Original Research Hamstring Muscle Volume as an Indicator of Sprint Performance Sergi Nuell, 1 V´ıctor Illera-Dom´ınguez, 1 Gerard Carmona, 2,3 Paul Macadam, 4 Mario Lloret, 1 Josep Maria Padull ´ es, 1 Xavier Alomar, 5 and Joan Aureli Cadefau 1 1 National Institute of Physical Education of Catalonia (INEFC), University of Barcelona (UB), Barcelona, Spain; 2 Football Club Barcelona (FCB), Barcelona, Spain; 3 Tecnocampus, College of Health Sciences, University of Pompeu Fabra, Matar ´ o-Maresme, Spain; 4 Sports Performance Research Institute New Zealand (SPRINZ), AUT University, Auckland, New Zealand; and 5 White Cross Clinic, Barcelona, Spain Abstract Nuell, S, Illera-Dom´ınguez, V, Carmona, G, Macadam, P, Lloret, M, Padull ´ es, JM, Alomar, X, and Cadefau, JA. Hamstring muscle volume as an indicator of sprint performance. J Strength Cond Res 35(4): 902–909, 2021—This study aimed to compare mechanical properties and performance during sprinting, as well as thigh muscle volumes (MVs), between national-level sprinters and physically active males. In addition, the relationships between thigh MVs and sprint mechanical properties and performance were investigated. Seven male sprinters and 9 actives performed maximal‐effort 40-m sprints. Instantaneous velocity was measured by radar to obtain theoretical maximum force (F 0 ), the theoretical maximum velocity (V 0 ), and the maximum power (Pmax). For MV assessment, series of cross-sectional images of each subject’s thigh were obtained by magnetic resonance imaging for each of the quadriceps and hamstring muscles and the adductor muscle group. Sprinters were faster over 10 m (7%, effect size [ES] 5 2.12, p , 0.01) and 40 m (11%, ES 5 3.68, p , 0.01), with significantly higher V 0 (20%, ES 5 4.53, p , 0.01) and Pmax (28%, ES 5 3.04, p , 0.01). Sprinters had larger quadriceps (14%, ES 5 1.12, p , 0.05), adductors (23%, ES 5 1.33, p , 0.05), and hamstrings (32%, ES 5 2.11, p , 0.01) MVs than actives. Hamstrings MV correlated strongly with 40-m sprint time (r 520.670, p , 0.01) and V 0 (r 5 0.757, p , 0.01), and moderately with Pmax (r 5 0.559, p , 0.05). Sprinters were significantly faster and had greater V 0 and Pmax than active males. Larger MVs were found in sprinters’ thighs, especially in the hamstring musculature, and strong correlations were found between hamstring MV and sprint mechanical properties and sprint performance. Key Words: hamstring muscularity, acceleration, sprint mechanical properties, sprint training Introduction In team sports, acceleration capacity is crucial for performance due to the short, multidirectional movements that these activities require. By contrast, in track and field events, with longer distances, sprint per- formance is determined by acceleration capacity and maximum ve- locity. Although many factors may influence sprint performance, recent literature has shown that the ability to produce large amounts of hor- izontal ground reaction forces (GRFs) is the strongest predictor of acceleration and sprint performance regardless of the performance level (2,27,32). The overall mechanical capability to produce and apply horizontal forces throughout the sprint is well described by the inverse linear force-velocity (F-V) and parabolic power-velocity (P-V) rela- tionships, which characterize the mechanical limits of the entire neu- romuscular system (32). These individual F-V relationships describe the changes in horizontal force generation with increasing running velocity and are summarized by the following variables: the theoretical maxi- mum force (F 0 ), the theoretical maximum velocity (V 0 ), and the max- imum power (Pmax) (24,32). As the relationship between these variables encompasses the entire capability of the neuromuscular sys- tem, it includes properties of individual muscles, morphological fea- tures, and neural mechanisms underpinning motor-unit drive. Moreover, these mechanical properties of the sprint integrate the ability of the given athlete to orient the GRF horizontally (32). Sprinting is a complex movement pattern in which the body mass of the athlete is propelled forward by propulsive forces produced by lower-limb joint torques (37). It is known that muscle volume (MV) is the major determinant of joint torque in humans (11), hence, it can be hypothesized that MV plays an important role in sprint performance. When comparing sprinters to the average population, the differences in muscularity are no- ticeable (1,14). Because larger muscles will produce greater force and power (1,11,14), extremely large muscles might be expected to be advantageous for sprint performance. However, caution should be used with this affirmation because enlargement of a muscle increases the moment of inertia of the segment and, hence, reducing the limb’s angular acceleration for a given joint torque (14,37). Many studies have investigated the relationship of muscularity and sprint performance (17,18,21,31) using muscle thickness or muscle cross-sectional area (CSA), often reporting positive relationships between these measures and sprint perfor- mance. Nevertheless, it is known that changes in muscle shape are not uniform along the muscle; this highlights the limitations of a single measure, which might not be sufficient to accurately quantify hypertrophic changes (9,14,37). Thus, adopting MV, instead of muscle thickness or CSA, as a measure of muscularity may provide a more accurate assessment of sprint adaptations and their relationship with performance (37). In the previous literature, no clear understanding has yet been reached regarding thigh musculature and its association with sprint performance. For example, Sugisaki et al. (36) and Tottori et al. (38) proposed that muscularity of the quadriceps and adductors is Address correspondence to Joan Aureli Cadefau, jcadefau@gencat.cat Journal of Strength and Conditioning Research 35(4)/902–909 ª 2021 National Strength and Conditioning Association 902 Copyright © 2021 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.