Muscle Function and Thickness Are Not Associated with Responsiveness to Post-Activation Performance Enhancement

Abstract

There is great variability in responsiveness to post-activation performance enhancement (PAPE). Factors such as greater expression of type II muscle fibers and experience with strength training are associated with greater responsiveness to PAPE. We investigated whether there is an association between markers of muscular function and morphology and responsiveness to PAPE in untrained individuals. Sixty-six active men (22 ± 2 years, 74 ± 11 kg, and 172 ± 27 cm) participated in the study. Their countermovement jump (CMJ) heights were measured before and four minutes after a pre-activation protocol consisting of five squats with a 5RM load. Isometric knee extensor peak torque (IPT), CMJ power (PO_CON), and thickness of the vastus lateralis muscle (VL_MT) of the participants were also determined in an unpotentiated condition. Change in CMJ height (ΔCMJ) following the pre-activation protocol was calculated and its associations with baseline CMJ height (CMJ_CON), PO_CON, IPT, and VL_MT were tested. Linear stepwise multiple regression models were also applied to screen for predictors of ΔCMJ among the dependent variables. No significant change (p = 0.28) in CMJ height was observed after the PA protocol (pre: 30.8 ± 5.1 cm; post: 31.0 ± 5.5 cm). No significant associations (p > 0.05) were found between ΔCMJ and IPT, VL_MT, PO_CON, and CMJ_CON (r = 0.29, 0.18, 0.09, and 0.01, respectively). Linear stepwise multiple regression analyses did not result in any significant models for ΔCMJ prediction. Although we confirmed the high individual variability in response to PA, no associations between neuromuscular performance/morphology and responsiveness to PAPE were found.

1. Introduction

Performance of explosive motor actions such as jumping and sprinting can be acutely potentiated after a bout of near-maximal exercise followed by a brief rest interval, and this phenomenon has been described as post-activation performance enhancement (PAPE) [1]. There are many different contexts in which PAPE can be applied, including controlled competitive settings and power training sessions [2,3,4,5,6]. The implementation of PAPE to improve power performance in the latter has been termed contrast or complex training [7].

Responsiveness to PAPE is highly variable, and different individuals present different magnitudes of explosive force enhancement using this method. It is a consensus that type II muscle fibers (i.e., the fast phenotype) are determinants of explosive force performance [8], and it is also known that the physiological mechanisms underpinning PAPE are expressed more in this type of fiber [9,10,11]. It is also known that trained individuals are more responsive to PAPE than their untrained counterparts [12]. Whether the increased responsiveness to PAPE observed in trained individuals is a product of specific training adaptations is yet to be determined, but the phenotypical overexpression and development of type II muscle fibers in this population might play a role in this phenomenon [13,14].

However, recent studies from our group presented conflicting evidence regarding the association between responsiveness to PAPE and indirect expression of type II muscle fibers or muscle strength. In [15] we showed that polymorphisms of the ACTN3 gene—which is associated with the expression of type II muscle fibers—do not influence responsiveness to PAPE in untrained individuals, who are less exposed to training-induced phenotypical adaptations in muscle fiber typology and strength. Furthermore, in [16] we showed, in untrained PAPE-responding men, that responsiveness to PAPE is compromised by plyometric exercise-induced muscle damage probably due to fiber-type specific damage, since it is known that type II muscle fibers are predominantly damaged following eccentric biased exercise [17]. Interestingly, however, no association was observed between maximal knee extension strength loss and decreased responsiveness to PAPE during countermovement jumps (CMJ) followed by 5RM of a barbell squat exercise [16]. These findings from our group contrast with those of Hamada et al. [11], which showed that type II muscle fibers are more responsive to post-activation potentiation.

Even though the current body of literature suggests that resistance training experience favors responsiveness to PAPE, it is not yet clear—especially considering our recent findings [15,16]—whether what makes an individual more responsive to PAPE is the expression and/or development of type II muscle fibers or other muscle characteristics, such as maximal strength and power (which does not rely entirely on fiber-type distribution, but also on neural parameters) and morphology. Furthermore, it is also not clear whether there are any easily assessable functional and/or morphological muscular parameters that might predict—in isolation or conjunctly—responsiveness to PAPE.

