**1. Introduction**

Soccer is the most practiced sport worldwide today, with approximately 265 million players across five continents, which is equivalent to 4% of the world's population [1]. Traditionally considered [2,3] a discontinuous and intermittent physical exercise that encompasses low-, medium-, and high-intensity activities, soccer generates physiological and metabolic demands [4] that have increased with the physical demands necessitated by the professionalization of this sport.

Researchers have studied the physiological variables of professional soccer players during matches, such as distance covered (10–13 km), [5] mean heart rate (HR; 165–170 bpm, equivalent to 85% of the maximum HR), or oxygen uptake (45–65% of VO2 max with telemetric controls) [6,7]. These variables are used to analyze energy metabolism, with some researchers reporting 70–85% aerobic metabolism use and the remainder as anaerobic metabolism [8]. However, this measurement is difficult because it depends on the duration of the high-intensity phases and the recovery period [9,10].

These physiological parameters have become more relevant as the requirements of professional soccer players have increased, and several cases of soccer players experiencing cardiovascular events during matches have been reported in recent years [11], with "sudden death" being the most remarkable both clinically and socially. In fact, between 25% and 49% of athletes who experienced sudden death in Spain were practicing soccer [12,13].

**Citation:** Pérez-Gosalvez, A.; García-Muro San José, F.; Carrión-Otero, O.; Pérez-Fernández, T.; Fernández-Rosa, L. Blood Pressure and Heart Rate Responses to an Isokinetic Testing Protocol in Professional Soccer Players. *J. Clin. Med.* **2022**, *11*, 1539. https://doi.org/ 10.3390/jcm11061539

Academic Editor: David Rodríguez-Sanz

Received: 19 January 2022 Accepted: 8 March 2022 Published: 11 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Therefore, many researchers have performed various functional assessment tests in the laboratory and controlled field tests that attempt to reproduce the conditions that a soccer player is subjected to during a match [8,14–16]. The most widely used field tests are categorized as aerobic, anaerobic, and specific, and the most referenced laboratory tests are ergospirometry, anaerobic tests, isometric/isotonic contractions, and isokinetic dynamometry [17].

Many researchers have conducted different functional assessment tests in the laboratory and controlled field tests that try to mimic, in as standardized and objective a manner as possible, the conditions that a football player is subjected to during a match [8,14,15,18]. Among all these tests, isokinetic dynamometry allows for an objective assessment of a football player's muscle function as well as his response to maximum intensity requirements [19,20]. Thus, isokinetic dynamometric systems have been used to perform specific strength training, rehabilitate postsurgical musculoskeletal processes [21,22], prevent muscle imbalances that are a risk factor against muscle injuries [23–25], and evaluate the muscle strength and power of the lower extremities in soccer players [26,27]. For this, isokinetic dynamometry has been considered the gold standard among the strength tests that can be performed on a soccer player [28].

The maximum requirement of this test is an adequate musculoskeletal state and a sufficient cardiorespiratory condition to satisfy the requirements; thus, it would be interesting to evaluate a soccer player's cardiovascular response during a usual isokinetic protocol. This protocol constitutes a controlled laboratory test, which is part of the usual physical assessment of soccer players, and generates metabolic, muscular, and cardiovascular demands that differ from those generated during maximal aerobic exercise. This cardiovascular response has previously been studied in incremental dynamic exercise in soccer players, with the treadmill test used to evaluate cardiorespiratory fitness; this method has also been used to detect possible cardiovascular functional risks [29]. Although the isokinetic assessment test is an indispensable requirement in the evaluation of the physical fitness of soccer players, there are no previous studies assessing the cardiac and blood pressure risk in these athletes.

However, to date, no reliable published studies with an adequate sample size have described how soccer players' BP and HR respond to the demands of an isokinetic testing protocol of the lower extremity musculature. Studies describing the cardiovascular response to isokinetic exercise in healthy or old adults are scarce [30,31] and possess high sample variability [32–35], which prevents the extrapolation of their results to a highly trained population capable of developing high levels of muscle strength and power in the lower extremities.

This study aimed to describe the BP and HR responses triggered by an isokinetic testing protocol in professional soccer players and compare cardiovascular parameters at completion of this isokinetic protocol with those during a treadmill test.

