Handbook of Human Motion pp 81100  Cite as
3D Dynamic Pose Estimation from MarkerBased Optical Data
Abstract
The desire to capture images of human movement has existed since prehistoric times (see chapter “Observing and Revealing the Hidden Structure of the Human Form in Motion Throughout the Centuries”). However, it is only since the late nineteenth century and the development of cameras able to capture multiple sequential images that the recording and quantitative analysis of movement has become possible. With modern cameras and high computational power now available, it is commonplace for researchers and clinicians to make detailed measurements, from which an estimation of the position and orientation (pose) of a human body during motion can be computed. This chapter focuses on the estimation of dynamic 3D pose based on optical motion capture systems that record the 3D location of markers attached to the body (see Fig. 1). In this chapter, we describe the estimation of the pose of a multibody model comprising segments that are connected by joints that constrain the direction and range of motion between those segments. There are three common deterministic solutions to the problem of pose estimation; direct, single body, and multibody. This chapter focuses on the two optimization methods, single body and multibody, that provide a deterministic and a discriminative solution to the problem of pose estimation. Unlike the direct pose estimation, these two approaches mitigate, to some extent, uncertainty in the data.
Keywords
Skeletal modeling Pose estimation Motioncapture Inverse kinematics Soft tissue artifact OptimizationIntroduction
To estimate the pose of the multibody model, the 3D location of reflective markers attached to the segments is recorded by one or more optical sensors. It is beyond the scope of this chapter to describe the algorithms for identifying these 3D locations from the optical sensors, but regardless of the optical technology, the resultant 3D locations are used consistently between approaches. The tracking of each segment (pose estimation during a dynamic trial) is accomplished by establishing the location of the markers in the segment’s anatomical reference frame to which they are attached, recording the location of these markers in each frame of a motion trial, and by satisfying the specified joint constraints. A fundamental assumption of the algorithms presented in this chapter is that segments are rigid and the markers attached to those segments are secured rigidly and do not move relative to the segment to which they are attached. The number of markers required, and the number of segments to which markers are attached, depends on the structure of the multibody model and the pose estimation algorithm being used. The most important concept within this methodology is observability. Observability is dealt with in more detail later in the chapter; however, in short, a system is observable if the data are sufficient to describe, uniquely, the pose of the model. If the markers were truly attached rigidly to the underlying skeleton, i.e., a marker’s coordinates in the AF were invariant during movement, and the segments of the multibody model were truly rigid, and the markers were never occluded, this would be a straightforward chapter as all the pose estimation methods described in the scientific literature and textbooks would yield reliable pose estimations, and we could choose the mathematically simplest approach.
Any marker that is attached to the skin, however, can move relative to the underlying skeleton (Cappozzo et al. 1996). This relative motion occurs as flesh between the marker and skeleton deforms during movements, and is commonly known in the biomechanics community as soft tissue artifact (STA). It is, as yet, challenging to mitigate STA through mathematical approaches because, while STA is systematic, it varies on a casebycase basis between individuals, between locations on the body, and between movements. Pose estimation algorithms that mitigate these “uncertainties” resulting from STA can improve the effectiveness of pose estimation dramatically.
The two pose estimation algorithms discussed in this chapter are common in the biomechanics community and are deterministic and discriminative. In other words, they rely solely on the structure of the multibody model and instantaneous data to estimate pose. This is in contrast with probabilistic pose estimation, in which prior information (e.g., models of STA or predictions based on the statistics of past performance) are incorporated into the pose estimation algorithm (see chapter “3D Dynamic Probabilistic Pose Estimation From Data Collected Using Cameras and Reflective Markers”).
State of the Art
Six Degree of Freedom (6DOF) Pose Estimation
This section describes an algorithm for six degree of freedom (6DOF) pose estimation, sometimes referred to as a segment optimization algorithm (Lu and O’Connor 1999) or singlebody optimization. To estimate the pose of a segment at each frame of data, the 6DOF algorithm requires that a set of not less than three noncollinear markers be attached to each segment. To clarify the need for three markers, we will describe the information available from 1, 2, or more markers on a segment. If a segment was to have a single marker attached to it, this marker would permit the estimation of translations of the segment along the three principal axes of the global reference frame (e.g., 3DOF). If a second marker was added, it would be possible to estimate rotations about two principal axes of the segment; however, rotations about an axis between the two markers would be undetectable (e.g., 5DOF). When a third marker is added, offset from the line between the first two, rotations about all three segmental axes become observable (e.g., 6DOF). Additional markers on a segment cannot increase the number of degrees of freedom but, as will be see below, can be useful in a leastsquares sense. This method is referred to as a 6DOF method because each segment (or joint) is considered to have six independent variables that describe its pose; three variables describe the location of the segment’s origin within the global reference frame (its position) and three variables describe the rotation about each of the principal axes of the segment (its orientation). In principle, each segment can be tracked independently of any other segment. This independence infers that there is no explicit linkage defined, i.e., there are no preconceived assumptions about the properties of any joint connecting segments. This means that the endpoints of a segment, and those of its the proximal and distal adjacent neighbors, are free to move relative to each other, based directly and solely on the recorded motion capture (MoCap) data (Cappozzo et al. 1995). This independent estimation of the pose of the segments requires that markers used to track one segment are not used to track any other segment. It is quite common, however, for one marker to be used as a tracking marker on two adjacent segments. For example, a lateral knee marker may be used as a tracking marker on the thigh and the shank. In this situation, the thigh and shank segments are still 6DOF because six variables describe the motion of a segment, but in this case the segments are not actually independent of each other.

