The primary findings of this study demonstrated that excellent movers possess more total lean body mass than poor movers, with subtotal lean mass normalized to body mass, as well as subtotal lean mass normalized to body mass and bone mineral content being significantly greater (p < 0.05); trunk and lower extremity lean mass normalized to body mass was also found to be significantly greater. While total percent body fat was not found to be significant, the variable showed a trend towards significance, with poor movers possessing more body fat than excellent movers. The significance of these variables likely plays a role in the expression of biomechanics variables such peak loading, asymmetry, and lower extremity lean mass.
This study also found that poor movers exhibited greater peak loading.
Individuals with poor movement profiles experienced greater peak vGRF normalized to subtotal lean mass and peak vGRF normalized to subtotal lean mass and body weight (p < 0.05). Contrary to our initial hypothesis there was no difference in loading symmetry between the movement profile groups (p > 0.05).
These findings support our hypotheses that excellent movers possess greater amounts of lean mass than poor movers (roughly 1.3 kg more), and that excellent movers demonstrate smaller peak loading values during the loading phase of a jump-landing task. There was also a strong trend towards significance for total percent body fat, with poor movers exhibiting more than excellent movers.
A study by Bell et al found that lean mass content significantly contributed to energy absorption during landing, stating that deficiencies in lower extremity lean mass
may reduce the ability to produce force and power while jumping.6 This study serves as a model that highlights the importance of lean mass and the impact of body composition on performance. Lean mass is an important indicator of performance and injury risk, since greater lean mass composition allows for better energy absorption during landing, thus reducing risk of injury. This can be explained by the fact that lean muscle plays a
significant role in impact forces, as muscle functions to dynamically dampen force, with all muscles playing a significant role in the management of peak vGRF.21 Another study by Montgomery et al also found that less lean mass may limit an individual’s ability to safely control their landing, which places them at an increased risk of injury. Likewise, Montgomery believed that stiffer biomechanics (ie poor movement profiles) could lead to more energy absorption by anatomical structures other than lean mass, such as bone and cartilage, thus placing the individual at an increased risk of injury.27 Murphy et al also determined that low muscle strength and muscle imbalances increase the risk of injury, with imbalance in athletes leading to higher rates of injury.29 Athletes with reduced quadriceps strength were more likely to sustain non-contact injuries.29 It can be inferred from these studies and our findings that increased lean mass plays a significant role in protecting an individual during the loading phase of a jump-landing task via energy absorption. This absorption by lean mass likely prevents surrounding structures from absorbing excessive force, and may reduce the risk of injury. This may also explain the difference in peak loading values between groups; poor movers with less lean mass likely subject their surrounding structures to greater force absorption.
While lean mass plays a significant role in performance, it is also important to note that various other factors likely contributed to the excellent movers’ overall
performance. For example, Myer et al found that neuromuscular training significantly improved performance and lower extremity biomechanics in female athletes. It would be useful to further study how neuromuscular training can benefit females that are at a higher risk of injury, since various studies have found that dynamic neuromuscular training can reduce differences in force absorption, active joint stabilization, muscle imbalances, and functional biomechanics.30 Neuromuscular training also has been shown to increase strength of structural tissues.30 Therefore, while our study demonstrated the importance of lean mass and its role in performance, other studies seem to conclude that neuromuscular control and training play a significant role in performance and injury prevention.
Loading asymmetry was not found to be significantly different between the two groups (p > 0.05). The literature states that loading asymmetry can lead to increased risk of injury. Greater loading asymmetry refers to a disparity in force distribution between limbs at impact; if there is asymmetry in the lower limb mass, then one limb is absorbing more vGRF than the other. A study by Bates et al concluded that an overloaded limb may be at an increased risk of injury.5 Asymmetry during a jump-landing task may also lead to performance deficits. However, our findings suggest that uninjured females, regardless of movement profile, do not seem to demonstrate a significant amount of asymmetry between limbs during such a task.
Limitations of this study may include slight variations in percent body fat and lean mass values as calculated by the DXA. Some subjects were reported to possess greater body fat percentage than they appeared to exhibit. Another limitation included imprecise frame values during data analysis of both initial contact and peak loading
during the jump-landing task. The software utilized frame values in increments of five, rather than exact increments of one. This could have an effect on the time between contact and peak load placed on joints during the task.
Further Study
Further research should be performed in order to continue to develop an
understanding of how these results can lead to injury and injury prevention. For example, studies that emphasize the impact of neuromuscular training on lean mass may prove useful, such as the one performed by Myer et al (2005). This study sought to determine whether neuromuscular training programs would be useful in altering movement profiles, thus improving one’s lean mass and reducing risk of injury.
While both groups demonstrated similar limb asymmetry magnitudes during the loading phase of the jump-landing task, the movement profile groups differed in the amount of impact load taken on. It is possible that the vGRF taken on may be the defining factor in injury risk as a result the ability of lean mass to dampen ground reaction forces. Further research may need to be done in order to determine whether asymmetry in lean mass is a contributing factor to overall performance and force absorption.
Conclusions
It appears that excellent movers expose their total mass to their peak vGRF, as do poor movers (raw vGRF data does not differ). However, upon normalizing vGRF data to respective body weight, we found a significant difference. These findings suggest that
excellent movers expose their mass to less force during a jump-landing task when compared with poor movers, whose peak loading was significantly higher. This could be due to the fact that excellent movers can generate more force due to their increased lean mass, and manage it more efficiently. In other words, excellent movers may have a greater capacity to mitigate forces that would otherwise be transferred to their joints (and potentially be damaging) during the loading phase of a jump-landing task as a result of their increased lean mass.