Measurement of Elastic-Like Behaviour in the Power
Squat
Michael Bird* & Jackie Hudson
*Health and Exercise Science Program Truman State University,
Department of Physical Education California State University, Chico USA
Bird, M. & Hudson, J. (1998) Measurement of Elastic-Like Behaviour in the Power Squat.
Journal of Science and Medicine in Sport •(2): 89-99
Because traditional procedures of evaluating elastic-like behaviour have yielded
mixed results, the purpose of this work was to explore two methods of measuring
elastic-like behavior in the power squat. The entire concentric time method was
based on traditional procedures. The initial concentric time method was developed
to examine elastic-like behavior for the beginning 0.2 s of concentric movement.
The present study compares a power squat performed maximally by nine subjects
at 70% of their 1 repetition maximum. Squats were performed with rebound (REB)
and without rebound (NRB). For the entire concentric time method only concentric
time was significantly greater (p < 0.05) in the NRB than the REB. For the initial
concentric time method the relative displacement, velocity, net work, & peak power
of the center of mass were significantly greater (p < 0.05) in the REB than the NRB.
Some subjects had theoretically infeasible negative results for elastic enhancement
in the entire concentric time method, but not the initial concentric time method. It
seems that measuring elastic-like behavior near the end of the movement can be
confounded by the constraints of the task. Based on its success, the initial
concentric time method appears to be more appropriate for measurement of elasticlike behavior in lifting.
Elastic-like b e h a v i o u r in h u m a n m o v e m e n t h a s b e e n d o c u m e n t e d for m a n y
activities w i t h r e b o u n d . In t h e s e s t u d i e s n u m e r o u s v a r i a b l e s a n d p r o c e d u r e s
have b e e n u s e d to reflect t h e benefits of p e r f o r m i n g with a r e b o u n d . M a n y
questions, however, r e m a i n u n a n s w e r e d . One l i m i t a t i o n of u n d e r s t a n d i n g elasticlike b e h a v i o r h a s b e e n t h e v a r i a t i o n in t h e m e t h o d s of its m e a s u r e m e n t .
The typical s c e n a r i o of g a i n i n g elastic benefit w a s s u m m a r i z e d b y C a v a g n a
(1977): "In s o m e exercises, kinetic a n d / o r g r a v i t a t i o n a l p o t e n t i a l energy of t h e
b o d y a r e a b s o r b e d b y c o n t r a c t e d m u s c l e s while t h e y a r e forcibly s t r e t c h e d . T h i s
m e c h a n i c a l e n e r g y is w a s t e d m o r e or l e s s c o m p l e t e l y a s h e a t if t h e m u s c l e is k e p t
active a t t h e s t r e t c h e d l e n g t h or if it is allowed to r e l a x after stretching. O n t h e
contrary, if t h e m e c h a n i c s of t h e exercise is s u c h t h a t s h o r t e n i n g of t h e m u s c l e
i m m e d i a t e l y follows stretching, a n a p p r e c i a b l e recovery of t h e w o r k d o n e o n t h e
m u s c l e c a n t a k e place" (p. 125). This recovery of w o r k is believed to b e p a r t i c u l a r l y influential in t h e initial m o m e n t s of t h e c o n c e n t r i c p h a s e of m o v e m e n t
( C h a p m a n , Caldwell, & Selbie, 1985).
M a n y different s p o r t s a n d m o v e m e n t s have b e e n u s e d to e x a m i n e t h e elasticlike benefits g a i n e d from a r e b o u n d . T h e m o s t c o m m o n l y u s e d t a s k h a s b e e n t h e
vertical j u m p ( A s m u s s e n & B o n d e - P e t e r s o n , 1974a; H u d s o n & Owen, 1985; Komi
& Bosco, 1978). O t h e r t a s k s t h a t involve t h e lower e x t r e m i t y i n c l u d e r u n n i n g
89
Measurement of Elastic-Like Behaviour in the Power Squat
(Asmussen & Bonde-Peterson, 1974b; Cavagna, Citterio, & Jacini, 1981),
repetitive deep knee bending (Thys, Faraggiana, & Margaria, 1972), knee
extensions (Bober, Putnam, & Woodworth, 1987), and plantar flexion of the ankle
(Shorten, 1987). Upper extremity tasks include pushing a pendulum with the
arms (Bober, Jaskolski, & Nowacki, 1980), elbow flexions (Pousson, Van Hoecke,
& Goubel, 1990), forearm rotations (Chapman, 1980; Chapman et al., 1985), and
the bench press (Wilson, Elliott, & Wood, 1991).
