Transference of Strength and Power Adaptation to Sports Performance

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2018-08-08 23:31:00
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SUMMARY
The training of horizontal propulsive force
generation is one aspect of many sports that is not easily simulated with traditional
gym-based resistance training methods, which principally work the leg
musculature in a vertical direction. Given that most motion involves an
integration of both vertical and horizontal force production, transference of
gym-based strength gains may be improved if exercises were used that involved
both vertical and horizontal force production.
INTRODUCTION
Running velocity over short distances is an
important factor for successful performance in most team sports (2,21,26).
Velocity is the product of stride length and stride rate or frequency, and to
increase velocity, at least 1, if not both, of these parameters must be
increased (23,24). From the deterministic model depicted in the Figure, it can
be observed that both stride length and frequency are products of the amount
and duration of force exerted. That is, the fundamental factors relating to
optimizing velocity are the application of force and the time over which it is
applied.
What is not apparent from the model is the
direction of force application that is most important. That is, is the
application of horizontal or vertical force of more importance to increase
velocity? Within the literature, there are differing views as to the
significance of each during sprint performance. Further ambiguity is added to
this issue when additional sport-specific factors need to be considered, such
as those encountered during contact situations in rugby and rugby league.
Therefore, it is not entirely clear which force component is more important in
affecting increased velocity within a sporting situation, such as rugby and
rugby league.
The velocity requirements of the sport also
need to be considered, such as the distances or durations over which players
are commonly required to sprint. In sports where average sprint distances range
from 10 to 30 m, it would appear that the ability to achieve maximum velocity
within the shortest time frame is more important than the maximal velocity
itself. That is, acceleration rather than maximum velocity would seem to be of
greater importance to many sportsmen and women. This leads to the question of
whether there are different directional requirements to force application when
considering maximum velocity and maximum acceleration.
This literature review addresses this contention
by (a) investigating the literature on horizontal force production and its
effect on velocity and acceleration, (b) investigating the literature on
vertical force production and its effect on velocity and acceleration, and (c)
suggesting future
 research directions.
图:Deterministic model of velocity
(adapted from Hay (7)).

