Skeletal muscle fiber type and TMS-induced muscle relaxation in unfatigued and fatigued knee-extensor muscles.

The force drop after transcranial magnetic stimulation (TMS) delivered to the motor cortex during voluntary muscle contractions could inform about muscle relaxation properties. Because of the physiological relation between skeletal muscle fiber-type distribution and size and muscle relaxation, TMS could be a noninvasive index of muscle relaxation in humans. By combining a noninvasive technique to record muscle relaxation in vivo (TMS) with the gold standard technique for muscle tissue sampling (muscle biopsy), we investigated the relation between TMS-induced muscle relaxation in unfatigued and fatigued states, and muscle fiber-type distribution and size. Sixteen participants (7F/9M) volunteered to participate. Maximal knee-extensor voluntary isometric contractions were performed with TMS before and after a 2-min sustained maximal voluntary isometric contraction. Vastus lateralis muscle tissue was obtained separately from the participants' dominant limb. Fiber type I distribution and relative cross-sectional area of fiber type I correlated with TMS-induced muscle relaxation at baseline (r = 0.67, adjusted P = 0.01; r = 0.74, adjusted P = 0.004, respectively) and normalized TMS-induced muscle relaxation as a percentage of baseline (r = 0.50, adjusted P = 0.049; r = 0.56, adjusted P = 0.031, respectively). The variance in the normalized peak relaxation rate at baseline (59.8%, P < 0.001) and in the fatigue resistance (23.0%, P = 0.035) were explained by the relative cross-sectional area of fiber type I to total fiber area. Fiber type I proportional area influences TMS-induced muscle relaxation, suggesting TMS as an alternative method to noninvasively inform about skeletal muscle relaxation properties.NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS)-induced muscle relaxation reflects intrinsic muscle contractile properties by interrupting the drive from the central nervous system during voluntary muscle contractions. We showed that fiber type I proportional area influences the TMS-induced muscle relaxation, suggesting that TMS could be used for the noninvasive estimation of muscle relaxation in unfatigued and fatigued human muscles when the feasibility of more direct method to study relaxation properties (i.e., muscle biopsy) is restricted.

Sex differences in neuromuscular and biological determinants of isometric maximal force.

AIM: Force expression is characterized by an interplay of biological and molecular determinants that are expected to differentiate males and females in terms of maximal performance. These include muscle characteristics (muscle size, fiber type, contractility), neuromuscular regulation (central and peripheral factors of force expression), and individual genetic factors (miRNAs and gene/protein expression). This research aims to comprehensively assess these physiological variables and their role as determinants of maximal force difference between sexes. METHODS: Experimental evaluations include neuromuscular components of isometric contraction, intrinsic muscle characteristics (proteins and fiber type), and some biomarkers associated with muscle function (circulating miRNAs and gut microbiome) in 12 young and healthy males and 12 females. RESULTS: Male strength superiority appears to stem primarily from muscle size while muscle fiber-type distribution plays a crucial role in contractile properties. Moderate-to-strong pooled correlations between these muscle parameters were established with specific circulating miRNAs, as well as muscle and plasma proteins. CONCLUSION: Muscle size is crucial in explaining the differences in maximal voluntary isometric force generation between males and females with similar fiber type distribution. Potential physiological mechanisms are seen from associations between maximal force, skeletal muscle contractile properties, and biological markers.

Downhill running increases markers of muscle damage and impairs the maximal voluntary force production as well as the late phase of the rate of voluntary force development.

PURPOSE: To examined the time-course of the early and late phase of the rate of voluntary force development (RVFD) and muscle damage markers after downhill running. METHODS: Ten recreational runners performed a 30-min downhill run at 10 km h-1 and -20% (-11.3°) on a motorized treadmill. At baseline and each day up to 4 days RVFD, knee extensors maximum voluntary isometric force (MVIC), serum creatine kinase (CK) concentration, quadriceps swelling, and soreness were assessed. The early (0-50 ms) and late (100-200 ms) phase of the RVFD, as well as the force developed at 50 and 200 ms, were also determined. RESULTS: MVIC showed moderate decrements (p < 0.05) and recovered after 4 days (p > 0.05). Force at 50 ms and the early phase were not impaired (p > 0.05). Conversely, force at 200 ms and the late phase showed moderate decrements (p < 0.05) and recovered after 3 and 4 days, respectively (p > 0.05). CK concentration, quadriceps swelling, and soreness increased (p < 0.05) were overall fully resolved after 4 days (p > 0.05). CONCLUSION: Downhill running affected the knee extensors RVFD late but not early phase. The RVFD late phase may be used as an additional marker of muscle damage in trail running.

