support a variety of studies. The 20’ X 30’ dimensions of the room allow for collection not only of postural tasks, but also gait activities. The lab contains a 28’ runway, which gives us the ability to perform various gait or sport specific activities. The lab also contains a seating apparatus used to isolate trunk motion from the pelvis and lower extremities. This apparatus enables the experimenters to isolate specific trunk musculature while the subject performs maximal voluntary isometric contractions (MVIC). This method gives the experimenters a way to assess trunk strength and to have reference values for normalization of EMG data (Figure 4).

The Ohio University Motor Control Lab is funded through The National Institutes of Health (R01-HD045512) and the Ohio University Research Council. The key focus of the studies conducted in the Motor Control Lab revolves around central nervous system control of multi-joint movements in a kinematically redundant system and the influence of orthopedic impairments on that control. Specifically, as we identify sets of rules that govern control of multi-joint movements in healthy participants, we can test for shifts or changes in these rules with various impairments. Consistent with this notion, we are currently studying individuals with sub-acute or chronic low back pain, or those who have recently recovered from an acute episode of low back pain. The goal of the research is to understand the mechanisms of recovery and recurrence in low back pain. Future studies will use these reaching paradigms to assess treatment interventions on reducing the rate of recurrence in low back pain. This will ultimately provide clinicians with better evaluation and treatment criteria when confronting patients with low back pain.

Dr. James Thomas, PhD, PT, associate professor in the School of Physical Therapy at Ohio University, is the director of the Ohio University Motor Control Lab. Other key individuals in the day to day operations include Dr. Daohang Sha, a post-doctorate fellow, and seven graduate research assistants who are currently clinical doctorate students in the School of Physical Therapy. The lab group is shown in Figure 5 below.

Figure 5 - Ohio University Motor Control Lab group. Back row: Dr Daohang Sha, James Odenthal, Tyler Bowersock, Kevin Swank, Stacey Moenter, Candice Kochman. Front row: Sara Clagg and Nikki Vander Wiele.