The aim of this study was, therefore, to screen for morphological/functional muscular parameters that can predict, either in conjunction or in isolation, responsiveness to PAPE of CMJ, which is the most frequently investigated explosive muscle action in PAPE studies. Our hypothesis was that neither maximal and explosive strength nor vastus lateralis muscle thickness would be associated with responsiveness to PAPE following a frequently adopted pre-activation protocol.

2. Materials and Methods

2.1. Participants

The current study consists of the conjunct analysis of three different experiments in which the same pre-activation protocol was adopted with the aim of inducing PAPE in untrained young men. Two of these experiments have been published elsewhere [15,16]. In total, 66 men (22 ± 2 years, 74 ± 11 kg, 172 ± 27 cm) volunteered to participate in the experiments. The inclusion criterion was to be a healthy young male. Exclusion criteria for the experiments included having practiced strength training of any type (e.g., weightlifting, calisthenics, Pilates) or any type of systematic training in the six months preceding recruitment and having suffered any musculoskeletal injuries in the previous year. The rationale for the inclusion of untrained individuals in the sample of the present study was to improve the heterogeneity of responsiveness to PAPE—which allows for better association analyses—while also making it possible to identify which of the investigated variables might determine responsiveness to PAPE without the bias of phenotypical changes conferred by resistance training. All participants read and signed informed consent terms approved by the institutional Ethics Review Board. All experimental procedures were conducted in agreement with the declaration of Helsinki on the use of humans as research participants and approved by the institution’s ethics committee (CAAE 86496518.4.0000.5465, 86264918.4.0000.5465, and 61544322.4.0000.5465).

2.2. Experimental Design

This study comprises a transverse analysis of the association between markers of muscle function/morphology and changes in CMJ jump height (ΔCMJ) following a pre-activation protocol. As such, the participants visited the laboratory on three distinct occasions (1) to provide their informed consent and relevant anthropometric information, and familiarize with dynamometry, squatting exercise, and CMJ; (2) to determine their five-repetition maximum (5RM) loads, further familiarize with CMJ, and determine dynamometry- and muscle morphology-related variables; and (3) to determine CMJ-related variables and responsiveness to PAPE.

On their first visit to the laboratory, participants performed a light warm-up consisting of five minutes of pedaling with a 25 W load (Excalibur, Lode, The Netherlands), followed by 10 unweighted squats. Five minutes following the warm-up, they performed three five-second maximal isometric contractions with the knee extensors of their dominant leg in an isokinetic dynamometer (System 3, Biodex Systems, Shirley, NY, USA) with a one-minute rest interval between contractions. They were allowed to rest for five minutes and then performed three sets of five barbell back squats with no load other than the bar itself, which weighed 10 kg. Each squat was carefully supervised by an experienced examiner who provided feedback on proper squat technique (participants were instructed to maintain an upright posture with their chest lifted, scapulae adducted, and knees tracking in line with their toes). They were instructed to squat until their knees were flexed at 90° and then return to a standing position. After familiarization to the squat exercise, they performed three sets of five CMJs, also receiving immediate feedback. For CMJs, they were instructed to rapidly squat until approximately 90° and jump as high as possible immediately after. The 90° knee flexion angle used for squats and CMJs was controlled visually by experienced examiners.

The second visit to the laboratory was conducted 48 h after the first visit. Prior to any procedures, thickness of the vastus lateralis muscle (VL_MT) was determined. The same warm-up performed in the first visit was repeated, followed by the same procedures for familiarization to CMJ. After that, they performed two maximal voluntary isometric contractions with their dominant knee extensors in the dynamometer to determine isometric peak torque (IPT). Fifteen minutes later, individual 5RM load in the barbell back squat was determined using a custom-built squat cage for safety purposes. The procedures for determination of the 5RM load were followed according to what has been established in the literature and are described in detail elsewhere [15,16].

In the third and last visit to the laboratory, responsiveness to PAPE was determined. The participants repeated the same warm-up performed in the previous visits prior to the experimental procedures. CMJ height was assessed 10 min following the warm-up (control condition—CON). Ten minutes after CON CMJ assessment, participants performed a pre-activation protocol consisting of five squats with a 5RM load. After four minutes, CMJ assessment was repeated (PAPE condition—PAPE). The timeline of the experimental procedures is illustrated in Figure 1.

