© National Strength & Conditioning Association
Volume 23, Number 3, pages 13–20
A Brief Review:
Explosive Exercises
and
Sports Performance
G. Gregory Haff, PhD, CSCS, and
Adrian Whitley, MS
Exercise Physiology Laboratory
Appalachian
Jeffery A. Potteiger, PhD, FACSM,
CSCS
Exercise Physiology Laboratory
Introduction
The ability to generate high power outputs in sport is often
a determinant of athletic success (43). In fact, it is believed that power
output-generating capabilities are among the most important factors in sports
performance, especially those that involve jumping and sprinting (31). The use
of resistance training modes and methods that have explosive exercise
components may enhance an athlete’s ability to generate high power outputs.
Explosive exercises generally utilize rates of force development that approach
near maximal or maximal values and potentiate an
athlete’s ability to generate high rates
of acceleration (40). The highest recorded rates of force development have been
demonstrated in male power athletes who employ explosive exercises of varying
loads in their training regimes (18, 25It appears that
explosive exercises
tend to enhance an athlete’s ability to generate high rates of
force development (18, 22, 25, 54), whereas slow exercises tend to impair this
ability (24, 25, 50). Almost any exercise can be performed explosively
depending upon the resistance used.
Several studies and review articles have reported evidence
and logical arguments for the use of
explosive exercises. These types of
exercises are marked by high-force, high-velocity movements and are used by
athletes who participate in strength and power sports (9, 15, 16, 24, 39, 42). Generally, we can define an explosive exercise as
having a maximal or near maximal initial rate of force development that is
maintained throughout a specified range of motion. These types of exercises are
marked by a rapid initiation of
force production and focus on movement
accelerations, which result in near maximal or maximal movement velocities at a
given resistance. Therefore, a conceptual continuum of explosive exercise can
be created (Figure 1). The type of explosive exercise
employed in the training program will then dictate the adaptive response of the
athlete and will ultimately affect the sports performance. It is likely that
improvements in sports performance through the use of explosive exercises may
be partially dependent upon the movement and velocity patterns required by the
sport and upon the training status of the athlete.
Neuromuscular Factors
When examining strength and the factors that are involved in
the production of muscular force, several factors can be delineated (Table 1).
The effectiveness of explosive exercises as training tools may be related to
their ability to affect these factors. Specifically, when
examining these factors, the body’s ability to recruit motor units or to
stimulate the rate coding mechanism is of critical importance to understanding
the effectiveness of explosive exercises in sports performance. Additionally, the
hypertrophic response to explosive exercises may add
further evidence to the effectiveness of explosive exercises as a training modality.
Motor Unit Recruitment and Rate Coding
When examining the neuromuscular system, the motor unit is
described as being composed of a
motor neuron and all the muscle fibers
it innervates (32). Motor units are generally composed of between 9 and 1,934
muscle fibers per motor neuron (13, 30). Muscle fibers that have a lower muscle
fiber to motor neuron ratio (9:1) are used to control fine movements, whereas
muscle with large ratios (1,934:1) are used in the performance of gross
physical movements (30, 51). The ability to regulate the amount of tension produced
by a muscle is clearly related either to the ability to recruit or to rate
coding of motor units (10, 11, 33, 35). Several
investigations have suggested that there is a sequential gradation of motor
unit recruitment that results in a recruitment of smaller to larger motor units
(6, 27). Often this concept is
termed the size principle. This
principle appears to hold true for both ramp and ballistic or explosive
voluntary and reflex contraction (5). Generally, it is believed that small
motor units, which tend to have lower thresholds and are predominantly composed
of Type I fibers, are recruited in response to lower force demands. When higher
forces are demanded, the higher-threshold motor units, which typically are made
up of Type II muscle fibers, are recruited (9). The fact that larger, more
powerful motor units are recruited only when high force or high power outputs
are demanded by the activity is of particular interest to understanding the
effectiveness of explosive exercises (43). Thus, in order to activate the larger
motor units, explosive exercises--which generally require high force and high
power outputs--are needed. In addition to stimulating
the recruitment of higher-threshold motor units, explosive exercises, which
require high contraction speeds, have the potential to alter the motor unit recruitment
pattern. These exercises may train higher-threshold motor units to contract
before or in concert with low-threshold motor units (10, 36, 49).
Therefore, the use of explosive exercises in a training program may result in
adaptations that allow the athlete to be able to recruit larger motor units
sooner or more efficiently. These findings may partially explain why there is a
high
degree of velocity specificity in resistance training (28).
Another strategy for increasing the amount of force generated is the activation
of the rate coding mechanism (34). Rate coding is often defined as occurring
when the frequency of neural impulses sent to motor neurons already activated
is increased (4). The rate coding process is unique in that the force generated
increases without additional motor units being recruited. The high force and
high power output demand of explosive exercises may also result in changes in
the muscles’ ability to rate code because of the ability of this type of
exercise to increase the frequency of stimulation of higher-threshold motor
units (9).
