The recovery process is of particular importance to athletes who are required to perform optimally over subsequent training sessions and competitions (Rattray et al., 2015) Cold water immersion (CWI) has become well-known and the most frequently used recovery method by specialists in sports sciences, both among high performance athletes and amateur athletes, who seek to minimize fatigue and accelerate recovery processes (Calleja-González et al., 2016; Ihsan et al., 2016). In this way, several reviews of these methods (Burgess and Lambert, 2010; Versey et al., 2013; Ihsan et al., 2016) as well as meta-analyses (Leeder et al., 2012; Hohenauer et al., 2015; Sánchez-Ureña et al., 2015; Machado et al., 2016), have demonstrated the beneficial effect of these techniques on recovery, while other meta-analysis studies report no significant effect in recovery (Murray and Cardinale, 2015; Higgins et al., 2017). The literature reports that CWI includes the following physiological effects: decreased skin temperature and internal temperature (Peiffer et al., 2010), decreased acute inflammation, localized edema, and thigh muscle volume level (Vaile et al., 2008), decreased muscular pain sensation (Rowsell et al., 2011; Pointon et al., 2012; Delextrat et al., 2013; Minett et al., 2014; Sánchez-Ureña et al., 2017) and increased parasympathetic activity after exercise favoring recovery processes (Al Haddad et al., 2010; Stanley et al., 2012). It also improves the perception of recovery (Brophy-Williams et al., 2011; Stanley et al., 2012) and decreases the perception of fatigue (Rowsell et al., 2011; Delextrat et al., 2013). These studies used continuous and intermittent CWI protocols, but few studies have made a comparison between both protocols.

Neuromuscular function is the capacity to perform mechanical work as a result of muscular and nervous system function; because of the relevance of this process, the evaluation of muscle properties is fundamental to accessing the effectiveness of the recovery protocols (Halson, 2014). In the particular case of neuromuscular function and fatigue, Halson (2014) indicates that neuromuscular function can be assessed through a series of tests ranging from maximum voluntary contraction, to speed tests, countermovement jumps, or any other test that expresses aspects such as maximum force, flight time, contact time, speed of execution, and other contractile properties. Several studies have reported that CWI can accelerate the recovery of neuromuscular performance, expressed in the ability to repeat counter movement jumps (CMJs) (Vaile et al., 2008), improves performance in CMJ (Ascensao et al., 2011; Sánchez-Ureña et al., 2017), and benefits the recovery of isometric strength and muscular power (Vaile et al., 2008). Other studies indicate that CWI has no effect on neuromuscular function (De Nardi et al., 2011; Pournot et al., 2011).

As far as contractile capacities are concerned, tensiomyography (TMG) is a non-invasive method to access muscle contraction properties; a monophasic quadrangular electrical stimulation (0–110 mA) is applied to superficial muscles to assess it involuntary mechanical response, this technique provides information about muscle stiffness or muscle tone, muscle contraction time, and fatigue (Rey et al., 2012). TMG uses involuntary muscle response, contraction time, and muscle deformation as neuromuscular function indicators. TMG parameters are contraction time (Tc), indicating the muscle contraction velocity; muscle radial displacement (Dm); muscle belly radial stiffness (De Paula Simola et al., 2015); and muscle contraction velocity at 10 and 90% of Dm (V10 and V90). These represent the velocity of the muscle belly radial deformation (De Paula Simola et al., 2015). TMG parameters have been used as fatigue indicators, which correlate with gold standards of neuromuscular function as plyometric jumps, muscle force, creatine phosphokinase (CPK), and others (García-Manso et al., 2012; Hunter et al., 2012; De Paula Simola et al., 2015, 2016). For example, higher Tc indicates lower muscular speed of response due to fatigue (Hunter et al., 2012), and lower Dm represents and evaluates muscle stiffness; therefore a lower Dm indicates a high muscle tone and an excess of rigidity in muscle structures. This behavior is dependent on the sport and its characteristics (Tous-Fajardo et al., 2010).

