Defining Movements
The first consideration when counting number of movements as a measure of performance efficiency is to understand the nature of the skills that are being evaluated. In this regard, the field of motor control kinesiology has typically classified actions as composed of combinations of movements that can be defined as either discrete or continuous. Discrete movements are those that have a recognizable beginning and end, like throwing a ball, turning a doorknob, or flipping a light switch; they are usually essential to skills that rely of the precise production of a distinct outcome. Continuous movements, on the other hand, have no recognizable beginning or end, and will continue until they are stopped arbitrarily by the performer. Skills composed of continuous movements can have precision constraints but are often concerned with the maintenance of an ongoing action. These skills include activities such as walking, swimming, and cycling.29 In the surgical-medical domain, the clinical technical performances of interest can more often than not be characterized as serial actions. This refers to skills that are made up of a number of discrete movements that must occur in a very particular order. These actions can appear continuous, but usually have distinct components with very definitive beginnings and ends.29 As such, any one clinical technical skill can be conceptualized in terms of the minimum number of discrete movements that would be needed for its successful execution. However, as the number of movements assessment construct suggests, this minimum is not always achieved.
That performances can contain movements in excess of the minimum required by the task is fundamental to the use of the number of movements metric as a measure of efficiency. Simply put, performances often contain errors or imperfect actions, which require corrections; and each erred movement and subsequent correction constitutes the production of additional movements. In this way, the hallmark error volume associated with novice performances has led to the natural assumption that new trainees will perform procedures with more movements, and therefore, more inefficiently. Given this position, the challenge for assessors of surgical technical skills is to determine where one movement within a procedure ends and the next one begins. In a motion capture and analysis protocol, the way that serial movements are usually disentangled from one another involves plotting the position function as a displacement profile and then differentiating and double-differentiating it to generate velocity and acceleration profiles respectively (Note: if the motion data is captured by accelerometer technology, then integrations are performed on the resulting profile to reveal velocity and displacement). From these profiles, assessors look for determinant characteristics within the action trajectories that indicate a new movement. Defining these characteristics becomes one of the most important decisions underpinning the effective use of the “number of movements ” metric.
Reflecting on the way that errors and corrections emerge in a motor performance can be helpful in setting the appropriate motion analysis parameters for determining the onset and offset of a movement. Consider, for instance, the types of errors that require a correction. For one, an action can require a correction because the performer selects and executes the wrong movement. This type of error occurs, for instance, when the laparoscopic surgery trainee forgets that the display screen is incongruently rotated with respect to the work space and ends up moving a grasper to the right instead of the left. To correct these types of errors, the ongoing movement must be terminated and replaced with an entirely new movement. Sometimes, this involves reversing direction to return to where the action started, or stopping to reassess the situation before initiating a new movement in search of corrective solution. With this type of error and correction in mind, skill assessors may set a zero crossing in the velocity-over-time profile as the end and start points for successive movements. However, this type of new movement determinant can be insufficient when one considers that a series of movements can be executed without the limb coming to a complete stop between each.
It is necessary to understand that noise in the neuromuscular system means that the production of movements is inherently variable,30 such that discrete precision actions usually require a subtle or not-so-subtle correction (or corrections) towards their conclusion in order to be successful even when the appropriate movement is selected and executed correctly by the performer.31 In this regard, new movement determinants based on zero crossings in the acceleration-over-time profile are also problematic as they ignore the refined and controlled nature of human motion.32, 33 Indeed, the findings from over a century’s worth of experiments on the accuracy of voluntary actions reveal that any one precision movement includes complimentary impulses that move the limb toward its goal and then integrate response-produced sensory feedback to correct the overall movement for accuracy.34 In this regard, the typical acceleration profile of a single discrete movement derived via motion capture techniques has 3 zero crossings, which characterize a large sinusoid (i.e., the initial impulse) that is followed by a second, smaller sinusoid (i.e., the corrective impulse).31, 35, 36 The idea is that because variability in movement execution is so inherent to motor performance that its management becomes a fundamental challenge for learners as they move along the continuum of expertise. That is, novice performers struggle to determine whether their approach to performance introduces too much variability to correct, while skill performers understand the inherent variability and develop strategies that allow them to anticipate the type of corrections required for their movements. In this way, a single expert movement often includes periods of deceleration and re-acceleration.31