The degrees of freedom problem is often posed by asking which of the many possible degrees of freedom does the nervous system control? By implication, other degrees of freedom are not controlled. We give an operational meaning to "controlled" and "uncontrolled" and describe a method of analysis through which hypotheses about controlled and uncontrolled degrees of freedom can be tested. In this conception, control refers to stabilization, so that lack of control implies reduced stability. The method was used to analyze an experiment on the sit-to-stand transition. By testing different hypotheses about the controlled variables, we systematically approximated the structure of control in joint space. We found that, for the task of sit-to-stand, the position of the center of mass in the sagittal plane was controlled. The horizontal head position and the position of the hand were controlled less stably, while vertical head position appears to be no more controlled than joint motions.

Driven by recent empirical studies, we offer a new understanding of the degrees of freedom problem, and propose a refined concept of synergy as a neural organization that ensures a one-to-many mapping of variables providing for both stability of important performance variables and flexibility of motor patterns to deal with possible perturbations and/or secondary tasks. Empirical evidence is reviewed, including a discussion of the operationalization of stability/flexibility through the method of the uncontrolled manifold. We show how this concept establishes links between the various accounts for how movement is organized in redundant effector systems.

We describe an uncontrolled manifold hypothesis, which suggests a particular solution for the notorious problem of motor redundancy. A body of recent experiments supports the uncontrolled manifold hypothesis and shows its ability to discover biological strategies of the coordination of apparently redundant motor systems. The hypothesis and associated computational apparatus have great potential for application in the areas of motor rehabilitation and motor skill acquisiton.

Some individuals can stabilize their knees following anterior cruciate ligament rupture even during activities involving cutting and pivoting (copers), others have instability with daily activities (non-copers). Movement and muscle activation patterns of 11 copers, ten non-copers and ten uninjured subjects were studied during walking and jogging. Results indicate that distinct gait adaptations appeared primarily in the non-copers. Copers used joint ranges of motion, moments and muscle activation patterns similar to uninjured subjects. Non-copers reduced their knee motion, and external knee flexion moments that correlated well with quadriceps strength. Non-copers also achieved peak hamstring activity later in the weight acceptance phase and used a strategy involving more generalized co-contraction. Both copers and non-copers had high levels of quadriceps femoris muscle activity. The reduced knee moment in the involved limbs of the non-copers did not represent "quadriceps avoidance" but rather represented a strategy of general co-contraction with a greater relative contribution from the hamstring muscles.

Background Patellofemoral pain syndrome (PFPS) is the most common overuse injury in runners. Recent research suggests that hip mechanics play a role in the development of this syndrome. Currently, there are no treatments that directly address the atypical mechanics associated with this injury. Objective The purpose of this study was to determine whether gait retraining using real-time feedback improves hip mechanics and reduces pain in subjects with PFPS. Methods Ten runners with PFPS participated in this study. Real-time kinematic feedback of hip adduction (HADD) during stance was provided to the subjects as they ran on a treadmill. Subjects completed a total of eight training sessions. Feedback was gradually removed over the last four sessions. Variables of interest included peak HADD, hip internal rotation (HIR), contralateral pelvic drop, as well as pain on a verbal analogue scale and the lower-extremity function index. We also assessed HADD, HIR and contralateral pelvic drop during a single leg squat. Comparisons of variables of interest were made between the initial, fi nal and 1-month follow-up visit. Results Following the gait retraining, there was a signifi cant reduction in HADD and contralateral pelvic drop while running. Although not statistically signifi cant, HIR decreased by 23% following gait retraining. The 18% reduction in HADD during a single leg squat was very close to signifi cant. There were also signifi cant improvements in pain and function. Subjects were able to maintain their improvements in running mechanics, pain and function at a 1-month follow-up. An unexpected benefi t of the retraining was an 18% and 20% reduction in instantaneous and average vertical load rates, respectively. Conclusions Gait retraining in individuals with PFPS resulted in a signifi cant improvement of hip mechanics that was associated with a reduction in pain and improvements in function. These results suggest that interventions for PFPS should focus on addressing the underlying mechanics associated with this injury. The reduction in vertical load rates may be protective for the knee and reduce the risk for other running-related injuries.