RegardleSs of the task, the advantages gained from moving with a rebound have
been assessed from the differences in the kinematic or kinetic variables of similar
motions performed with and without rebound. Although the concentric, or
shortening, phase of the activity was typically superior when performed with
rebound, it was partly dependent on the methods and variables used. Previous
methods have included measures of elastic energy, elastic enhancement, and
efficiency. Variables used to evaluate elastic-like behavior include velocity (Bober
et al., 1980; Chapman, 1980; Thys et al., 1972), work (Chapman, 1980; Thys,
Cavagna, & Margaria, 1975; Wells, 1967), power (Thys et al., 1972; Wilson et al.,
1991), force (Bosco, Tihanyi, Komi, Fekete, & Apor, 1982; Thys et al., 1975; Wilson
et al., 1991), and energy (Asmussen & Bonde-Peterson, 1974b; Hudson & Owen,
1985; Komi & Bosco, 1978).
Methods of calculating elastic-like benefit in any given task have also varied.
When evaluating elastic energy, researchers typically used the ratio of the
difference in concentric energy (with and without rebound) to eccentric energy for
the chosen variable (Asmussen & Bonde-Peterson, 1974a; Hudson & Owen, 1985;
Komi & Bosco, 1978). For elastic enhancement, researchers typically used the
ratio between the concentric values (with and without rebound) for the chosen
variable (Thys et al., 1972; Wilson et al., 1991).
Although the gains from rebound movements are thought to occur early in
concentric motion, most of the previously mentioned studies evaluated benefits at
the end of the concentric phase. For movements such as a vertical jump, an
evaluation of the chosen variable(s) at the end of the concentric phase was
appropriate because peak velocity occurred at or near the end of a brief thrust
phase and at or near full extension of the joints. For movements with a different
purpose, such as weight liking activities, a different approach may be needed. For
example, Wilson et al.'s (1991) study of the decay of elastic benefit in the bench
press used a finite amount of time based on the power generated during the lift.
An initial, finite time interval may be applicable for lifting types of activities
b e c a u s e they require greater time for the range of motion (ROM) to be completed
and peak velocities are reached well before the end of the ROM.
The power squat has not been previously used for the evaluation of elastic-like
behavior. It is an appropriate task for study because it can be performed with and
without rebound. In addition, we can use this task to advance our understanding
of elastic-like behavior through the manipulation of load. Eventually, we may
provide insight for the many athletes who train with the power squat.
The purpose of this work was to explore two methods of measuring elastic-like
behavior in the power squat. The entire concentric time method was based on
previous procedures used to study the jump. The initial concentric time method
was developed to examine elastic-like behavior in the early stages of concentric
movement.
90
Measurement of Elastic-Like Behav]our in the Power Squat
Methods
Subjects
Nine skilled college-age weight lifters (mean mass [_+S.D.] of 77.45 _+9.56 kg and
height of 1.75 _+ 0.07m) participated in this study. All subjects were in good
apparent health, with no reported history of chronic or recent acute back, knee,
or leg injuries. All lifters signed informed consent in accordance with the approved
standards set forth by the university. Each subject was allowed to perform
preferred stretching activities and warm-up lifts. The hip (greater trochanter),
knee (lateral epicondyle), and ankle (lateral malleolus) joints were marked on the
left side of the body with reflective tape. The b a r center also was similarly marked
with reflective tape.
TaSk
Each subject performed a power squat with the bar placed on the superior,
posterior t r u n k in the area of the spines of the scapulae. In keeping with standard
practice, each subject used a weight belt in performing all lifts. No other devices
used to enhance the lifter's capabilities, such as knee wraps or lifting suits, were
allowed. All lifters were encouraged to rest adequately between lifts to avoid
fatigue.
