![Typical force-time (solid black line) record for a countermovement jump between the onset of movement and take-off, with the associated velocity-time (dotted grey line in graph A) and displacement-time (dotted grey line in the graph B) presented and the unweighting (negative acceleration and negative direction), braking (positive acceleration but negative direction) and propulsion (positive acceleration [until force is below bodyweight] and positive direction) phases highlighted. The dash-dot black line represents bodyweight.](https://www.researchgate.net/profile/John-Mcmahon-8/publication/322540215/figure/fig2/AS:583756263997440@1516189747607/Typical-force-time-solid-black-line-record-for-a-countermovement-jump-between-the-onset_Q320.jpg)
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UNDERSTANDING THE KEY PHASES OF THE COUNTERMOVEMENT JUMP
FORCE-TIME CURVE
Submission Type: Article
Funding Statement: No funding was received for this article.
Conflict of Interest Statement: There are no conflicts of interest concerning this article.
Authors: John J. McMahon, PhD, CSCS*D,
1
Timothy J. Suchomel, PhD, CSCS*D,
2
Jason P.
Lake, PhD
3
and Paul Comfort, PhD, CSCS*D
1
Affiliations:
1
McMahon and Comfort are with the Directorate of Sport, Exercise and Physiotherapy,
University of Salford, Salford, Greater Manchester, UK.
2
Suchomel is with the Department of Human Movement Sciences, Carroll University,
Waukesha, WI, USA.
3
Lake is with the Department of Sport and Exercise Sciences, University of Chichester,
Chichester, West Sussex, PO19 6PE, UK.
Positions:
John J. McMahon is a lecturer in sports biomechanics and strength and conditioning in the
Directorate of Sport, Exercise and Physiotherapy at the University of Salford.
Timothy J. Suchomel is an assistant professor in the Department of Human Movement Sciences
at Carroll University.
Jason P. Lake is a reader in sport and exercise biomechanics and program leader of the MSc
Strength and Conditioning in the Department of Sport and Exercise Sciences at the University
of Chichester.
Paul Comfort is a reader in strength and conditioning and program leader of the MSc Strength
and Conditioning in the Directorate of Sport, Exercise and Physiotherapy at the University of
Salford.
Corresponding Author: Address author correspondence to John J. McMahon at
j.j.mcmahon@salford.ac.uk or telephone +44(0)161 295 3892
Preferred Short Title: Countermovement Jump Force-Time Curve Phases
Abstract Word Count: 98
Text Word Count: 3710
Number of Tables: 2
Number of Figures: 6
Abstract
The countermovement jump (CMJ) test is commonly conducted to assess
neuromuscular function and is being increasingly performed using force platforms.
Comprehensive insight into athletes’ neuromuscular function can be gained through detailed
analyses of force-time curves throughout specific phases of the CMJ, beyond jump height
alone. Confusingly, however, many different terms and methods have been used to describe
the different phases of the CMJ. This article describes how six key phases of the CMJ
(weighing, unweighting, braking, propulsion, flight, and landing) can be derived from force-
time records to facilitate researchers’ and practitioners’ understanding and application to their
own practice.
Keywords
Force Platform; Impulse; Phase Identification; Vertical Jump
Introduction
Force platforms are among the most frequently used biomechanical apparatus in the
field of Sports Biomechanics and strength and conditioning (S&C) research, but S&C
practitioners have historically sought cheaper field-based alternatives to test their athletes’
physical status. Affordable and valid commercial force platforms have been recently developed
(17, 27), meaning that S&C practitioners are more likely to utilize them in the future. A
common test included in athlete testing batteries and the associated scientific literature that is
performed on a force platform is the countermovement jump (CMJ). The CMJ is appealing
because it is quick to perform, non-fatiguing and requires minimal familiarization, yet it can
yield valuable insight into an athlete’s neuromuscular and stretch-shortening cycle (SSC)
capabilities (3, 4, 9, 21, 22).