R_{ AG } is a rotation matrix from AF to GF

O_{ AG } is the translation from AF to GF.
The 6DOF algorithm requires a minimum of three noncollinear tracking markers, but more can be accommodated because the 6DOF algorithm permits a solution for an overspecified system with an unlimited number of tracking markers on a segment. This overspecification means that, provided noise (or some features of STA) in the data is uncorrelated, the leastsquares algorithm will act to minimize the effects of the noise. If one or more tracking targets are missing in any frame(s), the overspecification still allows a calculated segmental pose, provided at least three noncollinear targets are present. The observability for a 6DOF method is straightforward because it is simply N ≥ 3, provided the locations of the markers are fixed in the AF, and are not collinear. In principle, tracking markers can be placed anywhere on a rigid segment. In practice, marker placement on an anatomical segment is a compromise between distributing markers over the entire surface of a segment and placing markers in areas that exhibit minimal STA (Cappozzo et al. 1997). As concluded in a review article by Cereatti et al. (2006), there have been attempts to modify the 6DOF algorithm in order to mitigate the effects of STA (Cappozzo et al. 1997; Andriacchi et al. 1998), but none of these approaches have proved satisfactory.
Pose Estimation Using a Technical Reference Frame (TF)
While this chapter is focused on estimating pose from marker data, it is convenient at this time to discuss briefly pose estimation from two other 6DOF sensors: electromagnetic sensors and Moiréphase tracking. It is beyond the scope of this chapter to describe the theory behind the sensor technology, but in summary, electromagnetic systems record the 6DOF pose of a sensor relative to an emitted electromagnetic dipole field. The Moiréphase tracking (MPT) 3D motion capture system (Weinhandl et al. 2010) is a singlecamera 3D motion tracking technology that tracks the 6DOF pose of a Moiré target (a lightweight, multilayer passive optical target; Weinhandl et al. 2010). The important idea to note is that these sensors describe their pose relative to an internal reference frame, not an anatomical frame. To put these 6DOF sensors in the context of markerbased MoCap (the focus of the chapter), we consider a slightly different approach to the 6DOF algorithm.
Using the same markers (m_{ i }) from one frame of data, and assuming that the transformation from TF to AF is invariant, we can identify (R_{ TA }, O_{ TA }) from vector calculus using the same methods used to define AF in the first place.

