Coordination and Symmetry Patterns During the Drop Vertical Jump in People With Chronic Ankle Instability and Lateral Ankle Sprain Copers

Discussion

To our knowledge, this is the first biomechanical analysis to detect movement and motor control pattern disparities between a group with CAI and LAS copers during a DVJ task. We analyzed the DVJ in separate phases in recognition of the dissimilar constraints imposed by the drop jump (phase 1) and drop landing (phase 2) components of this task.

During phase 1, the CAI group displayed increased preparatory or pre-IC hip flexion on their involved limb and a greater flexion moment pattern at this joint on their uninvolved limb ≃90 milliseconds post-IC compared with LAS copers. During phase 2, CAI participants again displayed an increase in hip flexion prior to and during IC. However, this increase was evident bilaterally in contrast to phase 1. Furthermore, they displayed greater extensor dominance at the hip ≃60 milliseconds post-IC on their uninvolved limb compared with the matched limb of LAS copers. These results are largely in agreement with our hypotheses, which were based on previously documented observations made on these groups earlier in the pathological process.^13,19 Finally, because there was no difference in the jump height achieved by the CAI group compared with the LAS coper group, we believe the comparison of the underlying energetics and kinematics in a process-oriented analysis between these groups to be appropriate.⁴⁰

An important consideration in the interpretation of these results is that the best method of analyzing temporal data is a continued source of controversy in the movement sciences.⁴¹ Time series data have natural temporal ordering, and this temporal dependence has previously been considered a primary challenge in movement analysis.⁴² Outside of nonlinear dynamic analyses (which are ideally suited to cyclical movement patterns)⁴³ to evaluate human movement, researchers can: (1) analyze discrete values (which are assumed to be representative of entire waveforms) with univariate statistics; (2) convert temporal data into discrete values via a principal components analysis, with subsequent utilization of univariate statistics; (3) adopt a multivariate statistical approach, such as parametric mapping or functional analysis of variance, using a differential equation to represent the curve; and (4) use multiple discrete statistical comparisons for multiple (temporal) data points. In the current study, we did not undertake a multivariate approach (option 3 above) due to the inaccuracy that a differential equation may have in characterizing an irregularly shaped curve. Our chosen method analysis (option 4 above) is potentially prone to an increasing risk of family-wise error due to the number of comparisons that have been made (each data point for the waveform was subjected to a t test). Therefore, our findings need to be considered in the context of the methods of analysis that produced them and those of other studies.

Two previously published studies are available that detail an evaluation of participants with chronicity following LAS injury in comparison with copers during a “stop-jump” task,^44,45 although this analysis was completed on their involved limb only. The stop-jump task is comparable to the DVJ task because both tasks possess energy amplification (phase 1) and attenuation (phase 2) stages and require the participant to complete the task by landing from a maximal vertical jump in a controlled manner.^8,44,45 Although temporal waveform data were not plotted for movement and motor control variables in these studies and only phase 1 of the stop-jump task was evaluated,^44,45 one of the presented findings is comparable and consistent with those of the current study: CAI group participants adopted a position of increased hip flexion at IC.⁴⁴ That they also displayed greater external rotation at the hip⁴⁴ was in contrast to our findings, as no such pattern was evident. These 2 studies^44,45 were among the first studies to use an LAS coper sample as a comparison group for the CAI group participants.

More recently, an International Ankle Consortium consensus statement has been published outlining the ideal experimental definition of CAI cohorts.¹² Participants with CAI in the aforementioned studies were actually grouped according to whether they had predominantly mechanical or functional insufficiency components of the heterogeneous CAI condition,^44,45 which is contrary to the subsequently published methodological guidelines.¹² In contrast, because the current analysis is part of a larger longitudinal investigation, both LAS coper and CAI groups' outcomes were only recently defined and confirmed, whereas the participants in the studies by Brown and colleagues were long established as being chronically impaired.^44,45 This duration of disability may have as yet unknown implications on the extent of their anomalous patterns of neuromuscular control. As such, and due to the differences in the nature of the prescribed task (and the methods of analysis used to quantitate this task), caution must be placed in the direct comparison of our findings with those of Brown and colleagues.^44,45

The moment-of-force profiles presented as part of the present analysis give an important indication of the loading associated with the development of chronic injury.^46,47 These profiles are of particular relevance because they identify the forces of amplification and attenuation required for the relevant phases of the DVJ. That between-group differences were only evident on the uninvolved limb highlights the potential capacity for people with CAI to exhibit bilateral deficits in neuromuscular control^48,49 and the importance of performing a bilateral experimental analysis. Indeed, the label of “uninvolved limb” is a misnomer on the basis of the current findings and perhaps should be replaced with “contralateral limb.”

