Alteration in Global Motor Strategy After Ankle Sprain
Alteration in Global Motor Strategy After Ankle Sprain
This study is the first to identify global motor strategy variables associated with performance on the SEBT. Furthermore, the results of this study demonstrate that military personnel with LAS do not use the same global strategy as healthy controls when performing a goal oriented task such as the SEBT. Their global motor strategy differed in almost all conditions (except for the uninjured limb in the medial direction), suggesting that the strategy is not only linked to the conditions where the injured limb is in stance, but also when the uninjured limb is used. This difference in global strategy is characterized by less lowering of the CoMgl and less variation in its vertical velocity. This finding highlights the importance of having a sound global motor strategy in order to perform well during the test and it also suggests that every effort should be made during rehabilitation to help recover such a strategy.
According to our results, the global strategy variables along the vertical axis seem to be very good indicators of a subject's performance on the SEBT. The chosen global strategy variables, derived from body CoM displacement, helped to characterize performance on the SEBT in terms of motor control abilities. Our findings also support the construct validity of the SEBT that maximal reach distance, as commonly measured in clinical setting, provides a valid estimate of global motor control in the studied population. The body CoM behaviour improved knowledge of how participants were organizing their movements during the critical period of stability at foot contact (central phase), allowing the transition between the two subtasks.
As supported by a systematic review and original works, performance on different motor tasks is often altered bilaterally after LAS even in acute LAS stage. These differences in performance between groups could indicate a change in motor strategies as observed in the current study. The strategy used by the healthy group, leading to better performance, can be supported by biomechanical advantages and constraints. First, a lower vertical position of the CoMgl allows a better control of CoMgl horizontal displacement by increasing overall body stability. Indeed, a smaller lever arm in an inverted pendulum decreases muscle force required to maintain balance. Moreover, a lower body CoMgl position in unipedal stance increases the projection of the reaching limb by a better orientation of the pelvis, and it also increases the effect of the trunk CoM to counteract the perturbation. Thus, by a greater lowering of the CoMgl, the physical performance is optimized but the motor control requirement is much higher as the person needs to simultaneously control more body segments.
Secondly, the group with LAS demonstrated a more cautious approach near foot contact compared to their peers without LAS, as shown by a smaller peak-to-peak velocity of the body CoM. This finding suggests that LAS subjects are planning the transition between the subtasks more cautiously as they try to reduce body velocity around this event. After foot contact, a lower velocity of straightening up may allow more time to adjust for the internal perturbation and therefore enhance stability. Several studies have shown that a quick displacement of a body segment away from the body can efficiently counteract a loss of CoM control. In the present study, the delay in straightening up after foot contact has been attributed to the maintenance of the lower limb away from the body. Finally, global strategies in the horizontal plane also seemed to differ between groups. These variables can show changes in motor control because they give an idea of how far the participant pushed CoMgl to the perceived limits of stability, which seemed to be further in the healthy group. Moreover, the horizontal excursion of CoMgl seemed to have different characteristics according to reaching directions in the healthy group compared to the LAS group. Indeed, the LAS group showed a more grouped horizontal excursion of CoMgl for different reaching directions. This may be the result of less adaptability of the central nervous system for the group with alterations of motor control (LAS group) as previously observed for static postural control.
The difference in global strategy of the military personnel with LAS was mainly explained by changes in segmental motor strategy such as a decrease in pelvis lowering during the task and a more cautious bringing back of the reaching limb. First, the LAS group, which showed the lowest performance at SEBT, seemed to lower the pelvis less by flexing the knee less, and sometimes also the ankle, compared to the healthy group. Other studies have also reported a decreased range of motion at the knee during poor performance at SEBT. In the LAS group a significant decrease of maximal ankle dorsiflexion was found near foot contact for the AM direction only. Interestingly, it is indeed in the AM direction that the largest amplitude of ankle dorsiflexion is required.
Secondly, the lower peak-to-peak vertical velocity of CoMgl for the LAS group seemed to be mostly explained by a more constant velocity of the reaching limb after the foot contact and a delayed straightening up. Indeed, this smaller variation of vertical velocity in the LAS group could be partially explained by a cautious return of the reaching limb, which may help to prolong the stability advantages related to a smaller lever arm in the inverted pendulum. During the return-from-MRD subtask, the return to baseline velocity for hip abduction was delayed for all conditions of the injured limb of the LAS group. A more constant velocity when bringing back the reaching limb towards the body allows time to adjust the movement to preserve stability, diminishes the magnitude of the internal perturbation after a critical stability period, and also allows the use of the reaching limb to distribute mass away from the pivot point to reduce acceleration of CoMgl. Moreover, a more constant velocity of the reaching limb after foot contact is less demanding in terms of motor control compared to the bell-curve velocity profile shown in the healthy group. Indeed, a bell-curve velocity around foot contact requires excellent muscle coordination and good movement planning.
