Muscle-tendon unit mechanobiological responses to consecutive high strain cyclic loading

In response to a mechanical stimulus, tendons have a slower tissue renewal rate compared to muscles. This could over time lead to a higher mechanical demand (experienced strain) for the tendon, especially when a high strain magnitude exercise is repeated without sufficient recovery. The current study investigated the adaptive responses of the triceps surae (TS) muscle-tendon unit (MTU) and extracellular matrix turnover-related biomarkers to repetitive high tendon strain cyclic loading. Eleven young male adults performed a progressive resistance exercise over 12 consecutive days, consisting of high Achilles tendon (AT) strain cyclic loading (90% MVC) with one leg once a day (LegT1) and the alternate leg three times a day (LegT3). Exercise-related changes in TS MTU mechanical properties and serum concentrations of extracellular matrix turnover-related biomarkers were analysed over the intervention period. Both legs demonstrated similar increases in maximal AT force (∼10%) over the 12-day period of exercise. A ∼20% increase in maximal AT strain was found for LegT3 (p<0.05) already after 8 consecutive exercise days, along with a corresponding decrease in AT stiffness. These effects were maintained even after a 48h rest period. The AT mechanical properties for LegT1 were unaltered. Biomarker analysis revealed no sign of inflammation, but altered collagen turnover and delayed increase in the collagen type I synthesis rate. Accordingly, we suggest that tendon is vulnerable to frequent high-magnitude and volume of cyclic mechanical loading, as accumulation of micro-damage can potentially exceed the rate of biological repair, leading to increased maximal tendon strai

Differences in run-up, take-off, and flight characteristics: successful vs. unsuccessful high jump attempts at the IAAF world championships

Studies previously conducted on high jump have yielded important information regarding successful performance. However, analyses in competitive scenarios have often disregarded athletes’ unsuccessful attempts. This study aimed to investigate the biomechanical differences between successful and unsuccessful jumps during competition. High-speed video footage (200 Hz) was obtained from 11 athletes during the 2018 Men's World Athletics Indoor Championship Final. From each athlete, one successful (SU) and one unsuccessful (UN) jump at the same bar height were included in the analysis, leaving seven athletes in total. Following whole-body 3D manual digitization, several temporal and kinematic variables were calculated for the run-up, take-off, and flight phases of each jump. During SU jumps, athletes raised the center of mass to a greater extent (p < 0.01) from take-off. Touchdown in SU jumps was characterized by a faster anteroposterior velocity (p < 0.05), lower backward lean (p < 0.05), and changes in joint angles for the stance and trail limbs (p < 0.05). Athletes also shortened the final contact time during SU jumps (p < 0.01) after producing a longer flight time in the final step of the run-up (p < 0.05). Elite-level high jumpers undertake a series of adjustments to successfully clear the bar after UN jumps. These adjustments reinforce the importance of the run-up in setting the foundations for take-off and bar clearance. Furthermore, the findings demonstrate the need for coaches to be mindful of the adjustments required in stance and trail limbs when looking to optimize feedback to athletes during training and competition

Running into Fatigue: The Effects of Footwear on Kinematics, Kinetics, and Energetics.

Recent studies identified a redistribution of positive mechanical work from distal to proximal joints during prolonged runs, which might partly explain the reduced running economy observed with running-induced fatigue. Higher mechanical demand of plantar flexor muscle-tendon-units, e.g., through minimal footwear, can lead to an earlier onset of fatigue, which might affect the redistribution of lower extremity joint work during prolonged runs. Therefore, the purpose of this study was to examine the effects of a racing-flat and cushioned running shoe on the joint-specific contributions to lower extremity joint work during a prolonged fatiguing run. On different days, eighteen runners performed two 10-km runs with near-maximal effort in a racing-flat and a cushioned shoe on an instrumented treadmill synchronized with a motion-capture-system. Joint kinetics and kinematics were calculated at 13 pre-determined distances throughout the run. The effects of shoes, distance, and their interaction were analyzed using a two-factor repeated-measures ANOVA. For both shoes, we found a redistribution of positive joint work from ankle (-6%) to knee (+3%) and hip (+3%) throughout the entire run. Negative ankle joint work was higher (p<0.01) with the racing-flat compared to the cushioned shoe. Initial differences in foot-strike patterns between shoes disappeared after 2 km of running distance. Irrespective of the shoe design, alterations in the running mechanics occurred in the first 2 km of the run, which might be attributed to the existence of a habituation rather than fatigue effect. While we did not find a difference between shoes in the fatigue-related redistribution of joint work from distal to more proximal joints, more systematical studies are needed to explore the effects of specific footwear design features. [Abstract copyright: Copyright © 2020 American College of Sports Medicine.

