Adaptation of ankle-knee interaction during repeated submaximal loading to exhaustion

![CDATA[Background: Neuromotor patterns which are different between tasks may be sensitive to subtle adaptations, especially if there are physiological changes such as fatigue or familiarisation. It is plausible that there may be a modulation of a neuromotor pattern during continuous tasks. Purpose: This study evaluated the ankle-knee joint interaction during repetitive submaximal effort loading to exhaustion. Methods: Seventeen volunteer, healthy and recreationally active participants (mean (SD)) 22.7 (2.5) years of age, 173.5 (6.7) cm in height, 72.7 (10.6) kg in body mass) provided written and informed consent for inclusion in this study which was approved by the University of Western Sydney Human Research Ethics Committee (H1074). Each participant completed a trial of on-the-spot single-leg hopping on the dominant leg to exhaustion in synchrony with an audible metronome (2.2 Hz) to a target hop height using tactile feedback. Synchronous recording (First Principles v1.2.4) and processing (Visual 3D) of force plate (low-pass Butterworth filter with 50 Hz cut-off), kinematics (4th order Butterworth filter with 8 Hz cut-off) (lower limb, pelvis and trunk) and surface electromyography (sEMG) (band pass filtered 50–500 Hz) of 7 lower limb muscles were made. Each trial was time normalised to 100% and the mean of 10 consecutive data points at 0, 20, 40, 60, 80 and 100% of the trial were determined to represent the dependant variables at each of these moments from start to finish of the trial. Dependant variables included ankle (A) and knee (K) joint angles (at initial contact (IC) and peak vertical ground reaction force (PvGRF)), ankle (AL) and knee (KL) excursion during loading and ankle-knee joint coupling (AKJC) in the frontal axis. A one-way repeated measure ANOVA was used to determine differences during the trials with Bonferroni correction used to reduce the risk of a type 1 error. Results: There was a significant decrease in AKJC (p < 0.001) demonstrated by greater AL as fatigue levels increased while hopping pace and height remained unchanged (p > 0.05). There was a significant increase towards ankle dorsiflexion of AKIC (p < 0.001) and APvGRF (p < 0.001) at the end of the trial and concomitant decrease in AL (p = 0.03). There was a converse significant decrease in KL (p < 0.001) even though there was a significant increase in KIC (p < 0.001) and KPvGRF (p = 0.028). Conclusion(s): Adaptations of ankle and knee joint angles at IC offset decreased KL (increased knee joint stiffness) and this may have allowed (i) a shift in optimal muscle activation characteristics between the prime movers (triceps surae and quadriceps femoris) and (ii) optimisation of the force-length relationship. Although there was synchronous adaption in the ankle and knee joints, this was not characterised by proportionate changes in neuro-mechanical changes at each joint or synergistic muscle group. Implications: Determination of the modulation of a gross motor pattern when motor performance remains unchanged is a factor which needs to be considered during training and rehabilitation during repetitive tasks which may be sensitive to the effects of fatigue and motor learning.]]