Which of the following is a good resistance training technique for developing explosive strength?

Could ‘explosive’ strength training deliver better results with less overall work than traditional resistance methods? John Sampson investigates

Athletic performance in many sports demands the development of muscle strength, which is required for other performance related characteristics, notably speed and power. Muscle strength is routinely developed through prolonged participation in a structured resistance exercise programmes. Yet despite extensive research in the area, the adaptive mechanisms contributing to maximal strength adaptation are not yet fully understood.

Skeletal muscle is an extremely sensitive and highly adaptive tissue; consequently almost any overload applied to the muscle will result in some form of adaptation (ie strength gain). In the case of athletes, even sub-optimal resistance training programmes can result in some positive adaptations. However, long-term adherence to such a resistance training programme is unlikely to result in optimal strength gains and in some cases may even lead to reduced performance capabilities and an increased risk of injury.

A number of key principles are applied during resistance exercise programmes. Legend suggests Greek athlete Milo of Croton lifted and carried a calf on his shoulders each day from birth until it became fully grown; as the animal grew in size so did his strength. This legend clearly demonstrates the importance of applying a progressively increasing external load!

The mechanical loading of muscle as a consequence of the external load is perhaps the most important consideration of any resistance-training programme. Research has consistently indicated that moderate to heavy loads are required in order to gain an increase in muscle size, muscle activity and muscle strength. Correspondingly, an extensive review of the literature and current guidelines published by the American College of Sports Medicine (ACSM) suggest relatively heavy loads that equal, or are in advance of 80% of a one-repetition maximum (1RM) are required in order to achieve optimal strength gains (1).

Resistance exercise programmes can be modified not only by the external load, but also by the speed of contraction, and level of induced fatigue. Altering resistance exercise programmes in just one of these ways will induce a distinct skeletal muscle response. However, the combined effects of adjusting training in two or more of these areas simultaneously will result in more complex physiological interactions that may either hinder or improve training related strength gains. Unfortunately, we still have insufficient evidence to fully understand the complex interactions between load, movement speed and the extent of muscular exhaustion induced by the level of work (eg completed number of sets and repetitions).

Optimum strength training protocol

Everybody knows that a structured resistance training programme results in increased muscle size (hypertrophy), and that a larger muscle has the potential to produce greater levels of force. When first starting out in resistance training you may have noticed increases in your muscular strength, but no increase in the size of your muscles. Strength gains without muscle size increase are generally attributed to an increased level of ‘muscle activation’ – ie better recruitment of motor units that ‘fire’ muscle fibres (see box 1).

Maximal activation of muscle fibres during resistance exercise is essential for maximal strength gains. When completing a set of resistance exercises, you’ll no doubt be aware of an increase in the difficulty of exercise as you complete an increased number of repetitions. This is because motor units fatigue and in an attempt to maintain the desired force output, more motor units are recruited. Consequently the level of muscle activity increases as the muscle attempts to maintain the required force to overcome the load. This explains why training to the point of repetition failure is seen as an important consideration in resistance training program design.

Repetition maximum loading regimes were first credited by Delorme (2) and later Delorme and Watkins (3) who conducted a series of investigations examining the effect of progressive-resistance training in exercise rehabilitation. This research proposed the completion of 10 repetitions using a load which was consequently heavy enough to result in ‘task failure’ (muscular exhaustion) on the 10th repetition. This loading technique was termed a 10-repetition maximum (10RM). Some 60 years later, repetition maximum loading regimes remain the dominant resistance-training model utilised across research literature and in gyms across the world. Task failure has therefore become closely affiliated with maximal strength adaptation and most resistance exercise programmes advocated by coaches and fitness trainers result in high levels of muscular exhaustion.However, research into the necessity for such high levels of induced fatigue is far from conclusive. Most resistance trainers apply the use of multiple sets in order to achieve maximal strength gains, yet there’s conflicting evidence surrounding the use of multiple versus single set strength training programmes. The concept of high levels of workload and induced fatigue as a prerequisite for strength adaptation is thus far from proven.

Increasing the number of sets performed in a resistance training session is not the only way to influence the level of muscular exhaustion. Research has compared the effects of allowing brief inter-repetition rest periods within a resistance exercise programme. Two of the studies in this area have again produced conflicting evidence (4,5).

Each of these studies examined two resistance exercise protocols that were performed against a relatively heavy external resistance:

One protocol induced high levels of fatigue by directing participants to complete repetitions without rest, to the point of task failure (muscular exhaustion);

The other protocol had participants performing the same number of repetitions but with a 30 second inter-repetition rest period, thus allowing time for recovery in between repetitions.

The strength gains resulting from this ‘high fatigue’ exercise regime were compared to those undergoing the lower fatigue protocol. One of the studies found that those in the high fatigue group achieved greater strength gains, while the other study observed no difference between the groups. This poses a dilemma; one set of results proposes an important role for high levels of muscle fatigue, yet the other suggests this is not necessary!

Repetition failure

The necessity of repetition failure has also been questioned. A recent study (6), compared the strength gains achieved in a group performing exercise sets to the point of repetition failure and those of a second group who performed the same total number of repetitions but over a greater number of sets (ie where repetition failure wasn’t induced). This research again suggested similar strength gains were achieved in both groups despite a less exhaustive stimulus applied in the group performing a greater number of sets.