2.3. Dependent Variables

CMJ performance was measured using a commercially available mobile phone application (My Jump 2, Carlos Balsalobre, Spain) that has been validated previously [18]. The participants performed three consecutive CMJs in a 10-s window following the instruction provided in the familiarization sessions. CMJ height and power output values were calculated for each jump based on flight time, individual body mass, and push-off distance, and the mean values obtained from the three CMJs was used for analyses. Jump height was estimated using the following equation:

$$h=\frac{1}{2}\times {\left(\frac{ft}{2}\right)}^2\times g$$

where ft is CMJ height (m), ft is flight time (s), and g is the acceleration due to gravity (m·s⁽⁻²⁾).

Mean power output produced during CMJ was estimated using the following equation:

$$PO=BM\times g\times \left(\frac{h}{PD}+1\right)\times \sqrt{\frac{g\times h}{2}}$$

where PO is mean power output (W), BM is body mass (kg), h is CMJ height (m), PD is push-off distance (m) (i.e., the difference between the height of the great trochanter of the femur in a fully erect and a 90°-squatted position), and g is the acceleration due to gravity (m·s⁽⁻²⁾).

ΔCMJ was used to determine responsiveness to PAPE, by subtracting the value obtained in the CON condition (CMJ_CON) from the value obtained in the PAPE condition. CMJ_CON and the power output produced in the CON condition (PO_CON) were also considered as skeletal muscle performance variables for analyses.

To determine IPT, participants were positioned in the dynamometer according to the guidelines provided by the manufacturer. Their hips and knees were flexed at 85° and 70°, respectively. Their trunk, hip and dominant thigh were firmly secured to the chair of the dynamometer using cushioned straps and the distal portion of their dominant leg was fixed to a lever attached to the load cell. After performing a specific warm-up consisting of five submaximal isometric contractions, participants were asked to try to extend their knees as forcefully as possible for five seconds twice, with a one-minute recovery interval in between. The greatest torque value obtained in the two maximal voluntary isometric contractions was considered as IPT and used for analyses.

VL_MT was determined at mid-distance between the greater trochanter and the lateral condyle of the femur by ultrasonography using a 5-cm probe with a 9.0 MHz frequency (ProSound 2, Aloka, Japan). Measurements were performed by an experienced examiner after five minutes of laid rest to allow for proper fluid accommodation. Saline gel was applied to the skin of the participants to amplify acoustic transmission. VL_MT was considered as the distance between the superficial and deep fasciae of the vastus lateralis following a perpendicular line traced between the skin and the femur. Three measurements were performed, and the mean value was used for analyses. The typical error and mean coefficient of variation of the specific examiner who conducted VL_MT assessments were 0.1 mm and 0.35%, respectively.

2.4. Statistical Analyses

Data normality was tested and confirmed using the Shapiro–Wilk test. Changes in CMJ height were compared between the CON and PAPE conditions using two-tailed Student’s t-test for repeated measures. The smallest worthwhile change in CMJ height was calculated by multiplying the standard deviation for CMJ_CON by a small effect size (i.e., 0.2) to screen for positive and negative responder within the sample. Associations between ΔCMJ and CMJ_CON, PO_CON, IPT, and VL_MT were described using Pearson’s product moment correlation tests.

A linear stepwise multiple regression model was performed to screen for which independent variable (i.e., CMJ_CON, PO_CON, IPT, and VL_MT) predicted ΔCMJ. Models including IPT and VL_MT were performed with samples sizes of 38 and 17 participants, respectively. The alpha level was adjusted using the Bonferroni correction. Significance levels were set as p < 0.05. Analyses were performed using the Statistical Package for Social Sciences (SPSS 20, IBM, Armonk, NY, USA) and figures were prepared using Prism (Prism 8.3, GraphPad Software, Boston, MA, USA). Data are expressed as means ± SD.

3. Results

Participants’ mean 5RM load was 94.7 ± 20.3 kg. No significant differences were found between CMJ height measured under the CON and PAPE conditions (30.8 ± 5.1 vs. 31.0 ± 5.5 cm, respectively; p = 0.28) (Figure 2A). The smallest worthwhile change for CMJ height in the present study was calculated as ± 1.03 cm, and the typical error and mean coefficient of variation were 0.45 cm and 1.4%, respectively. Of the 66 participants included in the sample, 18 showed decreases in CMJ height under the PAPE condition, 20 showed increases, and 28 did not present any change in CMJ height in the same condition (Figure 2B).

