Exercise-induced muscle damage (EIMD), which is commonly associated with eccentric exercise, leads to an increase in intramuscular proteins measured in the plasma (34,35) and the delayed onset of muscle soreness (40). However, for athletes, the greatest consequence of EIMD is the prolonged decrements in the ability to produce force (8,40), which can limit subsequent training and competition performance.
Interventions that may attenuate reductions in force producing capability after EIMD have been vastly studied. The use of carbohydrate–protein supplements as a suitable intervention has received increased attention (5,6,10–12,15,30,32–34,41–43) with equivocal results. Studies demonstrating a benefit of carbohydrate–protein milk supplements on limiting decrements in force have primarily measured concentric peak torque using isokinetic dynamometry (10–12,41). The relevance of these observations to athletes is questionable as such static movements rarely occur during many sports due to the lack of high velocities and the absence of the stretch shortening cycle.
Two recent studies (12,30) have assessed dynamic sporting movements after muscle-damaging exercise and the consumption of a carbohydrate–protein supplement, both with differing results. Cockburn et al. (12) observed an attenuation of decrements in reactive strength index for 48 h with the consumption of milk-based carbohydrate–protein immediately after muscle-damaging exercise. However, Roberts et al. (30) observed no difference between the consumption of placebo, carbohydrate supplement, or carbohydrate–protein supplement on repeated sprint ability measured on a cycle ergometer after a simulated rugby union match play. However, only modest increases in creatine kinase and myoglobin were observed, and all strength and power measurements returned to near preexercise values after 24 h in all trials, indicating minimal levels of muscle damage.
To ensure that interventions that are designed to enhance recovery are applicable to an applied sport setting, there is a need to investigate measures of muscle function that relate to the dynamic sporting movements observed in many sports. Field-based team sports such as soccer, rugby, and hockey involve dynamic sporting movements and are popular throughout the world (36). Furthermore, milk is a readily available and easily accessible product that has received increased interest as a sports drink (31). Semi-skimmed milk is the most popular in the United Kingdom (38), and previous studies have demonstrated it is beneficial for recovery from EIMD (10,11). Therefore, the aim of the current study was to investigate the effects of semi-skimmed milk consumed after muscle-damaging exercise on performance tests specific to field-based team sports.
Fourteen healthy male participants (age = 24 ± 4 yr, stature = 183.1 ± 7.1 cm, mass = 79.9 ± 8.4 kg; mean ± SD) who competed in semiprofessional (Northern League) soccer volunteered to take part in the study. After institutional ethical approval, the experimental procedures and the associated risks and benefits were explained; the participants then gave their written informed consent. Participants were fully familiarized with all testing procedures before commencing the study and had no previous experience in the bout of muscle-damaging exercise.
Participants were instructed to maintain their habitual diet throughout the study and to record their food intake. In addition to recording their diet throughout the study, participants were asked to record their diet 1 d before performing the baseline Loughborough Intermittent Shuttle Test (LIST) (25) and to repeat this the day before completing the LIST after muscle-damaging exercise. Participants were required to arrive at the laboratory in the morning after an overnight fast, in a rested state, having avoided strenuous physical activity, caffeine, alcohol, and anti-inflammatory drugs for at least 48 h and having not taken any nutritional supplements in the previous 6 months.
Participants were assigned to one of two independent groups: (i) 500 mL semi-skimmed milk (MILK) and (ii) 500 mL water (CON). An independent t-test revealed no group differences in baseline participant characteristics (age and body mass), except height (P < 0.05). However, body mass index was not significantly different between groups (P < 0.05).
Before any testing, participants were required to attend a familiarization session of all the performance tests and a 15-min session of the LIST. During this session, participants completed the multistage fitness test to determine the intensity of the LIST. All participants were required to attend the laboratory for 5 d. On the first visit, participants completed baseline performance tests (in the following order: countermovement jump height, reactive strength index, 15-m sprint, and agility time) and the LIST. During the baseline performance tests, baseline active muscle soreness was recorded. Participants then reported to the laboratory a week later for four consecutive days. The first day involved providing baseline blood samples and ratings of passive muscle soreness and completing the muscle damage protocol; immediately following this, participants consumed their allocated supplement. At 24, 48, and 72 h after muscle-damaging exercise, participants returned to the laboratory to complete baseline performance tests (in the same order as the familiarization), to rate perceptions of muscle soreness, and to provide blood samples. At 48 h, participants completed the LIST.
