Even though some authors found it possible to partially weight bear according to the prescribed limits the majority of studies showed that patients were not able to keep within the given load [2–5]. All studies that we are aware which reported the ability to accurately follow the instructions were performed with healthy subjects [6, 13, 14]. Therefore, their results can not accurately be assigned to patients since their inability to partially weight bear must be assumed. Chow et al.  reported that the patient's potential to partially weight bear was predictable by simply testing the left hand grip and performing a mental state test. Based on these findings we hypothesized that the ability to partially weight bear is associated with the psychomotor skills of the patient and therefore predictable with a motor performance test. Such a relationship should be searched for in this study. Statistical analysis showed a significant correlation of the average Fmax and the age as well as the patient's body weight. This disagrees with Chow et al.  who reported no significance for these factors. The evaluation of the results of several subtests of the MLS showed a significant correlation with the average Fmax. It was thereby not important whether the tests were performed on the right, the left or on both sides simultaneously. Subtests correlating with the SD Fmax could be used to predict how constant the patient is able to partially weight bear.
The total duration of the subtest line tracking and the number of hits of the subtest tapping seemed to be especially dependent on the compliance and could therefore be an indicator to determine who will take the instructions seriously. Since complying with the therapist's instructions is crucial for accurate partial weight bearing these tests might be very useful.
Comparing the two groups with each other showed that the results of several subtests differed significantly which indicates the possibility to predict the ability to partially weight bear. It is of importance, that patients in Group 2 not only loaded the concerned leg with Fmax > 180 N but also with a higher percentage of their own body weight and had an increased SD Fmax. This indicates that correct partial weight bearing is rather a matter of psychomotor control than of body mass.
Furthermore, a regression analysis was performed trying to calculate the average Fmax using the parameters of the MLS. The resulting formula with ten different factors turned out to be too complicated and arguable for clinical use.
The Pedar Mobile system used for force measurements has been validated by Hurkmans et al. [15, 16]. But there are still some problems associated with this system that have to be considered as limiting factors. The sensors of the insoles have a threshold value to minimize confounders. Forces below these values are not registered and therefore the measurements are slightly to low. Since this threshold value was the same for all patients comparison of the two groups was not influenced. The relation between the average Fmax and parameters of the MLS are also not affected because the loss of force is small and assumed about equal for all patients. Falsification of results caused by this confounder is therefore within reasonable limits. A small amount of force is further directed via the shoe and not via the insole to the ground resulting in an additional loss of force. To minimize these confounders Fong et al.  described a method to estimate the complete ground reaction force with pressure insoles. The described technique could not be adapted to this investigation because patients often showed an altered moving pattern loading only some parts of the insoles.
For this study, only short-term force measurements in presence of an investigator have been performed. It has to be assumed that patients are taking more care in partial weight bearing when accompanied than when unobserved. Since this study was designed to investigate the ability to accurately partially weight bear and to search for an association to the patient's psychomotor skills short-term measurements were sufficient. Since measurements were performed only a few days after surgery, pain could have been a further confounding factor leading to more accentuated unloading of the extremity. To minimize this effect, patients were only tested when they declared no pain. For organizational reasons psychomotor skills were tested after surgery. A certain influence of medication on the test results could not be excluded. To establish equal conditions all patients were tested postoperatively and the test was not performed when the patient was obviously influenced by medication.
Eventually, the patients' absolute strength or endurance of the upper limbs could have influenced their ability of correct partial weight bearing. We did, however, not consider the maximum load of each patient but averaged Fmax on a rather short walking distance of 20 m, thus minimizing the relevance of strength endurance. Additionally, it is likely that lowered absolute upper extremity strength has direct implications on the results of the MLS that is operated by using arms, hands and fingers.
Some authors state that motor control resources are specific and that transfer between skills is small [18, 19]. Thus, at first sight, a method specifically measuring the psychomotor skills of the lower extremities might seem favourable to draw conclusions on the ability of intentionally decreasing the load on one leg. Partial weight bearing on forearm crutches, however, is a complex interaction of all four extremities and the trunk. Finally, testing protocols involving the legs are just not applicable for patients with injuries of the lower extremities. Our results showed differences between Group 1 ("good" partial weight bearing) and Group 2 ("bad" partial weight bearing) especially for those subtests where both hands had to be coordinated simultaneously as this would be required for walking on crutches.
The key question of this study was whether some persons have more ability than others to coordinate the muscles of their trunk, arms and legs and adapt it to a new weight bearing situation. We, therefore, decided to use a standardized and validated procedure testing general psychomotor skills [7, 8]--the MLS.