Recent studies have shown that a comprehensive insight into athletes’ neuromuscular
function can be gained through detailed analyses of force-time curves throughout specific
phases (7, 16, 28, 29, 32) or the entire CMJ (3, 4, 9, 21, 22), when compared to measuring the
output of the jump alone (i.e. jump height [JH]). Therefore, it is important that practitioners
who wish to use force platform-based assessments of CMJ can recognize the constituent parts
of the CMJ force-time curve, and understand their relative contribution to CMJ performance
and how they can be manipulated through coaching and training. To achieve this, the different
CMJ phases must be identified using robust methodologies; this has not always been the case
in the research literature and so warrants discussion with a view to its practical application (28,
29). This may be because the identification of some CMJ phases requires the derivation of
other variables from force-time data, or due to the many different terms used across studies to
describe the different CMJ phases (5, 12, 14, 21, 22, 24, 25, 32-34).
As the use of force platform-based CMJ assessment across research and applied settings
appears likely to increase because of the increased availability of affordable force platform
systems, and because, as mentioned above, many different terms have been used to describe
the different CMJ phases in the literature (some of which are less obvious than others),
clarification of the key CMJ phases, along with simpler descriptions of them, seems timely.
The purpose of this article, therefore, is to outline the key CMJ phases using simple, but
accurate, terminology to facilitate the collection, understanding, and practical application of
CMJ force-time data by S&C researchers and practitioners.
Initial Assumptions
Before describing the CMJ phases, it is important that the reader is aware of the initial
assumptions related to the examples discussed in this article, given that they can significantly
influence the resultant force-time records. Firstly, the data presented in the figures contained
within this article are taken from a single athlete’s CMJ trial (apart from data shown in Figure
4 which includes two CMJ trials [of the three performed in total] from the same athlete). The
CMJ trials included in these Figures were collected as part of a recently published study from
the lead author’s laboratory (19), thus institutional ethics approval and written informed
consent was provided before testing. A summary of the data collection and analyses procedures
used to acquire the CMJ trials presented in this article is presented in Table 1. Please note that
the vertical and horizontal axes have been re-scaled across Figures 1, 3, 5 and 6 to align the
force-, velocity- and displacement-time records for ease of phase identification.
**INSERT TABLE 1 ABOUT HERE**
It is beyond the scope of the present article to discuss the limitations of force platform-
only analyses. If the reader is interested in further understanding the benefits and limitations of
force platform-only analyses, then we refer them to an excellent article by Linthorne (15).
Additionally, if the reader requires further information about different force platform
technologies (e.g. those which include strain gauges versus piezoelectric sensors) then we refer
them to an excellent applied article by Beckham et al. (2). An important point to note is that
researchers and practitioners should attempt to establish sources of error associated with
whichever force platform they use, in line with previous investigations (33, 34). It might be
that some force platforms demonstrate a poor ability to register rapid changes in force (1), for
example, and so the data may need to be treated, in the form of digital filtering, before being
analyzed. If of interest to the reader, a detailed discussion of various smoothing techniques, as
they apply to dynamic force-time records, can be read in the previously mentioned applied
article (2).
Weighing Phase
The first CMJ phase is the weighing phase (sometimes referred to as the silent period
(21, 22) or stance phase (34)), whereby the athlete is required to stand as still as possible (Figure
1), usually for at least 1 second (21, 22, 25, 26). The purpose of the weighing phase is self-
explanatory, it is to weigh the athlete, but its importance is possibly less obvious, and it is
therefore likely to be a phase which is overlooked by researchers and practitioners. Accurate
calculation of bodyweight (BW) is essential for two reasons: firstly, it is used to identify a
threshold to determine the onset of movement (to be discussed in the following section) and
secondly, it (or, rather, the body mass derived from it) is included in forward dynamics
procedures (Figure 2). BW is usually calculated as the average (mean) force reading over the
weighing phase (21, 22, 26), although some studies have subtracted the peak residual force
during the flight phase (when the force platform is unloaded) from the average force during the
weighing phase to account for signal noise (14, 34). It is likely that the latter approach is more
accurate as the noise in the force signal will vary from trial to trial, but while flight phase noise
will provide information about the signal noise per se, it cannot inform the
researcher/practitioner about ‘human noise’ during the weighing phase. The most important
consideration here is that a consistent approach to BW determination is applied to enable fairer
data comparisons between trials, sessions and athletes.
