Strength-building exercises are essential in the training and recovery of athletes.1 Strength is a physical quality that crucially influences competitive results in a wide range of sporting disciplines.2 Some studies claim that increasing muscle strength is indispensable to enhance velocity and resistance in lower-limb training programs.3–5 Several training methods have been reported with a view to optimizing the development of muscle strength.6

Explosive strength is the type of strength required in most sport disciplines and constitutes the basis for multiple technical maneuvers. In addition, it is a key factor in the performance of a significant number of athletes.7 In group sports, explosive strength is a crucial component of physical achievement as most games require athletes to make extremely fast and highly intensive movements at some point.8,9 The development of explosive strength in the knee musculature is essential for the enhancement of sprinting performance.10–12

One of the greatest breakthroughs in the area of strength-building in the last few decades has been the introduction of eccentric muscle exercises (EMT) in most training programs.13 Strength development programs are characterized by the central role played by muscle contractions but there is currently no agreement as to what type of muscle contraction is most conducive to the development of explosive strength.14

The main objective of this study analyzed different kinds of muscle training protocols divided into two main categories: dynamic muscle training, which included concentric muscle training (CMT), eccentric muscle training (EMT) and concentric-eccentric muscle training (C-EMT); and isometric muscle training (IMT). The purpose was to determine which of those protocols was most effective in enhancing explosiveness and improving athletic performance on four functional explosive strength tests: a vertical jump test, a horizontal jump test, a speed (over 60m) test and a jump force test.

Demographic variables were homogeneous across all 4 study groups (Table 1).

Both systolic (SBP) and diastolic (DBP) blood pressure was measured for all the subjects in the study.

The relative decrease of SBP and DBP following the 12-day training program was analyzed in the subjects assigned to the dynamic muscle training groups.

HR was measured before and after each exercise during the 12-day training program.

The functional explosive strength tests were administered before and after the 12-day training program (Table 2).

The relative improvement achieved by men was compared to that achieved by women for different variables analyzed showing in horizontal jump before training in men values of 194.5cm respect 146.77cm in women, being after training for men 201.46cm and 154.75cm for women. In vertical jump, the data before training were for men 47.29cm and 32.73cm, for women, corresponding after training to men 49.24cm and 34.55cm in women. Speed also showed improvement, in men before training 8.03s being 7.74s after training. In women, the speed was 10.11s before and 9.68 after training. Finally, in jump force, before training the data of men were 4155.07 and 4253.1 in women, and after 4253.1 in men and 2662.34 in women (Table 3).

At the end of the training program, comparisons were made to determine the differences between the groups. Specifically, comparisons focused on the improvements achieved by individual subjects and by the study groups on completion of the exercise sessions.

On the horizontal jump test, the comparison between the scores achieved before and after the training program proved statistically significant (p<0.001).

For the vertical jump test, statistically significant results were obtained when comparing the pre- and post-training scores of individual subjects but not when comparing the study groups themselves (p<0.001).

On the 60-m speed test, individual comparisons between pre- and post-training scores showed statistically significant differences but comparisons between the groups did not. Likewise, no statistically significant differences were observed when comparing the improvement achieved on vertical jumping with the improvement achieved on the 60-m speed test (p<0.001).

For the jumping variables, it was intended to know the relationship between having lost weight during the 12 days of muscle training and jumping more on the last day of assessment. The results obtained do not show statistically significant differences between the subjects who lost weight versus those who gained weight, both in the horizontal jump (p=0.920), nor in the vertical jump (p=0.775).

On the jump force test, individual differences between pre- and post-training scores were statistically significant, whereas the differences between the groups were not. The subjects’ HR was measured every day. Pre- and post-training HR values were compared daily for each subject. In addition, comparisons between day 1 and day 6 (pre- and mid-program) HR values and between day 1 and day 12 (pre- and post-program) HR values were drawn. The results obtained were: between day 1–6, the HR before training p=0.269 and after p=0.174; between day 1–12, HR before training p=0.237 and after p=0.063.

The vertical jump, the horizontal jump, the 60-m sprint and jump force were selected as variables for this study on account of the direct relationship that exists between them. In 2004, Wisloff demonstrated a close relationship between the vertical and the horizontal jump.24 In the present study, that kind of direct relationship between variables was not observed, which means that an improvement on one variable does not necessarily entail an improvement on the others.