The reason predicating the increased flexor and extensor patterns during phases 1 and 2 of the DVJ, respectively, may have links with off-loading strategies identified in these individuals in the acute phase of injury.^13,19 When these participants were evaluated in comparison with a noninjured control group within 2 weeks of sustaining their LAS, they exhibited greater extension dominance on their uninvolved limb joints compared with their involved limb during both phases of the DVJ. These motor patterns were coupled with RIM asymmetries when this cohort of participants were grouped together and compared with a noninjured control group. At both the 2-week and 6-month assessments that preceded the current report, participants with LAS exhibited significantly greater RIM asymmetry (approximately 13% compared with 5% in controls), with their uninvolved limb being subjected to significantly greater force per unit of time.^13,19 This finding led us to theorize that people who are early in the recovery process re-weight dependence from the injured limb onto the contralateral limb to fulfill the energy demands of the task.^14,15 On the basis of the current findings, this asymmetry persists (both groups displayed 10% asymmetry) but is not likely to predicate outcome (CAI or LAS coper) because no between-group differences were manifested.

As we previously alluded to, the increased flexor moment pattern on the contralateral hip during phase 1 of the present study may represent the persistence of a re-weighting strategy whereby this limb is “relied on” to produce the flexor pattern that precedes the extension required to initiate phase 2. However, because there was neither a between-group difference in the extensor pattern subsequent to this flexion moment nor any difference in the jump height achieved during the task, the product of this motor control process is not immediately evident. Similarly, during phase 2 of the DVJ, the increase in contralateral hip extensor pattern relative to that of LAS copers may represent an off-loading strategy to re-weight force attenuation on the limb not affected by the initial LAS and may have its roots in the motor control patterns that emerged immediately postinjury. The lack of a between-group difference in RIM or RIM symmetry makes it difficult to be conclusive regarding the value of this strategy.

Despite the exploratory nature of this experiment and the large number of resultant dependent variables in the current analysis, it is interesting to note that only at the hip joint did between group differences emerge. The crucial role that the hip plays in the combined dissipation of impact forces while preventing collapse of the lower extremity during landing is well established.^50–52 This role is illustrated in the present study by the sinusoidal trajectory of the hip's moment-of-force profile following ground contact during either phase of the DVJ; an initial extensor pattern is closely followed by a flexor pattern in the fulfillment of these combined roles. Similar to the findings of Brown et al⁴⁴ during the stop-jump task, the CAI group participants displayed greater hip flexion during phase 1 of the DVJ on their previously injured limb. Analysis of phase 2 results revealed similar patterns of increased hip flexion for this limb, in addition to the contralateral limb. In both cases, the increase in hip flexion began prior to IC, which suggests that this movement pattern is part of a preparatory strategy adopted by CAI participants to fulfill task constraints.^53,54 Hip muscle activation onset patterns and weakness of the hip musculature have previously been implicated in the presence of CAI.^55,56 This finding, in combination with the findings of the current study, is of particular relevance to clinicians, who must avoid the development of rehabilitation programs without consideration of neuromuscular control as a global concept. Either weakness or a change in activation patterns in the hip musculature may produce deviations in joint motion, which have the capacity to manifest elsewhere in the kinetic chain,²² including the contralateral side.⁵⁵ Should these deficits persist, the motor apparatus may enter a cycle of injury recurrence belied by anomalous neuromuscular control. On the basis of the current findings and those of Brown et al,⁴⁴ it appears that the hip may play an important role in the development of CAI; therefore, clinicians must consider the potential value of designing rehabilitation programs with joints proximal to the injured ankle in mind. It is likely rehabilitative success would be potentiated by a program that includes the entire kinetic chain, which may drive patients toward a coper path rather than a CAI path.

The issue, however, is that the accuracy of such speculations cannot be confirmed on the basis of the present research alone due to its design. Although this study was part of a longitudinal analysis designed to identify the sensorimotor predictors of CAI, the importance of the observed deficits remains unclear. Despite the consistency of the current results compared with our previous findings of the same cohort in the acute phase of injury¹³ and 6 months following this phase,¹⁹ caution must be placed in declaring their relevance to the development of CAI. That all of the reported deficits were observed in a single cohort of participants also threatens the generalizability of our results; however, as we have noted, the observed deficits are consistent with other CAI cohorts completing similar tasks.^44,45 A further limitation of the present study is that there was potential for statistical error, as the cohort was recruited at convenience with no a priori sample size calculation being undertaken. However, a post hoc power analysis of discretized kinematic and kinetic variables at the point of the greatest observed between-group difference revealed that we had strong observed power (values for observed power ranged from 0.8 to 0.9). In contrast, our nonsignificant findings were associated with low-to-moderate statistical power (values for observed power ranged from 0.15 to 0.5, with a mean value of 0.33 and a median value of 0.34). A post hoc sample size calculation based on the difference in mean and standard deviation values for the RIM variable revealed that we would have needed a minimum of 250 participants to achieve a level of power of 0.80. We believe that it would be difficult and unrealistic to recruit this number of participants. It is important to note, however, that the nonsignificant differences were associated with small effect sizes, indicating that these relationships are not likely to be clinically important.