Hypoxia Does Not Change Performance and Psychophysiological Responses During Repeated Cycling Sprints to Exhaustion With Short Exercise-to-Rest Ratio.

PURPOSE: To compare the acute performance and psychophysiological responses of repeated cycling sprints to exhaustion with a short exercise-to-rest ratio (1:6), between different effort durations and inspired oxygen fractions. METHODS: On separate visits, 10 active participants completed 6 repeated cycling sprint exercises to exhaustion with 3 different effort durations (5, 10, and 20 s) and 2 conditions of inspired oxygen (20.9% and 13.6%). Exercise-to-rest ratio was 1:6 for all trials (ie, 5:30, 10:60, and 20:120). Vastus lateralis muscle oxygenation (near-infrared spectroscopy), blood lactate concentration, and lower-limb and breathing discomfort, using ratings of perceived exertion, were measured. RESULTS: Number of sprints and peak power output decreased while blood lactate increased (all P < .001) during 5:30 compared with 10:60 or 20:120. No condition or interaction effects were reported for blood lactate and exercise-related sensation. Muscle deoxyhemoglobin increased (P < .001) and total hemoglobin decreased (P = .002) during sprint with increasing sprint duration (no condition or interaction). CONCLUSION: During repeated-sprint exercise to exhaustion with a short exercise-to-rest ratio, the psychophysiological responses did not differ between normoxia and moderate hypoxia, probably due to an extended recovery period. It means that hypoxia did not modify repeated-sprint exercise performance with a short exercise-to-rest ratio. The sprint duration was the primary underlying factor of the observed differences in performance and muscle oxygenation reported between the repeated-sprint exercise sessions.

The repeated bout effect influences lower-extremity biomechanics during a 30-min downhill run.

The repeated bout effect in eccentric-biased exercises is a well-known phenomenon, wherein a second bout of exercise results in attenuated strength loss and soreness compared to the first bout. We sought to determine if the repeated bout effect influences changes in lower-extremity biomechanics over the course of a 30-min downhill run. Eleven male participants completed two bouts of 30-min downhill running (DR1 and DR2) at 2.8 m.s-1 and -11.3° on an instrumented treadmill. Three-dimensional kinematics and ground reaction forces were recorded and used to quantify changes in spatiotemporal parameters, external work, leg stiffness, and lower extremity joint-quasi-stiffness throughout the 30-min run. Maximum voluntary isometric contraction (MVIC) and perceived quadriceps pain were assessed before-after, and throughout the run, respectively. DR2 resulted in attenuated loss of MVIC (P = 0.004), and perceived quadriceps pain (P < 0.001) compared to DR1. In general, participants ran with an increased duty factor towards the end of each running bout; however, increases in duty factor during DR2 (+5.4%) were less than during DR1 (+8.8%, P < 0.035). Significant reductions in leg stiffness (-11.7%, P = 0.002) and joint quasi-stiffness (up to -25.4%, all P < 0.001) were observed during DR1 but not during DR2. Furthermore, DR2 was associated with less energy absorption and energy generation than DR1 (P < 0.004). To summarize, the repeated bout effect significantly influenced lower-extremity biomechanics over the course of a downhill run. Although the mechanism(s) underlying these observations remain(s) speculative, strength loss and/or perceived muscle pain are likely to play a key role.HighlightsA 30-min downhill running bout increased contact time and reduced flight time transitioning to an increased duty factor.Lower-extremity stiffness also decreased and mechanical energy absorption increased over the course of the first 30-min downhill running bout.When the same bout of 30-min downhill running was performed three weeks later, the observed changes to lower extremity biomechanics were significantly attenuated.The findings from this study demonstrated, for this first time, a repeated bout effect for lower extremity biomechanics associated with downhill running.

The Oxidative-Glycolytic Balance Influenced by Sprint Duration Is Key during Repeated Sprint in Hypoxia.

