The Influence of Concussion History and Progressively Increasing Cognitive Load on Jump Landing and Cutting Reaction Time, Biomechanics, and Task Demands

Participants

Participants were included if they self-reported being physically active for at least 90 minutes per week and were 18 to 30 years old. Concussion history was self-reported and collected with the National Institutes of Health common data element form.²¹ A concussion was defined as a “traumatic brain injury caused by a direct blow to the head, neck or body resulting in an impulsive force being transmitted to the brain” per the 6th International Consensus Statement.²² All mechanisms were included.

Concussion history participants were excluded if they self-reported being admitted to the hospital postconcussion or reported ≥13 symptom severity on the Sport Concussion Assessment Tool symptom inventory.²³ To mitigate the risk of participants with persistent symptoms (ie, postconcussion syndrome) the cutoff of ≥13 was chosen based on normative data that athletes report an approximately 3-symptom severity when not concussed (eg, baseline, preseason).²³ The reliable change parameter (ie, the cut point for determining clinical impairment) was 10 from the same study.²³ Control group participants were excluded if they self-reported experiencing a concussion in their lifetime. All participants were excluded if they self-reported having attention-deficit/hyperactivity disorder, attention deficit disorder, uncorrected vision problems, history of neurological disease or seizures, structural brain lesions (eg, stroke), currently using antidepressants, currently experiencing a high fever, or undergoing immunosuppressive therapy. We did not exclude individuals based on MSKI history; however, we did exclude if participants were currently experiencing an MSKI.

Groups were matched by age (±2 years), sex, and body mass index (±2 kg/m²). One of the concussion history participants did not feel comfortable performing the cutting task with his or her current style of shoes and is only included in the jump landing analyses. The matched control was also removed from the cutting analysis.

Demographics

Demographics included age, sex, height, mass, the Sport Concussion Assessment Tool symptom inventory, the Godin Leisure Activity Questionnaire, the Lower Extremity Functional Scale (a self-reported MSKI form), and the dominant limb.^24–26 The self-reported MSKI form was adapted from prior research.¹⁴ The original form asked participants to report whether they had sustained any MSKI to the lower extremity or back within the past 5 years.¹⁴ We asked participants to report traumatic injuries across their lifetime (eg, any fractures, muscle tears, ligament tears). Dominant limb was defined as which limb participants preferred to kick a soccer ball for distance.

The Tampa Scale of Kinesiophobia-11 (TSK-11) was also collected but after jump landing and cutting because it was used in the larger 2-day study.^28,29 The TSK-11 is reported as a demographic variable and has shown good to excellent test-retest reliability (intraclass correlation coefficient [ICC] = 0.87) and convergent validity (r = 0.60).^28,29

Single-Task Serial Subtraction

Participants performed a baseline single-task serial subtraction by 3s and 7s from a random integer between 90 and 200.³⁰ Trials lasted 20 seconds. One practice trial and 3 recorded trials were completed per condition (6 total trials). Participants were instructed to subtract as quickly and accurately as possible. Participants were audio-recorded to be scored offline.

Serial 7s and 3s were chosen because researchers have shown no differences in gait biomechanics between the commonly used assessments of serial subtraction, spelling words backward, and reciting months in reverse.¹⁸ However, serial 7s has been proven to be more difficult than serial 3s during single-task conditions.³⁰ Since the most recent Sport Concussion Assessment Tool 6 only requires serial subtraction during gait for dual-task conditions, we opted for compared serial 3s and 7s.³¹

Single- and Dual-Task Jump Landing and Cut

A 30-cm box was placed half the participant’s height away from 2 force plates.³² Participants stood on the box and assumed an athletic position when the researcher said, “Get set.”²⁷ An audible buzzer was randomly played 2 to 5 seconds after the “get set” cue. The buzzer sound cued the participant to jump forward toward 2 force plates embedded in the ground. Participants were instructed to react as quickly as possible. For the jump landing, participants landed with both feet simultaneously on the 2 force plates (1 foot per plate). Immediately upon landing, participants jumped straight up into the air as high as possible. For the cut, participants only used their nondominant limbs. Participants were asked to land on their nondominant limb and cut 45° in the opposite direction. All participants were given at least 1 practice trial for all conditions and continued to practice until they reported feeling comfortable.