Evidence of different sensitivity of muscle and tendon to mechano-metabolic stimuli

This study aimed to examine the temporal dynamics of muscle-tendon adaptation and whether differences between their sensitivity to mechano-metabolic stimuli would lead to non-uniform changes within the triceps surae (TS) muscle-tendon unit (MTU). Twelve young adults completed a 12-week training intervention of unilateral isometric cyclic plantarflexion contractions at 80% of maximal voluntary contraction until failure to induce a high TS activity and hence metabolic stress. Each participant trained one limb at a short (plantarflexed position, 115°: PF) and the other at a long (dorsiflexed position, 85°: DF) MTU length to vary the mechanical load. MTU mechanical, morphological, and material properties were assessed biweekly via simultaneous ultrasonography-dynamometry and magnetic resonance imaging. Our hypothesis that tendon would be more sensitive to the operating magnitude of tendon strain but less to metabolic stress exercise was confirmed as tendon stiffness, Young's modulus, and tendon size were only increased in the DF condition following the intervention. The PF leg demonstrated a continuous increment in maximal AT strain (i.e., higher mechanical demand) over time along with lack of adaptation in its biomechanical properties. The premise that skeletal muscle adapts at a higher rate than tendon and does not require high mechanical load to hypertrophy or increase its force potential during exercise was verified as the adaptive changes in morphological and mechanical properties of the muscle did not differ between DF and PF. Such differences in muscle-tendon sensitivity to mechano-metabolic stimuli may temporarily increase MTU imbalances that could have implications for the risk of tendon overuse injury

Improving Trip- and Slip-Resisting Skills in Older People: Perturbation Dose Matters

Aging negatively affects balance recovery responses after trips and slips. We hypothesize that older people can benefit from brief treadmill-based trip and slip perturbation exposure despite reduced muscular capacities, but with neuropathology, their responsiveness to these perturbations will be decreased. Thus, to facilitate long-term benefits and their generalizability to everyday life, one needs to consider the individual threshold for perturbation dose. This is a non-final version of an article published in final form in Exercise and Sport Sciences Review

Retention and generalizability of balance recovery response adaptations from trip-perturbations across the adult lifespan

For human locomotion, varying environments require adjustments of the motor system. We asked whether age affects gait balance recovery adaptation, its retention over months and the transfer of adaptation to an untrained reactive balance task. Healthy adults (26 young, 27 middle-aged and 25 older; average ages 24, 52 and 72 years respectively) completed two tasks. The primary task involved treadmill walking: either unperturbed (control; n=39) or subject to unexpected trip perturbations (training; n=39). A single trip perturbation was repeated after a 14-week retention period. The secondary transfer task, before and after treadmill walking, involved sudden loss of balance in a lean-and-release protocol. For both tasks the anteroposterior margin of stability (MoS) was calculated at foot touchdown. For the first (i.e. novel) trip, older adults required one more recovery step ( P=0.03) to regain positive MoS compared to younger, but not middle-aged, adults. However, over several trip perturbations, all age groups increased their MoS for the first recovery step to a similar extent (up to 70%), and retained improvements over 14 weeks, though a decay over time was found for older adults ( P=0.002; middle-aged showing a tendency for decay: P=0.076). Thus, although adaptability in reactive gait stability control remains effective across the adult lifespan, retention of adaptations over time appears diminished with aging. Despite these robust adaptations, the perturbation training group did not show superior improvements in the transfer task compared to aged-matched controls (no differences in MoS changes), suggesting that generalizability of acquired fall-resisting skills from gait-perturbation training may be limited

Positive Work Contribution Shifts from Distal to Proximal Joints during a Prolonged Run

