Abstract
s study aimed to assess the effect of a downhill-running (DR) bout on muscle damage biomarkers. It also examined whether training background and gait kinematics may influence DR-induced muscle damage and strength loss. Thirty-six experienced trail runners (25 men, 11 women), participants of a 106 km ultra-trail, performed a 5 km DR bout at 15% decline and at an intensity equivalent to their first ventilatory threshold. Muscle damage biomarkers (creatine kinase, lactate dehydrogenase, and myoglobin) were analyzed before and 30 min after the DR protocol, and also before and after the UT race. Isometric strength was assessed before and after DR, and gait parameters were recorded during DR. All muscle damage biomarkers increased following DR (d= 0.19 to 1.85). Lactate dehydrogenase concentrations after the race and DR were associated (r= 0.64). Athletes who habitually performed downhill repetitions showed reduced creatine kinase (182±73 U/L vs. 290±192 U/L;p< 0.05; d = 0.64) and greater squat strength retention (4±10% vs.−9.1±16.8%;p<0.05;d= 0.87). Ankle plantar flexion and squat strength retention were inversely correlated with vertical oscillation
biomarkers increased following DR (d= 0.19 to 1.85). Lactate dehydrogenase concentrations after the race and DR were associated (r= 0.64). Athletes who habitually performed downhill repetitions showed reduced creatine kinase (182±73 U/L vs. 290±192 U/L;p< 0.05; d = 0.64) and greater squat strength retention (4±10% vs.−9.1±16.8%;p<0.05;d= 0.87). Ankle plantar flexion and squat strength retention were inversely correlated with vertical oscillation (r=−0.44) and step length (r=−0.37), respectively. In summary, lactate dehydrogenase response to a short DR bout could indicate an athlete’s readiness to handle ultra-trail-induced muscle damage, although further research is needed to confirm it. In addition, despite the exploratory nature of the study, regularly performing downhill intervals and adopting a more terrestrial gait pattern appear to soften strength loss and muscle damage response to DR. Keywords:lactate dehydrogenase; downhill running intervals; downhill running gait; vertical oscillation; step length 1. Introduction The rate of speed decrease during an ultra-trail (UT) race is usually greater on descents and flat sections than on ascents [1]. Minimizing speed loss on downhill sections toward Sports2026,14, 12 https://doi.org/10.3390/sports14010012
Sports2026,14, 12 2 of 12 the end of UT races appears to be a trait of top performers [2]. It has been suggested that faster finishers can maintain greater relative performance in downhill sections not only because of their superior aerobic capacity, but also, most importantly, because of less exercise-induced muscle damage (EIMD) and fatigue [2]. In view of this perspective, slower runners would self-regulate to a lesser intensity in downhill sections to prevent more severe musculoskeletal damage and reduce pain perception [2,3]. After the Ultra Trail du Mont-Blanc or Western States Endurance Run, mean post-race creatine kinase (CK) concentrations can exceed 15,000 U/L [4,5]. Much of this muscle damage occurs during downhill sections, as the eccentric phase of the stretch-shortening cycle is intensified [6]. Muscle damage is thus proposed as a main performance-limiting factor in mountain running [7–9]. Even in flat marathons, greater resistance to EIMD may help maintain speed in the second half of the race, as runners who slow down the most tend to show the greatest post-race muscle damage, while those with more even pacing have reduced muscle damage markers [10]. Indeed, it has recently been recommended to assess muscle damage from downhill running (DR) to gauge athletes’ tolerance to UT-specific muscle damage and explore training strategies to mitigate it [11]. Prior exposure to DR has been shown to reduce markers of muscle damage and strength loss, creating a repeated bout effect (RBE) in untrained individuals and recre- ational runners [12,13]. Four weeks (10 sessions) of DR training in previously untrained individuals also promoted neuromuscular adaptations typical of high-intensity eccentric resistance training [14]. However, it is unclear whether regularly performing DR intervals in experienced trail runners (TR) provides extra protection against muscle damage and strength impairment. In experienced TR, high-intensity DR causes strength losses in knee extensors and plantar flexors similar to those after a UT race [15], and much greater than those after uphill running at similar oxygen uptake [16]. Therefore, trail runners who regu- larly perform DR repetitions may experience less muscle damage and strength loss after DR. Strength training, especially isometric exercises at