For dual-task conditions, participants performed serial subtraction by either 3s or 7s starting from an integer between 90 to 200. Participants were instructed to subtract as quickly and accurately as possible and to perform the motor task to the best of their abilities. Participants started counting while on the box, continued counting when performing the movement, and continued to count until the researcher said, “Stop.” The research team let the participant complete or attempt approximately 2 to 3 words after landing from the jump landing and running after the cut before saying, “Stop.”

The jump landing and cutting tasks were block randomized such that all conditions of the jump landing were completed before moving on to the cutting or vice versa. Single-task conditions were always completed first. Serial 3s and serial 7s were randomized after single-task conditions. All trials of 1 condition were completed before moving on to the next condition. Three jump landings and 3 cuts off the nondominant limb were collected for each cognitive condition (9 jump landing trials; 9 cut trials). Failed trials were discarded and repeated. Failed trials included stepping off the box before the light, not landing with the whole foot inside the force plate, not landing with both feet simultaneously on the force plates (jump landing only), not sprinting to the cones during the cut, clear and obvious avoidance of the serial subtraction, and forgetting to continue subtracting after landing or while sprinting to the cones.

National Aeronautics and Space Association Task Load Index

The National Aeronautics and Space Association (NASA) Task Load Index was administered after each of the 6 conditions. For example, the NASA Task Load Index was administered immediately after completion of the jump landing with no counting, jump landing with serial 3s, and jump landing with serial 7s.

Data Reduction and Processing

Cognitive and dual-task effect calculations are described in Table 1. During jump landing and cutting, the serial subtraction duration is between the start of counting and when the research team said, “Stop.” Dual-task effect was calculated as described in Equation 1. Negative dual-task effect represents worse performance during the dual-task versus single-task condition (ie, dual-task cost), and a positive dual-task effect represents better performance during the dual-task versus single-task condition (ie, dual-task benefit)³³: $$\left(\frac{\left[\text{dual-task performance}\right]-\left[\text{single-task performance}\right]}{\left[\text{single-task performance}\right]}\right)\times 100,$$(1) $$\left(\frac{\text{Correct Responses}}{\text{Trial Time}}\right),$$(2) $$\left(\frac{\text{Correct Responses}}{\text{Attempts}}\right)\times 100.$$(3)

Table 1.Cognitive and Spatiotemporal Outcomes of Interest Collected During Single- and Dual-Task Conditions

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For jump landing and cutting, retroreflective markers (approximately 14 mm in diameter) were placed bilaterally on the acromioclavicular joints, iliac crests, greater trochanters, and anterior superior iliac spines. Additionally, markers were placed bilaterally on the medial and lateral femoral epicondyles, medial and lateral malleoli, calcaneus, fifth metatarsal, and second metatarsal of both limbs. A marker was also placed on the sternal notch.⁷ A cluster of noncolinear markers (3/4 markers per cluster) was placed on the posterior superior iliac spines and sacral body and placed bilaterally on the nondominant thigh, shank, and foot.⁷ Marker position data were sampled at 240 Hz with a 10-camera Qualisys motion capture system (MIQUIS; Qualisys Systems). Force plate data were sampled at 2400 Hz with 2 Bertec force plates.

Raw marker position data and force data were exported to Visual 3D software (C-Motion Inc) for analysis. All data (marker and force) were processed with a fourth-order, low-pass Butterworth at 10 Hz like in prior research.^27,34,35 Marker and force data were filtered at the same frequency to reduce joint moment artefacts.^36,37 The anterior and posterior superior iliac spines defined the pelvis, hip joint centers were estimated using the Bell Method, knee joint centers were estimated using the midpoint between medial and lateral femoral epicondyles, and ankle joint centers were estimated using the midpoint between the medial and lateral malleoli. Euler/Cardan angles (YXZ rotation sequence) were used to calculate the hip, knee, and ankle angles. Hip motions were defined as the thigh relative to the pelvis, knee motions were defined as the shank relative to the thigh, and ankle motions were defined as the foot relative to the shank. Trunk motion was defined relative to the lab (absolute angles). Rotation about the y, x, and z axes was defined as flexion/extension, abduction/adduction (trunk lateral bending), and internal/external rotation, respectively. Outcomes of interest were calculated during the eccentric portion of the task (initial ground contact [when vertical ground reaction force exceeded 10 N] to the lowest point of the center of mass).

