Prior to this study, the relationship between knee extensor strength and sprint performance with each relation to quadriceps femoris size in sprinters had not been explored. In this study, we determined that higher fast-velocity isokinetic knee extensor strength variables are correlated with better personal best 100-m sprint time in sprinters. In contrast, all quadriceps femoris size variables did not correlate with personal best 100-m sprint time. Moreover, we found that the correlation between absolute fast-velocity isokinetic knee extensor strength and personal best 100-m sprint time in sprinters was independent of the size of the quadriceps femoris. Furthermore, we determined that absolute fast-velocity isokinetic knee extensor strength was the most predictive variable for the personal best 100-m sprint time in sprinters. These findings suggest that the fast-velocity knee extensor strength may be an important variable for superior 100-m sprint performance in sprinters. Therefore, this study is the first to determine the positive relationship between knee extensor strength and sprint performance in sprinters independently of the quadriceps femoris muscularity.
Several previous studies have reported that higher isokinetic knee extensor strength is related to better sprint performance in sprinters [6, 7]; however, prior to the present study, further understanding of this relationship had not progressed in recent years. Alexander et al. [6] reported a negative correlation between fast-velocity isokinetic knee extensor strength (i.e., 230º/s) and personal best 100-m sprint time in 14 male sprinters. Dowson et al. [7] also reported negative correlations between absolute and relative strengths of three velocity isokinetic knee extensions (i.e., 60 to 240º/s) and 15-m sprint time in 24 male athletes, including 8 sprinters. Furthermore, they reported that these correlations were stronger with fast-velocity (i.e., 150 and 240º/s) than with slow-velocity (i.e., 60º/s). These previous findings suggest that isokinetic knee extensor strength, especially during faster-velocity contraction, may be related to better sprint performance in sprinters. In the present study, we determined that absolute and relative knee extensor strengths during fast-velocity isokinetic contraction, but not during isometric and slow-velocity isokinetic contractions, are negatively correlated with better personal best 100-m sprint time in 58 sprinters. Therefore, with a relatively large sample size of sprinters, the present findings corroborate the results in previous studies [6, 7].
Using ultrasonography (US), Kubo et al. [27] reported that absolute muscle thickness (MT) of the anterior thigh (i.e., quadriceps femoris) is negatively correlated with personal best 100-m sprint time in sprinters. Monte and Zamparo [28] also reported that absolute MTs of the four quadriceps femoris muscles are negatively correlated with personal best 100-m sprint time in sprinters. Nevertheless, MRI is known to be a more appropriate apparatus to measure muscle size than US [29,30,31]. Using MRI, our previous studies reported that absolute and relative (i.e., normalized to body mass) middle CSAs of the quadriceps femoris did not correlate with personal best 100-m sprint time in sprinters [15, 19]. Moreover, Sugisaki et al. [18] analyzed MRI-measured MV, which is the most appropriate marker of muscle size [29, 30], and reported no correlations between absolute and relative quadriceps femoris MVs and personal best 100-m sprint time in sprinters. Similarly, some previous studies have reported that MRI-measured absolute and/or relative quadriceps femoris MVs did not correlate with sprint performance in sprinters [10, 16, 17]. In the present MRI study, we determined that although a trend against significance negative correlation was observed between absolute middle quadriceps femoris CSA and personal best 100-m sprint time, other quadriceps femoris size variables did not correlate with personal best 100-m sprint time in sprinters. Therefore, the present findings corroborate the results of previous studies [15,16,17,18,19]. Altogether, we suggest that greater quadriceps femoris may not be an essential morphological factor for achieving better 100-m sprint performance in sprinters.
Because muscle strength (i.e., joint torque) is theoretically expressed as the product of muscle force and joint moment arm (MA) dimension, the magnitude of the knee extensor strength is determined not only by the quadriceps femoris size but also by the knee extensor MA. Previous studies have reported a positive correlation between muscle strength and MA of knee extensors in untrained individuals [11,12,13,14]. Furthermore, our previous study determined that the knee extensor MA is negatively correlated with personal best 100-m sprint time in sprinters [15]. This may be due to the potential relationship between knee extensor strength and MA; however, no study has examined this relationship in sprinters. If this relationship is observed for sprinters, it may help our understanding of the present findings that higher knee extensor strength is related to better 100-m sprint performance in sprinters independently of the quadriceps femoris size.
