1. Introduction
Explosive strength is of great importance for performance in several power-demanding sports, such as sprints, long and high jumps, athletic throws, and martial arts. Explosive strength may be evaluated with the rate of force development (RFD; i.e., the slope of the tangent line of the force/torque-time curve during an explosive muscle contraction [
1,
2]), which has been linked with explosive sports performance. Indeed, lower body isometric leg press RFD at 100, 150, 200, and 250 ms from the onset of contraction, is associated with performance in track and field throwers, where the time for the final delivery of the implement is between 150–250 ms [
3]. Taekwondo is also considered to be a powerful sport. For example, one of the most effective kicks in taekwondo, the roundhouse kick to the head, is developed between in approximately 250 ms (time from the instant that the striking foot leaves the floor until it reaches the objective and achieves the maximum impact force) [
4,
5,
6]. This kick is characterized as a high velocity unloaded movement, in contrast to track and field throws where a relatively heavy external throwing implement is used. Thus, it might be postulated that RFD might be equally important for taekwondo athletes as for throwers. However, RFD in taekwondo athletes has not been described before.
RFD is influenced by different factors at early (<100 ms) and late phases (>100 ms) from the onset of muscle contraction [
2,
7,
8]. The early phase of RFD is thought to be determined mainly by the neural drive to the muscle, as well as the muscle fiber type composition [
2,
7,
8,
9]. Late phase RFD is largely influenced by maximal strength and most likely muscle mass, as suggested by studies reporting parallel changes in maximal muscle strength and contractile RFD 150–250 ms from the onset of contraction [
1,
2,
7,
8,
10,
11,
12,
13]. Nevertheless, little is known about the correlation between muscle mass and RFD in power athletes. Recently, a close relationship was reported between isometric leg press late RFD (>100 ms) and lean body mass in young aged track and field throwers, while early RFD (<100 ms) was not related with their lean body mass [
14]. Taekwondo athletes are expected to have lower lean body mass when compared to track and field throwers [
15], although direct evidence is lacking on this issue. This may suggest a lower late RFD and lower maximum isometric force (MIF) and/or torque (MIT) in taekwondo athletes as compared to that of throwers, although this remains to be evaluated.
Muscle architectural characteristics, fascicle length, pennation angle, and muscle thickness, may also influence RDF [
2,
14,
16]. Previous studies indicate that pennation angle and muscle thickness seems to affect late RFD [
2,
14,
17,
18]. Fascicle length is thought to reflect the number of sarcomeres in series in a muscle and the increase in this number has been suggested to contribute to higher shortening velocity and muscle power [
19]. Longer fascicles are typically found in agonist muscles of faster sprinters [
20,
21,
22], while studies in humans have reported longitudinal muscle growth in response to power training [
23,
24,
25,
26,
27]. Recently, a correlation was found between vastus lateralis fascicle length and late RFD (100–250 ms) in young aged track and field throwers [
14]. In addition, previous study indicates that in experienced power trained athletes, vastus lateralis fascicle length have stronger contribution/effect on sprinting, jumping, and throwing performance as compared to pennation angle and muscle thickness [
28]. However, the relationship between muscle architecture and early and late RFD in athletes is still debatable, while there is no such information for taekwondo athletes.
The aim of the study was to describe the relationship between lower-body lean mass and RFD and to compare lower-body RFD in taekwondo athletes and track and field throwers, the latter having higher lean body mass when compared to taekwondo athletes. For this purpose, maximal isometric leg press and leg extension RFD curves were recorded from taekwondo athletes and track and field throwers of national and international level. It was hypothesized that (1) lower body lean mass would be associated with RFD in both athletic populations, but also (2) that they would differ in early and late RFD characteristics. Muscle architecture was also evaluated to provide further insight into the muscle morphology origin of the anticipated differences in RFD between the two athletic groups.