 
HORIZONTAL VERSUS VERTICAL FORCE PRODUCTION
DETERMINANTS OF VELOCITY
Velocity is the product of stride rate or
frequency and stride length, and to increase velocity, at least 1, if not both,
of these parameters must be increased without a proportionately similar or
larger decrease in the other (9,20,23,24). If velocity is simply the product of
the frequency and length of a runner’s stride (Figure), it would be possible to
attain faster top running velocities simply by increasing the frequency of
steps. Weyand (24) reported that at top velocity during level treadmill
running, stride frequencies were 1.16 times greater for a runner with a top
velocity of 11.1 versus 6.2 m··s-1 (1.8-fold range) (r2 = 0.30). However, when the same
researchers investigated the individual variation at top velocity on -6-degree
decline and degree incline treadmill inclinations, no significant difference in
stride frequency (4.38±0.08 versus 4.34±0.08 steps·s-1,respectively) was
observed despite a significant difference in top velocity (9.96±0.30 versus 7.10 ± 0.31 m·s-1 respectively). Hunter (9) reported that step rate was
not significantly related to sprint velocity (r=-0.14), as did Brughelli (3)
who reported a trivial correlation between maximum running velocity and stride
frequency (r = 0.02). Heglund and Taylor (8) suggested that the range of stride
frequencies used at different velocities tends to be narrow; however, these
results were based on animal studies using quadrupeds ranging in body size from
a mouse to a horse.
Stride frequency is directly influenced by
the stride time, which in turn is compromised of swing time or flight time and
contact time or stance time (20). That is,
Stride frequency =1/(flight time + stance
time) change
Given swing time comprises the majority of
the total stride time at top velocity (approximately 75% of stride time for
maximum velocities of 6.2 to 11.1 m·s-1),the relatively weak relationship between top velocity and maximal
stride frequency may be the result of runners with different top velocities
repositioning their legs in similar periods. That is, similarities between
minimum swing times minimize the extent of possible variation in maximal stride
frequencies. Regression relationships presented by Weyand (24) showed that
minimum swing times were only 8% (0.03 seconds) shorter for a runner with a top
velocity of 11.1 versus 6.2 m·s-1(r2 = 0.06) during level treadmill running. In contrast, swing times at the slower
velocities attained during inclined running were actually 8% shorter than those
of the faster decline running (0.331 ± 0.005 and 0.359 ± 0.004 seconds, respectively). This difference, however, was
attributed to interruption of the limb’s arc because of the inclination of the
running surface rather than differences in velocity.
If it is indeed the case that both fast and
slow runners and fast and slow running velocities present similar swing times,
then differences in maximal stride frequencies between fast and slow runners
may result from the contact portion of the stride being shorter in faster
runners and velocities. Brughelli (3) reported a low correlation between
maximum running velocity and contact time (r = 0.14); however, this is in
contrast to other research. Nummela (20) reported that maximal running velocity
had a significant negative relationship with ground contact times (r=-0.52). In
support of this finding, the contact times at maximum velocity observed by
Weyand (24) were significantly shorter for the faster decline running compared
with those for the slower incline running (0.098 ± 0.003 and 0.130 ± 0.004 seconds, respectively).
Kyrolainen (12) reported that as running velocity increased from 3.45 to 8.25 m·s-1, contact times shortened from 0.227 ± 0.011 to 0.115 ± 0.007 seconds. Munro (18)
also reported a decrease in contact time as running velocity increased (0.27 ± 0.020 seconds at 3.0 m·s-1 seconds at 5.0 m·s-1 and 0.199 ± 0.013 ). It would seem that
an increase in velocity because of an increase in stride frequency may be
attributable to a decrease in the time the athlete is in contact with the
ground.
As stated previously, if velocity is simply
the product of the frequency and length of a runner’s stride (Figure), then it would
also be possible to attain faster top running velocities simply by increasing
the stride length. Weyand (24) reported that during level treadmill running,
stride lengths at top velocities were 1.69 times greater (4.9 versus 2.9 m) for
a runner with a top velocity of 11.1 versus 6.2 m·s-1 (r2= 0.78). It was also reported that stride lengths during maximal
velocity decline running (4.6 ± 0.14 m at 9.96 ± 0.30 m·s-1) were significantly greater than
those of maximal velocity incline running (3.3 ± 0.10 m
at 7.10 ± 0.31 m·s-1)。This is in agreement with other researchers (3,9) who reported
significant correlations between maximum running velocity and stride length (r
= 0.66 and r = 0.73, respectively).
Stride length is the sum of the takeoff,
flight, and landing distance. However, Weyand (24) reported that contact
lengths did not differ between fast and slow runners with regression equations
indicating that contact lengths were only 1.10 times greater for a runner with
a top velocity of 11.1 versus 6.2 m·s-1 (r2=
0.30). Furthermore, when these results were analyzed within groups of men and
women, it was reported that contact lengths varied little or not at all in
relation to top velocity. Nummela (20) reported that an increase in stride
length was related to an increase in both vertical force (r = 0.58) and
horizontal propulsion force (r = 0.73), suggesting that an increase in stride