**INSERT FIGURE 1 ABOUT HERE**
**INSERT FIGURE 2 ABOUT HERE**
The reason for the suggested weighing duration of 1 second is largely a consequence
of a study conducted by Street et al. (33) which showed that weighing durations of ≥ 1 second
leads to a ≤ 1% overestimation of JH (calculated based on the impulse-momentum relationship)
when compared to the maximum weighing duration of 2 seconds (34). A ≤ 1% overestimation
of JH is considered acceptable (26, 33), thus we recommend that the weighing phase should
last for ≥ 1 second. It is essential, however, that the athlete remains stood upright and as still
as possible during the weighing phase as it is vital that center of mass (COM) velocity and
displacement equal zero at the onset of movement for the numerical integration method to be
accurate (34). Street et al. (33) showed that, when subjects were stood still, integrating the
force-time data using the trapezoid rule (i.e. commencing forward dynamics procedures)
anywhere from 0-1.5 seconds (the time range tested) before the onset of movement had no
meaningful effect on JH. It would seem fine, therefore, for force-time data integration to
commence from the start of the weighing phase. It is important to note, however, that a 0.5%
error in body mass (BW · gravitational acceleration
-1
) during the weighing phase can induce
errors in JH, although this error is diminished when integrating force-time data over a sufficient
duration and frequency (34). From a practical perspective, to ensure that at least a 1 second
weighing phase is achieved, starting data collection on the word “2” during a “3, 2, 1, jump”
command works well based on the authors’ research and experience.
Unweighting Phase
The second CMJ phase is the unweighting phase, whereby the athlete commences a
countermovement by first relaxing the agonist muscles (18), resulting in combined flexion of
the hips and knees, including some dorsiflexion. The unweighting phase begins at the onset of
movement which is usually identified as the instant at which BW is reduced below a set
threshold value of force. The threshold value of force used to identify the onset of movement
has varied across studies (6), but the most recent “criterion” method was suggested by Owen
et al. (26). This method identifies the onset of movement as the instant when vertical force is
reduced by a threshold equal to 5 times the standard deviation of BW (calculated in the
weighing phase), hence the importance of standing still during the weighing phase (i.e. to
minimize the standard deviation of BW and thus increase the sensitivity of the onset of
movement threshold). Owen et al. (26) recommended going back 30 milliseconds from the
onset of movement because they identified that movement would have already started by this
instant and thus COM velocity would not equal zero. A 5-10 millisecond error in identifying
the onset of movement has little effect (≤ 0.1%) on the derived COM velocity and displacement
calculations, however, due to a lesser rate of change in force at this stage when compared to
take-off and landing (see later sections) (14). Although misidentifying the onset movement
may have little effect on forward dynamics procedures, it would likely have a more profound
effect on time-related variables (e.g. time to take-off, time to peak force etc.) and thus
associated metrics such as rate of force development (6) and reactive strength index modified.
It would be prudent, therefore, to at least adopt a consistent threshold across trials, sessions and
athletes for comparative purposes.
**INSERT FIGURE 3 ABOUT HERE**
The unweighting phase continues from the onset of movement through to the instant at
which force returns to BW (12, 14, 18, 21, 22, 24, 32), on the ascending aspect of the force-
time curve (Figure 3a-b). Therefore, the unweighting phase, as the name implies, comprises
the entire area of the force-time curve (before take-off) that is below BW (Figure 3a-b). The
instant at which force returns to BW coincides with the instant at which peak negative COM
velocity is achieved (Figure 3a). Plotting the velocity-time curve alongside the force-time curve
is visually useful, therefore, in revealing the exact point at which the unweighting phase ends
(Figure 3a). For a given athlete (whose body mass is constant during a CMJ trial), a greater
unweighting net impulse will lead to a greater peak negative COM velocity (as impulse [force
× time] = Δ momentum [mass × Δvelocity]) which will then require an equally large net
impulse to be applied in the subsequent braking phase to reduce momentum to zero (14, 24).
The unweighting phase is important, therefore, as it influences the rate and magnitude of force
production required in the braking phase which, in turn, will likely influence SSC function
(14).
