Key words exercise - post activation potentiation - fatigue - power
Introduction
Short track speed skating is characterized by a succession of explosive efforts where
performance is strongly correlated with the strength characteristics of the leg extensors
of the athletes [11 ]. In this sport, the ability of an athlete to produce high power and accelerate at
the start of the race can significantly affect the rank position entering the first
turn, which is a discriminating factor of the race outcome on short distances [18 ]. The development of training or conditioning methods to enhance explosive muscular
power of short track speed skaters, especially in the early phase of the race, could
thus positively impact race outcomes.
Earlier studies have shown that the performance of a muscle is affected, at least
in part, by its contractile history (“conditioning contractions”, CC) that can induce
post-activation potentiation (PAP) [35 ]
[38 ]. PAP is known to be a phenomenon by which the performance of a muscle or muscle
group, during explosive movements patterns requiring maximal strength and power, is
enhanced by its prior contractile activity [38 ]. Fatigue and PAP resulting from CC can coexist [31 ], but optimal performance following CC occurs when positive effects of PAP offset
the negative effects induced by fatigue [22 ]
[35 ]
[38 ].
Previous results examining the application of PAP to enhance performance in acute
voluntary contractions report conflicting findings with regard to its effects on performance
[6 ]
[9 ]
[38 ]
[40 ]
[41 ]. Many factors have been shown to influence the effects of PAP, namely the time interval
between the CC and exercise performance, the type of muscle fibres mainly recruited,
the mode and intensity of the CC, the characteristics of the subsequent exercise and
the training status of the subjects considered [8 ]
[22 ]
[34 ]. For example, elite athletes show a greater improvement in explosive performance
than recreationally trained subjects, probably due to a higher proportion of fast-twitch
fibres [6 ]
[9 ]
[16 ]. High-intensity CC, mostly recruiting fast-twitch fibers, appear to be more effective
in activating the mechanisms of PAP [13 ]
[35 ].
Although the time interval between CC and the following exercise appears to be critical
to maximize performance [8 ]
[34 ]
[38 ], little is known about the duration of the potential beneficial effects of PAP during
long-lasting, fatiguing and repeated explosive exercises. While previous studies have
reported the effects of PAP on a subsequent acute exercise consisting of a single
jump, a set of consecutive jumps or a short cycle sprint [7 ]
[10 ]
[12 ]
[13 ]
[40 ], no study focused on the effects of PAP on multiple sets of consecutive jumps. Indeed,
investigating explosive exercises over multiples sets would be helpful to better understand
the lasting effects of PAP and its practical outcomes in sports like short track speed
skating where the performance is characterised by the repetition of explosive efforts.
The present study aimed to assess the effect of a lower body CC on subsequent repetitive
sets of fatiguing maximum repeated jumps in elite short-track speed-skating athletes.
The objective was twofold: first determine the effect of CC on peak power output (PO)
produced during vertical repeated jumps and second, how this effect is sustained over
sets of repeated vertical jumps. It was hypothesized that prior lower-body CC would
result in a higher mechanical power, especially during the first sets of repetitive
maximum jumps, and that this potentiating effect would disappear in the subsequent
sets.
Materials and Methods
Subjects
Five elite short-track speed-skating athletes volunteered for participation in the
study (3 men and 2 women, age: 20.2±3.1 years, height: 1.71±0.10 m, mass: 67.6±11.6 kg;
mean±SD). All participants were training at the Montreal National Training Center
and were competing in national and international competitions. Before participation,
written informed consent was obtained from each adult athlete and from the parents
of each minor athlete. The protocol was in accordance with procedures approved by
the University of Montreal ethics committee in health research (certificate number:
13-061-CERES-D) and meets the ethical standards of Sports Medicine International Open
[17 ].
Testing protocol
The testing protocol consisted of 9 sets of 6 maximal consecutive jumps with sets
interspaced by a 5 s unweighted isometric squat with the trunk and knees flexed at
45° and 90°, respectively. Over a 1000 m race, skaters typically perform 9 laps on
a 111.12 m oval track with ~12 strides (~6 per turn) and ~2 gliding phases per lap.
Each set of 6 jumps were initiated from an isometric squat position, with the trunk
and knees flexed at 45° and 90° respectively. The 6 jumps were performed consecutively
with no time interval between them (which involves a stretch shortening cycle for
each jump preceded by another one.)
