Muscle co-activation and its influence on running performance and risk of injury in elite Kenyan runners

Relationship between metabolic cost, muscle moments and co-contraction during walking and running

BACKGROUND

The metabolic cost of locomotion is a key factor in walking and running performance. It has been studied by analysing the activation and co-activation of the muscles of the lower limbs. However, these measures do not comprehensively address muscle mechanics, in contrast to approaches using muscle moments and co-contraction.

RESEARCH QUESTION

What is the effect of speed and type of locomotion on muscle moments and co-contraction, and their relationship with metabolic cost during walking and running?

METHODS

Eleven recreational athletes (60.5 ± 7.1 kg; 169.0 ± 6.6 cm; 23.6 ± 3.3 years) walked and ran on a treadmill at different speeds, including a similar speed of 1.75 m.s-1. Metabolic cost was estimated from gas exchange measurements. Muscle moments and co-contraction of ankle and knee flexors and extensors during the stance and swing phases were estimated using an electromyographic-driven model.

RESULTS

Both the slowest and fastest walking speeds had significantly higher metabolic costs than intermediate ones (p < 0.05). The metabolic cost of walking was correlated with plantarflexors moment during swing phase (r = 0.62 at 0.5 m.s-1, r = 0.67 at 1,25 m.s-1), dorsiflexors moment during stance phase (r = 0.65 at 1.25 m.s-1, r = 0.67 at 1.5 and 1.75 m.s-1), and ankle co-contraction during the stance phase (r = 0.63 at 1.25 and 1.75 m.s-1). The metabolic cost of running at 3.25 m.s-1 during the swing phase was correlated with the dorsiflexors moment (r = 0.63), plantarflexors moment (r = 0.61) and ankle co-contraction (r = 0.60).

DISCUSSION AND CONCLUSION

Fluctuations in metabolic cost of walking and running could be explained, at least in part, by increased ankle antagonist moments and co-contraction.

The Relationship Between Running Biomechanics and Running Economy: A Systematic Review and Meta-Analysis of Observational Studies

BACKGROUND

Running biomechanics is considered an important determinant of running economy (RE). However, studies examining associations between running biomechanics and RE report inconsistent findings.

OBJECTIVE

The aim of this systematic review was to determine associations between running biomechanics and RE and explore potential causes of inconsistency.

METHODS

Three databases were searched and monitored up to April 2023. Observational studies were included if they (i) examined associations between running biomechanics and RE, or (ii) compared running biomechanics between groups differing in RE, or (iii) compared RE between groups differing in running biomechanics during level, constant-speed, and submaximal running in healthy humans (18-65 years). Risk of bias was assessed using a modified tool for observational studies and considered in the results interpretation using GRADE. Meta-analyses were performed when two or more studies reported on the same outcome. Meta-regressions were used to explore heterogeneity with speed, coefficient of variation of height, mass, and age as continuous outcomes, and standardization of running shoes, oxygen versus energetic cost, and correction for resting oxygen or energy cost as categorical outcomes.

RESULTS

Fifty-one studies (n = 1115 participants) were included. Most spatiotemporal outcomes showed trivial and non-significant associations with RE: contact time r = - 0.02 (95% confidence interval [CI] - 0.15 to 0.12); flight time r = 0.11 (- 0.09 to 0.32); stride time r = 0.01 (- 0.8 to 0.50); duty factor r = - 0.06 (- 0.18 to 0.06); stride length r = 0.12 (- 0.15 to 0.38), and swing time r = 0.12 (- 0.13 to 0.36). A higher cadence showed a small significant association with a lower oxygen/energy cost (r = - 0.20 [- 0.35 to - 0.05]). A smaller vertical displacement and higher vertical and leg stiffness showed significant moderate associations with lower oxygen/energy cost (r = 0.35, - 0.31, - 0.28, respectively). Ankle, knee, and hip angles at initial contact, midstance or toe-off as well as their range of motion, peak vertical ground reaction force, mechanical work variables, and electromyographic activation were not significantly associated with RE, although potentially relevant trends were observed for some outcomes.