An analysis of the values obtained before and after the study for each of the variables considered shows that it was the C-EMT group that experienced the greatest improvement, although differences with the other groups were not statistically significant. It must be borne in mind that both the pre-training and post-training values recorded in all four groups for the different variables were very similar. In addition, differences between pre- and post-training values were scarce. In line with other authors, the present study came to the conclusion that combined muscular training (C-EMT) is the most effective type of training for improving explosive strength performance. Dudley et al. claim that training sessions that combine concentric and eccentric exercises are cumulatively more effective than training consisting solely of isolated concentric and eccentric actions.25

For their part, Friedmann et al. compared two groups of athletes for 6 weeks and found that the group who combined concentric and eccentric exercises obtained the best results on vertical jump tests, suggesting that combined training results in an enhanced development of explosive strength.26 The present study did not find a statistically significant relationship between weight loss and greater explosive strength improvements on the vertical jump, which differs from the findings by Viitasalo et al.27

Notable differences were found in the vertical jump scores between males and females. Most of the evidence indicates that the mean absolute upper and lower trunk strength scores obtained by women are lower than those observed in men. Nonetheless, this difference takes on different forms as a function of the type of muscle movement performed. In this respect, the results obtained by males and females following eccentric exercises exhibit more similarities than those observed following concentric exercises.4,28

In the present study, males achieved a relative explosive strength gain of 3.45% on horizontal jump tests, whereas the improvement achieved by women was 5.5%. Fort the vertical jump, the relative gain obtained by men was 3.96%, while women obtained an improvement of 5.27%. On the 60-m speed test, the relative improvement attributed to males was 3.61%, while women increased their baseline values by 4.25%. For jump force, men recorded a 2.30% improvement while women reached 3.59%. In line with our findings, previous studies show that dynamic muscle training is equally beneficial for both genders, with a slight advantage in favor of women.29,30

However, this advantage in favor of women seems to disappear with isometric muscle training. In fact, in these exercises’ men, probably on account of their greater muscle mass, usually obtain higher scores than women. In this study, men obtained significantly better scores than women on the vertical jump test. This could be since women have 10% more adipose tissue than men in the lower limbs, which means that muscles in that area are more strained as a result of the extra load they have to bear. It has not been possible to confirm this hypothesis as the only report in the literature that has found a similar advantage of men over women did so for upper extremities.29 In any event, the most striking differences between males and females were observed in the dynamic muscle training groups and not in the IMT group and could be attributed to the effect of testosterone, which stimulates the nervous system and favors the phenotyping of rapid fibers.31

The heart rate and blood pressure values observed were remarkable. Following dynamic muscle training, a series of adaptative changes were recorded, which consisted in a reduction in HP both at rest and after exercise32 and a decrease in BP.33 A reduction was observed in the mean HR values of all groups from the day before the start of training until the day after cessation.

Nevertheless, the finding that resting HR decreases as the training session progresses, reported in this article and in the above-mentioned studies, only seems to hold when muscle training is performed in short sessions at low-to-moderate intensity.34 Conversely, the effects of long-term muscle training are unclear, with some studies even reporting no observable changes in the subjects’ HR.35,36

MBP values also decreased in all study groups, with the most notable differences corresponding to SBP in the C-EMT group.

Isometric activity has traditionally been associated with extreme responses in hypertensive individuals. Brook et al.,37 showed that this type of muscle training could become a useful non-pharmacological tool in the treatment of subjects with high BP. These authors reported a decrease of nearly 6mmHg following isometric training, supporting the recent inclusion by the American Heart Association of isometric exercise as BP-lowering treatment. Even if there were no hypertensive subjects in the patient sample used for this study, both SBP and DBP values in the IMT group also diminished.

At the level of dynamic muscle exercises, there is significant evidence to suggest that a certain reduction of resting SBP and DBP occurs following a bout of muscle training.38,39 This reduction ranges between 2 and 4mmHg for SBP and DBP respectively, which entails a 2% reduction of SBP and a 4% reduction of DBP.39

All three dynamic muscle training groups in this study also experienced a reduction of resting MBP. As to the percentage of BP reduction, results were similar to those reported by Bailon et al.40

All study groups exhibited a decrease in both HR and BP values. As dynamic muscle training was performed standing up and the static training was carried out sitting down, it could be inferred that the reduction in HP and BP values would exhibit differences depending on which group subjects were in. For SBP, DBP and MBP no statistically significant differences were observed between performing the muscle-building exercises in a standing or a sitting position. However, when the HR parameter was analyzed, statistically significant differences were found between dynamic and static muscle exercises in favor of the latter. The most significant decrease in HR occurred in the group that trained in a seated position. Some authors have explained this finding by arguing that activities performed sitting down or lying are associated with a decrease in hydrostatic pressure, which facilitates venous return to the heart and requires less effort thereby reducing the HR.27

Concentric-eccentric dynamic muscle training produced the best results on the analyzed variables. On the 60-m speed variable, the results of all three dynamic muscle training groups were better than those obtained by the isometric group, albeit the difference was not significant.

Gender-wise, it was the women who obtained the greatest improvement on their initial scores. This said, the men showed a better vertical jump performance than women, with the latter obtaining better results on horizontal jump tests and ran the 60m distance faster than men.

During the sessions, the HR experimented a progressive short-term reduction, which was particularly striking in the dynamic muscle training groups. As regards blood pressure, MBP values diminished across all study groups.

The authors declare that they don’t have any conflict of interests.