In experienced TR, high-intensity DR causes strength losses in knee extensors and plantar flexors similar to those after a UT race [15], and much greater than those after uphill running at similar oxygen uptake [16]. Therefore, trail runners who regu- larly perform DR repetitions may experience less muscle damage and strength loss after DR. Strength training, especially isometric exercises at long muscle lengths, has shown similar protective effects in untrained individuals [17,18]. No studies, however, have examined this effect in experienced trail runners. The aim of this study was four-fold: (i) to assess the immediate effect of a DR bout on muscle damage biomarkers in experienced TR; (ii) to explore whether muscle damage after a 5 km DR bout correlates with damage after a UT race; (iii) to explore whether DR-induced muscle damage and strength loss differ among athletes who typically perform strength training and DR intervals; (iv) to explore possible links between DR-gait and DR-induced muscle damage and strength loss. Our hypotheses were: (1) muscle damage following a 5 km DR bout would correlate with that after a UT race; (2) athletes who regularly perform strength training and downhill repetitions would suffer less post-DR strength loss and muscle damage; (3) DR-induced muscle damage and strength loss would be associated with smaller step length and lower vertical oscillation. 2. Materials and Methods 2.1. Participants Considering the effect size found in a previous work (large, >0.8) who assessed the change in plasma myoglobin (Mb) immediately after a DR protocol similar to ours [19], a sample size of 19 participants was deemed appropriate to find significant within-group differences in the present study (1-tailedα< 0.05, 1-β> 0.95) (Gpower, version 3.1.9.7, Universität Düsseldorf, Düsseldorf, Germany). Thirty-six experienced TR (25 men and 11 women) finally joined the study. The inclusion criteria were: participation in the 2025 Penyagolosa Trails CSP race and completion of at least two races longer than 65 km. The exclusion criteria were: having any cardiac or renal disease and taking any medication https://doi.org/10.3390/sports14010012
joined the study. The inclusion criteria were: participation in the 2025 Penyagolosa Trails CSP race and completion of at least two races longer than 65 km. The exclusion criteria were: having any cardiac or renal disease and taking any medication https://doi.org/10.3390/sports14010012
Sports2026,14, 12 3 of 12 on a regular basis. The research took place in the months leading up to and during the 2025 PenyagolosaTrails CSP race. The racetrack was 106.1 km long, starting at 65 m and ending at 1280 m above sea level, with total positive and negative elevations of 5584 m and 4369 m, respectively. All participants were fully informed about the procedure and provided written consent. Participation was voluntary, with the option to withdraw at any time. A questionnaire was used to collect demographic, training, and competition information. The investigation was conducted in accordance with the Declaration of Helsinki and received approval from the Research Ethics Committee of the University Jaume I of Castellon (reference number CEISH/103/2024). The study was registered at ClinicalTrails.gov with code NCT06969898 (www.clinicaltrials.gov, accessed on 5 May 2025). 2.2. Experimental Overview This study was part of a larger research project. In brief, participants visited the laboratory twice, two weeks apart. Body composition was assessed at the start of each visit. On the first visit, 6–8 weeks before the race, participants completed a cardiopulmonary exercise test (CPET) and familiarized themselves with the downhill protocol scheduled for the second visit. The second visit, 4–6 weeks prior to the race, began with a blood draw and muscle morphology assessment, followed by isometric strength tests and an uphill walking economy test. Participants then performed the DR protocol. Afterward, they repeated the isometric strength and uphill walking economy tests. Muscle morphology was reassessed, and a second blood sample was collected after the DR. This paper focuses on muscle damage biomarkers, training background, and DR-gait kinematics data. Participants attended both visits after fasting for more than six hours and maintained their regular mixed macronutrient diet the day prior to testing. Body Mass Index (BMI), fat mass percentage (%FM), and lean body mass percentage (%LBM) were assessed using a bioelectrical impedance weight scale (Tanita BC-780MA, Tanita Corp., Tokyo, Japan). Measurements were taken while participants wore minimal clothing (running shorts and a t-shirt), in accordance with the manufacturer’s guidelines. Skin and electrodes were thoroughly cleaned and dried before assessment. On the