As another explanation in terms of morphological factors, the muscle fascicle length may be involved in determining muscle strength [11, 14, 32]. Blazevich et al. [11] reported that the fascicle length of the quadriceps femoris muscle (i.e., vastus lateralis) was a positive predictive variable for the fast-velocity isokinetic strength, but not the isometric and slow-velocity isokinetic strengths, of knee extensors in untrained individuals. Additionally, previous studies have reported that the fascicle length of the quadriceps vastus lateralis is negatively correlated with better 100-m sprint performance in sprinters [33, 34]. Therefore, in addition to the knee extensor MA, muscle fiber fascicle length of the quadriceps femoris may also be an important morphological factor that contributes to our understanding of the present findings, especially by showing the relationship between fast-velocity isokinetic knee extensor strength and 100-m sprint performance in sprinters.
In terms of physiological factors, the muscle fiber composition is known to be an important factor for determining muscle strength [35,36,37]. Previous studies have reported that the fast-twitch fiber composition of the quadriceps vastus lateralis is positively correlated with isokinetic knee extensor strength [35,36,37]. In particular, this relationship is observed for faster-velocity contraction more than for slower-velocity contraction of knee extension [35,36,37]. Furthermore, previous studies have reported that the fast-twitch fiber composition of the quadriceps vastus lateralis is positively correlated with sprint performance in athletes, including sprinters [38, 39]. Therefore, in addition to the morphological factors, these previous findings may help our understanding of the present findings.
The results of this study showed that absolute strengths of the two velocity isokinetic knee extensions were higher in sprinters than in body size-matched non-sprinters, whereas no such a difference was observed for absolute isometric knee extensor strength. Similar results were also observed for the relative knee extensor strengths normalized to body mass. In a previous study, Miller et al. [10] reported that although absolute and relative isometric strengths of knee extension were higher in sub-elite sprinters than in untrained individuals, these strength variables did not differ between elite sprinters and untrained individuals. Their findings suggest that higher isometric knee extensor strength may not be required for successful sprinters. Thus, sprinters may be specifically characterized by a higher isokinetic strength, but not isometric strength, of knee extensors. Additionally, in the present study, we found that effect sizes of the differences in the two velocity isokinetic knee extensor strengths between sprinters and non-sprinters were relatively larger with fast-velocity than with slow-velocity (i.e., 0.60 [medium] and 0.41 [small], respectively), indicating that higher isokinetic knee extensor strength of sprinters may be conspicuous for fast-velocity contraction than for slow-velocity contraction. These findings may be a natural adaptation because sprint training is performed with fast dynamic movements. In addition to this reason, previous studies have reported that the knee extensor MA dimension and quadriceps femoris fascicle length were longer in sprinters than in non-sprinters [15, 33, 34]. Moreover, the fast-twitch fiber composition of the quadriceps femoris is higher in sprinters than in non-sprinters [40, 41]. Furthermore, the magnitude of neural activity (i.e., electromyographic activity) during muscle contraction is known to be a major determinant of muscle strength [14, 42]. Indeed, previous studies have reported that neural activity level during isokinetic contraction may be higher in sprinters than in non-sprinters [43, 44]. These findings may contribute to our understanding of the difference in isokinetic knee extensor strength, especially during fast-velocity contraction, between sprinters and non-sprinters.
Kubo et al. [27] reported that the absolute quadriceps femoris MT did not differ between sprinters and body size-matched non-sprinters. Our previous study also reported that no difference for the absolute and relative middle quadriceps femoris CSAs between sprinters and body size-matched non-sprinters [15, 19]. In contrast, the present study observed a trend against significance with a greater absolute quadriceps femoris MV in sprinters than body size-matched non-sprinters. Furthermore, the relative quadriceps femoris MV normalized to body mass was greater in sprinters than in non-sprinters, which corroborates the result of a study by Miller et al. [10]. These present results might be mainly because of a greater proximal quadriceps femoris CSA in sprinters than non-sprinters. Abe et al. [45] reported that MTs at the proximal and middle regions of the quadriceps femoris were greater in sprinters than in non-sprinters; however, body mass was heavier in sprinters than in non-sprinters. Several previous studies have reported that absolute and relative MVs of the rectus femoris, but not of other three quadriceps femoris muscles, were greater in sprinters than in body size-matched non-sprinters [17, 46]. Composition of the rectus femoris CSA relative to a total of the four quadriceps femoris CSAs is higher at the proximal region than at the middle and distal regions [47]. Therefore, greater MV and proximal CSA of the quadriceps femoris in sprinters than non-sprinters observed in this study may be attributed to a specific hypertrophy of the rectus femoris among the quadriceps femoris n sprinters.