Participants were provided with 500 mL of semi-skimmed milk (Rock Farm Dairy, Durham, UK), which provides whey and casein protein, CHO in the form of lactose, and contained 1.7% fat. Previous research demonstrated that 500 mL of milk consumed immediately after muscle-damaging exercise has the same beneficial effect as 1000 mL for limiting decrements in isokinetic muscle function and increases in creatine kinase (11). The control group was provided with 500 mL of water.
Muscle damage was induced in the hamstrings using isokinetic dynamometry (Cybex Norm; Cybex International, New York, NY). Participants completed 6 sets of 10 repetitions, with a 90-s rest between sets, of unilateral eccentric–concentric knee flexions at a speed of 1.05 rad·s−1. This protocol has been previously used in similar studies (for full details, refer to previous work [10–12]).
Multistage Fitness Test
The multistage fitness test (29) was completed to gain predictive measures of V˙O2max to calculate the running speeds required to elicit 55% and 95% of V˙O2max for performance of the LIST.
Muscle Soreness Measurement
Participants were required to rate passive soreness (when standing) and active soreness of the hamstrings (during all performance measures) on a visual analog scale. The level of perceived muscle soreness in the hamstrings, combined for both legs, was rated from 0 (no pain/soreness) to 10 (pain/soreness as bad as it could be). This scale has been previously used to determine muscle soreness (10–12).
Vertical jump height is commonly used when assessing performance in team-based field sports (14,19) and has been shown to decrease after muscle-damaging exercise (8). Using a force plate (Kistler Instrumente AG, Winterthur, Switzerland), participants were required to place their hands on their hips to minimize the impact of arm swing and, in one flowing movement, to bend their knees to approximately 90° and then jump vertically for maximum height. The participant’s jump height was calculated from flight time, and the mean of two jumps was used for the analysis. The coefficient of variation for this protocol from reliability trials conducted in Northumbria University Laboratories is 1.9%.
Reactive strength index.
Fast reactive strength index is a measure of an athlete’s ability to use the stretch-shortening cycle (44) and provides a measure of dynamic muscle actions that can be related to sports involving running and jumping. The assessment of reactive strength index has been previously used in similar studies (for full details, refer to previous work ).
Speed is a fitness prerequisite for field-based team sport athletes (14,19) and has been shown to decrease after EIMD (16). To assess this parameter, a 15-m sprint was used, as it has been reported to be the average distance per sprint covered during soccer match analysis (37). Timing gates (Brower Timing Systems, Draper, UT) were placed at 5-m intervals to obtain split times. Participants were instructed to sprint through all timing gates as fast as possible from a standing start positioned 30 cm behind the first timing gate. Participants completed this twice with a rest period of 1 min and 20 s, and the mean of two performances was used for analysis. The coefficients of variation for this test, calculated from reliability trials conducted at Northumbria University, are between 0.5% and 1.5%.
During field-based team sports, players are continually involved in sudden directional changes in order to be effective during a match (23), termed agility. Agility has been shown to be reduced after EIMD (16). The t-test involves a series of forward, backward, and lateral movements and acceleration and deceleration to navigate a T-shaped course marked by cones. These movements are involved during field-based team sports, and this test has been shown to measure a combination of components, including leg speed, leg power, and agility, and may be used to differentiate between levels of sports participation (28). Participants were required to forward sprint for 9.14 m, side step to the left for 4.57 m, side step to the right for 9.14 m, side step to the left for 4.57 m, and back peddle 9.14 m through the timing gates. Participants were instructed to complete this in the quickest time possible, adhering to the following rules: must face forward at all times; during side stepping, legs must not cross; each side cone must be touched using the hand; and when returning to the central cone, participants must arrive level with it before running backward through the timing gates. Nonadherence to these rules resulted in that measurement being removed from analysis. A mean of two t-tests was used for analysis, and the coefficient of variation for this protocol calculated from reliability trials conducted in Northumbria University laboratories is 0.6%.
Loughborough Intermittent Shuttle Test
The LIST is a 90-min shuttle test, involving both high-intensity and endurance exercise, designed to elicit the physiological demands of field-based team sports (refer to previous work for full details ). The LIST has been shown to be reproducible (25) and therefore is a valuable tool to investigate the effects of muscle damage on field-based team sports.