3. Results
Taekwondo athletes had lower body mass (46.8%,
p = 0.000,
η2 = 0.791), as well as total lean mass (35.1%,
p = 0.000,
η2 = 0.877) and bone mineral density (10.8%,
p = 0.002,
η2 = 0.473,
Table 1) when compared to throwers. Also, taekwondo athletes had 14.6%, (
p = 0.030,
η2 = 0.254) lower CMJ height, lower CMJ power (69.8%,
p = 0.000), lower CMJ power relative to body mass (12.8%,
p = 0.015), lower CMJ maximum velocity (14.6%,
p = 0.037,
η2 = 0.258), lower leg press, and leg extension maximum isometric force/torque (63.7% and 63.8%,
η2 = 0.643 and
η2 = 0.765, respectively,
p < 0.05,
Table 2) when compared to throwers. Even when CMJ power was expressed relative to lean body mass, taekwondo athletes performed lower than throwers (28.4%,
p = 0.015,
η2 = 0.335,
Table 1). Taekwondo athletes had lower leg press isometric force (19.4%,
p = 0.007,
η2 = 0.643) and leg extension torque (13.4%,
p = 0.036,
η2 = 0.261,
Table 2) relative to total lean body mass as compared to throwers. Taekwondo athletes had significantly lower VL and GM muscle thickness (23.6% and 15.6%,
η2 = 0.398 and
η2 = 0.248,
p < 0.05, respectively), as well as VL fascicle angle (33.6%,
p = 0.001,
η2 = 0.511), when compared to throwers.
No significant differences were found between groups in RFD until the first 50 ms of the force/torque-time curve, either in leg press or in leg extension (
η2 = 0.009–0.158,
Table 2). Yet, taekwondo athletes had lower leg press RFD and lower sequential RFD later in the force/torque-time curve (80 ms = 30.2%, 100 ms = 35.3%, 150 ms = 37.7%, 200 ms = 46.4%, 250 ms = 48.6%,
p < 0.05,
η2 = 0.440–0.616,
Figure 1A and
Figure 2A), as well as in leg extension (80 ms = 47.8%, 100 ms = 51.0%, 150 ms = 49.6%, 200 ms = 51.5%, 250 ms = 48.7%,
p < 0.05,
η2 = 0.532–0.771,
Figure 3A and
Figure 4A) as compared to throwers.
Taekwondo athletes had higher leg press RFD at 30 ms and 50 ms relative to their lean body mass (30 ms = 39.8%,
p = 0.016,
η2 = 0.329, 50 ms = 23.7%,
p = 0.043,
η2 = 0.246,
Figure 1B) when compared to throwers. Also, when RFD was expressed relative to VL muscle thickness, taekwondo athletes had higher leg press RFD at 30 ms (difference between groups for RFD
30ms = 31.7%,
p = 0.049,
η2 = 0.235) as compared to throwers, while throwers had higher leg press RFD at 200 and 250 ms (difference between groups for RFD
200ms = 20.1%,
p = 0.048,
η2 = 0.237, and RFD
250ms = 21.7%,
p = 0.021,
η2 = 0.307,
Figure 1C) when compared to taekwondo athletes. Additionally, no significant difference was observed for leg press sequential RFD relative to lean mass or VL thickness between taekwondo and throwers (
Figure 2B,C). Leg extension sequential RFD was lower for taekwondo athletes at 30–50 ms relative to lower body lean mass as compared to throwers (68.2%,
p = 0.000,
η2 = 0.727), while leg extension sequential RFD expressed per VL thickness was lower for taekwondo at 30–50, 50–80, 80–100, and 150–200 ms, respectively (difference between groups for RFD
30–50ms = 73.7%,
p = 0.000,
η2 = 0.684, RFD
50–80ms = 22.1%,
p = 0.010,
η2 = 0.368, RFD
80–100ms = 18.5%,
p = 0.013,
η2 = 0.343, RFD
150–200ms = 21.4%,
p = 0.024,
η2 = 0.297,
Figure 4B,C).
Taekwondo athletes had higher leg press isometric force production (IFP) expressed per percentage of maximum volunteer contraction (%MVC) than throwers at all time windows (30 ms = 98%,
p = 0.002,
η2 = 0.497, 50 ms = 55%,
p = 0.001,
η2 = 0.519, 80 ms = 23%,
p = 0.009,
η2 = 0.371, 100 ms = 19%,
p = 0.012,
η2 = 0.350, 150 ms = 17%,
p = 0.007,
η2 = 0.391, 200 ms = 11%,
p = 0.022,
η2 = 0.304, and 250 ms = 9%,
p = 0.002,
η2 = 0.473,
Figure 5A). Additionally, in leg extension taekwondo athletes had higher IFP per %MVC only at 250 ms (250 ms = 9%,
p = 0.008,
η2 = 0.380,
Figure 5B).