Reaction time was calculated as the participant’s first movement after the audible buzzer sound. First movement was when the sacral body marker moved more than 3 cm in either the sagittal or transverse plane relative to the mean marker position for 0.5 seconds during the “get set” phase before movement.³⁸

Biomechanics variables of interest included reaction time, vertical ground reaction force, vertical loading rate (first derivative of the vertical ground reaction force slope), trunk flexion angle, trunk lateral bending angle, hip flexion angle, hip adduction angle, knee flexion angle, knee abduction angle, external knee flexion moment, external knee abduction moment, and ankle dorsiflexion angle.²⁷ Vertical ground reaction force and vertical loading rate were normalized to body weight (BW) and BW per second (BW/s), respectively. All joint moments were calculated with standard inverse dynamics, resolved in the proximal segment coordinate system, and normalized to a product of BW and height (BW × HT).

For the cut, trunk lateral bending was calculated as the largest displacement toward (away from straight up and down [0°]) the nondominant (planted) limb.²⁷ Since jump landing is primarily a sagittal plane movement, we did not calculate trunk lateral bending. We only performed the analysis for the nondominant limb because prior researchers have shown the nondominant limb to be most influenced by concussion history.^27,39

Statistical Analysis

An α level of ≤.05 was established a priori. Age, height, mass, and body mass index were compared between groups with independent samples t tests and Hedge g effect sizes. The Godin Leisure Activity Questionnaire, Lower Extremity Functional Scale, Sport Concussion Assessment Tool symptom inventory (total symptoms and symptom severity), and TSK-11 were compared with Mann-Whitney U tests and Cliff’s δ effect size. Dominant limb (right/left), sex (male/female), and traumatic injury history (yes/no) were compared between groups with a Fisher exact test and odds ratio effect size. Response rate and response accuracy during single-task serial subtraction were compared with a 2 (group [concussion, no concussion]) × 2 (cognitive load [serial 3s, serial 7s]) mixed-model analyses of variance.

For Aim 1, we used separate 2 (group [concussion, no concussion]) × 3 (cognitive load [no counting, serial 3s, serial 7s]) mixed-model analyses of covariance for each biomechanics outcome and task (jump landing, cut). We covaried for mean-centered months since the most recent concussion.^27,40 For Aim 2, we used separate 2 (group [concussion, no concussion]) × 2 (dual-task cost [serial 3s, serial 7s]) mixed-model analyses of variance for dual-task cost cognitive outcomes during jump landing and cutting. For Aim 3, we used a 2 (group [concussion, no concussion]) × 3 (cognitive load [no counting, serial 3s, serial 7s]) mixed-model analysis of variance to compare NASA Task Load Index scores for jump landing and cutting.

We included the mean center months since the most recent concussion as a covariate in Aim 1 analysis because time since concussion has been shown to influence landing biomechanics.⁴¹ No significant correlation was found between months since the most recent concussion and dual-task cost outcomes for jump landing or cutting (P range = .093–.993). No relationship with single- or dual-task NASA Task Load Index scores was found for either jump landing or cutting (P range = .405–.907; Supplemental Table 1). Serial 3s percent correct dual-task cost during jump landing significantly correlated with months since the most recent concussion (r = −0.509, P = .015), but including mean-centered months since the most recent concussion in the analysis did not alter the results.

Greenhouse-Geiser corrections for sphericity were used when needed. When post hoc testing was necessary for interactions and main effects (eg, cognitive load), we used false discovery rate corrections. Partial η² (η_p²) effect sizes were used for analyses of variance and covariance and were interpreted as small (≤0.06), medium (0.06–0.13), and large (≥0.14). Hedge g effect size was interpreted as small (<0.50), medium (0.50–0.80), and large (>0.80). Cliff’s δ was interpreted as small (≤0.33), medium (0.34–0.47), and large (≥0.48).

Aim 1: Single- and Dual-Task Biomechanics

The ICC_3,k’s (95% CI) for the jump landing reaction time for the no counting and serial 7s conditions were 0.870 (0.779, 0.924) and 0.824 (0.700, 0.897), respectively, indicating good test-retest reliability.