According to the evidence, mechanical muscle alteration has been related to increased muscle stiffness, less activation of the muscle fibers, and exercise induced-muscle damage (EIMD) (García-Manso et al., 2012; Hunter et al., 2012; De Paula Simola et al., 2015). EIMD has provided an explanation for the muscle damage response following a series of eccentric contractions; these will cause various outcomes, such as prolonged loss of muscle strength and delayed-onset muscle soreness (DOMS) (Ferreira-Junior et al., 2014). The day-to-day reliability of TMG parameters has been investigated and reported as high, with values between ICC = 0.84 and ICC = 0.95 (Tous-Fajardo et al., 2010; Rey et al., 2012; Simunic, 2012; Ditroilo et al., 2013; De Paula Simola et al., 2015). Only one study has analyzed the effect of CWI on the tensiomyographic indicators (García-Manso et al., 2011). It reported that significant decreases in muscle radial displacement, but no significant differences in contraction time variable, and the acute effect of CWI on these indicators was discussed without using a pre-CWI fatigue protocol. Based on the evidence, this study’s aim was to compare the effectiveness of two CWI protocols on neuromuscular function of recovery at 24 and 48 h post-exercise.

Figure 2 shows the contrast between groups by measurement moment for the CMJ variable and the main effect into each group. The results showed no significant interaction between group and measurement moment (F_(6,111) = 0.43, p = 0.85, ωp2 = 0). After the main effects analysis, there was a significant difference between measurement moments (F_(3,35) = 14.13, p < 0.001, ωp2 = 0) as follows: pre-CG CMJ results were significantly -9.3% lower (31.2 ± 7 vs. 34.4 ± 5.8) than 0 h, -7% (32 ± 5.7 vs. 34.4 ± 5.8) and at the 48-h measurement point (p = 0.028, p = 0.030, respectively); the pre-continuous CWI group CMJ results were significantly -12.2% lower than 0 h (32.3 ± 6.7 vs. 36.8 ± 5), -6.5% lower (34.4 ± 6.2 vs. 36.8 ± 5) at 24 h and -6.5% lower (34.4 ± 5.9 vs. 36.8 ± 5) than 48-h measures (p = 0.004, p = 0.018, p = 0.033, respectively); the pre-intermittent CWI, CMJ results were significantly -14.8% lower at 0 h (31.2 ± 4 vs. 36.6 ± 5) (p = 0.001), -6.8% lower (34.1 ± 6 vs. 36.6 ± 5) at 24 h (p = 0.015), and -10.4% lower at 48 h (32.8 ± 6 vs. 36.6 ± 5) (p = 0.001). There was no main effect of group (F_(2,37) = 0.40, p < 0.67, ωp2 = 0).

Supplementary Table 1 shows the TMG rectus femoris intergroup contrasts by measurement moment. There was no TMG rectus femoris interaction between groups or measure moments for any of the TMG BF variables. These results suggest that the CWI protocol does not contribute to the TMG recovery of the rectus femoris as a neuromuscular function indicator.

Supplementary Table 2 shows the TMG biceps femoris inter groups contrast by measure moment. There was no TMG biceps femoris interaction between groups or measure moment for any of the TMG BF variables. These results suggested that the CWI protocol does not contribute to the TMG recovery of the biceps femoris as a neuromuscular function indicator.

The present study aimed to analyze the effects on recovery of two CWI protocols, including a continuous protocol (12 min of immersion at 12 ± 0.4°C) and an intermittent protocol (6 min × 2 min immersion at 12 ± 0.4°C + 1 min out of the water at room temperature) after exercise, compared to passive recovery. The results obtained in the present study indicate that immersions in cold water do not contribute significantly to the recovery of the muscular function evaluated by tensiomyographic indicators and functions such as CMJ jumping ability, regardless of the protocol used, no significant differences were found between groups on any of the variables analyzed.

The above findings were different from those reported by Sánchez-Ureña et al. (2017) (CCWI, 12 min at 12°C vs. ICWI, 4 min × 2 min: 1 min at 12°C) and Vaile et al. (2008) (CCWI, 14 min at 15°C), who identified significant differences between groups at 24 and 48 h after treatment in CMJ performance. However, these coincided with reports from most studies where this variable was analyzed, in which there was also no significant difference in CMJ jump height at post-immersion measurements 24 and 48 h later [De Nardi et al., 2011 (CCWI, 8 min at 15°C vs. ICWI, 2 min × 2 min: 2 min at 15°C) and Pournot et al., 2011 (CCWI, 15 min at 10°C)]. In particular, in the study carried out by Ascensao et al. (2011) (CWI, 10 min at 10°C), significant differences were reported between the CG and CWI group at 24 h, but in this case the values of the group of immersions were even smaller than the group of thermo-neutral immersions; that is, they reported a detrimental effect of the immersions in cold water. Recent meta-analytic studies report a low and negative effect of CWIs on the ability to jump at 24 h (ES = -0.30 [-0.96, 0.35]) (Murray and Cardinale, 2015). On the other hand (Higgins et al., 2017) also report that CWI did not show significant effect sizes in variables such as the CMJ jump at 24 h (p = 0.05, 95% CI -0.004 to 0.578).