Braking Phase
As mentioned above, the third CMJ phase is the braking phase, whereby the athlete
decelerates (i.e. “brakes”) their COM. Hence this phase commences from the instant of peak
negative COM velocity (see above) through to when COM velocity increases to zero. This
coincides with the bottom of the countermovement (i.e. the peak negative COM
displacement/deepest part of the squat) (12, 14, 21, 22), as shown in Figure 3b. The braking
phase has been called the “stretching phase” (14, 24, 32) and “eccentric phase” (20-22) in some
previous studies, whereby it is assumed that the leg extensor muscle-tendon units are actively
stretching to decelerate body mass but one cannot assume that all active muscles are stretching
(i.e. medial gastrocnemius may actually shorten (15) during this phase). Also, one should
remember that the analysis of vertical force-time data, and additional variables that are derived
via the integration of said data, provides insight into linear COM kinetics and kinematics only
and does not inform us of joint or muscle-tendon unit behavior.
As mentioned earlier, the net impulse applied in the braking phase is equivalent to the
net impulse in the unweighting phase (14, 24) because the net impulse required to stop a given
mass travelling at a given velocity (in this case, the peak negative COM velocity) is
proportional to the net impulse that was applied to reach said velocity from the start (i.e. when
the athlete is standing still during the weighing phase). As a given net impulse can be achieved
by applying a large force over a short time or a small force over a long time and variations in
between, the shape of the net impulse produced in the braking phases will depend on the
strategy employed by the athlete (20-22, 24, 32). If the athlete attempts to minimize braking
phase time, as may be the focus if they are instructed to perform the CMJ as fast as possible,
they will have to produce a large braking force to match the unweighting phase net impulse
and reduce momentum to zero (Figure 4). Such a strategy will, therefore, be characterized by
a taller (large force) and thinner (short time) active net impulse and a higher rate of force
development in the braking phase (20-22, 24, 32).
**INSERT FIGURE 4 ABOUT HERE**
Propulsion Phase
The fourth CMJ phase is the propulsion phase (12, 24, 32), which has also been referred
to as the concentric (17-19) or push-off (18, 30) phase in some studies , whereby athletes
forcefully extend their hips, knees and ankles to propel their COM vertically. This phase
technically begins when a positive COM velocity is achieved (12, 14, 18) but a velocity
threshold of 0.01 m·s
-1
has been recently used with success to identify the onset of the
propulsion phase for large (full squad) data sets (20-22). The force platform sampling
frequency and the rate at which the athlete transitions from the braking to propulsion phase
will likely determine whether an amortization phase can be identified (time delay between zero
and 0.01 m·s
-1
COM velocity) but as no study has explored this to date, the amortization phase
will not be considered in this article. By this definition, the force at the onset of the propulsion
phase is determined by the force at the end of the braking phase (presumably minus any force
“lost” in the amortization phase) and the peak force attained for each of these phases occur
within a very short time of one another (usually towards the very end of the braking phase and
the very start of the propulsion phase). The shape of the propulsion force-time curve (certainly
during the early part of this phase) will, therefore, likely be influenced by the braking peak
force, with a large braking peak force requiring a large propulsion force to be applied quickly
to minimize time spent transitioning between phases and to reaccelerate body mass sooner.
The propulsion phase continues through to the instant of take-off (see next section).
Plotting the displacement-time curve alongside the force-time curve in this phase is visually
useful as it shows how vertical COM displacement becomes positive when it exceeds the zero
COM displacement that was set when the athlete was stood upright and still during the
weighing phase (Figure 3b). It can be assumed, therefore, that the peak positive COM
displacement gained in the propulsion phase reflects the COM displacement achieved through
plantarflexing the ankles (Figure 3b), as the athlete should adopt a neutral ankle angle (90°)
during the weighing phase when standing upright. This is sometimes referred to as contact
height (difference between height of COM at take-off and standing) (34) and informs one of
how much extra COM displacement an athlete generates via a forceful plantarflexion, which
may be a limiting factor for some (14). It is also interesting to note that peak COM velocity is
attained before rather than at take-off which coincides with the instant at which BW is reached
again on the descending aspect of the force-time curve and when zero COM displacement is
achieved (Figure 3a). At this point the COM begins to decelerate (12, 24, 32), probably due to
the shank and foot segments adding to the effective mass being accelerated at this point,
although positive COM displacement continues through to the next phase. Some researchers
have split the propulsion phase into three sub-phases (two acceleration sub-phases, between
onset of propulsion and when peak COM velocity is attained, and the previously mentioned
deceleration sub-phase) which could also be considered (24, 32).