CONCLUSIONS

Running biomechanics can explain 4-12% of the between-individual variation in RE when considered in isolation, with this magnitude potentially increasing when combining different variables. Implications for athletes, coaches, wearable technology, and researchers are discussed in the review.

PROTOCOL REGISTRATION

https://doi.org/10.17605/OSF.IO/293 ND (OpenScience Framework).

An exploration of muscle co-activation during different walking speeds and the association with lower limb joint stiffness

The aim of this study was to determine the muscle co-activations and joint stiffnesses around the hip, knee, and ankle during different walking speeds and to define the relationships between muscle co-activation and joint stiffness. Twenty-seven healthy subjects (age: 19.6 ± 2.2 years, height: 176.0 ± 6.0 cm, mass: 69.7 ± 8.9 kg) were recruited. Muscle co-activations (CoI) and lower limb joints stiffnesses were investigated during stance phase at different walking speeds using Repeated Measures ANOVA with Sidak post-hoc tests. Correlations between muscle co-activations, joints stiffnesses, and walking speeds were also investigated using Pearson Product Moment correlations. The results indicated that the hip and ankle joints stiffness increased with walking speed (p < 0.001) during the weight acceptance phase, and positive correlations were seen between walking speed and Rectus Femoris (RF) and Biceps Femoris (BF) CoI (p < 0.001), and a negative correlation was seen between walking speed and tibialis anterior (TA) and lateral gastrocnemius (LG) CoI (p < 0.001) during the weight acceptance phase, and the RF/BF CoI during pre-swing. These results provide new information on the variations in muscle co-activation around the hip, knee and ankle joints and their association with joint stiffness, and on the responses of stiffness and muscle co-activation to walking speed. The techniques presented could have further application and help our understanding of the effects of gait retraining and injury mechanisms.

Time course of muscle activation, energetics and mechanics of running in minimalist and traditional cushioned shoes during level running

The study aimed to compare the ankle muscles activation, biomechanics and energetics of running in male runners during submaximal level run using minimalist (MinRS) and traditional cushioned (TrdRS) running shoes. During 45-min running in MinRS and TrdRS, the ankle muscles pre- and co-activation, biomechanics, and energetics of running of 16 male endurance runners (25.5 ± 3.5 yr) were assessed using surface electromyography (tibialis anterior and gastrocnemius lateralis), instrumented treadmill and indirect calorimetry, respectively. The net energy cost of running (C_r) was similar for both conditions (P = 0.25) with a significant increase over time (P < 0.0001). Step frequency (P < 0.001), and total mechanical work (P = 0.001) were significantly higher in MinRS than in TrdRS with no evolution over time (P = 0.28 and P = 0.85, respectively). The ankle muscles pre- and co-activation during the contact phase did not differ between the two shoe conditions (P ≥ 0.33) or over time (P ≥ 0.15). In conclusion, during 45-min running, Cr and muscle pre- and co-activation were not significantly different between MinRS and TrdRS with significantly higher step frequency and total mechanical work noted in the former than in the latter. Moreover, C_r significantly increased during the 45-min trial in both shoe conditions along with no significant change over time in muscle activation and biomechanical variables.

Healthy Running Habits for the Distance Runner: Clinical Utility of the American College of Sports Medicine Infographic

ABSTRACT

Healthy running form is characterized by motion that minimizes mechanical musculoskeletal injury risks and improves coactivation of muscles that can buffer impact loading and reduce stresses related to chronic musculoskeletal pain. The American College of Sports Medicine Consumer Outreach Committee recently launched an infographic that describes several healthy habits for the general distance runner. This review provides the supporting evidence, expected acute motion changes with use, and practical considerations for clinical use in patient cases. Healthy habits include: taking short, quick, and soft steps; abdominal bracing; elevating cadence; linearizing arm swing; controlling forward trunk lean, and; avoiding running through fatigue. Introduction of these habits can be done sequentially one at a time to build on form, or more than one over time. Adoption can be supported by various feedback forms and cueing. These habits are most successful against injury when coupled with regular dynamic strengthening of the kinetic chain, adequate recovery with training, and appropriate shoe wear.