The present study confirms that each tested direction brings unique information with respect to segmental motor strategies. The non-redundancy was shown by the adaptability and differences in segmental motor strategies as a function of reaching direction along the vertical axis and in the horizontal plane. For example, the AM direction lowering, contrary to the PM direction, is better estimated by the pelvis lowering than the relative trunk lowering. This can be explained by the fact that the trunk can counteract the reaching limb less in the anterior direction by a smaller available range of motion at the hip and the back and by less powerful muscles controlling the movement in AM direction than in PM reaching against gravity. In fact, a recent study has shown differences in muscle activation patterns on the SEBT according to reaching directions.
From a clinical point of view, the performance during the SEBT has a significant meaning, as it reflects the motor control abilities during single leg stance, the sub-phase of walking and running where LAS or giving way usually occurs. Previous studies have shown bilateral decrease in reaching distance when executing the SEBT and bilateral modifications in the characteristics of CoP displacements, thereby supporting the hypothesis of central nervous system reorganization. However, the maximal reach distance and the measures of CoP displacement during single stance, the latter not being readily accessible to the clinician, do not provide any information on the nature of the dynamic control impairments. In the present study, we used kinematic data to describe the alterations in global and segmental strategies of persons with a LAS. Our findings not only give further support to the construct validity of the SEBT as a clinical measure of motor control, but they also indicate how alterations in motor control take place. They also provide visual cues that give insight on how movement should be retrained. As reaching with one limb while standing on the opposite foot may also be used as a challenging rehabilitation exercise, it might be wise for a clinician to ask the patient to reach for an object located on the lateral side of the stance foot to force a lateral displacement of the CoMgl towards the limits of the base of support. One could also stimulate the lowering of the CoMgl by asking the patient to further bend the lower limb while reaching as far as possible with the free limb.
In the present study, several variables were measured using a fairly small sample size of participants, and this may have limited the possibility of showing significant differences in some global strategy variables, especially the ones in the horizontal plane where a high inter-participant variability and differences in foot length could have had an impact. Also, while we ensured that both groups had similar personal characteristics, participants in the two groups were not individually paired. Therefore, it was not possible to control for side of injury and limb dominance in our comparative analyses. As previously mentioned, considering the high proportion of LAS injuries to the dominant limb, it was decided to use the dominant limb of the participants in the healthy group for intergroup limb comparisons. By doing so, the differences in the uninjured limb of the LAS group may have been slightly overestimated. Finally, it is important to mention that the calculated global body CoM is an estimate of true body CoM. Anthropometric segmental components of the body of each participant were not incorporated into the calculation of body CoM but were taken from population averages. This could have led to a slight over- or under-estimation of CoM position, even if theoretically it should have affected both groups equally.
Discussion
This study is the first to identify global motor strategy variables associated with performance on the SEBT. Furthermore, the results of this study demonstrate that military personnel with LAS do not use the same global strategy as healthy controls when performing a goal oriented task such as the SEBT. Their global motor strategy differed in almost all conditions (except for the uninjured limb in the medial direction), suggesting that the strategy is not only linked to the conditions where the injured limb is in stance, but also when the uninjured limb is used. This difference in global strategy is characterized by less lowering of the CoMgl and less variation in its vertical velocity. This finding highlights the importance of having a sound global motor strategy in order to perform well during the test and it also suggests that every effort should be made during rehabilitation to help recover such a strategy.
Global Motor Strategy Differences
According to our results, the global strategy variables along the vertical axis seem to be very good indicators of a subject's performance on the SEBT. The chosen global strategy variables, derived from body CoM displacement, helped to characterize performance on the SEBT in terms of motor control abilities. Our findings also support the construct validity of the SEBT that maximal reach distance, as commonly measured in clinical setting, provides a valid estimate of global motor control in the studied population. The body CoM behaviour improved knowledge of how participants were organizing their movements during the critical period of stability at foot contact (central phase), allowing the transition between the two subtasks.
As supported by a systematic review and original works, performance on different motor tasks is often altered bilaterally after LAS even in acute LAS stage. These differences in performance between groups could indicate a change in motor strategies as observed in the current study. The strategy used by the healthy group, leading to better performance, can be supported by biomechanical advantages and constraints. First, a lower vertical position of the CoMgl allows a better control of CoMgl horizontal displacement by increasing overall body stability. Indeed, a smaller lever arm in an inverted pendulum decreases muscle force required to maintain balance. Moreover, a lower body CoMgl position in unipedal stance increases the projection of the reaching limb by a better orientation of the pelvis, and it also increases the effect of the trunk CoM to counteract the perturbation. Thus, by a greater lowering of the CoMgl, the physical performance is optimized but the motor control requirement is much higher as the person needs to simultaneously control more body segments.