All lifters performed a squat with a rebound (REB) and a squat without a
rebound (NRB). The REB condition has both eccentric and concentric motions
and the NRB condition has only concentric motion. The REB and NRB conditions
were controlled to ensure that the thrust distances were as similar as possible.
An adjustable safety bar was positioned within a standard power rack to represent
the lowest position of the crouch. In this way, the ROM of the knee joint was
limited in all lifts to reach a minimum of 90 ° of flexion.
The REB was performed to a depth consistent with the adjusted bar. For the
NRB the lifters performed a downward movement from an upright position, as in
a normal squat, b u t were allowed to rest the bar on the rack without changing
body position or stance. This was done to minimize muscle fatigue that would
result from holding the isometric position at the bottom position under load.
Based on the findings of Wilson et al., the bottom position was held between five
and six seconds to ensure that stretch benefits would be dissipated as heat
(Wilson et al., 1991).
Each lifter performed from one to three lifts with a rebound to determine their
one repetition maximum (1RM) in the power squat. After a sufficient recovery
time, the lifters performed the power squat at 70% of their 1RM (mean 70% mass:
118.3 + 37.1 kg) for both REB and NRB conditions, the order of the lifts being
randomly determined. Both of the REB and NRB conditions were performed at
maximum effort. That is, the subject was asked to perform the lift with as m u c h
force and velocity as possible in both the REB and the NRB. This request was
consistent with procedures used by Wilson et al. (1991). Because lifters often
perform submaximal lifts at submaximal velocities, an acclimation period was
used during warm-up.
Data Collection and Reduction
Subjects were videotaped with a Panasonic camcorder (model PV-330D) at a rate
of 60 Hz. Blur was eliminated through the use of a 2000.s -1 high-speed shutter.
The camera was positioned perpendicular to the sagittal plane of the subject.
Data were reduced from video to digital horizontal and vertical coordinates
91
Measurement of Elastic-Like Behaviour in the Power Squat
Variable
REB
Mean
(_+S.D.)
NRB
Mean
(_+S.D.)
Repeated Measures
ANOVA of REB vs.
NRB p-value
Concentric Time (s)
0.593
(0.186)
0.793
(0.106)
0.013"
Relative Displacement of
COM(%)
71.08
(6.131)
75.31
(3.470)
0.090
Maximum Velocity of COM
(m's -1)
0.898
(0.246)
0.968
(0.171)
0.489
Net Work by Four Segment
Model (J)
635.9
(176.1)
624.6
(174.0)
0.893
Peak Power of Four Segment
Model (W)
1947
(655.6)
1808
(515.7)
0.627
* = significant a t alpha = 0 . 0 5 level
Table 1:
Means, standard deviations, and p-values for entire concentric time results. REB values
represent the squat with rebound and NRB values represent the squat without rebound.
The ANOVA results are based on the comparison of the variable for the REB and NRB
conditions.
representing relevant joint and b a r l a n d m a r k s via a Peak Performance Motion
Analysis System. Data were digitized with both a n automatic digitizing program
and m a n u a l manipulation of a cursor. Points digitized were top of head, shoulder,
elbow, wrist, hip, knee, ankle, toe, and b a r center. In the event of a m a r k e r not
being located, the point was manually digitized for that field. Several points, such
as the top of the head, elbow, and shoulder were digitized manually because
m a r k e r placement was impractical or impossible. The location of the shoulder
was estimated based on the orientation of the lifter with respect to the bar.
Digitized data were adjusted to a fixed origin within the field to eliminate frame
shifting and vibration errors inherent to the c a m e r a and data collection procedure.
Variable Calculation
A four segment model was used to represent the rigid link system of the h u m a n
body (Plagenhoef, Evans, & Abdelnour, 1983). Segments in this model were: (1)
the head, neck, trunk, and arms; (2) the thighs; (3) the s h a n k s and feet; and (4)
the bar. This model was used for calculation of the radii of gyration and moment
of inertia for each segment as well as the center of m a s s (COM) of the subject. A
quintic spline program was incorporated for eliminating r a n d o m errors in the
digitized coordinates (Woltring, 1986). Effectiveness of the spline routine was
assessed through visual examination of the second derivative and the residual
pattern of the difference between the raw and smoothed data points (Zernicke,
Caldwell, & Roberts, 1976). Sufficient filtering of the data resulted in a residual
plot without pattern and a second derivative with a definite pattern. The desired
contour of the second derivative pattern was neither "too smooth" nor "too coarse".