length is achieved by increasing both vertical and horizontal ground reaction
forces (GRF). These results would tend to suggest that the predominant
mechanism used by runners to achieve greater stride length is through greater
application of GRF. That is, stride length is determined by the product of
force exerted during foot-ground contact and the duration of the applied force
(23,24).
It would appear that the major determinants
of velocity are the forces applied to the ground and the time of foot-ground
contact. That is, the attainment of greater velocity requires the application
of greater support forces during briefer contact periods. GRF can be broken
down into 3 components; however, typically, the horizontal (anterior-posterior)
and vertical components are of most interest (10). Mero and Komi (14) have
shown a relationship between running velocity and average net resultant force
(vertical and horizontal), when related to body weight (r = 0.65), but there
are numerous hypotheses regarding the relative importance of various GRF
components to sprint performance. It has been shown that faster running
velocity are associated with increased vertical force production, although a
relationship to horizontal force production has also been shown
(3,10,12,18,20). This section investigates the relationship of both components
and suggests future directions for research in this area.
VERTICAL FORCE PRODUCTION
It has been theorized that during constant
velocity running, there is no or very little horizontal resistance to overcome
and that the propulsive forces that increase the body’s forward velocity before
takeoff simply offset the braking forces that decrease the body’s velocity on
landing (18,24). Furthermore, it is the vertical portion of stride that needs
assistance because of the need to overcome gravity; therefore, applying greater
forces in opposition to gravity would increase vertical velocity on takeoff,
translating to an increased running velocity.
Weyand (24) reported that an increase in
vertical force production was the predominant mechanism used by runners to
attain faster top velocities. Regression equations showed that at top velocity,
mass-specific forces applied to oppose gravity were 1.26 times greater for
faster runners compared with those for slower runners (r2=0.39).
Furthermore, when comparing the same subject at different velocities,
significant differences in vertical forces were observed between the faster top
velocities achieved during decline running and the slower top velocities of
incline running (2.30 ± 0.06 and 1.76 ± 0.04 body weight (BW), respectively). Munro reported that as
running velocities increased from 3 to 5 m·s-1,peak vertical GRF (relative to body weight) increased from 1.40 ± 0.11 to 1.70 ± 0.08 BW. Similar findings
were reported by Nigg (19) whereby vertical forces were found to significantly
increase as velocity increased from 3 to 6 ms21 (1331 ± 225 to 2170±489 N, respectively). Using the subject’s
reported mean body weight, these equate to estimated values of 1.9 and 3.0 BW,
respectively. Similarly, Kyrlainen (12) demonstrated changes in the GRF as
velocity increased from 3.45 to 8.25 m·s-1. Maximal
vertical force values increased from 1665 ± 219 to 2134± 226 N. As results were not reported by gender, relative values were
not able to be calculated. Arampatzis (1) also reported an increase in maximum
vertical GRF (N/kg) between velocities of 2.5 and 6.5 m·s-1, although values were not presented. These findings
support the theory of an increase in running velocity being achieved through an
increase in vertical GRF
HORIZONTAL FORCE PRODUCTION
In contrast to the above, it has been
suggested that the critical factor in maximal sprint running is an increase in
horizontal propulsive forces. To maintain velocity, the horizontal propulsive
force must be equal to the braking force; however, to increase velocity, the
propulsive force must be greater than the braking force (10,15,20), suggesting
that horizontal propulsive forces play an important role in velocity
development and acceleration.
Using multiple linear regression, Hunter (10)
found that relative propulsive impulse explained 57% (r2= 0.57) of the
variance in sprint velocity, whereas relative vertical impulse did not explain
any further variance in sprint velocity. These findings are supported by those
of Nummela (20) who also reported a significant correlation between maximal
running velocity and mass-specific horizontal forces during the propulsion
phase (r = 0.66). Once again, mass-specific vertical force was not found to be
related to the maximal running velocity. Munro (18) reported that propulsive
impulses, normalized by body weight, increased 79% from 0.14 ± 0.01 to 0.25±0.2 BWI as velocity increased
from 3.0 to 5.0 m·s-1. Over the same
velocity range, vertical GRF only increased 21%. Kyrolainen (12) also
demonstrated changes in the GRF with increasing velocity. As velocity increased
from 3.45 to 8.25 m·s-1, maximal forces in
the horizontal direction increased 175% from 235± 42 to
675± 173 N, whereas vertical forces only increased 30%.
As mentioned previously, the estimation of relative values was not possible
because of the nonseparation of results by gender. Increases in horizontal
forces were also reported by Brughelli (3). As running velocity increased from
40 to 100% of maximum, relative horizontal forces increased 105% from 0.21 ± 0.02 to 0.43 ± 0.06 N/kg, whereas vertical
forces only increased 18%. These findings seem to suggest that horizontal force
production is more important than vertical force production in allowing an
increase in running velocity.