For the jump landing reaction time, no significant group × cognitive load interactions (F = 0.47, P = .592, η_p² = 0.011) or group main effects (F = 2.68, P = .109, η_p² = 0.059) were found, but a significant cognitive load main effect was found (F = 36.05, P < .001, η_p² = 0.456; Table 4; Supplemental Table 2). All participants had a faster reaction time during the no counting than the serial 3s (mean difference [95% CI] = 0.18 [0.12, 0.24] seconds, P < .001, Hedge g = 1.187) and serial 7s (mean difference [95% CI] = 0.18 [0.14, 0.22] seconds, P < .001, Hedge g = 1.526) conditions. No reaction time difference was found between serial 3s and serial 7s (mean difference [95% CI] < 0.01 [−0.04, 0.05] seconds, P = .924, Hedge g = 0.011).

Table 4.Jump Landing Outcomes Compared Between Group and Cognitive Load (Covariate Adjusted Means [95% Confidence Intervals])

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For all other jump landing biomechanics variables, no significant group × cognitive load interactions (P range = .092–.992), group main effects (P range = .109–.656), or cognitive load main effects (P range = .276–.940; Table 4; Supplemental Table 2) were found.

For the cut reaction time, no significant group × cognitive load interactions (F = 0.54, P = .575, η_p² = 0.014) or group main effects (F = 2.18, P = .148, η_p² = 0.053) were found, but a significant cognitive load main effect was found (F = 38.06, P < .001, η_p² = 0.494; Table 5; Supplemental Table 3). Specifically, all participants had a faster reaction time during the no counting than the serial 3s (mean difference [95% CI] = 0.11 [0.07, 0.14] seconds, P < .001, Hedge g = 0.910) and serial 7s (mean difference [95% CI] = 0.15 [0.10, 0.19] seconds, P < .001, Hedge g = 1.261) conditions. Additionally, participants displayed significantly faster reaction time during serial 3s than serial 7s (mean difference [95% CI] = 0.04 [0.01, 0.08] seconds, P = .002, Hedge g = 0.319).

Table 5.Cutting Outcomes Compared Between Group and Cognitive Load (Covariate Adjusted Means [95% Confidence Intervals])

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For the cut vertical loading rate, no significant group × cognitive load interactions (F = 2.93, P = .061, η_p² = 0.067) or group main effects (F = 2.26, P = .141, η_p² = 0.052) were found, but a significant cognitive load main effect was found (F = 7.83, P < .001, η_p² = 0.160; Table 5; Supplemental Table 3). Specifically, all participants displayed a lesser vertical loading rate during the no counting than the serial 3s (mean difference [95% CI] = 2.96 [0.34, 5.58] BW/s, P = .011, Hedge g = 0.220) and serial 7s (mean difference [95% CI] = 4.15 [1.44, 6.86] BW/s, P = .001, Hedge g = 0.328) conditions. No difference between serial 3s and serial 7s was found (mean difference [95% CI] = 1.19 [−1.10, 3.49] BW/s, P = .202, Hedge g = 0.088).

No other significant group × cognitive load interactions (P range = .061–.960), group main effects (P range = .122–.974), or cognitive load main effects (P range = .111–.981) were found for the cutting task.

Aim 2: Dual-Task Effect Cognitive Outcomes

All participants, on average, had improved cognitive performance during dual-task (serial subtraction while jumping or cutting) versus single-task (baseline serial subtraction) conditions, resulting in a dual-task benefit and not a dual-task cost. For the dual-task effect response rate during jump landing, no group × cognitive load interactions (F = 1.43, P = .238, η_p² = 0.032) or group main effects (F = 0.09, P = .763, η_p² = 0.002) were found, but a significant cognitive load main effect was found (F = 11.70, P = .001, η_p² = 0.214). Interestingly, a smaller dual-task benefit response rate was found for serial 3s than serial 7s (mean difference [95% CI] = 21.29 [8.74, 33.85], P = .001, Hedge g = 1.179; Figure 1A), indicating that the response rate increased during the more difficult counting task in a dual-task context.

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For the response accuracy during jump landing, no significant group × cognitive load interactions (F = 1.15, P = .289, η_p² = 0.026), group main effects (F = 0.46, P = .503, η_p² = 0.010), or cognitive load main effects (F = 2.54, P = .118, η_p² = 0.056) were found.