To explain the behavior of the results obtained, Takeda et al. (2014) indicate that CMJ has usually been considered an indicator of neuromuscular performance, because after exercise-induced muscle damage, a decrease in CMJ is a result of impaired neuromuscular function and efficiency, due to the reduction of both the frequency and the intensity, by which the nerve impulse reaches the muscle. However, Pruscino et al. (2013) and Higgins et al. (2017) point out that the recovery of neuromuscular function depends not only on issues related to reducing the damage induced by exercise, but is also influenced by other physiological factors, such as muscle activation, muscle coordination, and level of fiber recruitment in the motor plate by the nervous system. Nevertheless, some studies (Rowsell et al., 2011; Higgins et al., 2013) indicate that recovery of EIMD is influenced by aspects such as the level of training and adaptive capacity.

On the other hand, a number of studies (Buchheit et al., 2011; Rowsell et al., 2011; Higgins et al., 2013) mention that the CMJ may not be sensitive enough to evaluate the recovery of neuromuscular function in trained athletes as these are influenced by psychological factors, such as a high level of motivation, competitiveness, exercise tolerance and pain tolerance, thereby allowing them to perform adequately when making maximum efforts in implementing a CMJ test.

To confirm the findings of this study, further trials are needed where these variables are analyzed; indicators of muscle function at the maximum voluntary contraction and torque, or biochemical indicators associated with muscle damage, such as CPK and lactate dehydrogenase (LDH), should be measured in addition to the subjective indicators of late-onset muscular pain (DOMS).

On the other hand, the implementation of recovery protocols performed in this study have no effect on mechanical fatigue indicators measured by the TMG, among which are Tc, Dm, V10, and V90. Dm behavior has been associated with an increase in muscle tone (García-Manso et al., 2012), and decrease in muscle fiber activation as a response to exercise induced fatigue (Hunter et al., 2012; De Paula Simola et al., 2015). In contrast, the present study shows a trend for a small decrease in the Dm behavior in the CG and García-Manso et al. (2011) reported significant differences in Dm during ICWIs (4 min × 4 min at 4°C) compared to a CG (no exercise was performed prior to the CWI/control interventions).

As for Tc (p = 0.89 in RF, p = 0.39 in BF) neither the study by De Paula Simola et al. (2015) nor the present study reported any differences over time after the exercise intervention. In contrast, De Paula Simola et al. (2015, 2016) reported that Dm, V10, and d V90 were able to detect fatigue after eccentric exercise. Despite this, in the present study, both CWI protocols were not effective in promoting superior recovery to the mechanical properties of the rectus femoris and the biceps femoris.

A possible explanation of why TMG indicators and the CMJ differ in the behavior of muscular fatigue at 24 and 48 h within the groups is that the TMG test was measured only in a muscle (rectus and biceps femoris) but the CMJ test included other muscles in the jump technique; the fatigue in these other muscles may explain the discrepancy between the TMG variables and CMJ. In future studies, it will be necessary to analyze other muscles, such as the gluteal and vastus femoris for example.

Both CWI, continuous and intermittent protocols, were ineffective for promoting superior recovery of CMJ performance and TMG muscle mechanical responses.

A single CWI recovery protocol, regardless of whether it is intermittent or continuous for 12 min, is not capable of recovering the functional and mechanical muscle properties of rectus femoris nor biceps femoris after a strenuous eccentric exercise in active men. It is necessary to explore the role of frequency and protocols (time immersion and water temperature) of CWI application during the hours and days after strenuous exercise, and it would be of much practical interest to inquire about the effects of different recovery techniques on the mechanical functions of the muscle.

BS-U: designed the research study, conducted the experiments, acquired and analyzed the data, and wrote the final version of the manuscript. DR-V: designed the research study, acquired and analyzed the data, and wrote the final version of the manuscript. RG-V: designed the research study and acquired and analyzed the data.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.