PURPOSE: This study investigates the effects of normobaric hypoxia on repeated sprint exercise (RSE) with different balance between oxidative (phosphocreatine and oxidative pathway) and glycolytic contributions. Therefore, performance and psychophysiological responses were compared during RSE to exhaustion with the same exercise-to-rest ratio (1:2) but different sprint durations (5, 10, or 20 s) either in normoxic (RSN) or hypoxic (RSH; F io2 = 0.13) conditions. METHODS: On separate visits, 10 active participants completed in random order three cycling RSN (5:10; 10:20 and 20:40) and three similar RSH sessions to exhaustion. Vastus lateralis muscle oxygenation was recorded by near-infrared spectroscopy. Blood lactate concentration, limb and breathing discomfort, and ratings of perceived exertion were measured. RESULTS: Total sprint number was smaller in hypoxia than in normoxia for 5:10 (20.8 ± 8.6 vs 14.7 ± 3.4; P = 0.014) and 10:20 (13.7 ± 6.3 vs 8.8 ± 2.5; P = 0.018) but not 20:40 (5.6 ± 1.9 vs 5.6 ± 2.5). The fatigue index was larger in hypoxia only for 5:10 (-43.5%, P < 0.001). Irrespective of condition, blood lactate concentration increased with the sprint duration with higher values for 20:40 than 5:10 (13.1 ± 2.7 vs 11.5 ± 2.2 mmoL·L -1 ; P = 0.027). Limb and breathing discomfort and ratings of perceived exertion did not differ in all RSE. Muscle oxygenation was mainly impacted by sprint duration (i.e., main effect of sprint duration on [HHb] min, [tHb] max, Δ[HHb], and Δ[tHb]) but not by hypoxia. The normoxia-to-hypoxia percentage decrease for total sprint number for 5:10 was correlated with the highest power output over 5 s ( R2 = 0.55; P = 0.013) and 10 s ( R2 = 0.53; P = 0.016). CONCLUSIONS: Hypoxia impairs repeated sprint ability when the oxidative but not the glycolytic contribution is substantial. The oxidative-glycolytic balance, influenced partly by sprint duration, is key during repeated sprint in hypoxia.

Downhill running affects the late but not the early phase of the rate of force development.

PURPOSE: This study aimed to evaluate the acute changes in the knee extensors maximum voluntary isometric contraction force (MVIC), rate of force development (RFD), and rate of EMG rise (RER) following a bout of downhill running. METHODS: MVIC and RFD at 0-50, 50-100, 100-200, and 0-200 ms were determined in thirteen men (22 ± 2 yr) before and after 30 min of downhill running (speed: 10 km h-1; slope: - 20%). Vastus lateralis maximum EMG (EMGmax) and RER at 0-30, 0-50, and 0-75 ms were also recorded. RESULTS: MVIC, RFD0-200, and EMGmax decreased by ~ 25% [Cohen's d = - 1.09 (95% confidence interval: - 1.88/- 0.24)], ~ 15% [d = - 0.50 (- 1.26/0.30)], and ~ 22% [d = - 0.37 (- 1.13/0.42)] (all P < 0.05), respectively. RFD100-200 was also reduced [- 25%; d = - 0.70 (- 1.47/0.11); P < 0.001]. No change was observed at 0-50 ms and 50-100 ms (P ≥ 0.05). RER values were similar at each time interval (all P > 0.05). CONCLUSION: Downhill running impairs the muscle capacity to produce maximum force and the overall ability to rapidly develop force. No change was observed for the early phase of the RFD and the absolute RER, suggesting no alterations in the neural mechanisms underlying RFD. RFD100-200 reduction suggests that impairments in the rapid force-generating capacity are located within the skeletal muscle, likely due to a reduction in muscle-tendon stiffness and/or impairments in the muscle contractile apparatus. These findings may help explain evidence of neuromuscular alterations in trail runners and following prolonged duration races wherein cumulative eccentric loading is high.

Performance fatigability and recovery after dynamic multi-joint maximal exercise in elbow flexors versus knee extensors.