Flight Phase
The fifth CMJ phase is the flight phase, whereby the athlete leaves the force platform
with the intention of attaining maximal positive COM displacement (i.e. maximal JH). As
eluded to above, the flight phase commences at the instant of take-off (when force falls below
a set threshold) and ends at the instant of touchdown (when the athlete contacts the force
platform again and force rises above a set threshold). As with the determination of the onset of
movement at the beginning of the unweighting phase, many force thresholds have been used
to identify take-off and touchdown in the literature. A threshold of force equal to 5 times the
standard deviation of flight force (when the force platform is unloaded), taken over a 300-
millisecond portion of the flight phase, has been successfully used to identify take-off and
touchdown in recent work (20-22). Incorrectly identifying the instant of take-off by as little as
2-3 milliseconds can lead to approximately a 2% variation in velocity and displacement (14),
with a 3 millisecond misplacement leading to a 0.9 cm absolute error in JH estimates using the
take-off velocity method (recommended ‘gold standard’: JH = v2 ÷ 2g, where v = velocity and
g = gravitational acceleration) (34). In relative error terms, a force threshold of 6 N and 10 N
above true zero (when the 0.7 – 2 N signal noise was accounted for) led to a 1% and 1.5%
overestimation in JH, respectively (33). These errors, although considered small, can be
reduced further by collecting force-time data at a sufficiently high sampling and integration
frequency (34) (see above and Table 1). Therefore, one should consider these potential sources
of error wisely when collecting and analyzing their force-time data and, again, apply a
consistent threshold across trials, sessions and athletes.
**INSERT FIGURE 5 ABOUT HERE**
At the instant when maximal positive COM displacement is achieved (Figure 5a),
which coincides with a momentary COM velocity of zero (Figure 5b), the athlete descends
back towards the force platform and the instant at which the selected force threshold is
exceeded denotes the instant of touchdown and, thus, the end of the flight phase. It is worth
noting in this section that using the duration of the flight phase (i.e. flight time) to estimate JH
(18) is based on the assumption that the apex of the jump (peak positive COM displacement)
occurs at half of the duration of the flight phase but this only holds true if COM height is the
same at the instant of take-off and touchdown (14). Consequently, any alterations in joint
geometry, as may be achieved by flexion of ankles, knees or hips before touchdown, will affect
this calculation. It is suggested, therefore, that practitioners and researchers should use the take-
off velocity method (impulse-momentum theorem, see above) of estimating JH (25), assuming
that they adhere to the aforementioned data collection and analyses criteria, where possible. If
the flight time method must be used, then it is important to instruct athletes to avoid flexing
ankles, knees and hips during landing. Finally, COM displacement (work-energy theorem) can
be used to calculate jump height too (18) if correct data collection and analyses procedures are
adopted, as numerical double integration is very sensitive to accurate body mass determination
(34).
Landing Phase
The sixth and final CMJ phase is the landing phase, whereby the athlete applies a net
impulse that will match the propulsion impulse to decelerate the COM from the velocity at
which it contacts the force platform at through to zero. As mentioned for the braking phase, the
net impulse required to stop a given mass travelling at a given velocity depends upon the
magnitude of the velocity. Landing velocity will mainly depend on JH, with greater JH leading
to greater landing velocity. Therefore, for an athlete of a given body mass, a greater landing
velocity will require the application of a larger net impulse. Unlike the braking phase, however,
there is little need for athletes to decelerate quickly during the landing phase when the CMJ is
being tested as single trials. Consequently, athletes are often instructed to “absorb” the landing
by flexing the hips, knees, and ankles, thereby applying a net impulse characterized by a smaller
force being applied over a longer duration (similar to that shown for the braking phase in Figure
4). Athletes who are not given, or do not adhere to, this instruction will produce larger peak
landing forces. The landing phase is considered to have ended when COM velocity reaches
zero again (Figure 6a) which coincides with the peak negative COM displacement achieved
during this phase (Figure 6b). The landing phase can also be split into two sub-phases (impact
[between touchdown and peak force] and stabilizing [between peak force and peak negative
COM displacement]) for additional information about landing strategy/ability.