Two weeks before the test sessions, subjects performed a familiarization session with
the testing protocol, 5 min after having completed the same directed 5-min dynamic
warm-up, including stationary jogging, high-knees, and half-squat exercises. Each
subject randomly performed the testing protocol (see [Fig. 1 ]) on 2 separate occasions (one week apart): with (experimental condition: COND) and
without (control condition: CTRL) preliminary voluntary CC 5 min before the execution
of the 9 sets. The participants were instructed to keep their hands on their hips
and to jump as high as possible throughout the testing protocol. Verbal encouragement
and verbal cues to correct knee or trunk positions were also provided as needed.
Fig. 1 Schematic representation of the experimental protocol. The CC were performed only
in COND, rest of equivalent duration was given in CTRL. Each set comprised 6 maximal
unweighted repeated jumps immediately followed by a 5 s isometric squat (black arrow).
Conditioning contractions
The CC consisted of two 3 s maximal unilateral isometric squats against a fixed barbell,
for each leg and alternately, resulting in 6 s of overall CC for each leg. Each leg
was thus alternately and maximally solicited, twice with about 5 s rest between repetitions
in order to switch leg.
Measurements
For each jump, we determined the average power output (PO) using the average velocity
during the push-off phase with a linear position transducer [14 ] (Tendo Sports Machine, Trencin, Slovakia) extending a string to a belt fastened
around the hips of the participants. We positioned the linear position transducer
on the ground between the subject’s feet and measured the linear displacement and
time during the execution of the jumps. The mean PO was computed and normalized to
body mass for each of the 6 repeated jumps.
Statistical analyses
Data are presented as mean±SD unless otherwise specified. A linear mixed model analysis
performed using R [30 ] (R Core Team (2014), R: A language and environment for statistical computing. R
Foundation for Statistical Computing, Vienna, Austria) and the lme4 [2 ] package was applied with the factors Condition (COND and CTRL) and Set (1–9) to
determine if there were significant differences between different time points and
treatment modalities. Residual analysis indicated that the model satisfied the normality
assumption required for the linear mixed model. Tukey’s post-hoc analysis was used
to identify specific sets where PO was different. The level of significance was set
at α=0.05. Effect size was estimated by computing partial eta squared (ηp
2 ), with 0.01, 0.13 and 0.26 representing a small, medium and large effect, respectively
[29 ].
Results
The statistical analysis revealed significant sets (p <0.001; ηp
2 =0.93), and conditions effects (p <0.01; ηp
2 =0.73), whereas the interaction effect was not significant (p =0.45). Mean and individual changes in PO for COND and CTRL conditions are shown in
[Fig. 2 ], [3 ], respectively. PO significantly decreased with sets, by 19.4±4.7% (from 15.0±1.9
to 12.1±1.5 W/kg: p <0.001) and 15.2±7.6% (from 14.3±2.0 to 12.1±1.3 W/kg: p <0.001), between the first and last set in COND and CTRL, respectively. Overall, PO
was greater in COND than in CTRL (13.7±0.70 and 13.2±0.69 W/kg respectively, p <0.01), which corresponds to an increase of 3.77±3.82% in COND compared to CTRL during
the whole sets. Qualitatively, [Fig. 3 ] shows that for all the participants, the PO was greater in the COND than the CTRL
condition during the 3 first blocks. During the subsequent blocks, the PO was greater
in the COND than in the CTRL condition in 73% of the cases.
Fig. 2 Mean and SE of average power output in CTRL (open circles) and in COND (black circles)
over the 9 consecutive sets of repeated jumps.
Fig. 3 Individual power output in CTRL (open circles) and in COND (black circles) over the
9 consecutive sets of repeated jumps (subjects a , b , c , d and e ).
Discussion
The present study investigated the effects of lower-body maximal unilateral isometric
CC (2 × 3 s, total of 6 s for each leg), on the average PO in 9 sets of 6 consecutive
jumps interspersed with isometric squats (5 s) in elite short-track speed-skating
athletes. Our main finding was that maximal unilateral isometric CC lead to a significant
increase of PO.
Previous studies required participants to perform several sets of exercises after
a CC, but participants were allowed to rest between sets. This was done to investigate
the optimum timing of CC to maximize potentiation [37 ], which was shown to have a large interindividual variability [4 ]. For example, Kilduff et al. [24 ] showed that the optimal recovery time to maximize the effect of PAP on countermovement
jump performance was 8 min in professional rugby players. More recently, Bogdanis
et al. [4 ] had national-level track and field athletes perform 10 countermovement jumps following
either isometric, eccentric or concentric CC with varying recovery durations (from
15 s up to 21 min). These authors reported a very high intersubject variability in
the optimal recovery time, which partly depended on the type of contraction. However,
after the isometric CC, all subjects reached their peak performance between 2 and
8 min, 80% between 4 and 6 min, whereas 50% of them performed best after 4 min of
recovery time. Thus, in the present study, we chose 5 min as a potential optimal recovery
time between the CC and the 1st set of repeated jumps. As in track and field athletes [4 ], this delay was appropriate to increase the PO produced during the present 9 sets
of 6 maximal consecutive jumps protocol in short-track speed-skating athletes.