When all athletes were considered as a group, lean mass of the lower extremities was significantly correlated with leg press RFD
250ms (
r = 0.89,
p = 0.000,
Figure 6A) and leg extension RFD
250ms (
r = 0.86,
p = 0.000,
Figure 6B). In taekwondo athletes, lean mass of the lower extremities was significantly correlated with average RFD during the CMJ (
r = 0.710,
p = 0.041) as well as leg press RFD
100ms (
r = 0.680,
p = 0.047), RFD
200ms (
r = 0.730,
p = 0.035), RFD
250ms (
r = 0.810,
p = 0.016), and leg press maximum isometric force (
r = 0.83,
p = 0.008). Also, in taekwondo athletes, lean mass of the legs was significantly correlated with leg extension RFD
250ms (
r = 0.760,
p = 0.026) and leg extension maximum isometric torque (
r = 0.800,
p = 0.011). In contrast, no significant correlations were observed between lean body mass and performance for the throwers.
Vastus lateralis muscle thickness was significantly correlated with leg extension RFD
80ms (
r = 0.780,
p = 0.018) while GM muscle thickness was correlated with leg press maximum isometric force (
r = 0.670,
p = 0.049), in taekwondo athletes. VL fascicle length was significantly correlated with leg press RFD
250ms (
r = 0.67,
p = 0.05) in throwers. The linear combination of VL muscle thickness and VL fascicle length could significantly predict the power performance in CMJ and the RFD between 80–250 ms, as well as maximum isometric force/torque for both leg press and leg extension (
N = 18,
Table 3).
4. Discussion
The aim of the study was to describe the relationship between lower-body lean mass and RFD and to compare lower-body RFD in taekwondo athletes and track and field throwers, the latter having higher lean body mass when compared to taekwondo athletes. The main findings of the present study were that (1) lower body lean mass was significantly correlated with RFD in power-trained athletes having large lean body mass differences, (2) taekwondo athletes had greater early RFD when this was expressed relative to total lean mass or vastus lateralis muscle thickness, as compared to track and field throwers, and (3) throwers performed better in late RFD when compared to taekwondo athletes both in absolute values as well as relative to total lean mass or vastus lateralis thickness. Unfortunately, in the current study neither the neural components nor muscle fiber composition were assessed, therefore the contribution of these components to the early phase of force rise was not explored. However, it is reasonable to assume that longitudinal taekwondo training adaptations induced in response to fast muscle actions performed with relative low external resistance (e.g., body mass, [
15]) might have resulted in increased early RFD. Taekwondo is a combat sport that depends on both agility-speed and power production. Anecdotal data from coaches suggests that during competition athletes must use their lower limbs as fast as possible to achieve a successful attack while often earning a point with a reflex-respond kick to the opponent’s offence, which is a teaching strategy, an action that it has to be completed instantaneously. According to these, it seems that early RFD might be of significance for these athletes, while their lower legs’ lean mass could, at least partly, explain their high early RFD relative to their musculature in comparison to track and field throwers.
Conversely, the delivery phase in track and field throws ranges between 150–250 ms [
3,
39,
40,
41]. Chronic exercise training adaptations may have increased the late RFD ability of these athletes rendering them to perform better relative to their musculature as compared to taekwondo athletes, as found in the present study. Throwers had higher maximum isometric force relative to lean body mass which suggests an improved recruitment of their muscle tissue after the initial milliseconds of a maximum contraction. This might be the result of chronic resistance training adaptations in these athletes as opposed to the taekwondo athletes who do not regularly use high external resistances during training [
15]. However, given that both taekwondo athletes and throwers have similar critical time periods to develop their maximum force or power, it would be of interesting, future studies to investigate if a training program, similar to that of the throwers, could increase the performance of Taekwondo athletes.
When RFD was expressed relative to total lean mass and legs lean mass, taekwondo athletes had higher early RFD than throwers in leg press but not in leg extension (no significant difference between groups for the leg extension). Although leg extension is commonly used to assess RFD, it is not regularly performed during training by either taekwondo athletes or track and field throwers. Thus, specific neural adaptations may not have developed for both groups for the leg extension, which might explain these results. In contrast, the multi-articular leg press exercise is a movement regularly performed by athletes either as specific leg press or in a broader sense in jumping, squatting, etc. Neural adaptations may be more applicable in leg press, which might explain the closer link with RFD relative to muscle mass/thickness.
