Cognitive Load Reaches Into the Drop Jump
A simple arithmetic task changed the biomechanics of a drop jump in healthy young adults. Under dual-task conditions, jump height fell, reactive strength index dropped, ground contact time lengthened, and vertical ground reaction force weakened across key parts of the movement. The pattern matters because the drop jump is often treated as a clean test of lower-limb power, yet this study shows how quickly attention can alter what the force plate records.
Investigators compared single-task drop jumps with jumps performed while participants solved simple arithmetic problems. Thirty-five healthy young adults completed both conditions in randomized order, allowing the same motor task to be examined with and without a concurrent cognitive demand. Nineteen force-time metrics spanning braking, propulsion, and landing were analyzed, alongside within-session reliability, Bayesian statistics, and statistical parametric mapping of the full force waveform. The design reached beyond a single jump-height readout and instead asked where, when, and how divided attention reshaped the movement.
One sentence captures the core finding: divided attention reduced force production during both the eccentric and concentric phases of the jump.
Braking and Propulsion Both Lost Force
The dual-task condition lowered jump height and reactive strength index, while ground contact time increased. Jump height fell with p < 0.001 and Hedges’ g = 1.40, and reactive strength index also fell with p < 0.001 and Hedges’ g = 1.04. Ground contact time rose modestly but significantly, with p = 0.025 and Hedges’ g = −0.57. The Bayesian results pointed in the same direction, with very strong evidence for the RSI effect and supportive evidence for the longer contact time.
The waveform analysis sharpened the picture. Vertical ground reaction force was significantly lower during 29.2–34.3% of ground contact in the braking phase and during 52.0–79.1% of ground contact in the propulsion phase. Those are not trivial windows. Braking is where the neuromuscular system stores elastic energy and prepares for reversal, while propulsion is where that stored energy must be converted into upward motion. When cognitive load interferes with either phase, the athlete has less opportunity to absorb load efficiently and less force available to rebound.
This is the kind of interference that can escape a coarse summary metric. A jump can still “look” like a jump while the underlying force profile deteriorates in ways that matter for power expression and movement quality.
Landing Mechanics Shifted Without Uniform Instability
The landing phase also changed, but not in a simple monotonic way. Vertical ground reaction force was lower during 1.3–6.3% of landing, when force was first rising after contact, and higher during 15.5–19.6%, when impact force was being attenuated. In plain terms, the loading curve was reshaped rather than uniformly amplified or uniformly blunted. The task did not just suppress output; it altered how force was distributed across the earliest moments after touchdown.
That profile is biologically plausible. A concurrent cognitive task can compete for attentional resources needed for feedforward motor preparation, and the drop jump depends heavily on anticipatory control before the feet even hit the ground. Once landing begins, the nervous system must rapidly coordinate joint stiffness, reflex responses, and voluntary adjustments. If attention is partly occupied by arithmetic, the timing and magnitude of these responses can shift enough to change the force waveform.
The study sits close to prior dual-task work in jump-landing mechanics, but it adds a broader phase-specific view. Earlier studies often focused on overall jump height, early landing intervals, or selected joint variables. Here, the combination of discrete metrics and waveform analysis shows that the interference stretches from braking into propulsion and into the early landing sequence.
Most Metrics Stayed Reliable Despite the Cognitive Challenge
Reliability did not collapse under divided attention. For three-trial mean scores, 17 of the 19 metrics showed within-session intraclass correlation coefficients between 0.812 and 0.930 across both conditions. In practical terms, most measures remained stable enough within a session to support interpretation, even when cognitive load was added. The study therefore separates two issues that are often conflated: whether a metric is reliable, and whether it is resistant to cognitive interference.
A useful implication follows from that split. A metric can remain reproducible and still be sensitive to attentional state. That is exactly what happened here. The force-time measures were generally reliable, yet several of the key outputs still shifted when participants were asked to solve arithmetic problems at the same time.
The study also tested two variables linked to dynamic postural control after landing: spring-like correlation and vertical time to stabilization. Neither differed significantly between conditions, and the Bayesian analysis offered weak to moderate evidence for the absence of an effect. Those null findings matter because they suggest that not every post-landing control measure is equally vulnerable to the same cognitive load. Some aspects of stabilization may be harder to perturb, or the chosen task may have been too mild to destabilize them in healthy young adults.
What This Means for Athlete Testing
The paper’s central warning is simple: standardized physical testing is not always cognitively standardized. In sport, athletes rarely execute explosive movements in silence and without decision-making demands. They are tracking opponents, reading space, and processing instructions while they move. A laboratory jump performed under single-task conditions may therefore overestimate the force output or underrepresent the attentional cost of competition-like movement.
That has consequences for athlete monitoring and readiness assessment. If a coach compares jump metrics across sessions without controlling the cognitive context, apparent changes may reflect attention as much as neuromuscular status. If the same athlete performs one test while mentally fresh and another while solving arithmetic or making tactical decisions, the force plate no longer measures only the lower limb.
The study also argues for a broader use of waveform analysis in sports biomechanics. Scalar outcomes such as jump height and reactive strength index are useful, but they compress the movement into a single number. Continuous analysis of the force trace can reveal phase-specific changes that align more closely with motor control and elastic energy use. In this case, the waveform showed where the cognitive task intruded: early braking, late braking, mid-propulsion, and the first moments of landing.
The sample was healthy and young, the cognitive task was arithmetic, and the experiment was confined to a within-session design. Those features define the boundary of the evidence. What remains unresolved is how larger or more sport-specific cognitive loads would affect trained athletes, injured athletes, or repeated testing across days. The present data still draw a clear line: adding even a simple mental task can measurably alter the mechanics of a drop jump, and the change appears where force generation and rapid control are most vulnerable.