A number of research studies have therefore suggested similar strength gains can be achieved despite a reduction in the level of induced fatigue. An important point to make here however is that all groups ultimately performed the same relative amount of work, thus the efficiency of exercise was no different between groups (see box 2).

Some researchers have advocated the use of single set training programmes, which they believe increases exercise efficiency without compromising strength gain but this is an area of much contention. However, an important interaction between repetition speed and the level of induced fatigue may exist. Repetitions can be performed quickly or slowly and both methods have been used across research in the area.

A recent review of purposefully ‘slow training’ discusses the mechanical effects of such training (7). In short, the researchers highlight that repetitions performed slowly increase the time it takes to complete each muscle contraction over any given range of motion (effectively increasing the time a muscle is subjected to tension). However, they also point out that as a function of increasing the time under tension, the load must decrease. Considering the well defined relationship between external loading and muscular adaptation, this appears to directly contradict the well accepted notion that associated resistance training adaptations are proportional to load.

Increasing the time spent performing a muscular contraction is not the only way to increase total time a muscle is under tension. The same effects can be achieved by increasing the number of completed repetitions. Both methods increase the level of muscular exhaustion and will eventually lead to task failure. However, neither effectively increases the efficiency of resistance exercise.

Explosive contractions

In contrast to purposefully slow training, repetitions can also be performed as fast as possible. Such training is often termed as ‘explosive’ or ‘ballistic’. Remember, in order to stimulate muscle and achieve maximal strength gains during resistance exercise, you need to achieve maximal muscle activation. Explosive muscle contractions result in the rapid generation of force, increase the rate of motor unit firing and reduce motor unit recruitment thresholds.
Explosive muscle contraction can lead to superior activation of muscle. However, in order to perform an explosive movement, the external load needs to be reduced and (as we have discussed) a relatively heavy external load is required in order to gain maximal strength adaptation. Proponents of purposefully slow training have claimed that this makes explosive training less efficient.

It is, however, possible to attempt an explosive contraction against a heavy external load. High levels of force production are required whenever you attempt to initiate a high-speed movement. This is due to inertia; if you attempt to accelerate a mass very rapidly, much more force must be generated to overcome inertia compared to a slower movement with less acceleration.

Performing explosive contractions against relatively heavy loads is also likely to increase power related performance characteristics, while resulting in equal strength related adaptations as performing contractions against the same load at lower speeds (9).

The combination of a heavy external load combined with maximum contraction speed has also found favour in one-set resistance exercise training programmes. For example, researchers in Australia examined the effect of one and three sets of resistance exercise performed at either fast or slow speeds on maximal strength adaptation over a six-week resistance exercise programme with each set resulting in task failure (10). The results of this research highlighted that one set of heavy load exercise performed at fast speeds resulted in similar strength gains as three sets of exercise performed at slower speeds. Furthermore, no additional benefit was observed from performing three sets at the faster speeds. Thus a ‘heavy load, explosive contraction’ may be a key performance related variable in this area.

New research

The idea that ‘heavy load, explosive contraction’ resistance exercise can increase the efficiency of exercise has also been supported by research from our laboratory at the University of Wollongong, Australia. We manipulated the level of completed work (and thus the level of induced muscle fatigue) and the speed at which repetitions were performed.
One exercise group performed four sets of resistance exercise using a relatively heavy external load, resulting in repetition failure after approximately six repetitions. Meanwhile, a second group were also asked to perform four sets of exercise against the same relatively heavy external load, but we imposed a work reduction on this group by asking them to perform only four repetitions. We also asked this group to complete repetitions as fast as possible (ie explosively), whilst repetition speed in the first (task failure) group was controlled to a four-second cadence; a two-second muscle-shortening phase (normally associated with lifting a weight) and a two-second muscle-lengthening phase (normally associated with lowering a weight).

After 12 weeks of resistance exercise, we found similar strength gains in both groups. These findings are significant; not only did the second (explosive) group performed 30% less total work than the first, they also achieved the same strength gains without working to failure (see figures 1 and 2). Moreover, the similar gains in strength between groups were also accompanied by similar increases in muscle size and muscle activity, suggesting no benefit in any area of strength adaptation from a more exhaustive exercise routine performed by the first group.

References:

1. Med Sci Sp Ex (2002) 34, 364-380
2. J. Bone Joint Surg (1945) 27, 645-667
3. Arch Phys Med (1948) 29, 263-273
4. Br J Sp Med (2002) 36 370-375
5. Med Sci Sp Ex (1994) 26, 1160-1164
6. J Appl Physiol (2006) 100, 1647-1656
7. J Sp Sci Med (2008), 7, 299-304
8. J Appl Physiol (1993), 74, 359-368
9. J Am Geriatr Soc (2002), 50, 655-662
10. Med Sci Sp Ex (2005) 37, 1622-1626

Which type of resistance training is used to develop explosive strength?

Which of the following is a training technique used to develop explosive power?

What is explosive strength?

Which approach is recommended for strength training safety?