It is worth noting the differences in
methodologies used by the various studies. Results from studies using motorized
(24) and nonmotorized (24) treadmills have been presented alongside those
obtained from ground running (1,10,12,18–20). Although it may be questionable
as to whether constant velocity running on a motorized treadmill is an accurate
way of deducing cause and effect for over ground running, of greater interest
may be the conclusion presented by Weyand (24) reporting that an increase in
vertical force production was the predominant mechanism used by runners to
attain faster top velocities when only vertical force production was measured.
This is also true of Arampatzis (1) and Nigg (19) who reported that vertical
forces were found to significantly increase as velocity increased. Of the
studies who measured both vertical and horizontal force, Kyro¨la¨inen (12) and
Munro (18) reported increases in both components with an increase in velocity,
whereas Hunter (10) and Nummela at al. (20) reported significant relationships
only with the horizontal forces.
VERTICAL VERSUS HORIZONTAL
When the vertical and horizontal components
are compared, it is apparent that the magnitude of the vertical forces is the
larger of the two. Munro (18) reported that at velocities ranging from 3.0 to
5.0 m·s-1 ,peak
vertical GRF are typically 5–10 times greater than the peak horizontal forces.
At 3.0 and 5.0 m·s-1, horizontal propulsive
impulses were 10 and 15% of average vertical GRF, respectively. From the
results presented by Kyro¨ la¨inen (12) at 3.45 and 8.25 m·s-1, horizontal forces were 14 and 32%, respectively, of
vertical GRF. This apparent difference in magnitude is also supported by
Brughelli (3) who reported that at 40, 65, and 100% of maximum velocity,
relative horizontal forces were 9, 12, and 18%, respectively, of relative
vertical forces, which can be attributed to vertical acceleration, that is,
9.81 m·s-2
Although there does appear to be a
difference between vertical and horizontal force production, it seems that the
magnitude of this difference decreases as velocity increases. If horizontal
components of GRF are expressed as a percentage of the vertical component, then
an increase in the reported percentage would imply that the horizontal
component has increased proportionally more so than the vertical component. This
increase in the percentage contribution of the horizontal component of GRF as
speed increases is evident in the studies by Munro (18), 10% at 3.0 m·s-1 increased to 15% at 5.0 m·s-1,Kyrolainen (12), 14% at 3.45 m·s-1 increased to 32% at 8.25 m·s-1 and Brughelli
(3), 11% at 40% of maximum velocity increased to 19% at 100% of maximum
velocity。
In addition to a nonuniform increase in the
2 main components of GRF, it is also evident that the increases in vertical
forces with increasing velocity may not be linear. Although Munro (18) and Nigg
(19) indicated that the increases in the vertical GRF were linear with
increasing velocity in the rangeof3– 6 m·s-1,
and Keller (11) noted; similar linear increases up to 3.5 m·s-1,above these velocities, the
relationship has been reported to be nonlinear, and in some cases, there is no
further increase in vertical forces. Brughelli (3) reported that as running
velocity increased from 40 to 65% of maximum velocity, relative horizontal
forces increased 38% (0.21 ± 0.02 to 0.29 ± 0.03 N/kg) and relative vertical forces increased 17% (1.98 ± 0.23 to 2.31 ± 0.18 N/kg). However, as
running velocity increased from 65 to 100%, relative horizontal forces
increased a further 48% (0.29 ± 0.03 to 0.43 ± 0.06 N/kg), whereas relative vertical forces remained relatively
constant and only increased 1% (2.31 ± 0.18 to 2.33 ± 0.30 N/kg). These findings are similar to those of Nummela (20) who
also reported that relative vertical force remained constant after
approximately 65% maximum velocity. It was observed that vertical force
increased with the increasing velocity until the velocity of 7 m·s-1;thereafter, the velocity was increased without
further increase in vertical force. As mentioned previously, Keller (11)
reported a linear increase in relative vertical forces at lower velocities
(1.23 ± 0.10 BW at 1.5 m·s-1 to 2.45 6 0.28 BW at 3.5 m·s-1);
however, as velocity increased from 3.5 to 6 m·s-1,
there were no significant increases in relative vertical forces (2.45 ± 0.28 to 2.38 ± 0.28 BW, respectively).
Furthermore, a decrease was observed at the highest velocity of 8.0 m·s-1 (1.89 ± 0.49 BW), although
this only represented values for 3 trials from 1 subject at this high velocity.
Of interest are the findings of Hunter (10) who also reported that the
relationship between relative vertical impulse and sprint velocity showed signs
of nonlinearity. In this case, however, it was shown that after a certain
magnitude, any further increases in relative vertical impulse did not
correspond to an increase in sprint velocity. Although these results were only
reported in graphical form, they would seem to suggest that a ceiling effect
may exist with regard to vertical force production, that is, past a certain
point, velocity is no longer increased by increasing vertical GRF.
It has been shown that to reach faster
maximum running velocities increases in both vertical and horizontal GRF are
required. Although it appears that the vertical component is the larger of the
2 GRFs, it is suggested that running velocity is more dependent on horizontal
than on vertical force as the velocities increase toward maximal. This is
evident given that linear relationships were not observed between vertical
force and running velocity at higher velocities. The significance of the
