Acute effects of voluntary isometric contractions at maximal shortening vs. ballistic stretching on flexibility, strength and jump

Design

A randomized controlled repeated measures design was used to evaluate and compare the acute effects of VICAMS and BS on active range of motion, maximal isometric strength and vertical jump. After 2 weeks of familiarization, participants were randomly assigned into three groups: a BS group (N = 20), a VICAMS group (N = 20) and a control group (CG) (N = 20). Each group consisted of 10 males and 10 females. In order to assess the acute effects, before and after the intervention, AROM, MVIF and CMJ were determined. AROM and MVIF were measured for hip flexion (HF) and knee flexion (KF). The CG was re-measured 20 min after the first measurement but did not undergo any intervention.

Participants

Sixty healthy, recreationally active participants (30 females and 30 males; age 46.4 ± 5.5 years old; height 1.70 ± 0.17 m; weight 76.0 ± 14.9 kg; body mass index (BMI) 25.5 ± 2.5 kg/m²) were voluntarily enrolled in this study. All participants were instructed not to engage in physical activity the day before the test.

Inclusion criteria were: (1) to be between 35 and 55 years old; (2) to practice non-competitive physical activity at least twice a week. Exclusion criteria was: having a musculoskeletal injury or disease that prevented the correct performance of the tests. All participants received a detailed explanation of the study procedures and signed the informed consent form. The study was approved by the Research Ethics and Animal Experimentation Committee of the University of Alcalá (CEID/2022/4/083) and conducted in accordance with the principles established by the Declaration of Helsinki.

Testing procedures

To ensure a high level of reliability, all measurements were performed by the same researcher, who has more than 5 years of experience. The temperature in the laboratory was kept constant at around 23 °C. AROM and MVIF were measured with a calibrated handheld digital goniometer-dynamometer model Micro-FET3 (Hoggan Scientific, Salt Lake City, Utah, USA). This device provides reliability and ease of measurement (Bohannon, 2012). CMJ was measured with a 420 × 590 mm Chronojump contact platform (Boscosystem), which allows reliable and cost-effective measurement of jump height and changes in physical performance (Pueo, Penichet-Tomas & Jimenez-Olmedo, 2020).

Prior to the measurements the participants performed a standard warm-up of five raises of each knee, five abductions of each hip, five extensions of each hip, five bends of each knee and 10 heel raises at a time. All in a standing position, slow and controlled, and reaching the position of maximum muscle shortening. The same exercises were then repeated in a ballistic manner. To finish, they did five squats and three CMJ. The measurements were then performed in the following order: (1) AROM (3-min pause); (2) MVIF (3-min pause); (3) CMJ (5-min pause); (4) Intervention (5-min pause); (5) AROM (3-min pause); (6) MVIF (3-min pause); (7) CMJ. A scheme of the study design is shown in Fig. 1.

Active range of motion (AROM): Measurements of bilateral lower extremity AROM were carried out according to the methods described by Norkin & White (1986). Thus, HF was measured in the supine position and KF in the prone position. The correct starting position was then established with the goniometer, coinciding with the anatomical position. The subject was then asked to perform the movement in a slow and controlled manner to the maximum possible joint range. At this final position, the measurement was recorded. Three measurements were taken for each joint position and recorded from the digital readout on the goniometer. The highest of the three trials was used for data analysis. After each measurement, the limb was returned to the starting position. A 10-s rest was provided between measurements to minimize the effect of fatigue, and 30 s between positions to accommodate the posture of the participant and the investigator. The researcher observed whether compensatory movements occurred when executing the movement. If the participants made compensatory movements, they were corrected immediately. In cases where the participant was unable to correct the compensatory movement, the angle was measured at the point prior to compensation.