Each sprint throughout the LIST was timed using timing gates (Brower Timing Systems) set up at 0 and 15 m, which were used to calculate the overall mean sprint time during 90 min of the LIST. Heart rate was continually measured throughout the LIST using a telemetry heart rate monitor (Polar Electro, Oy, Finland). Lastly, after each 15-min cycle, participants were asked to rate their perceived levels of exertion on a rating of perceived exertion (RPE) scale (7). A mean of six cycles was used for analysis. During the LIST, participants were allowed to drink water ad libitum, the volume of which was recorded. During subsequent testing, participants were asked to consume the same volume.
Blood Sample Collection and Analysis
Serum CK and Mb concentrations were determined in duplicate from blood samples collected via venipuncture from a forearm vein into a serum gel monovette (9 mL). Total CK activity was analyzed using kinetic UV tests (Olympus analyzer; Olympus Diagnostica, GmbH, Hamburg, Germany). This method is based on the International Federation of Clinical Chemistry reference method. Olympus Diagnostica reported the intra- and the interassay coefficients of variation for this system at 0.64%–2.37% and 3.2%–4.55%, respectively. Myoglobin was analyzed using an assay kit (Myoglobin Enzyme Immunoassay Test Kit; Oxford Biosystems Ltd., Wheatley, Oxon, UK). Absorbance was read using an Anthos 2010 Microplate reader (Anthos Labtec Instruments, Salzberg, Germany). Intra- and interassay coefficients of variation for Mb assays were 3.9%–6.6% and 5.2%–11.8%, respectively.
The current study used statistical analysis that reports the uncertainty of outcomes as 90% confidence intervals, making probabilistic magnitude-based inferences about the true values of outcomes using the methods described by Batterham and Hopkins (4). The authors have previously used this method to determine the effect of the independent variable on the dependent variables (11,12). Each dependent variable was analyzed using a published spreadsheet (17) to determine the effect of the independent variable as the difference in the change between each group. The analysis of dependent variables was conducted on log-transformed values to overcome heteroscedastic error (24), except muscle soreness data. This variable was not log-transformed because it is inappropriate due to interval scaling (24). Participant descriptive data and muscle soreness data are presented as absolute mean ± SD. Mean values derived from the analysis of log-transformed variables were back-transformed to provide mean percentage change and percentage SD, except intramuscular protein values, which were reported as factors due to the large percentage changes.
For the calculation of the chances of benefit and harm, the smallest worthwhile or important effect for each dependent variable was the smallest standardized (Cohen) change in the mean: 0.2 times the between-subject SD for baseline values of all participants (4). Practical inferences were drawn using the approach identified by Batterham and Hopkins (4). Quantitative chances of benefit and harm were assessed qualitatively: <1%, almost certainly not; 1%–5%, very unlikely; 5%–25%, unlikely; 25%–75%, possibly; 75%–95%, likely; 95%–99%, very likely; and >99%, almost certainly (18).
P values for the interaction effect between time and group, determined using a one-way ANOVA with repeated measures, have also been stated. An independent t-test was used to determine P values for LIST measurements.
There were no differences between groups in total energy intake or macronutrient content of the diets.
Both groups demonstrated an increase in passive muscle soreness and active muscle soreness, for all performance measures, up to 48 h. At 72 h, muscle soreness began to return to baseline levels. For changes in all measures of muscle soreness, effects at both time points were unclear. The P values for all main interaction effects were 0.643, 0.860, 0.744, 0.973, and 0.944 for muscle soreness measured passively and during a countermovement jump, drop jump, 15-m sprint (Fig. 1), and t-test, respectively. A summary of the statistical analysis is shown in Table 1.
A summary of the statistical analysis for all muscle performance measures is shown in Table 2.
Baseline jump height values for the control and milk groups were 32.8 cm and 33.2 cm, respectively. Jump height decreased for both groups for 72 h. Changes between baseline and 48 h for the control and milk groups were −6.9% ± 6.3% and −5.5% ± 6.0%, respectively. Between baseline and 72 h, changes in jump height were −9.0% ± 9.2% and −7.0% ± 3.4% for the control and milk groups, respectively. All effects for both time points were unclear. The P value for the main interaction effect was 0.695.
Reactive strength index.
Reactive strength index at baseline was 124 and 126 cm·s−1 for the control and milk groups, respectively. At 48 h, reactive strength index decreased; at 72 h, it reached a plateau. Changes between baseline and 48 h for the control and milk groups were −18.2% ± 20.1% and −22.8% ± 22.6%, respectively. Between baseline and 72 h, changes in reactive strength index were −16.3% ± 12.5% and −21.5% ± 14.8% for the control and milk groups, respectively. All effects for both time points were unclear. The P value for the main interaction effect was 0.223.