Table 1
Factors Related to
Force Generating Capabilities
1. Motor unit
recruitment and activation patterns
2. Rate coding
3. Synchronization
4. Neural inhibition
5. Muscle
cross-sectional area
6. Motor unit type
Note: Modified from
Stone (43).
Generally, it is believed that there is an
interplay between rate coding and motor unit recruitment in the body’s
ability to generate force (9). The interplay of these 2 force-generating
mechanisms may be related to the size and fiber-type composition of the muscle
(9). Research evidence suggests that homogeneous muscles such as the adductor pollicis (7291% Type I fibers) rely primarily on motor unit
recruitment from 050% of their maximal voluntary contraction (MVC) (29). Rate
coding becomes the primary mechanism for increasing force production in this
muscle at intensities greater than 50% of its MVC (29). A different pattern of
recruitment and rate coding may be experienced with larger heterogeneous (both
Type II and Type I fibers) muscles, such as the deltoid and biceps brachii (7, 8, 29). With these larger muscle groups, there is
an initial reliance on rate coding of the low-threshold motor units which are
primarily composed of Type I fibers and small motor neurons (5, 6). Tension
development between 30 and 90% of MVC is primarily determined by the increased
recruitment of motor units (8, 29, 35). During this
period of increased motor unit recruitment, it is important to note that the
low-threshold units are the first to be recruited, but as the tension (force
and power output) increases, any additional force is generated by recruiting
higher-threshold motor units (4, 7, 8, 29). Increased rate coding of these higher-threshold
motor units is then needed to generate forces that approach 100% of MVC. Therefore,
it is important to note that maximal or near maximal forces can only be
generated through the increased recruitment or rate coding of higher-threshold motor
units. Because of their high force- and power output-generating capabilities,
explosive exercises appear to be the optimal mechanism for inducing
sport-specific changes in motor unit recruitment
and rate coding.
Hypertrophic Factors
When examining the hypertrophic effects
of explosive resistance exercise training, it appears that hypertrophy is
associated with Type II muscle fibers (24). This may be related to the
preferential activation of higher-threshold motor units, which are
predominantly composed of Type II muscle fibers (10, 36, 49).
Explosive exercise training will lead to greater increases in neural activation
during integrated electromyographic activity and rate
of isometric force production when compared to heavy resistance training (21,
24). Conversely, heavy resistance training appears to stimulate the hypertrophy
of both Type I and Type II fibers, with type II fibers experiencing a greater
rate of hypertrophy (20, 24). Thus, it is likely that alterations in maximal strength
are probably related to the combined effects of hypertrophic
factors, whereas rate of force development may be associated with alterations
in neural activation (24, 39). However, it is
likely that hypertrophy of Type II fibers
can result in some alterations in the rate of force development (21).
It is also likely that the training experience or status of
the athlete will exhibit a significant effect on the hypertrophic
and neural adaptation to explosive exercises (24, 39). Generally,
it is believed that untrained subjects will experience rapid gains in strength
during the first 2-3 months (39). These gains are largely associated with
neural adaptations to a training program (39). After
this 2- to 3-month period, additional strength gains will be related to hypertrophic factors (39).
Explosive Exercise and Power
Häkkinen and Komi
(20) have defined power as an explosive production of force. Generally,
explosive strength is related to maximal power output, which is best
characterized by brief muscle actions, which result in high-velocity movements
(31, 37). Maximal power output is generally related to strength or maximal
force production but is somewhat different and not completely dependent upon this
variable (31). Cross-sectional data clearly suggest that high levels of leg and
hip strength are present in athletes who possess superior maximal power outputs
as assessed by vertical jumping (21, 45, 47). This relationship is strengthened
by longitudinal studies, which assess increases in 1 repetition maximum squat
and vertical jump performance (45, 48).
It has been suggested that a continuum
of explosive exercise modalities exists (Figure 1) and that depending upon the
mode selected and method of application, different adaptations may occur. Typically,
low-speed/high-force resistance training, which is usually undertaken at
relative intensities of 80% or greater, can markedly increase maximum strength
(3, 19), power, and speed gains when compared to training with light weights
(41, 53). These adaptations may be potentiated when
high levels of muscular force are required and when there is a conscious
intention to create fast movements (2). This type of heavy strength training is
typically undertaken by power lifters (bench, squat, and deadlift)
and can result in high-power outputs when compared to nonlifting
controls (9). However, if heavy strength training is maintained for long
periods of time (months to years), the rate of isometric force and power
production can be impaired (26, 43).
Table 2
Exercise Power
Outputs
Jerk
Snatch
Clean
Deadlift
Squat
Bench press
Note: Modified from Stone (43).