**INSERT FIGURE 6 ABOUT HERE**
Practical Applications
Six key CMJ phases can be identified from force-time curves (weighing, unweighting,
braking, propulsion, flight, and landing – see Table 2) but accurate determination of these
phases depends on sufficient sampling and numerical integration frequency, a precise
determination of BW and the force and velocity thresholds used to determine the start and end
of each phase. It is essential that these factors are considered by researchers and practitioners
to ensure that these CMJ phases can be correctly identified and thus yield valid information
about athletes’ neuromuscular and SSC capabilities. Researchers and practitioners can use the
information presented here, therefore, to guide their own analyses and interpretation of
athletes’ CMJ force-time curves. Comprehensive discussion of the application of these data are
beyond the scope of this article. However, understanding the contribution of each phase to CMJ
performance enables practitioners to hone in on specific areas that may require development,
thus providing far greater insight into the athlete’s capacity to accelerate their body mass.
Specifically, practitioners should focus on gaining an understanding of what the shape of the
force-time curve means (8), and how the forces applied during and the length of the different
phases can be altered, both through coaching and specific training. Finally, it is suggested that
these simple, but relatively obvious, names for the different CMJ phases should be used by
S&C researchers and practitioners to promote consistency and clarity across the profession.
**INSERT TABLE 2 ABOUT HERE**
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Internacional de Medicina y Ciencias de la Actividad Física y el Deporte, Publish
Ahead of Print.
32. Sole CJ, Mizuguchi S, Sato K, Moir GL, and Stone MH. Phase characteristics of the
countermovement jump force-time curve: A comparison of athletes by jumping ability.
J Strength Cond Res, Publish Ahead of Print.
33. Street G, McMillan S, Board W, Rasmussen M, and Heneghan JM. Sources of error in
determining countermovement jump height with the impulse method. J Appl Biomech
17: 43-54, 2001.
34. Vanrenterghem J, De Clercq D, and Cleven PV. Necessary precautions in measuring
correct vertical jumping height by means of force plate measurements. Ergon 44: 814-
818, 2001.
Figures
Figure 1: Typical force-time record for a countermovement jump with the weighing phase
highlighted.
Figure 2: A brief description of how acceleration, velocity and displacement are derived from
the CMJ net force-time record, a process commonly referred to as forward dynamics. Please
note that the net force acting on the athlete’s center of mass is calculated by subtracting the
athlete’s bodyweight from the original vertical ground reaction force record.
Figure 3: Typical force-time (solid black line) record for a countermovement jump between
the onset of movement and take-off, with the associated velocity-time (dotted grey line in graph
A) and displacement-time (dotted grey line in the graph B) presented and the unweighting
(negative acceleration and negative direction), braking (positive acceleration but negative
direction) and propulsion (positive acceleration [until force is below bodyweight] and positive
direction) phases highlighted. The dash-dot black line represents bodyweight.
Figure 4: Example force-time record for a countermovement jump, between the onset of
movement and take-off, performed by the same athlete (body mass = 71.8 kg) who achieved
an almost identical unweighting and braking phase net impulse 95-96 N·s but whose braking
phase net impulse was characterized by a larger force and shorter time in trial 1 (black line) vs.
trial 2 (grey line). PF = peak force and PT = phase time.
Figure 5: Typical force-time (solid black line) record for a countermovement jump between
just before and after the instants of take-off and touchdown, respectively, with the associated
displacement-time (dotted grey line in the graph A) and velocity-time (dotted grey line in graph
B) presented and the flight phase highlighted. The dash-dot black line represents bodyweight
and the vertical grey line represents mid-point of the flight phase where peak center of mass
displacement and zero center of mass velocity is achieved.
Figure 6: Typical force-time (solid black line) record for a countermovement jump between
just before and after the instants of touchdown and end of the landing phase, respectively. The
associated velocity-time (dotted grey line in graph A) and displacement-time (dotted grey line
in the graph B) are presented and landing phase highlighted.
Tables
Table 1: A summary of the data collection and analyses procedures used to acquire the
athlete’s* countermovement jump trials presented in this article.