#### **2. Materials and Methods**

#### *2.1. Participants*

A minimum sample size of 46 was deemed to be representative of the adult population using GRANMO version 7.12 [36], assuming a reference population of 1000, a 95% confidence interval level, an estimate of the standard deviation of 20, a precision of 6, and a 10% replacement rate. A total of 63 professional male outfield soccer players (age 22.6 ± 1.2 years; height: 179.2 ± 5.2 cm; weight: 72.9 ± 5.5 kg), including 23 defenders (age 22.3 ± 3.5 years; height 181.5 ± 4.4 cm; weight: 74.6 ± 4.1 kg), 25 midfielders (age 20.9 ± 2.5 years; height: 178.6 ± 4.7 cm; weight: 73.4 ± 5.1 kg), and 15 forwards (age: 19.7 ± 1.9 years; height: 176.8 ± 5.9 cm; weight: 70.2 ± 6.6 kg), participated in this study. They belonged to a professional soccer team during the measurement period, with 20 on the first-division team and 43 on the second-division team.

This study was performed during preseason, and players were recruited using purposive sampling. Prior to the study, the players were evaluated by a sports medicine specialist

who performed an anamnesis and clinical examination following the Union of European Football Associations recommendations [37] to confirm that the players met the conditions for professional sports practice. Subsequently, electrocardiogram, echocardiography, and spirometry were performed to rule out possible contraindications in performing the tests [38]. None of the participating players showed warning signs that contraindicated their inclusion in the study.

All players who had a current federation record as a professional soccer player and who were physically and medically fit to start the season were included. Players who had undergone surgery in the previous 12 months of any pathology in the lower extremities or who had suffered an injury in the lower extremities that would have forced them to suspend sports activity for at least 1 month were excluded. Players undergoing medical treatment with a drug that interfered with physical capacity and those who suffered from acute systemic diseases were also excluded.

All participants were fully informed about the protocol prior to participation and provided informed consent in accordance with the principles of the Declaration of Helsinki. The study was approved by the ethics committee of CEU San Pablo University (approval code, 238/17/18).

A repeated-measures design involving active soccer players was used to determine the BP and HR responses triggered by the isokinetic testing protocol. Then, we compared the HR and BP values at completion of this isokinetic protocol with those achieved during a treadmill test.

Both tests (isokinetic and treadmill) were performed during the preseason of the soccer teams; the usual schedule followed in the medical and functional examinations of the soccer players' sports club was respected. These athletes are evaluated annually using isokinetic assessment and treadmill ergospirometry of the lower extremities.

Once their medical history was recorded, the soccer players performed an isokinetic strength test in the Research Unit in the Physical Therapies Laboratory of the Faculty of Medicine of CEU San Pablo University. After completion, the data were collected, and the second test was conducted within 3 days. The treadmill test was performed in the exercise physiology laboratory of the School of Sports Medicine in Complutense University of Madrid in the recommended environment [39]. Each player's information was encrypted to guarantee anonymity.

#### *2.2. Isokinetic Testing Protocol Description*

The isokinetic test was selected, as it is a test included in the functional assessment of elite football players worldwide and validated by UEFA [33] and Spanish Association of Football Team Doctors [40]. For this reason, this population group must face this test as a regular evaluation element of their physical condition.

Initially, with the participant sitting with their feet on the ground, baseline measurements of BP and HR were recorded using a BTL-08 ABPM II portable and digital sphygmomanometer (BTL Industries Ltd. Hertfordshire, United Kingdom). Subsequently, without removing the BP cuff from the participant's upper limb, the device was placed inside a sheath that was attached to an adjustable strap placed around the waist of the participant; thus, the following measurements could be performed without replacing the sphygmomanometer.

The participants performed a general 10 min warm-up exercise on a Monark cycle ergometer (model 818E) at a moderate pace and resistance, immediately after which BP, HR, systolic blood pressure (SBP), diastolic blood pressure (DBP), rate pressure product (RPP) (RPP = HR × SBP), and mean arterial pressure (MAP) (MAP = DBP + (0.333 × [SBP − DBP])) were recorded.