Elbow flexors (EFs) and knee extensors (KEs) have shown differences in performance fatigability and recovery of neuromuscular function after isometric and isotonic single-joint fatiguing contractions. However, dynamic multi-joint movements are more representative of real-world activities. The aim of the study was to assess central and peripheral mechanisms of fatigability after either arm-cranking or cycling. Ten physically active men performed maximal incremental arm-cranking and cycling until task failure. Maximal voluntary isometric contraction (MVIC) and electrically evoked forces of both EF and KE were assessed before (PRE) and 1 (POST) and 20 (POST20) min after exercise. At POST, MVIC decreased similarly to 76 ± 8% and 81 ± 7% (both P < 0.001) of PRE for EF and KE, respectively. MVIC force remained lower than PRE at POST20 for both EF and KE (85 ± 8% vs. 95 ± 3% of PRE, P ≤ 0.033), having recovered less in EF than in KE (P = 0.003). Electrically evoked forces decreased similarly from PRE to POST in EF and KE (all P > 0.05). At POST20, the ratio of low-to-high frequency doublets was lower in EF than in KE (75 ± 13% vs. 85 ± 10% of PRE; P ≤ 0.034). Dynamic maximal incremental exercise acutely induced similar magnitudes of MVIC and evoked force loss in EF and KE. However, at POST20, impaired MVIC recovery and lower ratio of low-to-high frequency doublets in EF than in KE suggest the recovery of neuromuscular function after dynamic maximal exercises is specific to and dependent on changes within the muscles investigated.

Reliability of relaxation properties of knee-extensor muscles induced by transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS)-induced relaxation rate reflects intrinsic muscle contractile properties by interrupting the drive from the central nervous system during voluntary muscle contractions. To determine the appropriateness of knee-extensor muscle relaxation measurements induced by TMS, this study aimed to establish both the within- and between-session reliability before and after a fatiguing exercise bout. Eighteen participants (9 females, 9 males, age 25 ± 2 years, height 171 ± 9 cm, body mass 68.5 ± 13.5 kg) volunteered to participate in two identical sessions approximately 30 days apart. Maximal and submaximal neuromuscular evaluations were performed with TMS six times before (PRE) and at the end (POST) of a 2-min sustained maximal voluntary isometric contraction. Within- and between-session reliability of PRE values were assessed with intraclass correlation coefficient (ICC2,1, relative reliability), repeatability coefficient (absolute reliability), and coefficient of variation (variability). Test-retest reliability of post-exercise muscle relaxation rates was assessed with Bland-Altman plots. For both the absolute and normalized peak relaxation rates and time to peak relaxation, data demonstrated low variability (e.g. coefficient of variation ≤ 7.8%) and high reliability (e.g. ICC2,1 ≥ 0.963). Bland-Altman plots showed low systematic errors. These findings establish the reliability of TMS-induced muscle relaxation rates in unfatigued and fatigued knee-extensor muscles, showing that TMS is a useful technique that researchers can use when investigating changes in muscle relaxation rates both in unfatigued and fatigued knee-extensor muscles.

Neuromuscular, biomechanical, and energetic adjustments following repeated bouts of downhill running.

PURPOSE: This study used downhill running as a model to investigate the repeated bout effect (RBE) on neuromuscular performance, running biomechanics, and metabolic cost of running. METHODS: Ten healthy recreational male runners performed two 30-min bouts of downhill running (DR1 and DR2) at a -20% slope and 2.8 m/s 3 weeks apart. Neuromuscular fatigue, level running biomechanics during slow and fast running, and running economy parameters were recorded immediately before and after the downhill bouts, and at 24 h, 48 h, 72 h, 96 h, and 168 h thereafter (i.e., follow-up days). RESULTS: An RBE was confirmed by attenuated muscle soreness and serum creatine kinase rise after DR2 compared to DR1. An RBE was also observed in maximum voluntary contraction (MVC) force loss and voluntary activation where DR2 resulted in attenuated MVC force loss and voluntary activation immediately after the run and during follow-up days. The downhill running protocol significantly influenced level running biomechanics; an RBE was observed in which center of mass excursion and, therefore, lower-extremity compliance were greater during follow-up days after DR1 compared to DR2. The observed changes in level running biomechanics did not influence the energy cost of running. CONCLUSION: This study demonstrated evidence of adaptation in neural drive as well as biomechanical changes with the RBE after DR. The higher neural drive resulted in attenuated MVC force loss after the second bout. It can be concluded that the RBE after downhill running manifests as changes to global and central fatigue parameters and running biomechanics without substantially altering the energy cost of running.

Internal Tibial Forces and Moments During Graded Running.

