Full Length ArticleForce-displacement differences in the lower extremities of young healthy adults between drop jumps and drop landings
Graphical abstract
Introduction
Human lower extremities are called upon to fulfil different roles; acting as springs, as force absorbing dampers, or as actuators (Raynor, Yi, Abernethy, & Jong, 2002) depending upon the goal of the task required, as well as external conditions. Biomechanical outputs such as end effector position, joint torque, or ground reaction force have many contributing factors. These factors include neural input to muscle fibers, mechanical properties of muscle, connective tissues and bone, and other mechanical variables, such as limb segment or interaction torques. Historically, Bernstein (1967) proposed that since there is such multiplicity of inputs which lead to eventual outputs (such as joint torque or ground reaction force), and the effects of those inputs vary depending upon intrinsic and extrinsic conditions; that the neural load of attempting to control each of those inputs through a feed-forward control system would be too great to allow for efficient movement. Therefore, he proposed that those inputs are adjusted based upon neural information of output (Bongaardt and Meijer, 2007, Zajac and Winters, 1990). This conclusion has been supported by subsequent research in neural control (Ito, 1996). It is therefore reasonable to suspect that the biomechanical goal of a lower extremity task; whether that goal be force storage and return, force damping, or force creation, will be reflected in the behavior of output variables measured during the performance of that task.
One category of motor task with the goal of force storage and return are those tasks in which the lower extremities (LEs) are called upon, individually or bilaterally, to act as “linear springs”. Foundational work in developing mathematical descriptions of the linear spring behavior in running and hopping was published by Blickhan (1989) and by McMahon and Cheng (1990). In both of these works, the mechanical behavior predicted through the researchers’ mathematical models coincided with what was empirically observed with humans running or hopping. Tasks requiring spring behavior of the LEs typically include high velocity bending of the hip, knee, ankle, and mid-foot (Farley et al., 1998, Ferris et al., 1998, Moritz and Farley, 2005, Moritz et al., 2004) such as running, hopping, and jumping. Each of these is an activity in which the primary biomechanical goal of the LE’s is to store the force of the landing in order to release it for immediate recoil (Farley et al., 1998). A common task used for researching this linear spring behavior is the “drop jump”, in which an individual hops from an elevated surface and immediately rebounds vertically upon landing (Ambegaonkar and Shultz, 2011, Earl et al., 2007, Myer et al., 2005). This is sometimes called a “countermovement jump” (Bobbert, Huijing, & van Ingen Schenau, 1987)
“Drop landings” may exemplify tasks with a different biomechanical goal than drop jumps (Ambegaonkar and Shultz, 2011, Earl et al., 2007, Myer et al., 2005). While researchers have shown that in drop jumps, the point of greatest force and greatest LE shortening, measured from the ground to the body’s center of mass (through bending of the trunk, hips, knees, ankles, and mid-feet) is simultaneous, except for some variability between trials and individuals (Farley et al., 1998, Ferris et al., 1998, Moritz and Farley, 2005, Moritz et al., 2004); in studies investigating the biomechanical behavior of the LEs in drop landings, researchers have concluded that during drop landings, rather than acting as springs, the LEs act as force dampers, absorbing rather than storing and returning the force of the landing (Kulas et al., 2006, Minetti et al., 1998, Puddle and Maulder, 2013). Examples of this type of task include a gymnast “sticking” a landing after a vault, a basketball player landing after catching a rebound, or a ballet dancer landing after performing a tour en l’air (Kulas et al., 2006).
In our literature review, we found few studies comparing drop jump to drop landings within subjects. One study which did the comparison was described by Ambegaonkar et al. (2011). They showed higher gastrocnemius and quadriceps post-landing amplitudes as measured by rectified electromyogram (EMG), and higher ground reaction forces in drop jumps compared to drop landings. These findings reflect the subjects’ requirement for greater knee and ankle muscle stiffness when the biomechanical goal is force storage and return. We found no studies described in the literature specifically comparing the sequence of maximum LE shortening relative to maximum ground reaction force between drop jumps and drop landings. A better understanding of how healthy humans adapt to different types of landings could benefit the physically active population by potentially shaping training methods and the design of force absorption products, such as athletic shoes and floors, to increase safety in activities including exercise, sports and dance (Dyhre-Poulsen et al., 1991, Ferris et al., 1998, Hackney et al., 2011, Hackney et al., 2011, Myer et al., 2005).
We hypothesized that for young, healthy adults during drop jumps, maximum ground reaction force and LE shortening would occur nearly simultaneously, as long as the countermovement jump is of amplitude sufficient that the take-off force exceeds the force of heel strike (Ball, Stock, & Scurr, 2010). By contrast, for drop landings, the point of maximum force, which is the result of heel contact (Fukano et al., 2014, Seegmiller and McCaw, 2003), would occur measurably earlier than maximum LE shortening. We propose that this difference in this behavior of the LEs during landing reflects the contrasting biomechanical goals of the two tasks; force storage and return in drop jumps, and force damping, or absorption, in the drop landings (Ambegaonkar and Shultz, 2011, Hackney et al., 2011, Leukel et al., 2012, Minetti et al., 1998, Puddle and Maulder, 2013).
Section snippets
Subjects
The participants were 10 healthy young adults, (five men and five women), mean age = 24.4 ± 1.8 years, mean body mass = 74.8 ± 20.5 kg. (Data from 13 subjects were initially analyzed, but the data of three subjects were unusable due to significant differences between landing types for heights of their hops off of the platform. The criterion by which the data of those three subjects were excluded is described in detail in the ‘Data Reduction and Analysis’ section). Exclusion criteria were used both to
Results
The mean %dFmax for the drop jump trials for all participants whose data were included in the study was 73% ± 14%, and 47% ± 09% for the drop landing trials. In both conditions, and for all participants, the displacement was rising at the time of peak force. The mean within subject difference score between drop jump and drop landing was 26% ± 20%. This is to say that on average, the temporal point of maximum LE shortening in drop landings followed the point of maximum force 26% later than it did for
Discussion
For all ten participants, the %dFmax were greater in drop jumps than in drop landings. These findings indicate that in drop jumps, the point of maximum force and the point of maximum shortening occurred nearly simultaneously. In drop landings, the point of maximum LE shortening followed that of maximum force by a greater proportion. This supports our hypothesis that in healthy, young adults, in contrast to drop jumps; during the drop landings, the temporal point of greatest force occurs early
Conclusion
Our findings show that peak ground reaction force and peak LE linear displacement are much closer to simultaneous in drop jump, compared to drop landing, in which the peak LE displacement tends to follow peak force, in healthy, young adults. Our explanation for this is as follows; for drop jumps, the peak force was generated by the recoil of stored force from the muscles and connective tissues of the LE in order to generate the take-off force for the countermovement jump. In drop landing, the
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