Task Instructions Rationale
Stand upright
1
with arms akimbo
2
and
remain as still as possible
3
until given the
command “jump”
1
Center of mass displacement
throughout the jump is calculated in relation
to standing center of mass height
2
Arm swing can induce small alterations to
velocity and height of the center of mass at
take-off (10) and negatively (albeit slightly)
impact data reliability (11)
3
A still period of at least 1 second before
commencing the jump is required to
ascertain bodyweight and the onset of
movement threshold (26)
On the command “jump”, rapidly
1
squat to
your preferred depth
2
by flexing your hip,
knees and ankles
1
A rapid squat is encouraged to stimulate
the stretch-shortening cycle
2
It is difficult to standardize
countermovement depth (31) and altering
this will affect ‘natural’ force-time
characteristics (13, 23)
At the bottom of the squat, immediately
1
jump as fast and as high
2
as possible by
rapidly extending your hip, knees and
ankles
1
Minimal pause at the bottom of the squat
(termed the amortization phase) is
encouraged to help prevent a reduction in
stretch-shortening cycle utilization
2
Attaining a greater propulsion velocity by
applying a greater net impulse will lead to a
greater jump height
Upon landing, absorb
1
the forces by flexing
your hip, knees and ankles, again to achieve
your preferred squat depth
1
Stiffer landings lead to larger peak forces
which may place the athlete at undue risk of
sustaining a musculoskeletal injury
Data Collection
Rationale
The force platform used was a portable
piezoelectric
Kistler
1
type 9286AA
(Kistler
*The athlete tested was a male collegiate soccer player aged 22 years, with a body mass of 67.5 kg and a standing
height of 1.8 m; **Data analyses as it relates to countermovement jump (CMJ) phase identification is not included
in this table as it forms the main discussion points of the present article.
Instruments Inc., Amherst, NY, USA)
which was placed on flat ground and
zeroed
2
before each trial
1
Kistler produce research-grade force
platforms capable of detecting rapid
changes in force production
2
It is essential to zero the force platform
before each CMJ trial to reduce signal noise
Force-time data for each trial were sampled
at 1000 Hz
1
for 5 seconds
A minimum sample frequency of 1000 Hz
has been recommended for CMJ force-time
assessments (26, 33)
Data Analyses**
Rationale
Raw
1
vertical force-time data were exported
for further analyses in a customized
Microsoft Excel spreadsheet
2
(version 2016,
Microsoft Corp., Redmond, WA, USA)
1
Filtered force-time data can lead to
underestimations of CMJ height (33)
2
Commercial software packages can
automate most calculations, but this can also
be done using Microsoft Excel
Numerical integration (see Figure 2 for
procedural description) using the trapezoid
rule
1
began at the onset of data collection
2
at
a frequency of 1000 Hz
3
1
The trapezoid method of integration is
most commonly applied to CMJ force-time
data and although it can lead to a ≤ 0.3%
underestimation of CMJ height, higher
order integration methods (e.g. Simpson’s
rule) do not vastly improve accuracy (14,
18)
2
Street et al. (33) showed that integrating
force-time data anywhere from 0-1.5
seconds before the onset of movement had
no meaningful effect on CMJ height
3
Integration frequencies of at least 200 Hz
have been suggested (34), with 1000 Hz
(the highest frequency tested) leading to the
smallest errors in center of mass
displacement
Table 2: A brief description of the key phases of the countermovement jump force-time curve.
Phase Description
Weighing Begins at the onset of data collection, when
the subject is stood upright and still, and
lasts for at least one
second
Unweighting Begins at the onset of movement (when
force falls below a set force threshold) and
ends when bodyweight is reached again,
which coincides with peak negative center
of mass velocity
Braking Begins at the end of the unweighting phase
and ends when center of mass velocity
equals zero (the momentary pause at the
bottom of the countermovement)
Propulsion Begins when center of mass velocity
becomes positive and ends at take-off (when
the subject leaves the force platform)
Flight Begins at the instant of take-off (when force
falls below a set force threshold) and ends at
the instant of touchdown (when force rises
above a set force threshold)
Landing Begins at the instant of touchdown and ends
when center of mass velocity equals z
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