Biology subjects

Blazevich, A. J.

Publications and source records attributed to Blazevich, A. J..

6 recordsLinked to original sources

bioRxiv2022

Does fatigue influence joint-specific work and ground force production during the first steps of maximal accelerative running?

The rate of initial acceleration during the first steps of a maximal-effort (sprint) run often determines success or failure in prey capture and predator evasion, and is a vital factor of success in many modern sports. However, accelerative events are commonly performed after having already run considerable distances, and the associated fatigue should impair muscle force production and thus reduce acceleration rate. Despite this, the effects of running-induced fatigue on our ability to accelerate as well as the running technique used to achieve it has been incompletely studied. We recorded 3-D kinematics and ground reaction forces during the first three steps of the acceleration phase from a standing start before and after performing a high-speed, multi-directional, fatiguing run-walk protocol in well-trained running athletes who were habituated to accelerative sprinting. We found that the athletes were able to maintain their rate of initial acceleration despite changing running technique, which was associated with use of a more upright posture, longer ground contact time, increased vertical ground reaction impulse, decreased hip flexion and extension velocities, and a shift in peak joint moments, power, and positive work from the hip to the knee joint; no changes were detected in ankle joint function. Thus, a compensatory increase in knee joint function alleviated the reduction in hip flexor-extensor capacity. These acute adaptations may indicate that the hip extensors (gluteal and hamstring muscle groups) were more susceptible to fatigue than the ankle and knee musculature, and may thus be a primary target for interventions promoting fatigue resistance.

biophysics↗

bioRxiv2021

Maintenance of ankle power and decreased inter-limb asymmetry during sprint running in fatigued multi-speed athletes

The ability to shift from walking and jogging to sprinting gaits, even when fatigued after prolonged effort, would have been as useful to our hunter-gatherer ancestors as it is to modern athletes. During prolonged jogging, joint moment and work decrease in the distal (ankle) joint but increase at proximal (hip/knee) joints as fatigue progresses, and such adaptations might be expected to occur in sprinting. Fatigue is also thought to increase inter-limb kinematic and force production asymmetries, which are speculated to influence injury risk. However, the effects of running-related fatigue on sprint running gait have been incompletely studied, so these hypotheses remain untested. We studied 3-D kinematics and ground reaction force production in dominant (DL) and non-dominant (NDL) legs during both non-fatigued and fatigued sprinting in habitual but uncoached running athletes. Contrary to the tested hypotheses, relative between-leg differences were greater in non-fatigued than fatigued sprinting. When not fatigued, DL produced greater propulsive impulse through both greater positive and negative work being done at the ankle, whilst NDL produced more vertical impulse, possibly resulting from the greater hip flexion observed prior to the downwards acceleration of the foot towards the ground. Whilst few changes were detected in DL once fatigued, NDL shifted towards greater horizontal force production, largely resulting from an increase in plantarflexion (distal-joint) moments and power. After fatiguing running, therefore, inter-limb asymmetry was reduced during sprinting and no distal-to-proximal shift in work/power was detected. Speculatively, these adaptations may help to attenuate decreases in running speed whilst minimising injury risk. SignificanceThe ability to attain fast running speeds may critically determine success in tasks such as prey chase- and-capture in hunter-gatherer societies as well as success in modern sports competitions. At times, sprint running may have to be performed whilst fatigued from previous, longer-distance running, when speeds are reduced, and injury risk may be higher. Previous work indicated that fatigue prompts a proximal shift in joint work and power production and an increase in inter-limb asymmetry. On the contrary, we show that relative ankle positive and negative joint work was maintained in the face of fatigue and that inter-limb asymmetry was reduced in a group of runners experienced, but not formally instructed, in both long-distance and sprint running.

biophysics↗

bioRxiv2021

INTRINSIC MOTOR NEURONE EXCITABILITY IS REDUCED IN SOLEUS AND TIBIALIS ANTERIOR OF OLDER ADULTS

Age-related deterioration within both motor neurones and monoaminergic systems should theoretically reduce neuromodulation by weakening motor neuronal persistent inward current (PIC) amplitude. However, this assumption remains untested. Surface electromyographic signals were collected using two 32-channel electrode matrices placed on soleus and tibialis anterior of 25 older adults (70{+/-}4years) and 17 young adults (29{+/-}5 years) to investigate motor unit discharge behaviours. Participants performed triangular-shaped plantar and dorsiflexion contractions to 20% of maximum torque at a rise-decline rate of 2%/s of each participants maximal torque. Pairwise and composite paired-motor unit analyses were adopted to calculate delta frequency ({Delta}F), which has been used to differentiate between the effects of synaptic excitation and intrinsic motor neuronal properties and is assumed to be proportional to PIC amplitude. Soleus and tibialis anterior motor units in older adults had lower {Delta}Fs calculated with either the pairwise [-0.99 and -1.46 pps; -35.4 and - 33.5%, respectively] or composite (-1.18 and -2.28 pps; -32.1 and -45.2%, respectively) methods. Their motor units also had lower peak discharge rates (-2.14 and -2.03 pps; -19.7 and -13.9%, respectively) and recruitment thresholds (-1.50 and -2.06% of maximum, respectively) than young adults. These results demonstrate reduced intrinsic motor neurone excitability during low-force contractions in older adults, likely mediated by decreases in the amplitude of persistent inward currents. Our findings might be explained by deterioration in the motor neurones or monoaminergic systems and could contribute to the decline in motor function during ageing; these assumptions should be explicitly tested in future investigations.