This study has some limitations. In the present study, body mass was used mainly as a variable for normalizing the knee extensor strength and quadriceps femoris size variables. It is well known that sprinters have a lower body fat percentage and whole-body fat mass than non-sprinters [33, 48]. Thus, although we observed that body mass did not differ between the two groups, whole-body muscle mass might be greater in sprinters than in non-sprinters. This possibility may be partially indicated by the present result that the quadriceps femoris MV relative to body mass was greater in sprinters than in non-sprinters. Considering these findings, to compare the knee extensor strength and quadriceps femoris size variables between the two groups, it would be more appropriate to normalize these variables with whole-body muscle mass than with body mass. As an alternative to this concern, this study attempted to normalize the knee extensor strengths with quadriceps femoris MV as a regional muscle mass, which can be considered as the underlying strength of the knee extensors [12, 13]. In the results, the relative knee extensor strength during fast-velocity isokinetic contraction, but not during isometric and slow-velocity contractions, was greater in sprinters than in non-sprinters. This finding can be established the fact of a higher fast-velocity isokinetic knee extensor strength in sprinters than non-sprinters. Nevertheless, Abe et al. [49] reported that body fat percentage is positively correlated with personal best 100-m sprint time in sprinters. Furthermore, a series of studies by Abe et al. [33, 49,50,51] suggested that a lower body fat percentage is associated with better sprint performance in sprinters. Based on their findings, body fat percentage and/or whole-body fat mass would be required to clarify the relationship between fast-velocity knee extensor strength and personal best 100-m sprint time in sprinters. Therefore, the lack of body composition measurement in the present study is a major limitation.
In addition to the lack of body composition measurement, we suggested that the results of this study are affected by other morphological (i.e., joint MA dimension and muscle fascicle length), physiological (i.e., fast-twitch fiber composition), and neuromuscular factors. Thus, the lack of measurements related to these factors is also considered a limitation of this study. Furthermore, we did not measure biomechanical data during 100-m sprinting in sprinters. Thus, we cannot explain in detail the potential impact of the knee extensor strength on 100-m sprint performance. Nevertheless, the higher knee extensor strength appears to help increasing peak vertical ground reaction force during the stance phase while 100-m sprinting [1,2,3]. The increased peak vertical ground reaction force contributes to shortening contact time and increasing step frequency [1, 4]. These kinetic and kinematic variables are important biomechanical determinants for superior 100-m sprint performance [4, 5]. Although the present study determined significant correlations between fast-velocity knee extensor strength variables and 100-m sprint performance in sprinters, these correlations were low (i.e., r = −0.409 to −0.477). Because superior sprint performance is determined by various factors, further studies are needed to examine the effects of some factors underlying the relationships between fast-velocity knee extensor strength and sprint performance in sprinters.
As another limitation of this study, we did not measure sprint performance in non-sprinters. This measurement would be useful to understand the essential importance of knee extensor strength on sprint performance in various populations. Previous studies have reported a positive relationship between knee extensor strength and sprint performance in young term sport athletes [8, 9]. To the best of our knowledge, no study has examined this relationship in untrained individuals. Nevertheless, previous studies have reported that long-term resistance training increases sprint performance, potentially by enhancing the knee extensor strength, in recreationally active young individuals [52, 53]. This population was the same population to non-sprinters recruited in this study. Therefore, it is hypothesized that higher knee extensor strength would play an important role in achieving better sprint performance in various populations, including untrained population. Nevertheless, to test this hypothesis, further studies are needed to examine the relationship between knee extensor strength and sprint performance in untrained individuals.