Running economy and lower extremity stiffness in endurance runners: A systematic review and meta-analysis

Background: Lower extremity stiffness simulates the response of the lower extremity to landing in running. However, its relationship with running economy (RE) remains unclear. This study aims to explore the relationship between lower extremity stiffness and RE. Methods: This study utilized articles from the Web of Science, PubMed, and Scopus discussing the relationships between RE and indicators of lower extremity stiffness, namely vertical stiffness, leg stiffness, and joint stiffness. Methodological quality was assessed using the Joanna Australian Centre for Evidence-Based Care (JBI). Pearson correlation coefficients were utilized to summarize effect sizes, and meta-regression analysis was used to assess the extent of this association between speed and participant level. Result: In total, thirteen studies involving 272 runners met the inclusion criteria and were included in this review. The quality of the thirteen studies ranged from moderate to high. The meta-analysis results showed a negative correlation between vertical stiffness (r = -0.520, 95% CI, -0.635 to -0.384, p < 0.001) and leg stiffness (r = -0.568, 95% CI, -0.723 to -0.357, p < 0.001) and RE. Additional, there was a small negative correlation between knee stiffness and RE (r = -0.290, 95% CI, -0.508 to -0.037, p = 0.025). Meta-regression results showed that the extent to which leg stiffness was negatively correlated with RE was influenced by speed (coefficient = -0.409, p = 0.020, r 2 = 0.79) and participant maximal oxygen uptake (coefficient = -0.068, p = 0.010, r 2 = 0.92). Conclusion: The results of this study suggest that vertical, leg and knee stiffness were negatively correlated with RE. In addition, maximum oxygen uptake and speed will determine whether the runner can take full advantage of leg stiffness to minimize energy expenditure.

Altering muscle activity in the lower extremities by bipedal landing with different drop tasks and shoes

BACKGROUND: The ability of the lower-extremity muscle activation directly affects the performance and in turn interacts with the loading conditions of the muscle itself. However, systematic information concerning the characteristics of lower-extremity muscle during landings is lacking. In particular, the landing height and shoes are also important factors based on the actual situation, which could further contribute to understanding the neuromuscular activity and how biochemical response of the body tissues to double-leg drop landings. OBJECTIVE: The study was to investigate the effects of landing tasks on the activation of lower-extremity muscles and explore the relationship among movement control, landing heights, shoe cushioning, and muscle activities. METHODS: Twelve male basketball players were recruited to perform drop jump (DJ) and passive landing (PL) from three heights (30, 45, and 60 cm) while wearing highly-cushioned basketball shoes (HC) and less-cushioned control shoes (LC). EMG electrodes were used to record the activities of the target muscles (rectus femoris, vastus lateralis, biceps femoris, tibialis anterior, and lateral gastrocnemius) during the landing tasks. RESULTS: Pre- and post-activation activity of the lower-extremity muscles significantly decreased during PL compared with those during DJ (p< 0.05). No significant shoe effects on the characteristics of muscle activation and coactivation during DJ movements were observed. However, the participants wearing LC showed significantly higher muscle post-activation (p< 0.05) at the three drop heights during PL compared with those wearing HC. Coactivation of the ankle muscles was higher in LC than in HC during 30-cm PL (p< 0.05). CONCLUSIONS: The activation patterns of lower-extremity muscles can be significantly influenced by landing types. Highly-cushioned basketball shoes would help reduce the risk of injuries by appropriately tuning the muscles during the PL.