Maximal voluntary isometric force (MVIF): It was performed using the positions and techniques described by Andrews, Thomas & Bohannon (1996). Force measurements are expressed in kilograms. The same protocol was followed as in the previous measurements, obtaining three measurements and using the highest for subsequent analysis (Brinkrnann, 1994). This reduced the possible learning effect. To reduce the fatigue effect, subjects rested 15 s between each measurement (Sisto & Dyson-Hudson, 2007) and 30 s between positions (Arnold et al., 2010). The positions for correct measurement were explained to the participants prior to the test. In cases where the subject did not demonstrate maximal effort, did not follow instructions or the HHD recorded an error message, the test was repeated with adequate rest compensation. To measure HF, the participant lay supine and raised one leg with the hip flexed at 90° and the knee relaxed. The examiner placed the dynamometer on the anterior side of the leg at the level of the femoral condyles and ensured that the pelvis was stabilized during the measurement. For the KF, the participant sat on the stretcher with the hips and knees flexed at 90°. The dynamometer was placed on the posterior side of the leg, just proximal to the malleoli. Trunk stability was monitored during the measurement. Subsequently, participants were instructed to exert as much force as possible against the device while maintaining the position. Meanwhile, the researcher held the device as still as possible without breaking the position (make test), as this has been shown to be more reliable than the break test (Stratford & Balsor, 1994). The duration of the contraction was approximately 5 s (Thorborg et al., 2010) and the instructions given to the participant were: push, hold, relax. It has been shown that this time is sufficient to reach maximal force (Wang, Olson & Protas, 2002). Participants were asked not to make an explosive effort, but to increase force progressively until peak force was reached (Maffiuletti et al., 2016). The device pad and the examiner’s forearm were held perpendicular to the limb being measured. (Bohannon, 1986a; Andrews, Thomas & Bohannon, 1996; Wang, Olson & Protas, 2002).

Countermovement jump (CMJ): In the warm-up, participants were instructed to familiarize themselves with proper jumping technique and a balanced landing. The assessment was carried out following the Bosco protocol (Bosco, Luhtanen & Komi, 1983) and each jump was recorded in centimeters. For the measurement, three jumps were performed with 1 min rest in between to avoid the effect of fatigue (Read & Cisar, 2001). The highest value was selected as final value. The execution technique consisted of placing both hands at hip level, bending the knees to a self-selected depth and attempting to jump as high as possible in a single movement. As the participants were inexperienced in CMJ, a knee flexion close to 90° was suggested. Only correctly executed jumps were considered. The jump was repeated for any mismatch that could affect the measurement. Verbal encouragement was provided to help achieve maximum effort on each jump (Heishman et al., 2018). Similar and consistent instructions were given to each participant prior to the test, to limit possible effects on jumping (Young, Pryor & Wilson, 1995).

Intervention

Ballistic stretching protocol: Each subject performed three sets of 20 s on each leg for each exercise, with a recovery of 10 s each time. The total time of the intervention was approximately 18 min. A beat rate of 40 beats per minute measured by a metronome was set (Samuel et al., 2008; Oliveira et al., 2017). The subject’s foot had to touch the ground on each repetition for full range of motion, except for knee extension, which had to return to the starting position. He was instructed to perform the movement following the metronome rhythm and reaching the maximum possible muscle shortening without bouncing (Yamaguchi & Ishii, 2005; Sekir et al., 2010). Verbal instructions were given during the execution to correct the exercise in case the execution was incorrect. It was performed first with the right leg and then with the left leg.

The exercises performed correspond to the movements measured in the ROM, AROM and MVIF. The order followed was:

1. Hip flexion (HF). The subject stood with hands resting on the hips to avoid arm assistance. He was instructed to lift one leg forward with the knee flexed and return to the starting position.

2. Hip abduction (HABD). The subject stood with hands resting on a wall to help stabilize the body during the movement. He/she was instructed to separate the leg to the side with the knee extended and return to the starting position.

3. Hip adduction (HADD). The subject stood with hands resting on a wall to help stabilize the body during the movement. He was instructed to slightly delay the supporting leg and bring the free leg forward just in front. The free leg went from neutral to maximum adduction and then to the starting position.

4. Hip extension (HE). The subject stood with hands resting on a wall to help stabilize the body during the movement. The subject was instructed to move the leg backwards with the knee extended and return to the starting position.

5. Knee flexion (KF): The subject stood with hands resting on a wall to help stabilize the body during the movement. The subject was instructed to flex and extend the knee to return to the starting position.

6. Knee extension (KE): The subject lay supine with one hip and knee flexed to 90°, holding the leg with the hands above the knee to remove the force of the hip flexors. He was instructed to extend and flex the knee to the starting position.