The use of high-speed/lowforce movements can also result in increased gains in
power output (3, 19). When this type of training is used with squatting and
jumping motions at ~30% of maximal isometric force, superior performance gains
have been noted in sports that rely on speed or power output (52). These
enhancements have been hypothesized to result from improvements in intra- and intermuscular coordination during the performance of high-speed/low-force
movements (9, 19, 41). Additionally, contractile speed
has been suggested to be increased after short-term high-power training when
compared to isometric training (12). Based upon these data, it appears that both
neural and contractile mechanisms are affected by the use of high-speed/low-force
exercises. However, if athletes use only high-speed/low-force movements,
maximal strength levels will not be improved, which suggests that an alternate
training model may be needed (26).
Several authors have suggested that
the optimal adaptive explosive exercise stimulus for the muscle and nervous
system must come from the combination of high-force/low-velocity, low-force/ high-velocity,
and high-force/ high-velocity exercise movements (1, 26, 44).
Harris et al. (26) suggest that the combination of training modalities will
optimize performance gains. In this study, the utilization of a
combination of high-force/low-velocity (5 weeks) and low-force/high-velocity
training (4 weeks) produced optimal gains over a 9-week training period when
compared to high-force/ low-velocity and low-force/highvelocity
training. These data suggest that maximum strength development can increase
power production early in a training program and that a shift toward power development
training is necessary during a training cycle in order to optimize power
production. This type of training shift is one of the major tenets of the
theory of periodized training (1, 46).
Additional cross sectional data suggest
that Olympic-style weight-lifters (snatch, clean and jerk) whose training
programs are centered on the utilization of explosive exercises possess
comparable maximal strength levels with power lifters (31). This also suggests
that weightlifters are capable of jumping higher than power lifters and
generally produce superior maximal power outputs (31). This relationship is not
totally unexpected, because the highest power outputs are produced during the
performance of weightlifting exercises (Table 2) (43).
Traditionally, weightlifters use training techniques that
utilize slow-velocity/high-force movements and explosive
high-velocity/high-force movements during certain phase of training and that
ultimately result in improved power output production (14, 23).
Explosive Exercises
Generally, explosive exercises or speed
strength exercises result in the production of high power outputs (43). The
exercises most typically employed in this capacity are the Olympic-style
lifts—more specifically the snatch, clean, pulling motions, and various jerking
movements (Table 3). The clean and jerk and snatch lifts have the potential to
produce some of the highest average human power outputs (Table 2) (14, 16, 17). Clearly, when comparing the Olympic-style lifts to traditional
high-force/low-velocity exercises, higher power outputs are encountered. Thus,
the use of explosive lifts such as the Olympic-style lifts may partially explain
the differences in power output capabilities of different strength power
athletes (31, 43). Because these exercises stimulate improved power
output-generating capabilities, many have suggested that they will produce a
significant carry-over to other strength power sports (43). This suggestion is
generally based upon the belief that these exercises produce movement patterns,
velocity characteristics, and power outputs that are similar to those needed in
many sports performances.
Types of Explosive Exercises
Snatch (squat and
power)
Clean (squat and
power)
Pulls (clean and
snatch)
Jump squats
Speed squats
Jerks (push and
split)
Injuries and Explosive Exercises
Explosive exercises appear to be a safe means of maximizing
sports performance. However, many sports medicine and exercise professionals
express skepticism about the safety of this type of training. Recently, Pierce
et al. (38) have reported that no injuries that required medical attention or
that limited training occurred in a group of 15 girls and 55 boys who ranged in
age from
Examination of the injury rates of athletes participating in
training programs that require the performance of speed strength or explosive
exercises indicates that apparently very few injuries occur. In fact, a
longitudinal study that examined 4 years of weight training in college football
teams (American Injury Monitoring System) reported that the time lost from
injuries incurred during weight training amounted to 1% of the time lost from
injuries that resulted from participation in football (55). It is important to
note that the safety of speed strength exercises can be maximized by using
proper lifting technique (43). Developing proper technique habits and following
appropriate teaching progressions are essential during the learning and
performance of explosive exercises. Proper lifting technique typically requires
the athlete or participant to perform the exercises in a controlled fashion.
Additionally, the safety of the exercise can be maximized through careful
supervision by a knowledgeable strength and conditioning professional, who can
correct technique and lead the athlete through appropriate learning
progressions.
Conclusions
Explosive exercises can result in improvements in power production. It appears that the Olympic-style lifts have the greatest potential to affect power production. These lifts stimulate neuromuscular adaptations, which may potentially result in improved sports performance. Power production may also be maximized by using a combination of explosive exercise modalities in a periodized training program. Additionally, when these exercise are performed with appropriate technique and are supervised by a qualified strength professional, there is minimal risk of injury.