physiology↗

bioRxiv2020

Do Motoneurons Slow With Aging? A Systematic Review And Meta-Analysis With Meta-Regression

Nervous system maladaptation is linked to the loss of maximal strength production and motor control with aging. Motor unit discharge rates are a critical determinant of force production; thus, lower discharge rates could be a mechanism underpinning maximal strength and motor control losses during aging. This meta-analysis summarized the findings of studies comparing motor unit discharge rates between young and older adults, and examined the effect of distinct muscles and contraction intensities on the magnitude of discharge rates difference between these two groups. Eligible studies were combined in a meta-analysis, including tested contraction intensities and muscles in different levels, to investigate whether there were differences in discharge rates between younger and older adults. Motor unit discharge rates were higher in younger adults compared to older adults, with a pooled standardized mean difference (SMD) for all studies of 0.63 (95%CI= 0.27 to 0.99). Contraction intensity had a significant effect on the pooled SMD, with a 1% increase in intensity associated with a 0.009 (95%CI= 0.003 to 0.015) change in the pooled SMD. These findings suggest that the reductions in motor unit discharge rates, especially at higher contraction intensities, may be an important mechanism underpinning age-related losses in maximal strength production.

physiology↗

bioRxiv2020

Persistent Inward Currents Increase With The Level Of Voluntary Drive In Plantar Flexor Low-Threshold Motor Units

This study tested the hypothesis that estimates of persistent inward currents (PICs) in the human plantar flexors would increase with the level of voluntary drive. Twenty-one participants volunteered for this study (29.2{+/-}2.6 years). High-density surface electromyograms were collected from soleus and gastrocnemius medialis during ramp-shaped isometric contractions to 10%, 20%, and 30% (torque rise of 2%/s and 30-s duration) of each participants maximal torque. Motor units identified in all the contraction intensities were included in the paired-motor unit analysis to calculate delta frequency ({Delta}F) and estimate the PICs. Increases in PICs were observed from 10% to 20% ({Delta}=0.6 pps; p<0.001) and 20% to 30% ({Delta}=0.5 pps; p<0.001) in soleus, and from 10% to 20% ({Delta}=1.2 pps; p<0.001) but not 20% to 30% ({Delta}=0.09 pps; p=0.724) in gastrocnemius medialis. Maximal discharge rate increased for soleus and gastrocnemius medialis from 10% to 20% (respectively, {Delta}=1.75 pps, p<0.001; and {Delta}=2.43 pps, p<0.001) and 20% to 30% (respectively, {Delta}=0.80 pps, p<0.017; and {Delta}=0.92 pps, p=002). The repeated-measures correlation identified associations between {Delta}F and increases in maximal discharge rate for both soleus (r=0.64; p<0.001) and gastrocnemius medialis (r=0.77; p<0.001). An increase in voluntary drive tends to increase PIC strength, which has key implications for the control of force but also for comparisons between muscles or between studies when relative force levels might be different. These data indicate that increases in voluntary descending drive amplify PICs in humans and provide an important spinal mechanism for motor unit firing, and thus force output modulation.

neuroscience↗

bioRxiv2020

Passive muscle stretching reduces estimates of persistent inward current strength in soleus motor units

Prolonged ([≥]60 s) passive muscle stretching acutely reduces maximal force production at least partly through a suppression of efferent neural drive. The origin of this neural suppression has not been determined, however some evidence suggests that reductions in the amplitude of persistent inward currents (PICs) in the motoneurons may be important. The aim of the present study was to determine whether acute passive (static) muscle stretching affects PIC strength in gastrocnemius medialis (GM) and soleus (SOL) motor units. We calculated the difference in instantaneous discharge rates at recruitment and derecruitment ({Delta}F) for pairs of motor units in GM and SOL during triangular isometric plantar flexor contractions (20% maximum) both before and immediately after a 5-min control period and immediately after five 1-min passive plantar flexor stretches. After stretching there was a significant reduction in SOL {Delta}F (-25.6%; 95%CI = -45.1 to -9.1 %, p=0.002) but not GM {Delta}F. These data suggest passive muscle stretching can reduce the intrinsic excitability, via PICs, of SOL motor units. These findings (1) suggest that PIC strength might be reduced after passive stretching, (2) are consistent with previously-established post-stretch decreases in SOL but not GM EMG amplitudes during contraction, and (3) indicate that reductions in PIC strength could underpin the stretch-induced force loss. SUMMARY STATEMENTMotoneurons require an amplification mechanism to operate within the firing frequencies observed during normal motor behaviour. Here we present evidence that this amplification mechanism is reduced after passive muscle stretching.

physiology↗