Application of Leg, Vertical, and Joint Stiffness in Running Performance: A Literature Overview

Stiffness, the resistance to deformation due to force, has been used to model the way in which the lower body responds to landing during cyclic motions such as running and jumping. Vertical, leg, and joint stiffness provide a useful model for investigating the store and release of potential elastic energy via the musculotendinous unit in the stretch-shortening cycle and may provide insight into sport performance. This review is aimed at assessing the effect of vertical, leg, and joint stiffness on running performance as such an investigation may provide greater insight into performance during this common form of locomotion. PubMed and SPORTDiscus databases were searched resulting in 92 publications on vertical, leg, and joint stiffness and running performance. Vertical stiffness increases with running velocity and stride frequency. Higher vertical stiffness differentiated elite runners from lower-performing athletes and was also associated with a lower oxygen cost. In contrast, leg stiffness remains relatively constant with increasing velocity and is not strongly related to the aerobic demand and fatigue. Hip and knee joint stiffness are reported to increase with velocity, and a lower ankle and higher knee joint stiffness are linked to a lower oxygen cost of running; however, no relationship with performance has yet been investigated. Theoretically, there is a desired “leg-spring” stiffness value at which potential elastic energy return is maximised and this is specific to the individual. It appears that higher “leg-spring” stiffness is desirable for running performance; however, more research is needed to investigate the relationship of all three lower limb joint springs as the hip joint is often neglected. There is still no clear answer how training could affect mechanical stiffness during running. Studies including muscle activation and separate analyses of local tissues (tendons) are needed to investigate mechanical stiffness as a global variable associated with sports performance.

Pre-exhaustion Exercise Differentially Influences Neuromuscular Fatigue Based on Habitual Physical Activity History

ABSTRACT

Harlan, KG, Merucci, RB, Weaver, JJ, Windle, TC, and Malek, MH. Pre-exhaustion exercise differentially influences neuromuscular fatigue based on habitual physical activity history. J Strength Cond Res 35(3): 739-745, 2021-Although there is anecdotal evidence of a potential physiological benefit of pre-exhaustion exercise to enhance muscular recruitment, few studies have systematically examined the effect on neuromuscular activity. Moreover, a subject's habitual physical activity history may, in part, contribute to the muscle's response on a subsequent workbout after a single pre-exhaustion workbout. To date, no studies have examined the effect of pre-exhaustion exercise on the electromyographic fatigue threshold (EMGFT). The purpose of this study, therefore, is to determine whether pre-exhaustion exercise influences the EMGFT. Specifically, we were interested in determining whether or not there is a dichotomous response to pre-exhaustion exercise based on the individual's habitual physical activity history. Thus, we hypothesized that healthy active subjects would have reduced EMGFT values, whereas elite runners would have increased EMGFT values as a result of the pre-exhaustion exercise. Eight healthy college-aged men (mean ± SEM, age = 24.5 ± 0.3 years; body mass = 83.1 ± 3.0 kg; and height = 1.80 ± 0.02 m) and 9 elite runners (mean ± SEM, age = 23.4 ± 0.7 years; body mass = 70.3 ± 2.7 kg; and height = 1.79 ± 0.03 m) participated in current study. Each subject visited the laboratory on 2 occasions separated by 7 days and performed the single-leg knee-extensor ergometry test. For one of the visits, the subjects performed the Thorstensson test (50 continuous, concentric knee extensions) before the single-leg knee-extensor ergometry. The EMGFT was measured on both visits for all subjects. For healthy subjects, we found that the EMGFT was significantly reduced after performing the 50 isokinetic knee extensions (control: 27 ± 6 W vs. Thorstensson: 21 ± 6.0 W; p = 0.001), whereas for elite runners, there was no significant mean differences between the 2 visits (control: 38 ± 3 W vs. Thorstensson: 39 ± 2 W; p = 0.813). These results suggest that 50 repetition of isokinetic muscle action, as a method of pre-exhausting the quadriceps femoris muscles, may be influenced by the subject's habitual exercise history.

Muscle Activity of the Triceps Surae With Novel Propulsion Heel-Lift Orthotics in Recreational Runners

Background:

The triceps surae muscle has been identified with propulsion during running gait, and typical heel-lift orthotics (THOs) have been used to treat some sports injuries of this structural-biomechanical unit. The effects of a novel propulsion heel-lift orthotic (PHO) on surface electromyography (EMG) activity of the gastrocnemius during a full cycle of running have yet to be tested.

Purpose/Hypothesis:

We aimed to assess EMG changes in gastrocnemius medialis and lateralis muscle activity when wearing THOs, PHOs, or neutral sports shoes only (SO) during running. We hypothesized that EMG activity of the triceps surae muscle would be lower for PHOs than THOs or SO during running.