For the response rate during cutting, no significant group × cognitive load interactions (F = 3.95, P = .053, η_p² = 0.086) or group main effects (F = 1.58, P = .216, η_p² = 0.036) were found, but a significant cognitive load main effect was found (F = 19.17, P < .001, η_p² = 0.313). All participants had a smaller dual-task benefit response rate for serial 3s than serial 7s (mean difference [95% CI] = 22.08 [11.90, 32.25], P < .001, Hedge g = 0.399; Figure 1B).

For the response accuracy during jump landing, no significant group × cognitive load interactions (F = 1.52, P = .224, η_p² = 0.035), group main effects (F = 1.24, P = .272, η_p² = 0.029), or cognitive load main effects (F = 0.99, P = .325, η_p² = 0.023) were found.

Aim 3: NASA Task Load Index

For the NASA Task Load Index assessed after the jump landing task, no significant group × cognitive load interactions (F = 0.59, P = .526, η_p² = 0.013) or group main effects (F = 0.63, P = .433, η_p² = 0.014) were found, but a significant cognitive load main effect was found (F = 84.33, P < .001, η_p² = 0.657). All participants reported task difficulty or demands to be significantly lower for the no counting than the serial 3s (mean difference [95% CI] = 11.57 [7.15, 15.98], P < .001, Hedge g = 0.649), lower for the no counting than the serial 7s (mean difference [95% CI] = 21.96 [17.08, 26.83], P < .001, Hedge g = 1.147), and lower for the serial 3s than the serial 7s (mean difference [95% CI] = 10.39 [7.24, 13.55], P < .001, Hedge g = 0.505; Figure 2A) conditions.

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For the cutting task, no significant group × cognitive load interactions (F = 0.83, P = .427, η_p² = 0.019) or group main effects (F = 1.71, P = .198, η_p² = 0.039) were found, but a significant cognitive load main effect was found (F = 39.22, P < .001, η_p² = 0.483). All participants reported task difficulty or demands to be significantly lower for the no counting than the serial 3s (mean difference [95% CI] = 11.57 [7.15, 15.98], P < .001, Hedge g = 0.351), lower for the no counting than the serial 7s (mean difference [95% CI] = 21.96 [17.08, 26.83], P < .001, Hedge g = 0.836), and lower for the serial 3s than the serial 7s (mean difference [95% CI] = 10.39 [7.24, 13.55], P < .001, Hedge g = 0.564; Figure 2B) conditions.

Aim 1: Single- and Dual-Task Biomechanics

We unexpectedly did not find single- and dual-task biomechanics differences between individuals with and without a concussion history. This contradicts some of the prior research in which authors found individuals with a concussion history displayed greater knee abduction angles and lesser trunk flexion angles than controls as well as increased hip stiffness and decreased knee stiffness compared with pre-concussion during single-task movements.^27,40,42 Since dual-task conditions lead to more high-risk landing biomechanics profiles than single-task conditions among healthy, non-concussed individuals, we expected the combination of concussion history and dual-task conditions to exacerbate landing biomechanic differences.^7,10,43 Our theory of exacerbation was supported by prior research in which authors have shown a difference in dual-task cutting biomechanics between those with and without a concussion history.¹³ However, prior researchers did not account for confounding factors such as lower extremity function (Lower Extremity Functional Scale) and kinesiophobia or fear of movement (TSK-11). Since kinesiophobia relates to landing biomechanics, the lack of differences in our study may be why we saw no differences in landing biomechanics.⁴⁴

A potential factor affecting our results is the time since the most recent concussion. Authors of a previous study that found lesser trunk flexion than controls included recreationally active participants, with a median of 126 days post-concussion (approximately 4 months) in the concussion history group.⁴⁰ In contrast, in our study, we had a mean of approximately 44.7 months (median = 31.0) post-concussion. Lower extremity biomechanics are influenced by time since the most recent concussion; therefore, trunk biomechanics may also be influenced.⁴¹ Studying participants at later stages post-concussion is still important because we do not know when biomechanics and neuromuscular control impairments resolve post-concussion. For example, the risk for MSKI is increased up to 3 years post-concussion, but longer time frames have yet to be explored. Additionally, gait continues to be impaired 6.3 years post-concussion, indicating long-term neuromuscular control impairments.^45,46