R_{ TG } is a rotation matrix from TF to GF

O_{ TG } is the translation from TF to GF

R_{ TG } and O_{ TG } are computed as in Eq. 4.
There is a considerable benefit to the 6DOF approach to pose estimation, as it is straightforward to implement with results that are easy to understand. The 6DOF solution has no local minima, and requires no guidance from users. Notably, 6DOF estimates a pose that is an accurate representation of the data, which is useful for identifying local problems. An example of such a local problem would be the swapping of the names of two markers between trials, or even within a trial (something not uncommon when working with many passive reflected markerbased MoCap systems). Such mislabeling of markers will cause obvious discontinuities in the pose estimations of a 6DOF segment, which can be easily identified and corrected. The deterministic assumption that neither STA nor noisy marker data occur can result in pose estimations where the adjacent endpoints of segments are dislocated from each other or “merge” together. While these pose solutions reflect the true marker data, and thus highlight the presence of noise and/or STA, they can present estimations of pose that are anatomically impossible. To highlight the serious challenge of STA, if the entire set of markers translates in unison (e.g., through inertial forces or impact), the estimated pose of the segment can be quite wrong. There is, however, no information in the relative configuration of the tracking markers to indicate that anything has gone awry, so this artifact cannot be mitigated. The next section describing inverse kinematics discusses a deterministic approach to remove such an obvious artifact as joint disarticulation from the 6DOF model.
Inverse Kinematics (IK) Pose Estimation
The solution to the IK is the pose of a multibody model that best matches the MoCap data, in terms of a leastsquares criterion . In the Lu and O’Connor (1999) approach, the IK solution is found for each frame of data, independent of any previous or subsequent frames of data. Mathematically, van den Bogert and Su (2008) described this approach, based on the overall configuration of the multibody model, using a set of generalized coordinates q.
Weighting
The selection of the weights, α_{ i }, can be made pragmatically and heuristically, or rules may be used that allow the computation of an optimal set of weights. Without a priori information, it is usually best to set α_{ i } to 1, but on occasion, when estimating pose, the user may want to ensure that certain segments follow the tracking targets with a higher degree of accuracy than other segments. For example, the user may want the distance between the foot and the floor (or recorded ground reaction force) to remain similar to the values that would be obtained using a 6DOF method because 6DOF is likely the best local estimate of the pose of the foot. Likewise, data from some markers may not be considered representative of the pose because they are noisy, so the weight of these data can be reduced. In some cases, the marker may be known to have substantial STA relative to one of the degrees of freedom (generalized coordinates) and the influence of the marker on this generalized coordinate can be removed.
Observability of the Inverse Kinematics
As mentioned previously, the pose of a multibody model is observable if the data are sufficient to describe the pose uniquely. In the case of the 6DOF pose estimation, three or more rigidly attached, noncollinear targets are required to track each segment. When one target is placed on a rigid segment, three independent pieces of information can be obtained, the X, Y, and Z coordinates of the target. When a second target is placed on the segment, two further pieces of information are obtained. The number of new pieces of information for the second target is two, not three like the first target, because if we know the X and Y locations of the second target, then the Z coordinate is known because the distance between the first and second targets is fixed. Thus, two targets only supply five of the six unknowns. When a third target is added, one additional piece of information is supplied; note the third target only adds one new piece of information because the distance from the third target to the first target and the distance from the third target to the second target are fixed. Still with three noncollinear targets, we have sufficient information to fully solve the pose of a 6DOF segment.
With IK, not only is there the assumption of rigid segments, but there are also constraints added at the joints. A consequence of the joint constraints is that fewer than three markers may be sufficient to fully determine the pose of a segment. For example, a segment that has only one degree of freedom (e.g., one connected to a parent segment by a hinge joint) only requires one marker to fully determine the joint angle. It is not possible to just count markers, however, because if this one marker is coincident with the hinge joint, it does not provide any information and the pose is nonobservable. Therefore, the question of whether the markers provide sufficient information to determine the model’s pose is far more complex when joint constraints exist.
A straightforward approach to the problem would be to specify the number of targets required to track a segment, based solely on the type of joint connecting that segment to its parent. For example, Yeadon (1984) required two markers to track a segment connected to the parent via a ball joint (three degrees of freedom) or a universal joint (two degrees of freedom) and required only one marker when the segment was connected via a one degree of freedom hinge joint. Although this approach will guarantee that the system will likely be observable, if these requirements are met, it can be overly conservative and will occasionally consider the model to be unobservable, when in fact there is sufficient information available. For example, Schulz and Kimmel (2010) demonstrated that it is possible to track the pose of the thigh segment without actually placing any markers on the thigh. Yeadon’s method would declare this model to be unobservable. This is important because for many activities, the STA of markers on the thigh is detrimental to an accurate estimate of the pose and if Schulz’s assumption that the hip has three degrees of freedom and the knee has one degree of freedom is an accurate reflection of the movement, his approach could be useful for studying many activities.
