Kinematic and neuromuscular characterization of cognitive involvement in gait control in healthy young adults

The signature of cognitive involvement in gait control has rarely been studied using both kinematic and neuromuscular features. The present study aimed to address this gap. Twenty-four healthy young adults walked on an instrumented treadmill in a virtual environment under two optic flow conditions: normal (NOF) and perturbed (POF, continuous mediolateral pseudorandom oscillations). Each condition was performed under single-task and dual-task conditions of increasing difficulty (1-, 2-, 3-back). Subjective mental workload (raw NASA-TLX), cognitive performance (mean reaction time and d-prime), kinematic (steadiness, variability and complexity in the mediolateral and anteroposterior directions) and neuromuscular (duration and variability of motor primitives) control of gait were assessed. The cognitive performance and the number and composition of motor modules were unaffected by simultaneous walking, regardless of the optic flow condition. Kinematic and neuromuscular variability was greater under POF compared to NOF conditions. Young adults sought to counteract POF by rapidly correcting task-relevant gait fluctuations. The depletion of cognitive resources through dual-tasking led to reduced kinematic and neuromuscular variability and this occurred to the same extent regardless of simultaneous working memory (WM) load. Increasing WM load led to a prioritization of gait control in the mediolateral direction over the anteroposterior direction. The impact of POF on kinematic variability (step velocity) was reduced when a cognitive task was performed simultaneously, but this phenomenon was no modulated by WM load. Collectively, these results shed important light on how young adults adjust the processes involved in goal-directed locomotion when exposed to varying levels of task and environmental constraints.

Methods
Subjects:

Healthy young adults, kinematic data: N = 24, 12 men and 12 women, age = 21.67 +/- 2.28 years old.
Healthy young adults, EMG data: N = 20, 10 men and 10 women, age = 21.80 +/- 2.42 years old.

The testing session was composed of three blocks performed in a randomised order: (1) three N-back tasks in a seated position (single-task cognitive performance, STC), (2) walking under normal (congruent) optic flow (NOF), and (3) walking under perturbed (continuous mediolateral pseudo-random oscillations) optic flow (POF). In the latter two blocks, the walking tasks were performed under both single-task (STW) and dual-task (DTW) conditions (i.e. walking while performing the N-back tasks). Participants were asked to walk naturally while looking straight ahead. The treadmill speed was adjusted to their preferred walking speed. The blocks (2) and (3) began and ended with a STW condition while the three DTW conditions were performed in a randomized order between the two STW conditions (Schaefer et al., 2015). Under dual-task conditions, no task priority instructions were given (Schaefer et al., 2015). Each condition lasted for 3 minutes. Hence, a total of thirteen experimental conditions were performed.

"STC" = Single-task cognitive performance;
"STW" = Single-task walking;
"DTW" = Dual-task walking;
"NOF" = Normal optic flow;
"POF" = Perturbed optic flow;
"1b" = 1-back task;
"2b" = 2-back task;
"3b" = 3-back task.

[See paper for exact definitions / descriptions]

Foot placement kinematics (200 Hz) and surface electromyography (EMG) of soleus and gluteus medius (1000 Hz) were recorded. For kinematic variables, mean, standard deviation (variability), statistical persistence and step-to-step error correction were computed from gait time series in mediolateral and anteroposterior directions. For EMG variables, duration and variability of muscle activation were calculated from the full width at half maximum (FWHM) and the variance ratio, respectively. Performance on N-back tasks was assessed with mean reaction time for target trials (RT, in second) and d-prime (d’, a.u.).