Input parameters of the spline routine were adjusted manually or iteratively until
consistent results were found from digitized point to digitized point, trial to trial,
92
Measurement of Elastic-Like Behaviour in the Power Squat
and subject to subject. The number of knots used and the m e a n square error
were monitored for all trials and subjects to assess consistency.
Output from the spline routine was used to calculate position, velocity, and
acceleration data of all coordinates (including the COM). All subsequent variables
were calculated from these data. The eccentric phase was defined as beginning at
peak negative vertical velocity of the COM and ending at zero velocity. The
concentric phase was defined as beginning at zero vertical velocity of the COM and
ending at peak velocity. The relative displacement of the COM was calculated as
the amount of vertical displacement of the COM at the end of the concentric phase
relative to the total vertical displacement of the COM d u n n g the entire upward
motion. Work was calculated as the net change in mechanical energy of the four
segment model. Power was calculated as work divided by time. Peak values for
all variables were used for analyses. Elastic enhancement was calculated as the
ratio of the REB energy over the NRB energy.
The entire concentric time was defined as the time from the initiation of upward
movement of the COM until peak velocity of the COM. The initial concentric time
was defined as the time from the initiation of upward movement of the COM until
0.2 s later. The duration of 0.2 second was selected because it was less than the
briefest concentric time for all subjects and conditions. Further, this time is
consistent with the methods and finite times used in previous work (Chapman et
al., 1985; Wilson et al., 1991).
Statistical Methods
Statistical analyses were performed on each variable for both the entire concentric
and initial concentric time periods. Statistics were computed via SAS program on
a VAX mainframe computer. A repeated measures ANOVA was used to test main
effects for conditions (REB or NRB). All tests for significance were performed at
the alpha = 0.05 level.
Results
Means, standard deviations, and statistical results are provided for both the entire
concentric time and initial concentric time methods in Table 1 and Table 2. Time
can only be evaluated for the entire concentric time method because it is held
constant in the initial concentric time method of examining elastic-like behavior.
The time required for completion in the entire concentric time method was
significantly less in the REB than in the NRB. The 0.2 s initial concentric time
value represents 34% and 25% of the entire concentric time for the REB and NRB
respectively.
The range of relative displacements of the COM for the entire concentric time
was between 61% and 80% in the REB and between 71% and 83% in the NRB.
The relative displacements were not significantly different between conditions for
the entire concentric time method. The range of relative displacements of the COM
for the initial concentric time was between 9% and 34% in the REB and between
3% and 12% in the NRB. The relative displacements were significantly greater in
the REB condition than in the NRB condition for the initial concentric time
method.
For the entire concentric time the maximum velocities of the COM ranged from
0.627 m.s -] to 1.434 mos-1 in the REB and from 0.738 mos-1 to 1.269 m.s -1 in the
NRB. No differences were found between conditions for the entire concentric time
method. For the initial concentric time the velocities of the COM ranged from
93
Measurement of Elastic-Like Behaviour in the Power Squat
variable
REB
Mean
(+_S.D.)
NRB
Mean
(_+S.D.)
R e p e a t e d Measures
ANOVA o f REB vs.
NRB p-value
N/A
N/A
N/A
Relative Displacement of
COM(%)
21.18
(7.82)
6.45
(2.64)
0.0001 *
Maximum Velocity of COM
(m's -1)
0.453
(0.196)
0.145
(0.051)
0.0003 *
Net Work by Four Segment
Model (J)
140.6
(65.30)
38.70
(22.20)
0.0004 *
Peak Power of Four Segment
Model (W)
973.7
(508.8)
313.9
(144.5)
0.0018 *
Concentric Time (s)
* = significant at alpha = 0.05 level
Table 2:
Means, standard deviations, and p-values for initial concentric time results. REB values
represent the squat with rebound and NRB values represent the squat without rebound.