Secondly, the group with LAS demonstrated a more cautious approach near foot contact compared to their peers without LAS, as shown by a smaller peak-to-peak velocity of the body CoM. This finding suggests that LAS subjects are planning the transition between the subtasks more cautiously as they try to reduce body velocity around this event. After foot contact, a lower velocity of straightening up may allow more time to adjust for the internal perturbation and therefore enhance stability. Several studies have shown that a quick displacement of a body segment away from the body can efficiently counteract a loss of CoM control. In the present study, the delay in straightening up after foot contact has been attributed to the maintenance of the lower limb away from the body. Finally, global strategies in the horizontal plane also seemed to differ between groups. These variables can show changes in motor control because they give an idea of how far the participant pushed CoMgl to the perceived limits of stability, which seemed to be further in the healthy group. Moreover, the horizontal excursion of CoMgl seemed to have different characteristics according to reaching directions in the healthy group compared to the LAS group. Indeed, the LAS group showed a more grouped horizontal excursion of CoMgl for different reaching directions. This may be the result of less adaptability of the central nervous system for the group with alterations of motor control (LAS group) as previously observed for static postural control.
Segmental Motor Strategy Differences
The difference in global strategy of the military personnel with LAS was mainly explained by changes in segmental motor strategy such as a decrease in pelvis lowering during the task and a more cautious bringing back of the reaching limb. First, the LAS group, which showed the lowest performance at SEBT, seemed to lower the pelvis less by flexing the knee less, and sometimes also the ankle, compared to the healthy group. Other studies have also reported a decreased range of motion at the knee during poor performance at SEBT. In the LAS group a significant decrease of maximal ankle dorsiflexion was found near foot contact for the AM direction only. Interestingly, it is indeed in the AM direction that the largest amplitude of ankle dorsiflexion is required.
Secondly, the lower peak-to-peak vertical velocity of CoMgl for the LAS group seemed to be mostly explained by a more constant velocity of the reaching limb after the foot contact and a delayed straightening up. Indeed, this smaller variation of vertical velocity in the LAS group could be partially explained by a cautious return of the reaching limb, which may help to prolong the stability advantages related to a smaller lever arm in the inverted pendulum. During the return-from-MRD subtask, the return to baseline velocity for hip abduction was delayed for all conditions of the injured limb of the LAS group. A more constant velocity when bringing back the reaching limb towards the body allows time to adjust the movement to preserve stability, diminishes the magnitude of the internal perturbation after a critical stability period, and also allows the use of the reaching limb to distribute mass away from the pivot point to reduce acceleration of CoMgl. Moreover, a more constant velocity of the reaching limb after foot contact is less demanding in terms of motor control compared to the bell-curve velocity profile shown in the healthy group. Indeed, a bell-curve velocity around foot contact requires excellent muscle coordination and good movement planning.
Complementary Information From the Tested Reaching Directions
The present study confirms that each tested direction brings unique information with respect to segmental motor strategies. The non-redundancy was shown by the adaptability and differences in segmental motor strategies as a function of reaching direction along the vertical axis and in the horizontal plane. For example, the AM direction lowering, contrary to the PM direction, is better estimated by the pelvis lowering than the relative trunk lowering. This can be explained by the fact that the trunk can counteract the reaching limb less in the anterior direction by a smaller available range of motion at the hip and the back and by less powerful muscles controlling the movement in AM direction than in PM reaching against gravity. In fact, a recent study has shown differences in muscle activation patterns on the SEBT according to reaching directions.
Clinical Implications
From a clinical point of view, the performance during the SEBT has a significant meaning, as it reflects the motor control abilities during single leg stance, the sub-phase of walking and running where LAS or giving way usually occurs. Previous studies have shown bilateral decrease in reaching distance when executing the SEBT and bilateral modifications in the characteristics of CoP displacements, thereby supporting the hypothesis of central nervous system reorganization. However, the maximal reach distance and the measures of CoP displacement during single stance, the latter not being readily accessible to the clinician, do not provide any information on the nature of the dynamic control impairments. In the present study, we used kinematic data to describe the alterations in global and segmental strategies of persons with a LAS. Our findings not only give further support to the construct validity of the SEBT as a clinical measure of motor control, but they also indicate how alterations in motor control take place. They also provide visual cues that give insight on how movement should be retrained. As reaching with one limb while standing on the opposite foot may also be used as a challenging rehabilitation exercise, it might be wise for a clinician to ask the patient to reach for an object located on the lateral side of the stance foot to force a lateral displacement of the CoMgl towards the limits of the base of support. One could also stimulate the lowering of the CoMgl by asking the patient to further bend the lower limb while reaching as far as possible with the free limb.
Study Limitations
In the present study, several variables were measured using a fairly small sample size of participants, and this may have limited the possibility of showing significant differences in some global strategy variables, especially the ones in the horizontal plane where a high inter-participant variability and differences in foot length could have had an impact. Also, while we ensured that both groups had similar personal characteristics, participants in the two groups were not individually paired. Therefore, it was not possible to control for side of injury and limb dominance in our comparative analyses. As previously mentioned, considering the high proportion of LAS injuries to the dominant limb, it was decided to use the dominant limb of the participants in the healthy group for intergroup limb comparisons. By doing so, the differences in the uninjured limb of the LAS group may have been slightly overestimated. Finally, it is important to mention that the calculated global body CoM is an estimate of true body CoM. Anthropometric segmental components of the body of each participant were not incorporated into the calculation of body CoM but were taken from population averages. This could have led to a slight over- or under-estimation of CoM position, even if theoretically it should have affected both groups equally.
Source...