horizontal component seems to be logical because one cannot increase horizontal
velocity by increasing vertical force, but acceleration and deceleration of
running velocity is produced mainly by changing horizontal force. The next
section considers the contribution of vertical and horizontal force production
with regard to acceleration.
ACCELERATION
Although velocity is very important in most
sporting situations, acceleration is of relatively greater importance when
covering only short distances at maximal effort (6,23). Therefore, it would
appear that the ability to achieve maximum velocity within the shortest time
frame is more important than maximal velocity itself. That is, acceleration
becomes an essential focus when investigating the requirements of many sports.
As discussed previously, there are numerous
hypotheses regarding the relative importance of various GRF components to
sprint performance. The velocity-time curve can be divided into 3 phases,
acceleration, constant velocity, and deceleration (15), and many of these
hypotheses were intended to be the most applicable to the constant velocity
phase of a sprint (10). It has been suggested that during constant velocity
running, the propulsive forces that increase the body’s forward velocity before
takeoff simply offset the braking forces that decrease the body’s velocity on
landing (18,24). In contrast, acceleration is achieved by changing horizontal force
such that the propulsive force is larger than the braking force (20). This
leads to the question of whether there is a different directional requirement
to force application when considering peak velocity and peak acceleration.
When investigating vertical and horizontal
GRF characteristics, Mero (13) compared the acceleration phase of sprinting
(velocity = 4.65 m·s-1) with that of
previous work investigating maximal sprinting (velocity = 9.85 m·s-1) (16). The respective average vertical forces were equal
(431±100 N and approximately 563 N, respectively),
whereas the horizontal forces produced during the acceleration phase of
sprinting were about 46% greater than those produced during constant velocity
maximal sprinting (526 ±75 and 360 ±42 N, respectively). It should be noted that the average vertical
force from Mero (16) was estimated from the stated value (1,286 ± 61 N), which was inclusive of body weight, minus the mean subject
body weight (73.7 kg).
The vertical and horizontal values during
acceleration obtained from Mero at 4.65 m·s-1 can be expressed relative to body weight using the mean body weight and compared
with the norms reported by Munro (18) at corresponding velocities of 4.5 and
4.75 m·s-1. Again, it can be seen (Table.)
that the respective relative vertical forces during acceleration and constant
velocity were equal at comparable velocities, whereas the horizontal force
during acceleration was greater than those recorded during constant velocity.
These results suggest a greater emphasis on horizontal force during
acceleration than there is during constant velocity running.
Hunter (10) reported that both simple and
multiple regression results showed a relatively strong trend for faster
athletes to produce greater magnitudes of relative propulsive impulse (r2=
0.57). It was thought that athletes with the ability to produce higher
horizontal propulsive forces would undergo larger increase in horizontal
velocity during each stance phase, thereby accelerating faster. This finding
agrees with the research of Mero and Komi (14) who reported a positive
relationship between average resultant GRF during propulsion and sprint
velocity between 35-m and 45-m marks (r = 0.84) and with those of Mero (13) who
reported a high correlation between horizontal force production in the
propulsion phase and running velocity (r = 0.69). These results further
emphasize the importance of the propulsion phase during the acceleration phase
of sprinting.
Hunter (9) suggested that a high vertical
GRF, and therefore, a high vertical velocity of takeoff, had a positive effect
on step length; however, it also had a negative effect on step rate. In
addition, there was evidence of a strong negative interaction between step
length and step rate (r = 20.78). That is, those athletes who had a high step
rate tended to have a shorter step length and vice versa. It was thought that
more frequent ground contacts, via a low vertical GRF and short flight time,
would allow a greater opportunity to accelerate. If flight time is increased
during acceleration, as determined by a large relative vertical GRF, this would
correspond to a decrease in the percentage of time spent in contact with the
ground. Given an athlete can only influence their sprint velocity when in
contact with the ground, this would be a disadvantage (10). That is, the most
favorable magnitude of vertical GRF is one that creates a flight time only just
long enough for repositioning of the lower limbs. If the athlete can reposition
the limbs quickly, then a lower relative vertical GRF is sufficient, and all
other strength reserves should be applied horizontally. It is only when an
athlete cannot achieve or maintain a high step rate such as when fatigued, that
a greater relative vertical GRF becomes more important (10).
Therefore, during the acceleration phase of
a sprint, greater increases in horizontal propulsion are required to achieve
high acceleration (10). Consequently, it is proposed that it would be of
advantage to direct most training effort into producing a high horizontal GRF,
not vertical GRF.
Table
Horizontal and vertical forces during acceleration and constant
  velocity

