Healthcare 2021, 9, 318. https://doi.org/10.3390/healthcare9030318 www.mdpi.com/journal/healthcare Article Variations of Trail Runner’s Fitness Measures across a Season and Relationships with Workload Sérgio Matos 1,2,3, *, Filipe Manuel Clemente 2,4 , Rui Silva 2 , Joel Pereira 2,5,6 , Pedro Bezerra 2,6 and José María Cancela Carral 1 1 Faculty of Educational Sciences and Sports Sciences, University of Vigo, 36005 Pontevedra, Spain; chemacc@uvigo.es 2 Escola Superior Desporto e Lazer, Instituto Politécnico de Viana do Castelo, Rua Escola Industrial e Comercial de Nun’Álvares, 4900-347 Viana do Castelo, Portugal; filipe.clemente5@gmail.com (F.M.C.); rui.s@ipvc.pt (R.S.); joelpereira@esdl.ipvc.pt (J.P.); pbezerra@esdl.ipvc.pt (P.B.) 3 Douro Higher Institute of Educational Sciences, 4560-708 Penafiel, Portugal 4 Instituto de Telecomunicações, Delegação da Covilhã, 1049-001 Lisboa, Portugal 5 Unidade de Investigação e Treino em Trabalhos em Alturas e Atividades de Ar Livre, Escola Superior Desporto e Lazer, 4960-320 Melgaço, Portugal 6 The Research Centre in Sports Sciences, Health Sciences and Human Development, 5001-801 Vila Real, Portugal * Correspondence: sergioms@esdl.ipvc.pt Abstract: Trail running involves off-road running over different surfaces of positive and negative unevenness. Given these particularities and the associated physical demands, it is essential to un- derstand this relationship and how fitness levels influence performance. This study aimed to ana- lyze fitness level variations during different times of the season and establish a relationship between changes in fitness levels and accumulated load. Twenty-five trail running athletes (age: 36.23 ± 8.30 years) were monitored over 52 weeks. Three periods of assessment were implemented, while load between those periods was calculated. Athletes were monitored daily by global positioning sys- tems. The collected data included distance covered, duration, and rate of perceived exertion (RPE), which were used to obtain session-RPE. Additionally, maximal aerobic speed, vertical jump, and dynamic balance were tested periodically. Moderate inverse correlations were found between as- sessment 1 and 2 for total sRPE and vertical jump: countermovement jump (VJ: CMJ) (r = −0.349), and Y balance test: left posterolateral (YBT: LPL) (r = −0.494). Similar correlations were found be- tween assessment 2 and 3 for total sRPE and VJ: CMJ (r = −0.397), and vertical jump: drop jump (VJ: DJ) (r = −0.395). The results suggest that trail running coaches should monitor and assess dose– response relationships and possible anterior asymmetries of dynamic balance performance. Keywords: endurance sports; trail running; fitness levels; performance; training monitoring 1. Introduction Athletes of any sport aspire to achieve their best performance at the point of competi- tion through training [1]. Although the training process is crucial, several conditions can influence an athlete’s physiological, psychological, and biomechanical factors—even their genetics and age [1]. The dose–response effects resulting from training’s relationship with all associated factors can either enhance or hinder performance [2]. The complexity of the relationship between training and an athlete’s physical and physiological levels is consid- ered crucial throughout the training process [3]. In this sense, monitoring the training load is essential for determining the dose–response relationship between training and the athlete [4]. Thus, such monitoring can reveal the ideal balance that will improve the athlete’s per- formance [5]. Training load can be monitored based on either internal or external load [6]. External load is related to the work done by the athlete (e.g., distance, acceleration, and speed), while Citation: Matos, S.; Clemente, F.M.; Silva, R.; Pereira, J.; Bezerra, P.; Cancela Carral, J.M. Variations of Trail Runner’s Fitness Measures Across a Season and Relationships with Workload. Healthcare 2021, 9, 318. https://doi.org/10.3390/ healthcare9030318 Academic Editor: Pedram Sendi Received: 1 March 2021 Accepted: 10 March 2021 Published: 12 March 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and insti- tutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses /by/4.0/).