The ANOVA results are based on the comparison of the variable for the REB and NRB
conditions.
0.171 m.s -1 to 0.723 m.s -1 in the REB and from 0.059 m-s -1 to 0.219 m.s -1 in the
NRB. The velocities were significantly greater in the REB t h a n in the NRB for the
initial concentric time method.
Net work performed in the entire concentric time ranged from 400.4 J to 979.8
J in the REB a n d from 313.7 J to 940.7 J in the NRB. Net work performed during
the entire concentric time was not significantly different for either REB or NRB
condition. The net work performed in the initial concentric time ranged from 35.5
J to 252.6 J in the REB and from 14.1 J to 91.0 J in the NRB. Work performed
in the initial concentric time, however, was significantly greater in the REB
condition.
M a x i m u m power values for the entire concentric time ranged from 1106 W to
3320 W in the REB a n d from 923 W to 2313 W in the NRB. J u s t as in net work
performed, the entire concentric time h a d no differences in p e a k power produced
for either condition. The power values for the initial concentric time were about
50% a n d 17% of p e a k power entire concentric time values for the REB a n d NRB
respectively. Also similar to n e t work performed, the initial concentric time had
significantly greater power p r o d u c t i o n when a r e b o u n d was used.
Elastic e n h a n c e m e n t for the entire concentric time h a d a m e a n a n d s t a n d a r d
deviation of 3.2+ 14.8%. As evident by the s t a n d a r d deviation, some of the values
were negative. E n h a n c e m e n t values for the initial concentric time h a d a m e a n
and s t a n d a r d deviation of 327+213%. No negative values were present.
DiSCUSSiOn
Variables
As evident from the results, the entire concentric time a n d initial concentric time
m e t h o d s produced different outcomes for the respective variables. Time, which
94
Measurement of Elastic-Like Behaviour in the Power Squat
1.2"
.....
l.O.
i
REB Vertical Velocity
NRB Vertical Velocity I
0.8.
0.6g 0.4-
~
0.2-
~ 0,0~?.2.
.0.4 •
-0.6 "
-2.(I
Figure 1:
i
-1.5
-1.0
-0.5
0.0
0.5
1.0
Representative COM vertical velocity vs. time graph for the rebound and no-rebound
conditions, Time zero corresponds to the beginning of the concentric phase for both
lifts.
was measured only for the entire concentric time method, had results consistent
with those reported by others (Cavagna, Komarek, Citterio, & Margaria, 1971;
Harman, Rosenstein, Frykman, & Rosenstein, 1990}. Although the concentric
times of the vertical j u m p reported by H a r m a n et al. (1990} were about half as
long as concentric times for the power squat in the present study, they found that
the concentric time was about 10% greater without rebound. Cavagna et al.
(1971} found that the time of positive work was 55% greater with no prestretch of
the muscle. In the present study the NRB required 34% greater time than the
REB. It seems that for most tasks the use of a rebound will allow the performer
to complete the action more quickly than not using a rebound.
Time is an important variable because other variables are dependent on it or
influenced by it. Elastic-like behavior itself is dependent on time because of its
transient characteristics {Aruin, Prilutski, Raitsin, & Savel 'ev, 1978; Hill, 1961;
Wilson et al., 1991}. If too m u c h time elapses between the eccentric prestretch
and the concentric contraction of the muscle, the elastic energy stored is
dissipated as heat (Hill, 1961).
In the present movement the use of a rebound may help reduce concentric time
under moderate load by aiding the musculature in overcoming inertia at the
beginning of the lift. This is one reason for the use of a time factor for the
measurement of elastic benefit near the beginning of the concentric phase. The
use of a finite time interval may allow gains from a rebound to be more accurately
reflected both in this task and perhaps other tasks as well. Time was also
consequential for this task because the concentric phase typically ended well
before the upward thrust ended.