The amplitude and direction of the observed enhancement of performance could be, at
least partly, explained by the training experience of the short track speed skaters
involved in the current study. Indeed, previous research evidenced that trained subjects
may show greater PAP responses than untrained individuals [6 ]
[33 ]. For example, Chiu et al. [6 ] examined the effects of a pre-activation exercise (5 sets of parallel back squats
at 90% of 1RM, with 2 min rest between sets) on 2 consecutive sets of squat jumps
(rebound or concentric-only squat jumps) with various loads. There was a 1 min rest
period between each load and an additional 10 min between the 2 sets. The authors
reported that the heavy resistance CC elicited a significant increase in performance,
but only when the sample was split into 2 groups based on training experience, with
a significant positive impact only observed in the trained subjects. The training
experience of the short track speed skaters involved in the current study could thus
explain the increase in performance observed.
Furthermore, studies have so far reported conflicting results when using dynamic [26 ]
[38 ]
[41 ] or maximal isometric contractions [15 ]
[33 ]. This discrepancy could be due to the volume of CC, however Bogdanis et al. [4 ] compared an equivalent volume of different types of CC and found that both isometric
and eccentric contractions lead to higher countermovement jump performance in comparison
to a control condition (without CC), whereas concentric contractions had no impact.
Moreover, compared to the baseline value (best countermovement jump performance determined
after a standardized warm-up in each condition), performance was improved only in
the isometric condition. Our results are in line with these observations and corroborate
those of Miyamoto et al. [27 ], who reported that ~5-s isometric CC can enhance subsequent dynamic voluntary performance.
Although somewhat speculative, 2 arguments can be raised to support the use of a conditioning
protocol involving CC performed separately and alternately on each leg to improve
performance in both single and repetitive explosive efforts. Firstly, the reduction
in the base of support induced by unilateral isometric CC could potentially increase
the solicitation of hip muscle stabilizers [5 ]
[25 ]. This may lead to an improvement of performance through greater activation of hip
stabilizers and improved transmission of forces of the primary power producers. Secondly,
improvement in repeated jump performance following a unilateral CC protocol may be
related to the fact that maximum voluntary force produced by a subject with the muscles
of one limb is less when these muscles are active simultaneously with the homologous
muscles in the contralateral limb than when they are active alone [3 ]. With regards to previous studies on the bilateral deficit [32 ]
[39 ], it may be hypothesized that unilateral CC could have lead a greater activation
of the targeted motor units contributing to the increase in the average power output.
However, further studies should be conducted to draw definite conclusions about these
2 hypotheses.
The small sample size (n=5) in the current study is a limitation, as for investigations
with elite competitors of less populated sports such as rowing (Jensen et al. [23 ]; n=7), biathlon (Hesford et al. [20 ]; n=2), alpine skiing (Barelle et al. [1 ]; n=4), and short track speed skating (e. g., Haug et al. [19 ]; n=5, Hesford et al. [21 ]; n=6). But our results suggest that 2 ×3 s maximal unilateral isometric CC executed
by speed skaters, lead to a significant increase in bilateral power output, in a subsequent
exercise performed 5 min later, at least for a few seconds. Another limitation is
that the exercise did not represent a real speed skating condition, with a force application
mainly directed vertically whereas in speed skating the force application is directed
more horizontally. A high PO measured vertically associated with a high quality of
vertical-to-horizontal transfer can lead to high maximal horizontal forces that is
a primordial for short sprint acceleration performances [28 ]. However, in the present study we hypothesized that the CC did not affect the quality
of vertical-to-horizontal transfer of force, indicating that the observed increase
in PO could result in an improvement of sprint-acceleration performances during speed
skating. Even though further studies should be conducted to draw definite conclusions
in view of lower limbs’ force-velocity-power properties [36 ] during real speed-skating conditions, athletes in various sports could benefit from
this enhancement of performance, especially if it can help to achieve a strategic
lead at the beginning of a race, as is the case in short track speed skating [18 ].
The present study showed that a maximal unilateral isometric CC induced an increase
in mean power output during repetitive sets of vertical squat jumps. Such an improvement
could be beneficial in sports involving repetitive powerful efforts, such as encountered
in short-track speed-skating competition.