Correlational analyses revealed no significant associations between ΔCMJ and IPT (r = 0.29), VL_MT (r = 0.18), PO_CON (r = 0.09), and CMJ_CON (r < 0.01) (Figure 3, panels A, B, C, and D, respectively).

Linear stepwise multiple regression models were performed by including independent variables hierarchically according to their association with ΔCMJ. Four models were performed: (1) including only IPT (n = 38); (2) including IPT and VL_MT (n = 17); (3) including IPT, VL_MT, and PO_CON (n = 17); and (4) including all independent variables (n = 17). None of the proposed models were significant (

Table 1. Statistical results for the four linear stepwise regression models tested in the study. IPT: isometric peak torque; VL_MT: vastus lateralis muscle thickness; PO_CON: power output during countermovement jumps in the control condition; CMJ_CON: countermovement jump height in the control condition.

	F	p	R2	Adjusted R2
Model 1 Predictors: IPT	0.05	0.83	0.003	−0.063
Model 2 Predictors: IPT & VLMT	0.34	0.72	0.046	−0.09
Model 3 Predictors: IPT, VLMT & POCON	2.51	0.1	0.367	0.221
Model 4 Predictors: IPT, VLMT, POCON & CMJCON	2.05	0.15	0.405	0.207

).

4. Discussion

The aim of this study was to investigate functional and/or morphological parameters that might predict responsiveness to CMJ PAPE. It was found that a frequently adopted pre-activation protocol did not result in enhancements in CMJ height in a large sample (n = 66) of untrained men, with 20 of them responding positively, 18 responding negatively, and 28 not presenting any worthwhile change. Correlation analyses resulted in no significant associations between responsiveness to PAPE (i.e., ΔCMJ) and baseline neuromuscular performance (IPT, CMJ_CON, and PO_CON) or muscle morphology (VL_MT). Furthermore, linear stepwise multiple regression models revealed that no combination of these variables was able to predict improvements in CMJ height following pre-activation. Therefore, our hypothesis that it is not possible to predict responsiveness to PAPE based on performance and morphological parameters was confirmed.

A common feature of pre-activation protocols designed to induce PAPE is the performance of heavy-load resistance exercise a few minutes prior to the activity to be potentiated [19,20,21,22,23]. However, set configuration, loads, and the interval between pre-activation and the targeted activity vary greatly among studies [12,23]. In a meta-analysis, ref. [12] showed that trained individuals respond better to multiple-set pre-activation protocols followed by relatively short rest intervals, whereas untrained populations benefit the most from single-set protocols and longer rest intervals. The pre-activation protocol adopted in the present study was shown to be effective in improving CMJ height in both trained [19,24] and untrained [16] individuals. However, it should be noted that non-responders (n = 11) were excluded from the sample (n = 22) of our recent study using this protocol [16] for methodological reasons. In the present study, 30% of the untrained men included in the sample responded positively to the pre-activation protocol with the remaining 70% either responding negatively (27%) or not showing any worthwhile change in CMJ height (43%). It is therefore reasonable to state that responsiveness to PAPE is heterogeneous in untrained individuals.

Superior responsiveness to PAPE has been frequently reported for trained populations compared to untrained individuals [25,26]. Nonetheless, athletes have also been shown to be better PAPE responders compared to resistance trained individuals [27]. Based on these observations, it can be speculated that training-induced adaptations in muscle performance and/or morphology might be associated with responsiveness to PAPE. Based on this rationale, we analyzed a cohort of previous PAPE studies from our group [15,16] in conjunction with another experiment that has not yet been published) which used the same pre-activation protocol and population to screen for possible strength training-modifiable parameters that might explain responsiveness to PAPE. Although we agree that PAPE is an ergogenic phenomenon that is more ecologically valid for trained and athletic populations, the heterogeneity in responsiveness to PAPE observed in untrained populations—as is the case in the present study—is an interesting feature for correlational analyses.