We did find that the vertical loading rate increased during the cutting task from the no counting to serial 3s/7s (ie, single-task to either dual-task) conditions. Greater vertical ground reaction force coupled with shorter stance times (ie, vertical loading rate) during landing contributes to anterior cruciate ligament tears.⁴⁷ No difference between serial 3s and serial 7s was found in our study. Authors of a previous study that included vertical loading rate as an outcome during landing showed no single- and dual-task differences.¹⁰ These participants were also recreationally active participants and used serial 7s to introduce dual -tasking.¹⁰ Given that our effect sizes were small and not clinically meaningful, our results agree with the findings from this prior study.¹⁰

Overall, dual-tasking, as studied here, had limited effects on landing biomechanics. Additionally, the gradation effect we hypothesized going from the no counting to serial 3s to serial 7s conditions was not supported. This may be due to the complex nature of jumping and cutting. Although the task itself requires simultaneous counting and jumping or cutting, when participants leave the box, they may choose to forgo the counting and focus on landing. Despite task instructions encouraging counting throughout the movement and after landing, participants may have nevertheless not counted briefly to safely plan their landing, making the intended dual-task more like a single-task paradigm. Future researchers may consider an unanticipated type of dual-task in which a decision must be made midair after the jump. Additionally, as this was part of a larger 2-day study, participants had already received considerable practice counting during the gait (which occurred before jumping or cutting). Potentially, participants had reached their peak counting ability due to the amount of practice they received during single-task counting and dual-task gait conditions (15 total dual-task gait trials plus practice dual-task gait trials).

We found that reaction time increased (became slower) from single- to dual -task for both jump landing and cutting. However, no meaningful difference between serial 3s and 7s for either task was found. For jump landing, serial 3s versus 7s was nonsignificant and had a small effect size (P = .924, Hedge g = 0.011). For cutting, the P value was significant, but the effect size was small (P = .002, Hedge g = 0.319). Furthermore, the mean difference during the cutting of serial 3s versus serial 7s was 0.04 seconds, which is only 0.01 seconds greater than the standard error of measurement (SEM = 0.030) for the visual light box version of the test.³⁸ Dual-tasking negatively affecting reaction time is well reported for both visual and auditory computerized reaction time and functional reaction time.^38,48,49 Authors of other studies have also shown that, as complexity of any kind increases, so does reaction time; for example, individuals have slower reaction times during an unanticipated cutting task than an anticipated cutting task.⁴⁰

A possible reason for why we found single- and dual-task differences during functional reaction time and not the biomechanics goes back to the dual-task paradigm itself, as previously discussed. Functional reaction time was calculated when participants initiated movement to jump off the box. During this time, it was easy for participants to continue their counting and thereby have their attention split between the counting and the buzzer (the buzzer initiates their movement), whereas once the participants were in the air, the participants may have manipulated their counting to slow down or stop and solely focus on the landing. Another explanation for slower reaction time during the dual-task conditions may have been because participants were more cautious covering or jumping the distance to the ground and therefore prioritized their landing biomechanics due to the heightened risk during the dual-task condition. However, this seems unlikely, although not directly tested in our study because prior researchers using a similar functional reaction time test found that slower reaction time was related to more high-risk knee flexion angles during a land-and-cut task.³⁴ Based on this result, we would have expected to have seen more high-risk landing biomechanics as reaction time slowed under dual-task conditions.

Aim 2: Dual-Task Effect Cognitive Outcomes

Participants performed better (eg, more responses per second) during the jump landing and cutting than baseline, resulting in a dual-task benefit, not a dual-task cost. The dual-task benefit response rate for serial 3s was smaller than serial 7s during both jump landing and cutting. This is not surprising as serial 3s were meant to be easier than serial 7s; therefore, less room for improvement existed during the serial 3s when going from baseline (single-task) to dual-task conditions. The increased cognitive performance during the dual-task was surprising given that dual-tasking typically impairs performance on 1 or both tasks being performed.^8,11,20 Reaction time performance was impaired (ie, slower) for serial 3s and 7s for jump landing and cutting (see above discussion), while cognitive performance improved. Although participants were instructed to perform both tasks to the best of their ability, perhaps participants focused more on the serial subtraction than the motor task, adopting a cognitive-first strategy. It is also possible that participants were able to quickly count when standing on the box (preinitiation of the jump landing), slow down when in the air and during landing, and then quickly account again after landing, thereby artificially inflating their cognitive performance during the dual-task condition.

