Prior to this study, the relationship between knee extensor strength and sprint performance with relations to quadriceps femoris size in sprinters had not been explored. In the results of this study, we determined that higher absolute and relative values of fast-velocity isokinetic knee extensor strength correlated significantly with better personal best 100-m sprint time in sprinters. By contrast, despite significant correlations between knee extensor strength and quadriceps femoris size variables, all quadriceps femoris size variables did not correlate significantly with personal best 100-m sprint time. Moreover, partial correlation analyses revealed that, after adjusting for physical characteristics or all quadriceps femoris size variables, a correlation between absolute fast-velocity isokinetic knee extensor strength and personal best 100-m sprint time remained significant. Furthermore, a stepwise multiple regression analysis revealed that the fast-velocity isokinetic knee extensor strength was the most predictive variable for the personal best 100-m sprint time. These present findings suggest that the knee extensor strength may be related to 100-m sprint performance in sprinters. Therefore, this study is the first to determine the relationship between knee extensor strength and sprint performance in sprinters independently of the quadriceps femoris muscularity.
Alexander et al. [6] reported that higher fast-velocity (i.e., 230 º/s) isokinetic knee extensor strength correlated with better personal best 100-m sprint time in 14 male sprinters. Following their study, Dowson et al. [7] also reported positive correlations between absolute and relative values of three velocity isokinetic knee extensor strength and 15-m sprint velocity in 24 male athletes, including 8 sprinters, and these correlations were stronger with fast-velocity (i.e., 150 and 240 º/s) than with slow-velocity (i.e., 60 º/s). Using multiple regression analyses, the two previous studies further reported that the fast-velocity isokinetic knee extensor strength was one of predictive variables for the sprint performances. Nevertheless, these previous studies examined with small sample sizes of sprinters. In the present study, we determined that higher absolute and relative values of fast-velocity isokinetic contraction (i.e., 180 º/s), but not of isometric and slow-velocity isokinetic contractions, of the knee extension correlated significantly with better personal best 100-m sprint time in 58 sprinters. Furthermore, using a stepwise multiple regression analysis, we demonstrated that higher absolute fast-velocity knee extensor strength was the most predictive variable for better personal best 100-m sprint time of the sprinters. Therefore, with a relatively large sample size of sprinters, the present findings corroborate the results in the previous studies [6, 7].
Using ultrasonography (US), Kumagai et al. [19] reported that muscle thickness (MT) of the anterior thigh (i.e., the quadriceps femoris) was larger in higher-level sprinters with personal best 100-m sprint times of < 11.00 sec than in lower-level sprinters with persona best 100-m sprint times of > 11.00 sec. Moreover, Monte and Zamparo [20] reported that US-measured larger MTs of the quadriceps femoris muscles correlated with better personal best 100-m sprint time in sprinters. Kubo et al. [21] also reported such a positive correlation between US-measured quadriceps femoris MT and personal best 100-m sprint time in sprinters. Nevertheless, magnetic resonance imaging (MRI) is known to be a more appropriate apparatus to measure muscle size than US [22, 23]. Using MRI, our previous study determined that the mid-thigh CSA of the quadriceps femoris did not correlate with personal best 100-m sprint time in 32 sprinters [13]. Furthermore, Sugisaki et al. [14] analyzed MRI-measured MV, which is the most appropriate marker of muscle size [22, 24], and reported no correlation between quadriceps femoris MV and personal best 100-m sprint time in 31 sprinters. In the present study, using MRI, we determined that although a trend against significance correlation was observed between middle quadriceps femoris CSA and personal best 100-m sprint time, MV and other region CSAs (i.e., proximal and distal CSAs) of the quadriceps femoris did not correlate with personal best 100-m sprint time in a relative large sample size of sprinters. Therefore, the present findings corroborate the results of our and other previous studies [13, 14]. Altogether, we suggest that greater quadriceps femoris may not be an essential morphological factor for achieving better 100-m sprint performance in sprinters.