When all the athletes were considered together, lean body mass was significantly correlated with late RFD. The close link between muscle mass and late RFD has been predicted in earlier studies where training-induced increases in late RFD correlated with chronic increases in muscle mass [
1,
10,
11,
13]. Similar correlations were also found for the taekwondo athletes but not for the throwers in the current study. Significant correlations between lean body mass and late RFD
100–250ms have been recently reported in young well-trained track and field throwers [
14]. However, the athletes that participated in that previous study had lower muscle mass (approximately 57.1 ± 13.2 kg, total lean mass) when compared to the athletes that participated in the current study (total lean mass = 75.9 ± 3.4 kg). This may suggest that the impact of muscle mass on late RFD may not be linear when large muscle masses have been accomplished.
The role of muscle architecture in power performance and the RFD has been debated. Some studies have shown a link between fascicle length or fascicle angle and muscle power [
14,
21,
28,
42,
43], while other studies failed to reveal similar results [
28,
44,
45]. This discrepancy might be due to the differential effect of muscle architecture in the early and late RFD. In a recent study, early RFD during jumping (10–30 ms) has been linked with the gastrocnemius fascicle length [
46]. In the present study, neither vastus lateralis nor was gastrocnemius architecture correlated with early RFD. However, vastus lateralis fascicle length was significantly correlated with leg press late RFD in throwers. Moreover, late RFD could be predicted from the linear combination of vastus lateralis thickness and fascicle length when all athletes were considered together. This suggests that in power-trained individuals the combination of muscle hypertrophy (thickness) and the number of sarcomeres in series (fascicle length) might be vital for power production. Similar results were recently reported in a group of young track and field throwers where the linear combination of vastus lateralis muscle thickness and fascicle length could predict 53% of the shot put performance [
14]. Future studies should address this intriguing issue more systematically, perhaps in a larger number of power-trained individuals. Finally, in the present study, RFD was evaluated in testing positions that were different to the ultrasound collection position. Recent study report that performing ultrasonography evaluation of vastus lateris architecture with athletes in a lying position or in a different position when compared to those that they have during strength and power evaluations, does not allow for us to have a precise view of muscle configuration that is present during performance evaluations, and this may have some impact on the correlations between strength/power performances and architecture parameters [
18]. Unfortunately, the present study performed prior to this report. Thus, the lying position during the architecture evaluation of vastus lateralis, may have some impact on the results of the present study, providing lower or non-significant correlations as compared to those that may be found if the evaluations were performed in a position that is close to those of the performance evaluations [
18].
Unfortunately, it was not possible to evaluate the electromyographic activity in this study. This might have provided further insight into the current results, especially about the neural input in the early phase of the RFD curve. Another limitation of this study was the lack of information regarding the muscle fiber composition of the athletes. Previous studies have shown a higher percentage of vastus lateralis area that is covered with type II muscle fibers in track and field throwers [
47,
48]. Although similar data might be expected for the taekwondo athletes, to our knowledge there are no such experimental data. However, a possible difference between the percentage of the vastus lateralis area that is covered with type II muscle fibers between the two groups of athletes in the current study might explain the differences that are found in early and late RFD relative to lean mass or muscle thickness. Furthermore, the relative small number of athletes in this study as well as the testing procedures (e.g., lack of warm-up activities prior the RFD testing) may pose limitations regarding the generalization of the results.
5. Conclusions
In conclusion, the current results reveal a significant correlation between lower body lean mass and RFD in power-trained athletes having large inter-individual differences in lean body mass. The track and field throwers are stronger than taekwondo athletes and they have more power in countermovement jumping. When the RFD is expressed relative to lean body mass, taekwondo athletes perform better that the throwers in the initial 30–50 ms after the initiation of an explosive contraction, which might be related to a higher neural input. In contrast, track and field throwers perform better relative to their lean muscle mass in the late part of the RFD curve.
In practice, the current results suggest that taekwondo athletes and coaches should regularly measure the lower body RFD, especially the early part of the force rise (<100 ms), as well as the lean body mass to evaluate the lower body power adaptations induced with training. The importance of the early RFD in taekwondo athletes revealed in this study suggests that these athletes should implement explosive actions with moderate to low external resistance (e.g., drop jumps) in their training. In contrast, in sports demanding explosive movements against higher external loads, such as the track and field throws, training may mainly focus on increasing the later part of the force rise of the force-time curve (>150 ms) and laboratory testing of the RFD may also focus on this part of the curve. For these athletes, development of lean body mass seems to be an important factor for the late RFD.