At baseline, the time taken to cover 5 m was 1.14 and 1.11 s for the control and milk groups, respectively. Between baseline and 48 h, there were no clear effects of milk consumption (1.4% ± 3.2%) in comparison with the control (4.5% ± 7.4%) for attenuating increases in time to cover 5 m. This was the same for changes between baseline and 72 h, where the change in the control group was 4.8% ± 4.8% and in the milk group was 3.4% ± 5.3%. The P value for the main interaction effect was 0.748.
At baseline, the time taken to cover 10 m was 1.89 and 1.86 s for the control and milk groups, respectively. Between baseline and 48 h, there was a possible benefit of consuming milk (1.7% ± 1.8%) in comparison with the control (5.0% ± 4.8%) for blunting increases in the time to cover 10 m. The effect was unclear for changes between baseline and 72 h, where the change in the control group was 5.2% ± 2.1% and in the milk group was 4.0% ± 1.9%. The P value for the main interaction effect was 0.860.
At baseline, the time taken to cover 15 m was 2.57 and 2.53 s for the control and milk groups, respectively. Between baseline and 48 h, there were no clear effects of milk consumption (3.5% ± 2.5%) in comparison with the control (5.1% ± 4.0%) for attenuating increases in the time to cover 15 m. There was a possible benefit of consuming milk (2.6% ± 2.5%) in comparison with the control (5.9% ± 4.3%) for blunting increases in the time to cover 15 m between baseline and 72 h (Fig. 2). The P value for the main interaction effect was 0.720.
Baseline values for agility time were 10.61 and 10.31 s for the control and milk groups, respectively. Between baseline and 48 h, there were no clear effects of the milk group (2.0% ± 3.4%) in comparison with the control group (3.4% ± 3.5%) for attenuating increases in agility time. There was, however, a likely benefit of milk consumption (0.7% ± 3.9%) in comparison with the control (4.8% ± 3.1%) for limiting increases in agility time between baseline and 72 h. The P value for the main interaction effect was 0.086.
Loughborough Intermittent Shuttle Test
During the LIST, heart rate, RPE, and mean 15-m sprint time were recorded. Table 3 shows a summary of the statistical analysis.
The mean heart rate during the LIST before muscle-damaging exercise was 158 and 150 bpm for the control and milk groups, respectively. There was no clear effect of the milk group (4 ± 14 bpm) in comparison with the control group (−4 ± 9 bpm) for attenuating changes in mean heart rate. The P value for the main interaction effect was 0.533
The mean RPE during the LIST before muscle-damaging exercise was 16 and 15 for the control and milk groups, respectively. There was no clear effect of consuming milk (1 ± 1) in comparison with the control group (0 ± 2) for attenuating changes in mean RPE. The P value for the main interaction effect was 0.197.
Mean 15-m sprint.
The mean time to cover 15 m during 90 min of the LIST before muscle-damaging exercise for the control and milk groups was 2.93 and 2.86 s, respectively. There was a likely benefit of the milk group (0.0% ± 2.0%) in comparison with the control group (2.4% ± 1.9%) for attenuating increases in the mean time to cover 15 m during the LIST. The P value for the main interaction effect was 0.009.
Intramuscular proteins in the serum.
A summary of the statistical analysis for CK and Mb is shown in Table 4.
The mean baseline CK values for the control and milk groups were 321 ± 224 and 174 ± 40 U·L−1, respectively. Baseline CK values for the control group are relatively high, but as results are analyzed as the difference between groups in change over time, this would not alter results. For both groups, CK increased during the 72 h. There were no clear effects of the milk group (5.0 ×/÷ 3.1) in comparison with the control group (1.5 ×/÷ 2.3) for limiting increases in CK between baseline and 48 h. Between baseline and 72 h, there were no clear effects of the milk group (11.4 ×/÷ 7.0) in comparison with the control group (4.9 ×/÷ 5.2) for blunting increases in CK. The P value for the main interaction effect was 0.655.
Mean baseline Mb values were 41.8 ± 28.5 and 34.1 ± 8.7 ng·mL−1 for the control and milk groups, respectively. Both groups showed an increase in Mb up to 72 h. Between baseline and 48 h, changes in Mb were 1.4 ×/÷ 2.6 and 1.3 ×/÷ 3.3 for the control and milk groups, respectively. Changes between baseline and 72 h in the control and milk group were 3.1 ×/÷ 4.1 and 1.9 ×/÷ 3.4, respectively. There were no clear effects at any time point. The P value for the main interaction effect was 0.549.
