Generic muscle parameters are often used in muscle-driven simulations of human movement to estimate individual muscle forces and function. The results may not be valid since muscle properties vary from subject to subject. This study investigated the effect of using generic muscle parameters in a muscle-driven forward simulation on muscle force estimation. We generated a normal walking simulation in OpenSim and examined the sensitivity of individual muscle forces to perturbations in muscle parameters, including the number of muscles, maximum isometric force, optimal fiber length, and tendon slack length. We found that when changing the number of muscles included in the model, only magnitude of the estimated muscle forces was affected. Our results also suggest it is especially important to use accurate values of tendon slack length and optimal fiber length for ankle plantar flexors and knee extensors. Changes in force production by one muscle were typically compensated for by changes in force production by muscles in the same functional muscle group, or the antagonistic muscle group. Conclusions regarding muscle function based on simulations with generic musculoskeletal parameters should be interpreted with caution.
Ming Xiao and Jill Higginson
Debra G. George-Reichley and Jill S. Higginson
The understanding of individual muscle impairments that affect swing phase in stroke gait will lead to better rehabilitation strategies for this population. We used induced acceleration analysis to evaluate the potential each muscle has to accelerate the hip and knee joints of the swing limb, using kinematics from three stroke subjects and five healthy subjects. To determine the influence of altered limb position on muscle function, we augmented hip extension by 10° in swing phase for all subjects. We found that in early swing, healthy subjects had greater potential to accelerate the knee into flexion than stroke subjects, whereas stroke subjects had greater potential to accelerate the hip into flexion. Perturbing the hip flexion angle into greater extension increased the potential of biarticular muscles to flex the knee in swing phase. The potential of muscles to improve swing phase dynamics depends on the initial posture of the limb and highlights the importance of subject-specific evaluations in the design of appropriate therapeutic interventions.
Daniel M. Grindle, Lauren Baker, Mike Furr, Tim Puterio, Brian Knarr and Jill Higginson
Prolonged sitting has been associated with negative health effects. Walking workstations have become increasingly popular in the workplace. There is a lack of research on the biomechanical effect of walking workstations. This study analyzed whether walking while working alters normal gait patterns. A total of 9 participants completed 4 walking trials at 2.4 and 4.0 km·h−1: baseline walking condition, walking while performing a math task, a reading task, and a typing task. Biomechanical data were collected using standard motion capture procedures. The first maximum vertical ground reaction force, stride width, stride length, minimum toe clearance, peak swing hip abduction and flexion angles, peak swing and stance ankle dorsiflexion, and knee flexion angles were analyzed. Differences between conditions were evaluated using analysis of variance tests with Bonferroni correction (P ≤ .05). Stride width decreased during the reading task at both speeds. Although other parameters exhibited significant differences when multitasking, these changes were within the normal range of gait variability. It appears that for short periods, walking workstations do not negatively impact gait in healthy young adults.
Stephen John Thomas, Charles Buz Swanik, Thomas W. Kaminski, Jill S. Higginson, Kathleen A. Swanik and Levon N. Nazarian
Subacromial impingement is a common injury in baseball players and has been linked to a reduction in the subacromial space. In addition, it has been suggested that decreases in scapular upward rotation will lead to decreases in the subacromial space and ultimately impingement syndrome.
The objective of this study was to evaluate the relationship between acromiohumeral distance and scapular upward rotation in healthy college baseball players.
Posttest-only study design.
Controlled laboratory setting.
24 healthy college baseball players.
Participants were measured for all dependent variables at preseason.
Main Outcome Measures:
Acromiohumeral distance at rest and 90° of abduction was measured with a diagnostic ultrasound unit. Scapular upward rotation at rest and 90° of abduction was measured with a digital inclinometer.
Dominant-arm acromiohumeral distance at rest and 90° of abduction (P = .694, P = .840) was not significantly different than in the nondominant arm. In addition, there was not a significant correlation between acromiohumeral distance and scapular upward rotation at rest and 90° of abduction for either the dominant or the nondominant arm.
These results indicate that the acromiohumeral distance is not adapting in the dominant arm in healthy throwing athletes. In addition, a relationship was not identified between acromiohumeral distance and scapular upward rotation, which was previously suggested. These results may suggest that changes that are typically seen in an injured population may be occurring due to the injury and are not preexisting. In addition, scapular upward rotation may not be the only contributing factor to acromiohumeral distance.
Alan R. Needle, Thomas W. Kaminski, Jochen Baumeister, Jill S. Higginson, William B. Farquhar and C. Buz Swanik
Rolling sensations at the ankle are common after injury and represent failure in neural regulation of joint stiffness. However, deficits after ankle injury are variable and strategies for optimizing stiffness may differ across patients.
To determine if ankle stiffness and muscle activation differ between patients with varying history of ankle injury.
Fifty-nine individuals were stratified into healthy (CON, n = 20), functionally unstable (UNS, n = 19), and coper (COP, n = 20) groups.
Main Outcome Measures:
A 20° supination perturbation was applied to the ankle as position and torque were synchronized with activity of tibialis anterior, peroneus longus, and soleus. Subjects were tested with muscles relaxed, while maintaining 30% muscle activation, and while directed to react and resist the perturbation.
No group differences existed for joint stiffness (F = 0.07, P = .993); however, the UNS group had higher soleus and less tibialis anterior activation than the CON group during passive trials (P < .05). In addition, greater early tibialis anterior activation generally predicted higher stiffness in the CON group (P ≤ .03), but greater soleus activity improved stiffness in the UNS group (P = .03).
Although previous injury does not affect the ability to stiffen the joint under laboratory conditions, strategies appear to differ. Generally, the COP has decreased muscle activation, whereas the UNS uses greater plantar-flexor activity. The results of this study suggest that clinicians should emphasize correct preparatory muscle activation to improve joint stiffness in injury-rehabilitation efforts.
R. Tyler Richardson, Elizabeth A. Rapp, R. Garry Quinton, Kristen F. Nicholson, Brian A. Knarr, Stephanie A. Russo, Jill S. Higginson and James G. Richards
Musculoskeletal modeling is capable of estimating physiological parameters that cannot be directly measured, however, the validity of the results must be assessed. Several models utilize a scapular rhythm to prescribe kinematics, yet it is unknown how well they replicate natural scapular motion. This study evaluated kinematic errors associated with a model that employs a scapular rhythm using 2 shoulder movements: abduction and forward reach. Two versions of the model were tested: the original MoBL ARMS model that utilizes a scapular rhythm, and a modified MoBL ARMS model that permits unconstrained scapular motion. Model estimates were compared against scapulothoracic kinematics directly measured from motion capture. Three-dimensional scapulothoracic resultant angle errors associated with the rhythm model were greater than 10° for abduction (mean: 16.4°, max: 22.4°) and forward reach (mean: 11.1°, max: 16.5°). Errors generally increased with humerothoracic elevation with all subjects reporting greater than 10° differences at elevations greater than 45°. Errors associated with the unconstrained model were less than 10°. Consequently, use of the original MoBL ARMS model is cautioned for applications requiring precise scapulothoracic kinematics. These findings can help determine which research questions are suitable for investigation with these models and assist in contextualizing model results.