Study Design:

Controlled laboratory study.

Methods:

A total of 26 healthy, regular recreational runners of both sexes (mean age, 33.58 ± 6.02 years) with a neutral Foot Posture Index and rearfoot strike pattern were recruited to run on a treadmill at 9 km/h using aleatory THOs of 6 and 9 mm, PHOs, and SO while EMG activity of the gastrocnemius medialis and lateralis muscles was recorded over a 30-second period. Intraclass correlation coefficients were calculated to assess reliability.

Results:

The intraclass correlation coefficient values indicated near perfect reliability, ranging from 0.801 for 6-mm THOs to 0.959 for SO in the gastrocnemius lateralis muscle. EMG activity of the gastrocnemius lateralis muscle was greater for PHOs (25.516 ± 4.780 mV) than for SO (23.140 ± 4.150 mV) (P < .05), but EMG activity of the gastrocnemius medialis muscle did not show any statistically significant difference between conditions (23.130 ± 2.980 mV vs 26.315 ± 2.930 mV, respectively) (P = .3).

Conclusion:

A novel PHO may increase muscle activity of the gastrocnemius lateralis during a full cycle of running gait; consequently, its prescription to treat triceps surae muscle injuries is cautioned.

Clinical Relevance:

The prescription of novel PHOs could increase EMG activity, which has not been previously described.

Plasticity of muscle synergies through fractionation and merging during development and training of human runners

Complex motor commands for human locomotion are generated through the combination of motor modules representable as muscle synergies. Recent data have argued that muscle synergies are inborn or determined early in life, but development of the neuro-musculoskeletal system and acquisition of new skills may demand fine-tuning or reshaping of the early synergies. We seek to understand how locomotor synergies change during development and training by studying the synergies for running in preschoolers and diverse adults from sedentary subjects to elite marathoners, totaling 63 subjects assessed over 100 sessions. During development, synergies are fractionated into units with fewer muscles. As adults train to run, specific synergies coalesce to become merged synergies. Presences of specific synergy-merging patterns correlate with enhanced or reduced running efficiency. Fractionation and merging of muscle synergies may be a mechanism for modifying early motor modules (Nature) to accommodate the changing limb biomechanics and influences from sensorimotor training (Nurture).

Tracking of Time-Dependent Changes in Muscle Hardness After a Full Marathon

Inami, T, Nakagawa, K, Yonezu, T, Fukano, M, Higashihara, A, Iizuka, S, Abe, T, and Narita, T. Tracking of time-dependent changes in muscle hardness after a full marathon. J Strength Cond Res 33(12): 3431-3437, 2019-We sought to identify changes in individual muscle hardness after a full marathon and to track time-dependent changes using ultrasound strain elastography (SE). Twenty-one collegiate marathon runners were recruited. Muscle hardness (i.e., strain ratio, SR) was measured using SE for the rectus femoris (RF), vastus lateralis (VL), biceps femoris (BF) long head, tibialis anterior (TA), gastrocnemius medial (GM) head, and soleus (SOL) muscles at the following time points: pre (PRE), immediately post (POST), day-1 (D1), day-3 (D3), and day-8 (D8), after a full marathon. We found that the SR decreased after the full marathon (i.e., the muscle became harder), and that the lowest SR across all measured muscles was observed on D1. Although there was no difference in the magnitude of change in SR between the muscles of the thigh, that of the MG and SOL were significantly larger than that of the TA. Muscle hardness in the vastus lateralis, biceps femoris, and SOL recovered at D8 (i.e., nonsignificant difference from PRE), whereas recovery of rectus femoris and gastrocnemius medial hardness at D8 was not observed. Thus, the degree of change in muscle hardness does not occur uniformly within the lower extremity muscles. In particular, changes in muscle hardness of the TA after a full marathon are small compared with other muscles and time-dependent changes in each muscle vary during recovery. The features of muscle hardness identified in this study will be useful for coaches when mentoring runners on proper forms and for training advisers and therapists who seek to address deficiencies in running.