This study determined that, despite significant correlations were observed between knee extensor strength and quadriceps femoris size variables, the knee extensor strength, but not the quadriceps femoris size, was related to personal best 100-m sprint time in sprinters. Thus, the present findings suggest the presence of the morphological factor other than the quadriceps femoris size to regulate the knee extensor strength in sprinters. We previously determined that the knee extensor moment arm (MA) correlated with personal best 100-m sprint time in sprinters [13]. Because muscle strength (i.e., joint torque) is theoretically expressed as the product of muscle force and 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. Indeed, we and others previously reported a positive correlation between muscle strength and MA dimension of the knee extensors in untrained participants [10-12]; however, no study has examined such a relationship in sprinters. If this relationship is observed for sprinters, it may help our understanding of the present findings that higher knee extensor strength was related to better 100-m sprint performance, potentially by enhancing knee extensor joint torque while sprinting, which is attributed to greater knee extensor MA rather than quadriceps femoris size.
In the present finding based on the comparison between sprinters and non-sprinters, we found that absolute and relative values of two velocity isokinetic knee extensor strengths were significantly higher in sprinters than in body size-matched non-sprinters, whereas no such significant differences were observed for those of isometric knee extensor strength. Thus, sprinters may be specifically characterized by higher isokinetic strength, but not isometric strength, of the knee extensors. This may be simply because sprint training is performed with dynamic movements. In addition to this reason, the fascicles of the lower limb muscles are known to be longer in sprinters than in untrained participants [25]. Drazan et al. [26] reported that longer muscle fascicle of the gastrocnemius medialis correlated significantly with higher isokinetic plantar flexor strength in untrained participants, whereas a correlation between the gastrocnemius medialis muscle fascicle length and isometric plantar flexor strength was only a trend against significance. Furthermore, Blazevich et al. [10] reported that muscle fascicle length of the quadriceps vastus lateralis was one of the positive predictive variables for the fast-velocity isokinetic strength, but not isometric and slow-velocity isokinetic strengths, of the knee extensors. In the present study, effect sizes of the differences in 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 effect size] and 0.41 [small effect size], respectively). Therefore, the higher isokinetic knee extensor strengths, especially with fast-velocity, of sprinters may be attributed to their longer muscle fascicles of the quadriceps femoris. Additionally, because positive correlations between the fascicle lengths of the quadriceps femoris muscles and sprint performance have been observed in previous studies [19, 20], the features of the longer quadriceps femoris muscle fascicles in addition to the greater knee extensor MA of sprinters may contribute to interpret the relationship between fast-velocity isokinetic knee extensor strength and 100-m sprint performance obtained in the present study.
Kubo et al. [21] reported that MT of the anterior thigh did not differ between sprinters and body size-matched non-sprinters. Our previous study also reported that no difference for the mid-thigh quadriceps femoris CSA between sprinters and body size-matched non-sprinters [13]. By contrast, in the present study, we found a trend against significance with a greater quadriceps femoris MV in sprinters than that in body size-matched non-sprinters. Furthermore, the relative quadriceps femoris MV was significantly greater in sprinters than in non-sprinters. This might be mainly because of a greater proximal quadriceps femoris CSA in sprinters than that in non-sprinters. Abe et al. [25] reported that MTs at the proximal and middle regions of the anterior thigh were greater in sprinters than in non-sprinters; however, they recruited sprinters with a greater body size than non-sprinters. Ema et al. [27] reported that although relative quadriceps femoris MV normalized to body mass did not differ between sprinters and body size-matched non-sprinters, the relative MV of the rectus femoris, but not of other three quadriceps femoris muscles, was greater in sprinters than in non-sprinters. 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 [28]. Therefore, greater MV and proximal CSA of the quadriceps femoris in sprinters than those in non-sprinters observed in the present study may be attributed to a specific hypertrophy of the rectus femoris among the quadriceps femoris in sprinters.
Although we determined a positive correlation between fast-velocity isokinetic knee extensor strength and personal best 100-m sprint time in sprinters, we did not evaluate biomechanical data during 100-m sprinting, which is a major limitation of this study. Thus, our findings cannot explain in detail the potential impact of the higher knee extensor strength on 100-m sprint performance. The higher knee extensor strength appears to help increasing peak vertical ground reaction force during the stance phase while 100-m sprinting [1-3]. 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 in achieving superior 100-m sprint performance [4, 5]. To further clarify the present findings, further studies are needed to examine whether higher knee extensor strength would be related to better kinetic and/or kinematic variables during 100-m sprinting in sprinters.