testing. Body Mass Index (BMI), fat mass percentage (%FM), and lean body mass percentage (%LBM) were assessed using a bioelectrical impedance weight scale (Tanita BC-780MA, Tanita Corp., Tokyo, Japan). Measurements were taken while participants wore minimal clothing (running shorts and a t-shirt), in accordance with the manufacturer’s guidelines. Skin and electrodes were thoroughly cleaned and dried before assessment. On the second visit, prior to the DR protocol, participants consumed an energy bar or gel containing 60 g of carbohydrates [20]. Resistance training, DR, and vigorous running were prohibited for 48 h before testing, and any training was not allowed within 24 h. Upon arrival at the laboratory, all pre-trial standardization procedures were confirmed verbally with each participant. 2.3. Cardiopulmonary Uphill Incremental Test CPETs were performed on a treadmill (SK7990, BH Fitness, Vitoria, Spain) with a 20% constant slope. After a 3 min warm-up at 2.5 km·h −1 , corresponding to a vertical velocity of 500 m·h −1 , speed was increased 0.5 km·h −1 every minute (equivalent to a vertical velocity of 100 m·h −1 ) until volitional exhaustion [21]. We opted for a lower gradient than suggested in the original protocol (25%), that it was tested in highly trained males, because our sample consisted of males and females with greater heterogeneity in fitness levels. Accordingly, we aimed to avoid premature muscle fatigue that could, in turn, hinder the attainment of maximal oxygen uptake (VO2max). Participants were free to walk or run as they preferred. Expired gases and heart rate were collected continuously using indirect calorimetry (Quark CPET, COSMED ® , Rome, Italy) and a chest-strap heart rate monitor (H10, Polar Electro Oy, Kempele, Finland). The gas analysis system was calibrated (including replacing the sample line and turbine) before each test to improve stability and sensitivity of the instrumentation [22]. VO2max values were accepted when a plateau (an increase of <2 mL/kg/min) or a decline in VO2was reached despite increasing workloads. If these criteria were not met, a VO2peak value was taken, defined as the greatest VO2 https://doi.org/10.3390/sports14010012
test to improve stability and sensitivity of the instrumentation [22]. VO2max values were accepted when a plateau (an increase of <2 mL/kg/min) or a decline in VO2was reached despite increasing workloads. If these criteria were not met, a VO2peak value was taken, defined as the greatest VO2 https://doi.org/10.3390/sports14010012
Sports2026,14, 12 4 of 12 measured over a 30 s period. First and second ventilatory thresholds (VT1and VT2) were determined using the guidelines of Skinner and McLellan [23] by two independent researchers. Peak vertical velocity (Vvertpeak) and vertical velocities corresponding to VT1 and VT2 (VvertVT1, VvertVT2) were calculated as the speed of the last complete stage added to the multiplication of the speed increment by the completed fraction of the incomplete stage [24]. 2.4. Downhill Running Protocol The DR protocol consisted of 5 km at 15% constant decline, with the intention of mimicking descents typical of major UT races [11]. Negative gradient was achieved by elevating the rear part of the treadmill with a custom-made platform and verifying the inclination using a goniometer. Two researchers were on each side of the treadmill throughout the test as a safety precaution to minimize the risk of falling. During the first visit, following the end of the CPET, the speed for the DR protocol was individually set to coincide the HR with the HR at uphill VT1. Briefly, participants walked at 6 km·h −1 for 1 min, ran at 10 km·h −1 for 3 min, and afterwards speed was set at 300% of their uphill VvertVT1. This reference was based on pilot work and previous studies indicating that VT1 on a−15% gradient was≈2.8 times greater than at a 15% gradient in trained TR [25]. After a 1 min stabilization period for HR, speed was manually adjusted until the HR coincided (±5%) with the one at uphill VT1. This procedure also served as a familiarization for the second visit test. ‘Downhill-adjusted’ VT1speed was 13.3±1.6 km·h −1 (325±9% of uphill VT1speed). Step length (SL), step frequency (SF), ground contact time (GCT), and vertical oscillation (VO) were measured using the Stryd powermeter (Stryd Inc., Boulder, CO, USA) [26–28]. 2.5. Isometric Ankle Plantar Flexion and Half-Squat Strength Tests Participants were familiarized with procedures concerning strength assessment during an informative session prior to the investigation. Isometric maximal voluntary contraction (MVC) was measured with a force sensor (Chronojump, Barcelona, Spain) [29] held onto a bar using a custom-adapted Smith