To demonstrate how it is possible to calculate a general solution to the observability problem, consider the simple example of a single segment constrained to its parent (in this example, the ground) by a ball joint. This system can be fully described by three degrees of freedom: the Euler rotations, θ_{ x } , θ_{ y } , and θ_{ z }.
To simplify this equation, consider the state where θ_{ x } = 0, θ_{ y } = 0, θ_{ z } = 0
Since the determinant of the Jacobian is zero, it is not invertible and its rank is not full; thus, one target is not sufficient to estimate the pose of a segment connected to ground via a ball joint.
The Jacobian of the cost function now reduces to:
Jacobian of cost function = \( \left\begin{array}{ccc}0& {A}_{1_z}& {A}_{1_y}\\ {}{A}_{1_z}& 0& {A}_{1_x}\\ {}{A}_{1_y}& {A}_{1_x}& 0\\ {}0& {A}_{2_z}& {A}_{2_y}\\ {}{A}_{2_z}& 0& {A}_{2_x}\\ {}{A}_{2_y}& {A}_{2_x}& 0\end{array}\right \)
If targets A_{1} and A_{2} have coordinates (0, 0, A_{1z}) and (0, 0, A_{2z}) which they are collinear along the Z axis, we would expect the system to be unobservable as the targets will not register rotation about the Z axis. For this case:
Jacobian of cost function = \( \left\begin{array}{ccc}0& {A}_{1_z}& 0\\ {}{A}_{1_z}& 0& 0\\ {}0& 0& 0\\ {}0& {A}_{2_z}& 0\\ {}{A}_{2_z}& 0& 0\\ {}0& 0& 0\end{array}\right \)
Column 3 equals zero, not full column rank, and thus the system is not observable as expected.
This matrix has a rank = 3, which is full column rank and thus marker information (A_{1} and A_{2}) is independent and the model is fully observable.
Therefore, the general solution for observability in inverse kinematics reduces to determining whether the Jacobian for cost function of Eq. 10 has full rank. If it does, we have sufficient information to determine the pose of the model. Conversely, if the rank of the Jacobian of the IK cost function is not full rank, there is not enough information to determine a unique pose for the model.
IK Optimization Algorithms
In the general case, there is no analytic solution for the IK problem. We, therefore, summarize examples from two classes of implementation of a numerical solution to this optimization problem: direction search methods and global search methods.
Direction Search Methods (Newton’s Method)
To understand Newton’s method, consider a function f(q) that starts at an initial vector q_{0}, moves through a series of vectors q_{ k }, and converges to a solution at q_{ min }.
 1.
Compute the search direction
 2.
Determine the length of the next step
 3.
Use the results of steps 1 and 2 to obtain a new point q_{ k }. These steps are repeated until a minimum is found
After solving for the search direction, (q − q_{ k }), the next point in the search, q_{k + 1}, is found by moving in the direction of (q − q_{ k }). Ideally, the step size is determined by the magnitude of the eigenvalues of the movement to ensure that we obtain a sufficient decrease in the cost function, without taking excessively small steps. In practice, steps sizes that have worked for previous data sets are assumed to be sufficient. Once q_{k + 1} is obtained, it is checked against a termination criterion (is (q − q_{ k }) small). If the termination criterion is satisfied, then the minimum for the global IK problem is found. If the criteria is not met, the process is repeated, beginning at step 1 with q_{k + 1} acting as the new current value q_{ k }.
 1.
The Hessian must be symmetrical
 2.
The model gradient must be equal to the function gradient at the current step and at the previous step
 3.
The Hessian cannot change drastically between successive steps
The consequence of these assumptions is that convergence may be compromised. Unlike the 6DOF leastsquares solution, there are many possible solutions to the IK optimization as the solution space typically has many local minima. If the initial estimated position, q_{0}, is “close” to the global minimum, the solution will likely converge to the correct solution. The initial estimated position, or “seed,” is therefore critical to the success of the algorithm. For the first frame of data, it is possible to use a 6DOF solution as the seed. For subsequent frames, the seed for the optimization algorithm at any given frame is the state of the model at the previous frame. This could be problematic if the solution at the previous frame was an inappropriate local minimum, resulting in subsequent pose estimates diverging from the real solution due to being held in this local minimum. For example, the data collection volumes of most optical MoCap systems are smaller than the laboratory that they are in, and subjects often begin their movements outside the volume (for example, to perhaps ensure that they are at a constant speed while walking or running through the data collection volume). The first frame with complete data can often be relatively unreliable because it is captured near to the edge of the calibrated volume, and therefore the likelihood of the optimization solution becoming trapped in a local minimum increases. In order to avoid this, one potential improvement to the algorithm is to compute the solution both forward and backward, in the hope that one of the passes will provide a more optimal solution path.
Global Search Methods: Simulated Annealing
 1.
