Does isometric strength relate to dynamic strength?
Introduction
Greater maximal strength is linked to enhanced biomechanical factors which contribute to athlete
performance. (Suchomel, et al., 2016) Three key biomechanical factors associated with optimal sport
performance are peak force (PF), rate of force development (RFD) and one-repetition max (1RM)
strength. (Stone, et al., 2003) Isometric strength and dynamic strength are two aspects of strength
that can be used to assess the biomechanical factors that affect sport performance. Isometric testing
looks at peak force and rate of force development (Nuzzo, et al., 2008) whereas dynamic strength is
analysed using 1RM exercises. (Kawamori, et al., 2006) Previous research suggests that there is a link
between isometric strength and dynamic strength. (Rasch, 1957) A study by Mcguigan, et al., (2010),
found that the isometric maximum strength had a clear correlation with 1RM testing. Nevertheless,
in the same study RFD did not appear to correlate clearly with the other measures.
There is insufficient research available to explain the connection between isometric strength and
dynamic strength. Therefore, the aim of this study is to investigate if there is a relationship between
isometric strength and dynamic strength using measures of peak force, rate of force development and
one-repetition max strength. Isometric strength will be assessed using the isometric mid-thigh pull.
McGuigan & Winchester (2008), supported the use of isometric mid-thigh pulls stating that the
isometric mid-thigh pull delivers a swift and effective method of calculating isometric strength in
athletes. Dynamic strength was assessed using the countermovement jump, squat jump and 20 metre
sprints. This is because dynamic strength can be judged by jumping and 1RM tests. (Stone, et al., 2004)
Method
Participants
Fifteen male and female students from Middlesex University participated in the study. The
participants were aged 20.4 ± 1.5 years, height 176.4 ± 7.4 cm and mass 72.1 ± 9.5 kg. The gender,
age, height and mass were recorded in an Excel spreadsheet and each participant was numbered from
one to fifteen. Prior to the study, all the students completed a consent form to participate.
Procedure
The participants carried out a warm-up consisting of; a 5-minute cycle at 60 RPM, 10 forward lunges
(Left & Right), 10 lateral lunges (Left & Right), 6-inch worms and practice isometric mid-thigh pulls and
countermovement jumps. The testing battery consisted of two maximal isometric mid-thigh pulls, two
maximal countermovement, two squat jumps and two 20-metre sprints. The isometric mid-thigh pull,
countermovement and squat jumps were completed using force plates. Data for the 20 metre sprints
were collected using timing gates and a collection device. Each participant completed two maximal
sprints, after each sprint there was a two-minute rest period. All tests were carried out on the same
day at the same time.
Isometric strength was assessed using the isometric mid-thigh pull to analyse peak force and rate of
force development. Before commencing the mid-thigh pull, the bar was adjusted to the mid-thigh area
whilst the participants were in the set position. The participants were then given a three-second cue
and then instructed to pull upwards on the fixed bar secured within a power rack for five seconds with
maximum effort. Participants executed 2×5 second trials followed by a one-minute rest in between.
The two values were logged into the excel spreadsheet.
Dynamic strength was measured using the countermovement jump and squat jump to determine
1RM. Partakers were instructed to keep hands on hips throughout each jump. On the squat jump
partakers had to ensure there is no counter movement as this will emerge as a countermovement
jump in the results. Two trials of each jump were completed with a one-minute rest in between jumps.
The 20 metre sprints were also used to assess dynamic strength. The participants were required to
sprint through two sets of timing gates over a 20-metre distance. This exercise was repeated twice
with a two-minute break in between.
Data analysis
All data was analysed using bespoke templates in Excel. The data was exported from Excel into SPSS
where a Shapiro-Wilk test of Normality was carried out. Correlations between the data variables were
then evaluated using the Pearson’s correlation test where the significance of the data was judged
using the principle P ≤ 0.05. If the P value obtained is equal or less than 0.05, this suggests that the
data accepts the hypothesis. If the P value is greater than 0.05, this suggests the data rejects the
hypothesis.