were measured using the Stryd powermeter (Stryd Inc., Boulder, CO, USA) [26–28]. 2.5. Isometric Ankle Plantar Flexion and Half-Squat Strength Tests Participants were familiarized with procedures concerning strength assessment during an informative session prior to the investigation. Isometric maximal voluntary contraction (MVC) was measured with a force sensor (Chronojump, Barcelona, Spain) [29] held onto a bar using a custom-adapted Smith machine. Bar height for each participant and test was individually set and firmly anchored to the ground using chain and pins to impede any movement. For the ankle plantar flexion (PF) assessment, participants stood upright with their hips and knees fully extended and their ankles in a 0-degree position of plantar flexion. For the half-squat (SQ) assessment, they adopted a knee angle of 140 degrees in the ready position [30]. Each test was performed twice, and the best performance was retained for statistical analysis. A 1 min rest was provided between attempts. For each contraction, participants were instructed to exert maximal upward force against the bar, mimicking the pattern of a half-squat and a calf extension, respectively. Each contraction was initiated following a verbal prompt from the researcher, and consistent verbal encouragement was provided throughout the process. The maximum force produced was modeled using the inverse monoexponential function that better fitted the raw data. This fitting was made by adjusting the maximum force to the speed at which the maximum force was reached. This method was used to more accurately estimate MVC, accounting for variations in the rate of force development and potential signal noise, as per [31]. 2.6. Blood Sampling and Analysis Blood samples were collected from an antecubital vein by venipuncture. Collection was performed by experienced nurses using BD Vacutainer PST II tubes on four occa- sions: before and 30 min after the DR protocol -as previously suggested [11]-, before and 30 min after the race. Samples were centrifuged at 3500 rpm for 10 min and maintained at 4 ◦ C during transport to the Castellon Provincial Hospital Consortium, where they https://doi.org/10.3390/sports14010012
and 30 min after the DR protocol -as previously suggested [11]-, before and 30 min after the race. Samples were centrifuged at 3500 rpm for 10 min and maintained at 4 ◦ C during transport to the Castellon Provincial Hospital Consortium, where they https://doi.org/10.3390/sports14010012
Sports2026,14, 12 5 of 12 were processed using a Beckman Coulter DXC 700 AU analyzer (Beckman Coulter, Inc., Brea, CA, USA) [32]. The following blood variables were considered for analysis: lactate dehydrogenase (LDH), CK, and Myoglobin (Mb). Post-race values were adjusted using the Dill and Costill method [33], which employed hematocrit and hemoglobin to determine the magnitude of plasma volume changes in each participant after the race [33,34]. 2.7. Statistical Analysis Statistical analyses were performed using the Statistical Package for the Social Sciences software (IBM SPSS Statistics for Windows, version 30; IBM Corp., Armonk, NY, USA). Normality was checked using the Shapiro–Wilk test, and all variables met normality assumptions. Paired-samplest-tests were used to assess the effect of DR on muscle damage biomarkers (CK, LDH, and Mb). The pre-to-post DR change in PF, SQ, LDH, CK, and Mb was compared between participants who usually perform strength training (a minimum of 2 sessions a week) and downhill repetitions (a minimum of 1 session a week), and those who do not during the 3 months prior to the race, using unpaired-samplest-tests. Homogeneity of variance was verified by Levene’s test. CK values distribution as a function of downhill repetitions engagement did not meet the equality of variances assumption. Accordingly, a one-way ANOVA with the Welch statistic was used in that case. Lastly, Pearson product- moment correlations were computed to explore: (1) whether DR-induced muscle damage was correlated with post-race muscle damage in the finishers sample set;(2) whether muscle damage and strength loss (∆PF and∆SQ) provoked by DR were associated with running gait. The meaningfulness of the outcomes was estimated through Cohen’s d effect size and 95% confidence intervals (CIs). A Cohen’s d < 0.5 was considered small; between 0.5 and 0.8, moderate; and greater than 0.8, large. Likewise,correlations > 0.5 were considered strong, 0.3–0.5, moderate, and <0.3, small. The significance level was set at p< 0.05, and data are presented as means and standard deviations (±SD). 3. Results Participants’ characteristics, including demographic information, training and com- petition history, and data from the CPET, are presented in Table. All muscle damage biomarkers significantly increased from
Description
The study explores muscle damage in trail runners after downhill running.