Voluntary isometric contraction at maximal shortening protocol (VICAMS): The joint movements selected for this protocol were the same as in the BS protocol and in the previous measurements. For the performance of each exercise, the researcher guided the participant manually and verbally throughout the application. VICAMS were performed after the researcher brought the joint to the maximum possible muscle shortening. Prior to each exercise and coinciding with the breaks, the subject was shown where the main muscles responsible for performing the movement were located so that he/she could become aware of where he/she had to exert the force. In addition, the subject was palpated with two fingers at various points along the path of these muscles. The participant was asked to concentrate on the muscles that are activated to produce the movement, and to resist by trying to maintain a constant opposing force of as similar intensity as possible to the one he was receiving. From the position of maximum shortening, the researcher exerted force in the opposite direction to the movement and the participant tried to resist this force.

The total time of the intervention was approximately 18 min, almost the same as the Ballistic Stretching. Each voluntary contraction lasted 5 s. Nine repetitions of each movement were performed with 3 s rest between repetitions, 15 between each leg and 30 between exercises. The intensity of the force applied was divided into three levels: three repetitions at light intensity (20–30%); three repetitions at medium intensity (45–55%); and three repetitions at maximum intensity (100%). It was applied in that order. After each repetition, the shortening position was maintained and insisted to reach the maximum. In order to minimize stimuli in other directions, the researcher kept the forearm perpendicular to the segment where the force was applied (Leal, Martínez & Sieso, 2012). Verbal cues during execution were: contract, hold, relax. The participant was allowed to hold on to the stretcher to remain stable in case of unbalance. It was performed first with the right leg and then with the left leg.

The order of the exercises was like that of the Ballistic Stretches:

1. Hip flexion (HF). In the supine position, force was applied just above the anterior aspect of the knee in the direction of hip extension, keeping the knee flexed.

2. Hip abduction (HABD). In the supine position, force was applied just above the outside of the knee in the direction of hip adduction. The weight of the leg was held with the other hand to minimize the action of gravity.

3. Hip adduction (HADD). In order to achieve maximum muscle shortening, one leg was raised just above the other. In the supine position, force was applied just above the inside of the knee in the direction of hip abduction. The weight of the leg was held with the other hand to minimize the action of gravity.

4. Hip extension (HE). In prone position, force was applied just above the posterior aspect of the knee in the direction of hip flexion. The other hand was used to help keep the knee extended and to minimize the weight of the leg.

5. Knee flexion (KF). In prone position, the force was applied just above the posterior aspect of the ankle in the direction of knee extension.

6. Knee extension (KE): The subject was seated on the stretcher with the legs hanging down, hip-width apart. The force was applied just above the anterior aspect of the ankle in the direction of knee flexion.

Statistical analysis

Descriptive data are presented as mean and standard deviation (SD). Normality of data was checked with Kolmogorov Smirnov test. The sample size was calculated using GRANMO version 7.12 (Barcelona, Spain). Accepting a 5% significance level and 80% statistical power, 14 participants were required for every group. This sample size was necessary to obtain an improvement of 7° in AROM of hip flexion. The standard deviation considered was 6.4°, based on previous studies (López-Bedoya et al., 2013). Baseline characteristics of participants were compared using independent samples Student’s t-tests. Since there were no between-group differences at baseline, a two-way (group × time) repeated-measures analysis of variance (ANOVA) with the Bonferroni post hoc test was used to evaluate group by time interactions and within-group and between-group effects. Confidence interval (CI) 95% were obtained for all differences and effect sizes (ES) were obtained in case of significant differences, interpreted by using Cohen’s d, considering a small (0.1), moderate (0.3), large (0.5), very large (0.7), and extremely large (0.9) effect (Hopkins et al., 2009). International Business Machines (IBM) SPSS statistics, version 21.0 (Chicago, IL, 224 USA) was used, and statistical significance was set at p < 0.05.

Limitations

The current study has several limitations. Firstly, the profile of the participants means that the results cannot be generalized to athletes. Future research should focus on the acute and chronic effects that VICAMS can have on athletes on these same variables. Secondly, the need for a person to apply the resistances may reduce the independence of the participants. In future research, it would be interesting to use another type of resistance that can be performed without the need for another person. Thirdly, only the effect of VICAMS on the lower body has been investigated. Acute and chronic effects on the upper body should also be studied. Also, the use of jump mats instead of force plates is a limitation. In the field of vertical jump diagnostics, force plates are the gold standard (Rogan et al., 2015), although jumping mats are inexpensive and easier to transport. Furthermore, the study lacks ecological validity for use in athletes. Finally, another suggested line of research could be the effects of VICAMS on subjects undergoing rehabilitation.