Study

Running phase
21

Running velocity, m·s
Vertical force, BW
Horizontal force, BW

Mero (13)

Acceleration

4.65
1.60
0.73

Munro
  et al. (18)

Constant velocity

4.5
1.65
0.23

4.75
1.68
0.24

BW = body weight.

 
CONCLUSIONS/FUTURE RESEARCH DIRECTION
It is generally accepted that maximal
running velocity requires high force production (2,15,17). As such, strength
and power training methods are almost universally promoted as a means of
training to improve running velocity (2,5,23). Therefore, the relationship
between strength and power and velocity are of considerable interest in
attempting to identify possible mechanisms for the enhancement of running
performance (2,5,25,27).
It is also generally accepted that the more
specific a training exercise to a competitive movement, the greater the
transfer of the training effect to performance (5,21,22), and as such, athletes
who require power in the horizontal plane, engage in exercises containing a
horizontal component, whereas athletes who require power to be exerted in the
vertical direction, train using vertical exercises (4,21). Given that a variety
of training regimens are commonly used to improve muscular force output with
the ultimate goal of enhancing sprinting performance (21,23), it would seem
intuitive to focus on the enhancement of the forces, which are the most
important in improving velocity.
From the literature, although it is
apparent that force production is necessary in both the vertical and horizontal
planes, it is the horizontal forces that experience the greatest increase when
accelerating to maximal velocity. This becomes even more valid when the demands
of rugby, league, or American football are taken into consideration. That is,
the need to accelerate quickly over short distances, where increases in
horizontal propulsive forces are essential, and the need to overcome large
horizontal resistances, in the form of contact from opposing players. It would,
therefore, seem critical that a movement-specific approach be applied to the
design of strength and power resistance programs for such sports.
Currently, most gym-based resistance
programs focus on exercises that principally work the leg musculature in a
vertical plane. It is proposed that the transference of gym-based strength
gains may be improved if exercises were used that involve both vertical and
horizontal force production. That is, if successful performance requires force,
velocity, and power (product of force and velocity) in the horizontal plane,
improvements may be realized if the design of the resistance training program
focuses on horizontal movement-specific exercises as well as traditional
vertical exercises. To date, however, the effectiveness of a gym-based
lower-body resistance training program with a horizontal component has not been
investigated.

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沙发346088240宇宙冠军 2018-08-19 14:08
谢谢分享!!!!!!!!!!!!!!!!!!!!!!!
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