Our results show that changes in CMJ height following pre-activation are not associated with the manifestation of maximal strength (IPT), muscle power (PO_CON), and jump performance (CMJ_CON) assessed in an unpotentiated state. These markers are different manifestations of neuromuscular performance that have abundantly been shown to improve following strength training [28,29,30] and to be superior in athletic populations compared to untrained people [31,32,33]. They are also frequently associated with the expression of type II muscle fibers, which are the most responsive to PAPE [11,14]. Furthermore, VL_MT, a surrogate of quadriceps—one of the main muscles involved in force production during jumping—muscle mass, is another variable that is notably affected by strength training [34]. Although many other manifestations of neuromuscular performance and morphology exist and are affected by strength training, our data—obtained from untrained persons, which present greater heterogeneity in these markers compared to trained persons—suggest that lower-limb muscle performance and mass are not associated with changes in powerful actions following pre-activation. It could be speculated that the mechanisms underpinning greater responsiveness to PAPE in trained populations are related not to increased muscle strength, power and mass, but rather to other cellular and neural mechanisms that are affected by strength training.

As in many other physiological processes, PAPE is a multifactorial phenomenon that relies on several adaptive mechanisms triggered by the stress imposed by pre-activation [35]. Therefore, it is reasonable to assume that no neuromuscular variable should explain PAPE in isolation. Considering this, linear stepwise multiple regression models were applied to screen for the possibility that some of the investigated variables could conjunctively predict responsiveness to PAPE. However, none of the tested models—which were proposed hierarchically, according to the coefficients of correlation between each predictor and ΔCMJ—were found to be significant. Hence, we present evidence that the investigated neuromuscular variables might not predict responsiveness to PAPE in untrained males, even when analyzed in conjunction.

Improvements in maximal and explosive force and muscle mass are some of the many functional and morphological adaptations to strength training. Other adaptations, such as increased muscle-tendon stiffness [36], skeletal muscle angiogenesis [37], intramuscular glycogen and phosphocreatine content [38,39], selective type II fiber hypertrophy [40], and a shift towards greater expression of type II myosin heavy chain [13,14], are observed following strength training interventions. Some of these adaptations, which are not directly translated into changes in the independent variables investigated in the present study, might be more closely related to responsiveness to PAPE. For instance, myosin head phosphorylation—especially type IIx—has been frequently observed following pre-activation protocols, resulting in a transient increase in evoked force [41]. Whether or not this phenomenon impacts voluntary performance is still in debate [35,42,43], but trained individuals are known to express greater amounts of type IIx myosin heads [44], which could explain the greater responsiveness to PAPE (or at least post-activation potentiation) observed in this population. It is also proposed that transient increases in intramuscular water content and temperature might underpin improvements in neuromuscular performance following pre-activation [9,10], while the greater capillary bed promoted by strength training might facilitate these mechanisms. Finally, strength training-induced increases in glycogen and, especially, phosphocreatine content within the muscle might favor the balance between fatigue and recovery following pre-activation, promoting a wider and/or larger PAPE window. This is supported by data showing that creatine supplementation improves responsiveness to PAPE [45]. Although speculative, these considerations should be considered when designing future studies to investigate the mechanisms underpinning responsiveness to PAPE.

The present study has limitations that should be mentioned. The first is the lack of responsiveness to PAPE. However, although this might have indicated that the adopted protocol was not ideal to induced PAPE, the occurrence of responsive, unresponsive, and negatively responsive individuals might have enriched the analyses performed in this study. We also believe that replication of the same protocol in a large, trained sample would strengthen the finding that baseline performance and muscle morphology are not good predictors of responsiveness to PAPE. Additionally, although relatively large, considering other PAPE studies, the sample size of the present study might still not be adequate for the number of independent variables used in the linear stepwise multiple regression models as well as for some of the correlational analyses, especially considering the small sample size obtained for VL_MT (i.e., n = 16). Although the determination of CMJ height from flight time with the smartphone application used in the present study was shown to be valid and reliable [18], CMJ power output (i.e., PO_CON) determination might be more prone to greater error as it relies on several other estimations based on kinetic and kinematic estimates, instead of actual force and velocity data. However, transverse studies are not the best from which to infer causation. Therefore, longitudinal, controlled, training studies should better elucidate the question raised in the current investigation. Finally, the investigation of maximal strength (i.e., IPT) and muscle thickness in the knee extensors, exclusively, is a limitation, as jump performance is determined predominantly by the conjunct work of the hip extensors, knee extensors and plantar flexors.

We conclude that responsiveness to PAPE might not be predicted by baseline markers of neuromuscular performance, such as knee extensors maximal isometric strength, CMJ power output and CMJ height, nor by quadriceps muscle mass.