It is unclear how common it is for cognitive performance to increase or improve during jump landing, cutting, or other athletic tasks because authors of most studies have not reported the cognitive component. In fact, of the 8 studies we found in which dual-tasking (serial subtraction, flanker task, choice reaction time, etc) was used, authors of only 4 reported some form of cognitive outcome.^{7,11,43,50–54} Authors of 2 of the 4 studies did not compare cognitive outcomes to a baseline performance, and authors of 1 study descriptively reported the total numbers subtracted during dual-tasking but did not perform any statistics comparing it to baseline or report response rate or response accuracy.^14,43,51

Only Ness et al (2020) statistically compared the cognitive component (recognition of colored dots [DOTS], backward digit span [DIGITS]) of dual-tasking to a single-task cognitive (eg, baseline) condition.⁵⁴ The authors reported a decrease in cognitive accuracy during a dual-task hopping task for both DOTS and DIGITS compared with the single-task condition.⁵⁴ Unfortunately, it is hard to compare that study to our work given the differing cognitive and motor tasks performed.

Aim 3: NASA Task Load Index

We found that perceived task demands as measured by the NASA Task Load Index increased as cognitive load increased for both jump landing and cutting. No other dual-task and jumping, cutting, or athletic-task study authors have incorporated this assessment. One of the goals of this research was to introduce a hierarchy of difficulty for the cognitive component of dual-tasking for rehabilitation. Despite no clear biomechanics differences observed, a benefit may still exist for using the cognitive hierarchy (no counting, serial 3s, serial 7s) presented in our research due to the increasing, self-reported task demands.

Slowly increasing the task demands during dual-tasking may help improve patients’ confidence postinjury. However, based on our results, increasing the task demands (at least with serial subtraction) may not be optimal for challenging landing biomechanics, as no clinically meaningful biomechanics differences were found between cognitive load conditions. Inclusion of more subjective cognitive and task demand assessments for sports training and monitoring has been called for.⁵⁵ Previous researchers have begun to use the NASA Task Load Index for sports and military training protocols or paradigms, and our results support the concept of using the NASA Task Load Index to monitor workload and task demands for discrete movements useful in rehabilitation.^56–58 Future researchers need to confirm our findings among more acutely concussed and injured populations such as those with postanterior cruciate ligament tears.⁵⁹

Potential Confounding Demographic Factors

Some differences among our demographics may have influenced our results. First, although not significant, the concussion history group had a greater proportion of participants with traumatic musculoskeletal injuries. However, a preliminary comparison shows a mean difference in reaction time between injury history and controls of 0.04 seconds, which is equivalent to the SEM for the test.³⁸ Additionally, our preliminary results show no significant injury history main effect (P range = .295–.891, n_p² range = 0.004–0.025) or injury history × cognitive load interaction (P range = .094–0.276, n_p² range = 0.032–0.055) for jump landing and cutting. Also, no reported differences were found in the Lower Extremity Functional Scale, TSK-11, or Godin Leisure Activity Questionnaire. Second, a slightly higher total number of symptoms and symptom severity was found for the concussion history group. Symptoms influence movement during gait, but no researchers have focused on sport-like tasks.⁶⁰

Limitations

The first limitation of our study was the length of time postconcussion among our participants. Dual-task deficits typically resolve around 2 months for gait.⁶¹ However, few researchers have focused on dual-task sport-like task recovery postconcussion. It was still important to explore in our current study because we do not know when the increased risk for MSKI postconcussion subsides (the longest study to date is 3 years postconcussion).⁴⁵ Additionally, neuromuscular control deficits have been reported 6.3 years postconcussion during gait.⁴⁶ In other words, impairments still exist long term postconcussion but may differ by task and individual. Second, our results may not be generalizable outside of recreationally active individuals. We did not account for prior level of sport participation, which may have influenced our discussion of biomechanical differences across different populations. Third, we did not track the number of discarded or repeated trials during data collection. It is possible some participants completed more jump landings and cuts than others, leading to a concern of physical fatigue. However, participants were always given approximately 30 seconds between trials, less than 1 minute of rest between conditions (motor or cognitive), and were reminded that they could request extra rest if necessary. In the end, all participants had full and complete data for each condition for analysis.