The stress experienced by the tibia has contributions from the forces and moments acting on the tibia. We sought to quantify the influence of running grade on internal tibial forces and moments. Seventeen participants ran at 3.33 m/s on an instrumented treadmill at 0 deg, ±5 deg, and ±10 deg while motion data were captured. Ankle joint contact force was estimated from an anthropometrically-scaled musculoskeletal model using inverse dynamics-based static optimization. Internal tibial forces and moments were quantified at the distal 1/3rd of the tibia, by ensuring static equilibrium with all applied forces and moments. Downhill running conditions resulted in lower peak internal axial force (range of mean differences: -9% to -16%, p < 0.001), lower peak internal anteroposterior force (-14% to -21%, p < 0.001), and lower peak internal mediolateral force (-14% to -15%, p < 0.001), compared to 0 deg and +5 deg. Furthermore, downhill conditions resulted in lower peak internal mediolateral moment (-11%to -21%, p < 0.001), lower peak internal anteroposterior moment (-13% to -14%, p < 0.001), and lower peak internal torsional moment (-9% to -21%, p < 0.001), compared to 0 deg, +5 deg, and +10 deg. The +10 deg condition resulted in lower peak internal axial force (-7% to -9%, p < 0.001) and lower peak internal mediolateral force (-9%, p = 0.004), compared to 0 deg and +5 deg. These findings suggest that downhill running may be associated with lower tibial stresses than either level or uphill running.

Use of transcranial magnetic stimulation to assess relaxation rates in unfatigued and fatigued knee-extensor muscles.

We examined whether transcranial magnetic stimulation (TMS) delivered to the motor cortex allows assessment of muscle relaxation rates in unfatigued and fatigued knee extensors (KE). We assessed the ability of this technique to measure time course of fatigue-induced changes in muscle relaxation rate and compared relaxation rate from resting twitches evoked by femoral nerve stimulation. Twelve healthy men performed maximal voluntary isometric contractions (MVC) twice before (PRE) and once at the end of a 2-min KE MVC and five more times within 8 min during recovery. Relative (intraclass correlation coefficient; ICC2,1) and absolute (repeatability coefficient) reliability and variability (coefficient of variation) were assessed. Time course of fatigue-induced changes in muscle relaxation rate was tested with generalized estimating equations. In unfatigued KE, peak relaxation rate coefficient of variation and repeatability coefficient were similar for both techniques. Mean (95% CI) ICC2,1 for peak relaxation rates were 0.933 (0.724-0.982) and 0.889 (0.603-0.968) for TMS and femoral nerve stimulation, respectively. TMS-induced normalized muscle relaxation rate was - 11.5 ± 2.5 s-1 at PRE, decreased to - 6.9 ± 1.2 s-1 (- 37 ± 17%, P < 0.001), and recovered by 2 min post-exercise. Normalized peak relaxation rate for resting twitch did not show a fatigue-induced change. During fatiguing KE exercise, the change in muscle relaxation rate as determined by the two techniques was different. TMS provides reliable values of muscle relaxation rates. Furthermore, it is sufficiently sensitive and more appropriate than the resting twitch evoked by femoral nerve stimulation to reveal fatigue-induced changes in KE.

Effect of a Fatiguing Ultratrail on the Graded Energetically Optimal Stride Frequency.

PURPOSE: To study the consequences of a fatiguing ultratrail run of 6 hours on self-optimizing capability during uphill and downhill (DR) running. METHODS: The authors collected temporal stride kinematics and metabolic data in 8 (experimental group) male runners before and after the ultratrail run and in 6 (control group) male ultramarathon runners who did not run but stayed awake and performed normal, daily physical activities avoiding strenuous exercises over the 6-hour period. For each subject, preferred and optimal stride frequencies were measured, where stride frequency was systematically varied above and below the preferred one (±4% and ±8%) while running 3 conditions on level, 5% uphill, or 5% DR in a randomized order. RESULTS: Preferred and optimal stride frequencies across grade, group, and time showed no significant differences (P ≥ .184). Metabolic cost and the energetically optimum metabolic cost showed a grade × group × time interaction (P ≥ .011), with an ∼11% increase in the 2 variables only during the DR bouts (P ≥ .037). CONCLUSIONS: Despite maintaining similar dynamics of stride frequency adjustments during the DR bout, the experimental group was not able to optimize its gait. This suggests that the DR section of ultratrail runs can introduce a perturbing factor in the runners' optimization process, highlighting the need for incorporating DR bouts in the training programs of ultratrail runners to minimize the deleterious effects of DR on the energetically optimal gait.

Spinal contribution to neuromuscular recovery differs between elbow-flexor and knee-extensor muscles after a maximal sustained fatiguing task.