The participants were then asked to remove their shoes and sit on the IsoMed2000 strength-testing system (D&R Ferstl GmbH, Hemau, Germany) in an upright position, with 85◦ flexion at the hip. The participants were secured by straps around the chest, waist, and right thigh. The dynamometer lever arm was aligned with the participant's tibial

spine, 2.5 cm proximal to the right medial malleolus; after ensuring it was comfortable for the participant, it was fixed using a Velcro strap. The rotating axis of the dynamometer was aligned with the knee joint's axis of rotation (lateral epicondyle of the femur). After checking the strap tension, the participant was instructed to hold on to the hand grips on the side of the seat during the efforts [41], and the isokinetic testing protocol was initiated (Figure 1).

**Figure 1.** Positioning of a subject before starting the isokinetic testing protocol (**a**) and prior to the beginning of the contraction series with the left lower extremity (**b**).

Before starting the protocol, the right lower extremity (RLE) was weighed using the automatic limb weighing system of the dynamometer to adjust for the gravitational effect on torque.

A bilateral study protocol of continuous concentric/concentric contraction was followed at low (60◦/s), medium (180◦/s), and high velocities (240◦/s) of knee flexion and extension through the knee's range of motion, from 0◦ (full extension) to 90◦ (flexed), as recommended by the Spanish Association of Football Teams Physicians in its protocol for professional soccer players [40].

Prior to assessment at each angular velocity, the participants performed three to five submaximal contractions of increasing intensity (25%, 50%, and 80%), completing the established range of motion with both knee flexion and extension, to adapt the musculature to the effort requested later [42,43]. The participants were oriented to avoid the Valsalva maneuver and to breathe spontaneously throughout the movement [44].

The protocol series required the performance of 5 repetitions of flexion and extension at 60◦/s, 10 repetitions at 180◦/s, and 25 repetitions at 240◦/s, all at maximum intensity and always starting with the right leg. During the protocol, encouragements by verbal coaching and visual feedback were provided to all participants to help them concentrate on the quality and maximum intensity of their movements [45,46].

Immediately after completing each of the six established series of contractions, the investigator activated the BP monitor and asked the participant to rest and without speaking to check BP, HR, SBP, DBP, RPP, and MAP (approximately 30–40 s). After verifying that the device had recorded the measurements, muscle warm-up/adaptation contractions were started in the next series of protocol contractions. Between the third (right leg at 240◦/s) and fourth (left leg at 60◦/s) series, the time for measuring BP and HR was used to move the dynamometer lever to the opposite side of the participant. This procedure lasted 30–40 s to homogenize the rest time with that of the series performed in the right leg [47,48].

Once the final BP and HR measurements were recorded and checked (after the sixth series), the straps, attachment to the mobile arm, and sphygmomanometer cuff were removed, and the athlete was recommended to spend 5–10 min passively stretching the lower extremity musculature before leaving the investigation unit.

#### *2.3. Treadmill Ergospirometry Test Description*

Treadmill ergospirometry test was performed after the isokinetic test. The "incremental speed protocol" for ergospirometry commonly used in the School of Sports Medicine of Complutense University was followed in the functional assessment of the participants [49,50]. For this, a treadmill (H.P. Cosmos), gas analyzer (model Vmax, Sensor Medics), and electrocardiograph (Quest Exercise Stress System, Burdick Inc; Milton, WI, USA) were used. After checking the BP and HR of the participants at rest, they performed a 2 min warm-up exercise on the treadmill at 4 km/h following the usual protocol [51,52], with electrocardiographic control but no respiratory control.

The maximum treadmill test was started at an initial speed of 6 km/h for all participants. Every 2 min, the speed was increased by 2 km/h until the participant was exhausted, with the slope constant at 1% as in the warm-up exercise. BP and HR were measured within 30 s after completion of the exhaustion test. Recovery started at 5 km/h, and this speed was maintained until the participant's complete recovery.

#### *2.4. Statistical Analyses*

We used the Kolmogorov–Smirnov and Levene tests to assess the normality of the values and the equality of variances, respectively. We then performed univariate repeatedmeasures analysis of variance (ANOVA) to determine differences among SBP, DBP, MAP, HR, and RPP values. Finally, we used Student's paired *t*-test to assess possible differences between phases. Bonferroni's post hoc tests were applied for comparative analyses of between-group differences when a significant interaction was found. SPSS version 24.0 for Windows was used for statistical analysis (Statistical Package for the Social Sciences, Chicago, IL, USA). The results are expressed as the mean ± standard error, and the significance level was *p* < 0.05.

#### **3. Results**