The primary finding of this study was that consuming milk immediately after muscle-damaging exercise limited decrements in measures of dynamic sporting movements that are necessary for performance in field-based team sports. A benefit was observed for the time to cover 10 and 15 m, agility time, and mean 15-m sprint performance during the LIST. There was no benefit of milk consumption on active and passive muscle soreness, increases in intramuscular proteins in the serum, reactive strength index, countermovement jump height, and RPE and heart rate measured during the LIST.
The finding of attenuated muscle function is in agreement with previous research (10–12,41). However, previous research has measured muscle function via isokinetic dynamometry, which has limited external validity when extrapolating to a sporting context (3). The current study has demonstrated a benefit for dynamic sporting movements that are applicable to field-based team athletes and therefore have greater external validity.
The ingestion of milk may have limited increases in myofibrillar protein degradation that may have maintained the force transmitting and/or force generating protein structures. This would have allowed subsequent performance to occur at closer to optimal levels. These structural factors are only one hypothesized reason why decrements in muscle function are observed after muscle-damaging exercise and is a likely mechanism underlying the attenuation of one-off performance. A reduction in glycogen resynthesis has also been observed after muscle-damaging exercise (45). Muscle glycogen is important for performance in intermittent sports, and therefore, the impairment of glycogen resynthesis will limit performance (9). The LIST replicates the physiological demands of field-based team sports (25), and muscle glycogen is depleted during the LIST (26). Therefore, if muscle glycogen is primarily used during the LIST and its resynthesis is inhibited via muscle damage, the ability to perform repeated sprints will be reduced. The intake of milk may have limited these changes. The ability to resynthesize glycogen may have been inhibited by reduced glucose uptake into the muscle cell due to inflammatory processes (20), decreased insulin sensitivity due to disruption of the muscle cell membrane (13), and reduced glucose transport via GLUT4 (2). These processes may be positively influenced by the intake of milk as it contains both CHO and protein. However, this is purely speculative as there is a lack of available data investigating the effect of milk on these processes.
There was no effect of milk on RPE and heart rate during performance of the LIST. Muscle-damaging exercise has been shown to increase RPE during endurance exercise performance 48 h after exercise (39). An increase in RPE is implicated in reduced performance (39). RPE did not increase in either group; therefore, changes in performance were not due to the participant’s perceptions of exercise intensity. Heart rate did not change before and after muscle-damaging exercise; therefore, the participant’s ability to exercise at a relative intensity was not altered due to EIMD.
No benefit of milk for limiting increases in passive and active muscle soreness was observed. During measurement of either aspect of muscle soreness, the hamstrings were not isolated, which may have affected individual perceptions of muscle soreness. Furthermore, milk may not affect processes leading to delayed onset of muscle soreness. The lack of effect of carbohydrate–protein supplementation on creatine kinase and myoglobin is in contrast to previous research (5,10–12,32–34,41). However, creatine kinase is a highly variable indirect marker of EIMD (27), and myoglobin should be used with caution (35). As this study has a strong applied focus, this finding is of minor concern as changes in intramuscular proteins are likely to be functionally irrelevant.
In contrast to previous findings (12), there was no benefit of milk consumption on reactive strength index. There was also no benefit to countermovement jump height. During jumping activities, the hamstrings do not play a significant role in performance; therefore, if milk ingestion does limit myofibrillar damage, then an effect on jumping performance would not be observed. This is due to other major muscle groups masking the effects. During sprinting activities, the hamstrings are used to a greater extent than the quadriceps (21), which may explain why benefits to this aspect of performance were observed. However, there was no effect of milk consumption on the time to cover 5 m. The EMG activity of the hamstrings increases as speed increases (22); therefore, during the initial 5 m, other lower limb muscles may have a more predominant role in performance.
In conclusion, this study has demonstrated that 500 mL of milk consumed immediately after muscle-damaging exercise possibly limits decrements in one-off sprinting performance and likely reduces increases in agility time and the ability to perform repeated sprints during the physiological simulation of field-based team sports. Athletes experiencing muscle damage may be able to limit performance decrements during subsequent training or competition through the acute intake of milk. During the competitive season, this is important because in many field-based team sports, the match day does not always occur on the same day each week (1), and on many occasions several matches take place in 1 wk. The findings of this study can be used to help inform recovery strategies of coaches, athletes, and sport scientists. However, it must be acknowledged that performing the LIST at 48 h may have affected on the results at 72 h, which is a limitation of the study.
No funding was received for this work. The authors declare no conflict of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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