Data from studies of elbow-flexor (EF) or knee-extensor (KE) muscles suggest that a fatigue-related decrease in motoneuron excitability only occurs in EF. It is unknown how motoneuron excitability changes after sustained fatiguing maximal voluntary isometric contractions (MVICs) in EF and KE in the same participants. In two sessions, eight healthy men performed a 2-min MVIC of EF or KE to induce fatigue with brief MVICs before and six times after the 2-min MVIC. Electromyographic responses elicited by corticospinal tract stimulation at the transmastoid [cervicomedullary motor-evoked potential (CMEP)] or thoracic [thoracic motor-evoked potential (TMEP)] level were recorded from EF and KE, respectively. To account for muscle excitability, CMEPs and TMEPs were normalized to maximal M-wave (Mmax) elicited by peripheral nerve stimulation during each brief MVIC. Immediately after the 2-min MVIC, biceps brachii and brachioradialis CMEP/Mmax were 88% (SD 11%) (P = 0.026) and 87% (SD 12%) (P = 0.029) of pre-MVIC (PRE) values, respectively, and remained lower than PRE after 5 s of recovery [91% (SD 8%), P = 0.036 and 87% (SD 13%), P = 0.046, respectively]. No subsequent time points differed from PRE (all P ≥ 0.253). TMEP/Mmax for rectus femoris and vastus lateralis were not different from PRE at any time during the recovery period (all P > 0.050). A different recovery pattern in motoneuron excitability occurred in EF as it recovered by 60 s whereas KE motoneurons were unaffected by the fatiguing task. The present findings may contribute to better understand muscle-specific neurophysiological differences in spinal excitability.NEW & NOTEWORTHY By comparing the changes in motoneuron excitability in elbow-flexor and knee-extensor muscles after sustained fatiguing maximal voluntary contractions, this study shows that motoneuron recovery behavior depends on the muscle performing the exercise. A different recovery pattern in motoneuron excitability occurs in elbow flexors as it recovered by 60 s whereas knee extensors were unaffected by fatigue. This finding can help to increase understanding of the effect of a fatigue and subsequent recovery on neural processes.

Biomechanics of graded running: Part II-Joint kinematics and kinetics.

Compared to level running (LR), different strategies might be implemented by runners to cope with specific challenges of graded running at different speeds. The changes in joint kinetics and kinematics associated with graded running have been investigated, but their interactions with speed are unknown. Nineteen participants ran on an instrumented treadmill at five grades (0°, ±5° and ± 10°) and three speeds (2.50, 3.33 and 4.17 m/s), while 3D motion and forces were recorded. Three speed × five-grade repeated-measures ANOVA was used to analyze kinetic and kinematic variables. A speed × grade interaction was observed for hip range of motion (ROM). Downhill running (DR) at fastest speed did not reduce ROM at the hip, compared to LR. Compared to LR, it was observed that the hip joint was responsible for a greater contribution of energy generation while running at the fastest speed at +10°. Speed × grade interactions were also observed for the energy absorption, peak moment, and peak power at the knee. Contrary to LR, running faster during UR did not require higher peak power at the knee. Finally, DR at the fastest speed did not increase peak negative power at the knee compared to LR. This study demonstrates that ankle, knee, and hip joint kinetics depend on speed and grade of running, while the effect of grade on joint kinematics was not substantially modulated by speed.

Biomechanics of graded running: Part I - Stride parameters, external forces, muscle activations.

Biomechanical alterations with graded running have only been partially quantified, and the potential interactions with running speed remain unclear. We measured spatiotemporal parameters, ground reaction forces, and leg muscle activations (EMG) in nineteen adults (10F/9M) running on an instrumented treadmills at 2.50, 3.33, and 4.17 m·s-1 and 0, ±5°, and ±10°. Step frequency illustrated a significant speed × grade interaction (P < .001) and was highest (+3%) at the steepest grade (+10°) and fastest speed (4.17 m·s-1 ) when compared to level running (LR) at the same speed. Significant interaction was also observed for ground reaction forces (all P ≤ .047). Peak ground reaction forces in the normal direction increased with running speed during downhill running (DR) only (+9% at -10° and 4.17 m·s-1 ). Impulse in the normal direction decreased at fastest speed and steepest DR (-9%) and uphill running (UR) (-17%) grades. Average normal loading rate increased and decreased at fastest speed and steepest DR (+52%) and UR (-28%) grades, respectively. Negative parallel impulse increased and decreased at fastest speed and steepest DR (+166%) and UR (-90%), respectively. Positive parallel impulse decreased and increased at fastest speed and steepest DR (-75%) and UR (+111%), respectively. EMG showed comparable u-shaped curves across the grades investigated, although only a change in vastus lateralis and tibilias anterior activity was detectable at the steepest grades and fastest speed. Overall, running grade and speed significantly influences spatiotemporal parameters, ground reaction forces, and muscle activations.