For the entire concentric time method, the relative displacement of the COM did
not differ by condition. Although the range of values for the different conditions
indicated that the NRB had a tendency to go slightly higher, it was not statistically
significant. Unlike the entire concentric time method, the initial concentric time
95
Measurement of Elastic-Like Behaviour in the Power Squat
method did differ by condition. The greater relative upward movement of the lifter
in the REB for the initial concentric time method m a y indicate how elastic-like
behavior is evident near the beginning of concentric movement, where the
probable recoil of elastic elements occurs. The greater relative displacement of the
COM in the REB m a y be due once again to the rebound helping the m u s c u l a t u r e
overcome inertia at the beginning of the concentric phase and provide additional
displacement in the same a m o u n t of time.
On the average, the lifters achieved the s a m e velocity in the REB as they did in
the NRB for the entire concentric time. The m a x i m u m velocity in the NRB,
however, occurred after a longer concentric interval. (See Figure 1 for a
representative graph of COM velocity.) Unlike these results, the findings for the
initial concentric time indicated that the early velocities were higher for the REB
condition. This was consistent with relative displacement results. The advantages
of the rebound seemed more a p p a r e n t when a finite time interval was used and
benefit was measured closer to when it likely occurred. Later in the ROM, the
advantages gained from elastic-like behavior seemed to be obscured by task
limitations. That is, in the squat the lifters reach m a x i m u m velocity well before
completing the upward movement. The slowing (perhaps prematurely) of the
lifters' motion n e a r the end of knee and hip extension m a y confound the
m e a s u r e m e n t of benefits gained early in the concentric phase.
If the t a s k had different requirements, s u c h as m a x i m u m velocity at the end of
the t h r u s t phase (as in a vertical jump), then the benefit from elastic-like behavior
would be evident at that time as well. Presumably, the performers of those types
of t a s k s are able to continue to add to the greater early velocities and therefore
achieve greater end results. Also, while jumping with m a x i m u m velocity is not
unusual, lifting for m a x i m u m velocity at a given load is unusual. Lifters usually
perform submaximal lifts with s u b m a x i m a l velocities. Lifting with maximal
velocities is not unprecedented, b u t is different and in need of some acclimation
by the performer for safety reasons.
Because of these t a s k differences, procedures used in an entire concentric time
method, which are adequate for measuring elastic-like behavior in the jump, may
not be appropriate for the power squat. In particular, the entire concentric time
method m a y not be sensitive to the elastic contributions created n e a r the
beginning of the thrust. Therefore, the initial concentric time method might better
reflect the benefits of elastic-like behavior.
As with the previous two variables, the net work performed during the entire
concentric time did not differ between the REB and NRB, b u t net work performed
during the initial concentric time was significantly greater in the REB condition.
Other research (Cavagna et al., 1971) h a s suggested that net work would be 10%
greater with prestretch. Presently, the net work in the entire concentric time
method was 2% greater with prestretch and net work in the initial concentric time
method was 350% greater with prestretch.
Net work performed was influenced by the m a s s of the system (which was the
same for each condition), time (which was not limited for the entire concentric
time, b u t was for the initial concentric time), relative displacement (which had
results similar to net work), and velocity (which also had results similar to net
work). The results for the work performed within the initial concentric time
method suggest that rebound benefits work as it does other variables. In the
beginning of the concentric p h a s e the elastic-like behavior allows greater net work
96
Measurement of Elastic-Like Behaviour in the Power Squat
as a result of greater velocities and displacements. These greater values for net
work are believed to be the result of elastic elements and contractile elements
working together to produce greater results than the contractile elements
independently.
Power values were consistent with values hypothesized and found in previous
research by Wilkie (1960) and Garhammer (1980). A study by Cavagna et al.
(1971) found that average power was 70% greater with prestretch, but that
instantaneous power was not different. The present results indicated that the
REB condition had greater peak power on the order of 7% for the entire concentric
time method and 310% for the initial concentric time method. Maximum power
produced in the entire concentric time did not differ by condition, b u t at the end
of the initial concentric time the power produced by the lifter was greater in the
REB than in the NRB.
