# Lower Limb Joint Coordination and Coordination Variability During Landing: A Scoping Review
## 4 Discussion  
This scoping review synthesized findings from 18 studies investigating lower limb joint coordination and variability during landing across various populations and task conditions. Key findings indicate that coordination strategies differ based on injury status, training exposure, sex, age, fatigue, and task complexity (among healthy individuals). Injured populations, such as individuals with ACL reconstruction or chronic ankle instability, displayed either overly rigid or highly variable movement patterns, respectively. Training interventions improved coordination stability, while fatigue disrupted it. Female athletes tended to show more constrained coordination than males, and children exhibited higher  
Figure 2: Visual summary of the effects of gender, injury, age, training, and fatigue on lower limb coordination and coordination variability during landing. variability and less refined strategies than adults. Across healthy populations, a moderate level of coordination variability, coupled with in-phase joint motion (both joints portray similar movement--see Table 3), emerged as a characteristic of adaptive and stable landing strategies. These findings collectively underscore the role of coordination analysis in identifying movement deficiencies and informing intervention design. **Coordination Strategies and Injury Risk:** A unifying observation across many studies is that individuals at elevated injury risk often display either unusually rigid coordination or overly variable coordination. When synthesizing coordination patterns across populations, clear distinctions emerge. ACL-reconstructed individuals tend to display rigid, low-variability coupling, particularly at the knee, likely as a protective strategy, whereas those with chronic ankle instability often exhibit overly variable, inconsistent coordination, especially at the ankle and its coupling with proximal joints. In contrast, healthy individuals show moderate variability and in-phase motion, particularly during double-leg landings, representing a more adaptive control strategy. These contrasts suggest that both extremes, rigid or erratic coordination, may reflect compromised neuromuscular control, albeit with different underlying mechanisms. Furthermore, whereas ACL-injured athletes often show reduced inter-limb symmetry, CAI individuals exhibit more intra-limb inconsistencies. This comparison helps define a \"healthy variability range\", reinforcing that both over-constrained and overly variable coordination can elevate injury risk, albeit through distinct pathways. From a dynamic systems perspective, both extremes can be suboptimal [37]. Female athletes and ACL-reconstructed individuals frequently showed rigid or low-variability coordination patterns (especially involving knee motions), whereas those with chronic ankle instability showed excessive variability and inconsistent couplings. These patterns likely represent different mechanisms of risk. A rigid coordination pattern (low variability) may fail to accommodate perturbations--if something unexpected happens during landing, a person who always uses the exact same joint strategy might not be able to adjust in time, leading to ligament overload (as hypothesized for women in cutting tasks [3]. On the other hand, very high coordination variability (as in CAI) suggests a lack of reliable motor control--the person cannot produce the same coordination twice, hinting at neuromuscular deficits that could cause unpredictable force distributions and possible injury in any given landing [18]. Notably, the healthy, trained populations seem to reside in the middle ground: they have a predominant coordination pattern (in-phase flexion synergy) but with enough variability to adapt to slight changes, and they exhibit neither stark rigidity nor uncontrolled randomness. This aligns with the concept of an optimal variability window for skilled movement [3; 10].