Defining Off-road Running: A Position Statement from the Ultra Sports Science Foundation.

Off-road running continues to grow in popularity, with differing event categories existing, and terminologies are often used interchangeably and without precision. Trail running, mountain running, skyrunning, fell running, orienteering, obstacle course racing and cross-country running all take place predominantly in off-road terrain. Ultramarathon running refers to any running event over marathon distance conducted in any terrain and surface. Although some overlap may exist between these running events, mainly through the common denominator of off-road terrain, distinct features need to be recognised. As scientific interest in these activities grows, it becomes important to clarify these terms and develop a universal language for discussing these events. Similarly, off-road running athletes are generally not properly defined within the scientific literature, which makes intra- and inter-study comparisons difficult. The current position statement of the Ultra Sports Science Foundation highlights the different off-road running events and recommends clear reference to distance, surface, elevation change and altitude, type of event (continuous vs. staged), type of support, name and year of the event, governing body, and guidance on terminology. We further recommend to describe off-road running athletes by basic data, physiological determinants, training and competition characteristics in the scientific literature in order to facilitate and guide further research and practice.

Changes in spatio-temporal gait parameters and vertical speed during an extreme mountain ultra-marathon.

The aim of the present study was to investigate the effects of altitude and distance on uphill vertical speed (VS) and the main spatio-temporal gait parameters during an extreme mountain ultra-marathon. The VS, stride height (SH) and stride frequency (SF) of 27 runners were measured with an inertial sensor at the shank for two different altitude ranges (low 1300-2000 m vs high 2400-3200 m) of 10 mountains passes distributed over a 220 km course. There was a significant interaction (F(4,52) = 4.04, p < 0.01) for the effect of altitude and distance on VS. During the first passes, the mean VS was faster at lower altitudes, but this difference disappeared at a quarter of the race length, suggesting that neuromuscular fatigue influenced the uphill velocity to a larger extent than the oxygen delivery. The average VS, SH and SF were 547 ± 135 m/h, 0.23 ± 0.05 m and 0.66 ± 0.09 Hz. The individual VS change for each uphill portions was more strongly correlated with the changes in SH (r = 0.80, P < 0.001, n = 321) than SF (r = 0.43, P < 0.001, n = 321). This suggests a large effect of the knee extensors strength loss on the diminution of VS.

Step length and grade effects on energy absorption and impact attenuation in running.

Abstract We sought to examine the effect of step length manipulation on energy absorption and impact attenuation during graded running. Nineteen runners (10F, 9M) ran on an instrumented treadmill at three step lengths (preferred and ±10% preferred) at each of five grades (0°, ±5°, and ±10°) while 3D motion data were captured. Speed was held constant at 3.33 m/s and step length was manipulated by syncing cadence to a metronome. Manipulating step length altered energy absorption (p ≤ 0.002) and impact attenuation (p < 0.0001) across all grades. Energy absorption at the knee joint was most responsive to step length manipulations [Δ range (±10%SL-PrefSL) = 0.076-0.126 J/kg, p < 0.0001], followed by the ankle (Δ range = 0.026-0.100 J/kg, p = 0.001) and hip (Δ range = 0.008-0.018 J/kg, p < 0.006). Shortening step length reduced knee joint energy absorption at all grades with the smallest effect observed during uphill running (Δ ≥ -0.053 J/kg), while large reductions occurred during level (Δ = -0.096 J/kg) and downhill running (Δ ≥ -0.108 J/kg). Increasing step length resulted in greater knee joint energy absorption (p ≤ 0.037) across all grades of running. Impact attenuation was greatest at long step lengths (Δ = 2.708) and lowest at short step lengths (Δ = -2.061), compared to preferred. Overall, Step length influenced the energy absorption and impact attenuation characteristics of the lower extremity during level and graded running. Adopting a shorter step length may be a useful intervention to reduce knee joint loading, particularly during downhill or level running. Elongating step length placed a greater demand on the lower extremity joints, which may expedite the development of neuromuscular fatigue.