Results
Table 1: Relationship between Dynamic and Isometric strength.
There was no significant correlation between the SJ and Peak force (r= 0.097, p= 0.721) and RFD 1 (r=
0.096, p= 0.724) and RFD 2 (r=0.081, p=0.766). There was no correlation between CMJ and Peak force
(r= 0.188, p= 0.486), RFD 1 (r= 0.213, p= 0.429) and RFD 2 (r= 0.17, p= 0.53). There was also a weak
correlation between Speed and Peak force (r=0.231, p=0.389), RFD 1 (r= 0.277, p= 0.299) and RFD 2
(r= 0.249, p= 0.352).
Discussion
The results of the present study indicated that there was no significant correlation between peak force
and jump height for the CMJ and SJ (Table 1). This is backed up by Nuzzo, et al., (2008) who proposes
that isometric measures do not correlate strongly with CMJ performance. On the other hand a study
by Kraska, et al. (2009), concludes that the ability to generate peak forces relates to jump height.
Meaning the stronger athletes are, the higher they jump. Discrepancies in the findings of the current
study and Kraska, et al. (2009) could be due to the number of participants. Kraska, et al. (2009) carried
out the study using sixty-three athletes, whereas the current study only used fifteen athletes. By
increasing the number of participants there would be more data to compare which could influence
the significance of the data.
The results in Table 1 shows that the CMJ had a closer correlation to Peak force and RFD than the SJ.
A possible reason for this could be because the CMJ involves a concentric and eccentric contraction
whilst the SJ only uses a concentric contraction. The CMJ utilises the stretch shortening cycle. This is
when the muscle stretches just before contracting. Komi (2003), articulates that the concentric and
eccentric muscle contraction in the CMJ generates greater force than the concentric contraction in
the SJ. Hence why the CMJ has a closer relationship to Peak force and RFD than the SJ. Nevertheless,
it is possible that the SJ can have the same or greater force output as the CMJ. A study by McLellan et
al. (2011) implies that the ability to generate force improves jump height. This could be interpreted as
if force is generated quicker on the SJ, the peak force and RFD values could be higher than the CMJ.
There are differences in the findings of the current study and the study by McLellan et al. (2011). First
of all the study by McLellan et al. (2011) carried out three repetitions of the CMJ and SJ, while the
R P R P R P
SJ 0.097 0.721 0.096 0.724 0.081 0.766
CMJ 0.188 0.486 0.213 0.429 0.17 0.53
SPEED 0.231 0.389 0.277 0.299 0.249 0.352
PEAK FORCE RFD 1 RFD 2
current study performed two repetitions. Had an extra repetition been executed, there would be more
data available to analyse and make comparisons.
Overall, Speed had the greatest relationship to Peak force and RFD compared to the SJ and CMJ. An
investigation by Mero (1988), found that there is an association between force production and running
velocity. Johnson & Buckley (2001), rationalise this statement by suggesting that during a sprint, the
knee upholds the centre of mass height which allows the horizontal and vertical force produced at the
hip to be delivered to the ankle. This could explain as to why the results from this study showed a
stronger correlation between Speed, Peak force and RFD.
Conclusion
In the conclusion, it is highly likely that isometric strength compares to dynamic strength. The
isometric testing in this study assessed Peak force and RFD, these measures explore force output. The
ability to produce force quickly which is RFD. RFD impacts sprint performance. (Marques & Izquierdo,
2014) This is because force production time especially in the 20m sprint is short, consequently the
rate at which force is developed is crucial for optimal performance. Force x time = momentum which
is the force that keeps us moving particularly during sprints. Peak force has an influence on dynamic
strength in terms of jump performance. As the study by Kraska, et al. (2009) reiterated, the greater
maximum force generated the higher the athlete can jump. Therefore isometric strength can have a
major influence on dynamic strength.
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