Running-related muscle activation patterns and tibial acceleration across puberty

This study examined whether differences exist in tibial acceleration transients and electromyography (EMG) variables during running across female pubertal development. Sixty-four girls classified as pre- (n = 19), early/mid- (n = 22) and late/post-pubertal development (n = 23) ran in a laboratory whilst EMG data were recorded from quadriceps, hamstring and calf muscle groups, and acceleration transients from a triaxial accelerometer. The late/post-pubertal girls exhibited delayed vastus lateralis onset (mean difference (MD) = 0.02, 95% CI = 0.008, 0.34 ms)) compared to pre-pubertal girls, lower vastus lateralis pre-activation (MD = 7.02, 95% CI = 12.63, 1.42%) compared to early/mid-pubertal girls, and longer time to peak (TTP) anterior/posterior (A/P) tibial acceleration compared to pre-pubertal girls (MD = 0.02, 95% CI = 0.006, 0.03 s). By contrast, late/post-pubertal girls demonstrated earlier semitendinosus onset compared to both early/mid- (MD = 0.02, 95% CI = 0.03, 0.005 ms) and pre-pubertal girls (MD = 0.02, 95% CI = 0.04, 0.007 ms). No other between-group differences were found for peak A/P, vertical and TTP vertical tibial acceleration (p > 0.05). Subsequently, regression analysis revealed that EMG variables accounted for approximately 34% (R² = 0.34) of the variance in TTP A/P tibial acceleration. These findings highlight that neuromuscular recruitment patterns and kinetics differ across female pubertal development while running and should be further explored in the context of adolescent female musculoskeletal injuries.

Running Economy: Neuromuscular and Joint-Stiffness Contributions in Trained Runners

CONTEXT

It is debated whether running biomechanics make good predictors of running economy, with little known about the neuromuscular and joint-stiffness contributions to economical running gait.

PURPOSE

To understand the relationship between certain neuromuscular and spatiotemporal biomechanical factors associated with running economy.

METHODS

Thirty trained runners performed a 6-min constant-speed running set at 3.3 m·s-1, where oxygen consumption was assessed. Overground running trials were also performed at 3.3 m·s-1 to assess kinematics, kinetics, and muscle activity. Spatiotemporal gait variables, joint stiffness, preactivation, and stance-phase muscle activity (gluteus medius, rectus femoris, biceps femoris, peroneus longus, tibialis anterior, and gastrocnemius lateralis and medius) were variables of specific interest and thus determined. In addition, preactivation and ground contact of agonist-antagonist coactivation were calculated.

RESULTS

More economical runners presented with short ground-contact times (r = .639, P < .001) and greater stride frequencies (r = -.630, P < .001). Lower ankle and greater knee stiffness were associated with lower oxygen consumption (r = .527, P = .007 and r = .384, P = .043, respectively). Only  lateral gastrocnemius-tibialis anterior coactivation during stance was associated with lower oxygen cost of transport (r = .672, P < .0001).

CONCLUSIONS

Greater muscle preactivation and biarticular muscle activity during stance were associated with more economical runners. Consequently, trained runners who exhibit greater neuromuscular activation prior to and during ground contact, in turn optimizing spatiotemporal variables and joint stiffness, will be the most economical runners.

Bone health in elite Kenyan runners

Impact loading in athletes participating in various sports has been positively associated with increased bone mineral density (BMD), but this has not been investigated in elite Kenyan runners. Body composition and site-specific BMD measures quantified with dual x-ray absorptiometry were measured in 15 elite male Kenyan runners and 23 apparently healthy South African males of different ethnicities. Training load and biomechanical variables associated with impact loading, such as joint stiffness, were determined in the elite Kenyan runners. Greater proximal femur (PF) BMD (g · cm^-2) was higher (P = 0.001, ES = 1.24) in the elite Kenyan runners compared with the controls. Six of the 15 (40%) Kenyan runners exhibited lumbar spine (LS) Z-Scores below -2.0 SD, whereas this was not found in the apparently healthy controls. PFBMD was associated with training load (r = 0.560, P = 0.003) and ankle (r = 0.710, P = 0.004) and knee (r = 0.546, P = 0.043) joint stiffness. Elite Kenyan runners exhibit greater PFBMD than healthy controls, which is associated with higher training load and higher joint stiffness. Our results reaffirm the benefits of impact loading on BMD at a weight-bearing site, while a high prevalence of low LSBMD in the elite Kenyan runners is hypothesised to be the result of a mismatch between energy intake and high training load. Future research investigating energy availability in Kenyan runners and the possible association with musculoskeletal injury should be investigated.