It is possible that the lack of significance of entire concentric time power values
was because of the "maximum effort" required of the lifters. Using this effort may
suggest they generate m a x i m u m power. As a reflection of the interaction between
time, relative displacement, velocity, and work, power is an indication of the
relative intensity for the overall performance of the lifters. At the end of the initial
concentric time, however, the inability of the lifters to produce similar power
values in the NRB m a y be an indication of how rebound aids in producing
powerful movements. The initial increases in power output may be due to the
increased speed of the entire muscle shortening and consequently the speed of the
concentric movement (Thys et al., 1972). Had the movement context been
different, such as a vertical jump, then the additional power resulting from the
elastic-like behavior in the initial stages of concentric motion may have continued
to build to greater values.
The elastic enhancement results were unusual. Some negative values were
found for elastic enhancement in the entire concentric time method. These
results were theoretically infeasible. The negative values occurred as a result of
the net changes in the energies of the NRB being larger than the net changes in
the energies of the REB. Results for the entire concentric time method using
previously described procedures involving eccentric energy with the respective
concentric energies also would have revealed negative results for the same
subjects. The negative results were another indication of the confounded variable
measures for the entire concentric time method. Similar calculations for the initial
concentric time method resulted in no negative values. Values for the initial
concentric time had a large variance, but were not inconsistent with the variance
found in other studies (Hudson & Owen, 1985).
Measurement Duration
In the present study of the power squat different results were found for each of
the methods designed to measure elastic-like behavior. The traditional method of
using the entire concentric time was not successful or effective in reflecting the
elastic-like behavior of musculature in any m a n n e r except concentric time. The
initial concentric time method, however, was m u c h more successful and
consistent in reflecting the benefits of using a rebound within the initial moments
of the concentric phase.
In general, quick movements may be better suited for traditional measurements
of elastic-like behavior. This may be particularly true for those movements which
97
Measurement of Elastic-Like Behaviour in the Power Squat
require m a x i m u m speed near the end of the t h r u s t phase, as in a vertical jump.
In the present task the a m o u n t of displacement of the COM in the concentric
phase relative to the displacement of the COM for the entire thrust was free to vary
by condition and subject. This variability is impossible to control in this task
because of the risks involving the lack of integrity between the lifter and the
weighted bar. If the lifter were not concerned about the b a r leaving the stable
resting spot on the posterior, superior t r u n k (or worse yet returning to the same
position after projection) then the relative displacement would be closer to that of
a vertical j u m p type of task.
Another difference between squat and vertical j u m p requirements was the need
to stop all motion at the end of the t h r u s t p h a s e in the squat, b u t not in the jump.
This limitation regarding the constraints of the lift and the safety concerns of the
lifter requires slowing upward motion prior to the end of the upward thrust. The
differences in the lifters' performance for the entire concentric time method m a y
be due to this constraint. Each lifter m a y have varied slightly between conditions
with respect to where they began slowing, t h u s confounding benefits produced by
elastic-like behavior. In a j u m p the range through which work can be performed
and energy produced remains similar for conditions with and without rebound,
but in a squat slight variations in range m a y produce large variations in energy
produced due to the large potential energy values, thus confounding energy
m e a s u r e s for the entire concentric time method.
As stated previously, the use of the entire concentric time method in a power
squat type of task m a y confound the benefits resulting from rebound. The use of
a finite time interval for e n h a n c e m e n t m e a s u r e s is one method that avoids this
problem. For instance, a time interval m a y be superior to a displacement interval
because w h e n a finite time is used the potential energy and kinetic energy can
each vary according to the performance differences in the conditions (with or
without a rebound). Moreover, the initial concentric time method can be used to
compare any two tasks performed with and without a rebound.
When deciding on a time interval several factors should influence the chosen
value. The factors include the purpose of the movement, the concentric time
typically used, and the data collection frequency. The interval used in any
movement should be brief enough to allow all subjects to perform the t a s k without
u n d u e influence from any factor (e.g., fear or risk of injury) that might affect
subsequent variable measures. Depending on the movement, time limits should
be long enough to allow elastic-like behavior to be clearly shown, yet not so short
as to distort benefit measures. Previous research h a s related the a m o u n t of time
to power (Wilson et al., 1991). It can also be determined relative to the time of the
thrust p h a s e or it can be a constant value. In the present study a constant value
of 0.2 seconds seems to be effective in representing the elastic benefits of the
power squat.
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