Some new values that do not actually reduce the minimum value are allowed so that more of the solution space can be explored. (The allowed values are determined by the Metropolis criteria.)
 2.
After making many estimates, and observing that the cost function declines slowly, one lowers the temperature and thus limits the size of allowed values that are larger than the current minimum. After lowering the temperature several times, only more optimal values are accepted, and the optimization approaches the global minimum.
One of the biggest challenges to simulated annealing is that the algorithm is computationally expensive, and perhaps more problematically, it is not possible to determine if the current solution is actually a global minimum without continuing the optimization indefinitely. In other words, there is no threshold or criterion for identifying that the search is complete. The user must decide how many iterations to perform in the optimization and accept that the minimum found in that time period may not be the global minimum. Despite the computational cost (time), simulated annealing is a more robust algorithm than direction search algorithms. Despite the robustness, however, most IK users opt for direction search algorithms because of time constraints.
6DOF Versus IK
In many circumstances, the IK solution is likely to be more anatomically congruent and therefore preferable to the 6DOF solution, but the user must attend to the determination of the appropriateness of the selected joint constraints. For example, an experiment that was focused on understanding the kinematics of an injured knee, where translations and rotations occur as a result of the injury (e.g., anterior cruciate ligament damage), would likely not benefit from an IK solution where the constraints, and consequent prescribed motion of the knee joint, “hide” the pathology. Finally, it is well known that residual errors, i.e., differences between model predictions and marker measurements, computed by IK algorithms are reflections of noise in the marker data, soft tissue artifact, and inaccurate marker placement. A limitation of the IK algorithm, however, is that it has no straightforward mechanism to compensate for systematic noise, even though it can be used to identify its presence.
Future Directions
In this chapter, we have described the current state of deterministic pose estimation algorithms.
The future evolution of deterministic algorithms is quite limited. Begon et al. (2016), for example, has introduced an approach that removes STA without modeling STA but rather by ignoring information in markers that are considered unreliable. For many segments of the human body, STA has a particularly disastrous effect on the axial rotation of the segment. In other words, the markers rotate about the long axis of the segment (upper arm, forearm, thigh to name a few). Begon’s solution was to ignore any information in the marker that would reflect axial rotation by projecting tracking markers onto the long axis of the segment. These projected markers influence five of the degrees of freedom of a segment only. The long axis rotation is then estimated based on the pose of adjacentconstrained segment. The example given by Begon is movement of the upper arm, in which the axial rotation of the upper arm is estimated by constraining the elbow joint to have only two rotational degrees of freedom, and therefore the axial rotation of the upper arm is based on the pose of the forearm. There is some potential for improvements to deterministic pose estimation algorithms based on similarly clever rejection of data in isolated/idiosyncratic cases.
It is our believe that the future of markerbased pose estimation lies not in deterministic algorithms but in algorithms based on Bayesian Inference (Todorov 2007) (chapter “3D Dynamic Probabilistic Pose Estimation from Data Collected Using Cameras and Reflective Markers”) and algorithms based on optimal control theory (Miller and Hamill 2015) ( “Optimal Control Modeling of Human Movement”). Bayesian Inference allows a principled way to mitigate the effects of STA by modeling artifact and removing it. Optimal control theory is capable of generating motion independently of any recorded data based on generated simulated motion of the behavior based on some optimization criteria (e.g., minimum energy). The technique can be influenced by recorded data to ensure that the pose estimation is arbitrarily close to the recorded motion. Optimal control theory has the additional benefit of being able to generate solutions for unobservable, and even sparse, marker sets.
Lastly, it is important to consider algorithms for which the soft tissue artifact is considered important data reflective of an individual subject instead of an artifact to be removed. Michael Black’s laboratory at the Max Planck Institute for Intelligent systems has been developing pose estimation algorithms based on statistical shape models (Loper et al. 2015). Instead of defining pose based on the position and orientation of an underlying skeleton, this research has focused on modeling the surface geometry of the subject and estimating the pose of the surface. Based on highdensity surface scans of subjects performing movement, the statistical shape model is a parameterized surface that can be subsequently fit to sparse surface data (e.g., markers). These models are remarkably good at representing the surface of the body during motion. From a biomechanics perspective, a fundamental question is whether we can infer the multibody skeletal pose from this parameterized surface data.
CrossReferences

3D Dynamic Probabilistic Pose Estimation from Data Collected Using Cameras and Reflective Markers

Observing and Revealing the Hidden Structure of the Human Form in Motion Throughout the Centuries

ThreeDimensional Human Kinematic Estimation Using MagnetoInertial Measurement Units

ThreeDimensional Reconstruction of the Human Skeleton in Motion
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