Brain oxygenation declines in elite Kenyan runners during a maximal interval training session

PURPOSE

The purpose of this study was to characterise the cerebral oxygenation (Cox) response during a high-intensity interval training session in Kenyan runners, and to examine any relationship with running performance.

METHODS

15 Kenyan runners completed a 5-km time trial (TT) and a Fatigue Training Test on a treadmill (repeated running bouts of 1-km at a pace 5% faster than their mean 5-km TT pace with a 30-s recovery until exhaustion). Changes in Cox were monitored via near-infrared spectroscopy through concentration changes in oxy- and deoxy-haemoglobin (Δ[O₂Hb] and Δ[HHb]), tissue oxygenation index (TOI), and total hemoglobin index (nTHI).

RESULTS

The number of 1-km repetitions achieved by the participants was 5.5 ± 1.2 repetitions at a mean pace of 20.5 ± 0.7 km h^-1. Δ[O₂Hb] measured at the end of each running repetition declined progressively over the course of the trial (p = 0.01, ES = 4.59). Δ[HHb] increased during each running bout until the end of the Fatigue Training Test (p < 0.001; ES = 6.0). TOI decreased significantly from the beginning of the test (p = 0.013, ES = 1.83), whereas nTHI remained stable (ES = 0.08). The Cox decline in the Fatigue Training Test was negatively correlated with the speed at which the test was completed (p = 0.017; r = -0.61), suggesting that the best performers were able to defend their Cox better than those of lower running ability.

CONCLUSIONS

In conclusion, this study suggests that elite Kenyan runners cannot defend cerebral oxygenation when forced to exercise to their physiological limits. This emphasises the critical importance of pacing in their racing success.

Adjustments with running speed reveal neuromuscular adaptations during landing associated with high mileage running training

It remains to be determined whether running training influences the amplitude of lower limb muscle activations before and during the first half of stance and whether such changes are associated with joint stiffness regulation and usage of stored energy from tendons. Therefore, the aim of this study was to investigate neuromuscular and movement adaptations before and during landing in response to running training across a range of speeds. Two groups of high mileage (HM; >45 km/wk, n = 13) and low mileage (LM; <15 km/wk, n = 13) runners ran at four speeds (2.5-5.5 m/s) while lower limb mechanics and electromyography of the thigh muscles were collected. There were few differences in prelanding activation levels, but HM runners displayed lower activations of the rectus femoris, vastus medialis, and semitendinosus muscles postlanding, and these differences increased with running speed. HM runners also demonstrated higher initial knee stiffness during the impact phase compared with LM runners, which was associated with an earlier peak knee flexion velocity, and both were relatively unchanged by running speed. In contrast, LM runners had higher knee stiffness during the slightly later weight acceptance phase and the disparity was amplified with increases in speed. It was concluded that initial knee joint stiffness might predominantly be governed by tendon stiffness rather than muscular activations before landing. Estimated elastic work about the ankle was found to be higher in the HM runners, which might play a role in reducing weight acceptance phase muscle activation levels and improve muscle activation efficiency with running training.NEW & NOTEWORTHY Although neuromuscular factors play a key role during running, the influence of high mileage training on neuromuscular function has been poorly studied, especially in relation to running speed. This study is the first to demonstrate changes in neuromuscular conditioning with high mileage training, mainly characterized by lower thigh muscle activation after touch down, higher initial knee stiffness, and greater estimates of energy return, with adaptations being increasingly evident at faster running speeds.