PURPOSE: To investigate the joint-specific contributions to the total lower extremity joint work during a prolonged fatiguing run. METHODS: Recreational long-distance runners (RR; n = 13) and competitive long-distance runners (CR; n = 12) performed a 10-km treadmill run with near-maximal effort. A three-dimensional motion capture system synchronized with a force-instrumented treadmill was used to calculate joint kinetics and kinematics of the lower extremity in the sagittal plane during the stance phase at 13 distance points over the 10-km run. RESULTS: A significant (P < 0.05) decrease of positive ankle joint work as well as an increase of positive knee and hip joint work was found. These findings were associated with a redistribution of the individual contributions to total lower extremity work away from the ankle towards the knee and hip joint which was more distinctive in the RR group than in the CR group. This redistribution was accomplished by significant (P < 0.05) reductions of the external ground-reaction force (GRF) lever arm and joint torque at the ankle and by the significant (P < 0.05) increase of the external GRF lever arm and joint torque at the knee and hip. CONCLUSION: The redistribution of joint work from the ankle to more proximal joints might be a biomechanical mechanism that could partly explain the decreased running economy in a prolonged fatiguing run. This might be because muscle-tendon units crossing proximal joints are less equipped for energy storage and return compared to ankle plantar flexors and require greater muscle volume activation for a given force. In order to improve running performance, long-distance runners may benefit from an exercise-induced enhancement of ankle plantar flexor muscle-tendon unit capacities

Retention of gait stability improvements over 1.5 years in older adults:effects of perturbation exposure and triceps surae neuromuscular exercise

The plantarflexors play a crucial role in recovery from sudden disturbances to gait. The objective of this study was to investigate whether medium (months)- or long(years)-term exercise-induced enhancement of triceps surae (TS) neuromuscular capacities affects older adults' ability to retain improvements in reactive gait stability during perturbed walking acquired from perturbation training sessions. Thirty-four adult women (65 +/- 7 yr) were recruited to a perturbation training group (n = 13) or a group that additionally completed 14 wk of TS neuromuscular exercise (n = 21), 12 of whom continued with the exercise for 1.5 yr. The margin of stability (MoS) was analyzed at touchdown of the perturbed step and the first recovery step following eight separate unexpected trip perturbations during treadmill walking. TS muscle-tendon unit mechanical properties and motor skill performance were assessed with ultrasonography and dynamometry. Two perturbation training sessions (baseline and after 14 wk) caused an improvement in the reactive gait stability to the perturbations (increased MoS) in both groups. The perturbation training group retained the reactive gait stability improvements acquired over 14 wk and over 1.5 yr. with a minor decay over time. Despite the improvements in TS capacities in the additional exercise group. no benefits for the reactive gait stability following perturbations were identified. Therefore, older adults' neuromotor system shows rapid plasticity to repeated unexpected perturbations and an ability to retain these adaptations in reactive gait stability over a long time period, but an additional exercise-related enhancement of TS capacities seems not to further improve these effects. NEW & NOTEWORTHY Older adults' neuromotor system shows rapid plasticity to repeated exposure to unexpected perturbations to gait and an ability to retain the majority of these adaptations in reactive recovery responses over a prolonged time period of 1.5 yr. However, an additional exercise-related enhancement of TS neuromuscular capacities is not necessarily transferred to the recovery behavior during unexpected perturbations to gait in older adults

Differences in motor response to stability perturbations limit fall-resisting skill transfer

This study investigated transfer of improvements in stability recovery performance to novel perturbations. Thirty adults (20-53 yr) were assigned equally to three treadmill walking groups: groups exposed to eight trip perturbations of either low or high magnitude and a third control group that walked unperturbed. Following treadmill walking, participants were exposed to stability loss from a forward-inclined position (lean-and-release) and an overground trip. Lower limb joint kinematics for the swing phase of recovery steps was compared for the three tasks using statistical parametric mapping and recovery performance was analysed by margin of stability and base of support. The perturbation groups improved stability (greater margin of stability) over the eight gait perturbations. There was no group effect for stability recovery in lean-and-release. For the overground trip, both perturbation groups showed similar enhanced stability recovery (margin of stability and base of support) compared to controls. Differences in joint angle kinematics between treadmill-perturbation and lean-and-release were more prolonged and greater than between the two gait perturbation tasks. This study indicates that: (i) practising stability control enhances human resilience to novel perturbations; (ii) enhancement is not necessarily dependent on perturbation magnitude; (iii) differences in motor response patterns between tasks may limit transfer