The unique surface characteristics of sand, along with the accessibility of various natural (beach) and artificial (indoor and outdoor) sand surfaces around the world, make sand a popular and viable training resource in a range of sport settings. Specifically, sand could be used as an alternative training surface (to more traditional venues such as grass) by recreational and elite team sport athletes for not only conditioning purposes but also for recovery and rehabilitation sessions. Although there is currently a large amount of anecdotal evidence in support of sand training (18,20,24,32), specific data relating to the value of exercising on a sand surface remains largely unknown.
To date, the majority of existing research has examined the energetics of running on sand during short bouts of steady-state exercise (i.e., 10 minutes) (19,26,27,37), showing that running on sand requires approximately 1.2–1.6 times the total energy cost (EC) of running at comparable speeds on a firmer surface, such as grass. A higher EC when running on sand has been attributed to many factors, including a reduction in the recovery of elastic energy (37), a decrease in muscle-tendon efficiency, and an increase in work lost to the environment (19). In addition, research by Pinnington et al. (28) identified a significantly greater contribution of the lower leg muscles when running on sand, partly because of an increased need for stabilization around the hip, knee, and ankle joints during the stance phase of running gait. The increased EC experienced on sand also results in a greater lactate accumulation (2–3 times) compared with running on firmer surfaces at similar speeds (26,27). However, despite this evidence, the effect of running on sand at higher speeds (>11 km·h−1) and for longer durations (>10 minutes) is still unclear.
Additionally, it has also been shown that training on sand can reduce the stress placed on the musculoskeletal system during exercise, limiting the degree of exercise induced muscle damage and associated negative side effects such as increased muscle soreness and reduced performance capacity (16,23). Similarly, the high impact forces encountered during exercise on a firm surface can result in the destruction of red blood cells (RBCs), otherwise known as exercise-induced hemolysis (22,25,33). Research has demonstrated that heel strike is a major cause of hemolysis during running (22,33) and that training variables (such as the surface type trained upon) may influence the amount of hemolysis experienced (25). Exercise-induced hemolysis and inflammation has also been shown to increase with exercise intensity (13,25); however, it is still unclear as to what impact training surface may have on this response. Peeling et al. (25) failed to show any significant difference in hemolysis or inflammation between grass and bitumen training surfaces during endurance running. However, the peak deceleration forces (measured using a Clegg impact hammer) experienced on sand (26,27) are significantly lower than both grass and bitumen (25) and may potentially reduce the degree of impact force experienced at heel strike to a greater degree.
With this evidence in mind, the aim of this study was to investigate any benefits that may be associated with sand training in a team sport setting. Specifically, this study focused on the use of sand during a preseason conditioning program, where the higher relative EC and reduced loading force experienced during exercise on sand (vs. grass) may allow for a more optimal athlete preparation leading into the competitive season. Therefore, this study investigated the acute effects of sand vs. grass training surfaces on acute physiological outcomes and recovery profiles of team sport athletes after a common preseason interval running training session. It was hypothesized that the completion of such a training session on a sand (vs. grass) surface would result in greater training intensities and physiological responses. Furthermore, it was also hypothesized that sand training would lead to lower hemolytic and inflammatory responses, accompanied by a reduced degree of muscle damage and soreness, and a smaller decrement in 24 hours postexercise athletic performance.
Experimental Approach to the Problem
This study was designed to compare the effect of sand and grass training surfaces on the training intensities experienced during an interval training session, and the physiological, perceptual, and physical markers of recovery within the 24 hours postexercise period. To achieve this, the athletes were required to complete 5 separate testing sessions in a repeated measures design, including 3 performance trials, and 2 interval-training sessions (one each on SAND and GRASS). The athletes were part of a larger training squad and therefore participated in all 5 sessions together, with the 2 surface trials conducted in a randomly selected order. Initially, the athletes undertook a baseline testing session (BASE) for the collection of a number of performance measures, including vertical jump (VJ), a repeated sprint ability (RSA) test, and a 3-km running time trial (RTT). These measures were used as a reference point, allowing for an assessment of performance recovery at 24 hours postexercise after both of the surface trials. After the collection of these BASE measures, the athletes were required to complete the 2 surface trials (sand and grass) in consecutive weeks, with each of these trials followed 24 hours later by a repeat of the BASE performance tests. This arrangement allowed for a comparison of training surface effects on subsequent recovery and performance at 24 hours postexercise. In addition to these physical measures of recovery, various blood markers were examined to assess the level of muscle damage, inflammation and hemolysis present at the preexercise, postexercise and 24 hours postexercise time points to further examine any trial differences throughout the 24 hours postexercise period. Finally, for a comparison of the training intensities achieved on both sand and grass surfaces, various physiological and perceptual variables (blood lactate [BLa], heart rate [HR], and ratings of perceived exertion [RPEs]) were also measured.
Determination of sample size was attained via a power analysis using a customized computer software program (GPOWER Version 2.0, Department of Psychology, Bonn University, Bonn, Germany). The effect sizes used to generate the sample size estimation were attained from previous investigations exploring variables similar to the proposed research (1,17,25). For this study, a sample size of 9 was recommended to yield a power of 0.85 at a significance level of 0.05. Therefore, 10 well-trained team sport athletes were recruited for participation in this investigation. Seven male athletes (Age: 22.6 ± 2.9 years; Mass: 82.9 ± 6.3 kg; Height: 182.3 ± 4.5 cm) and 3 female athletes (Age: 19.7 ± 2.5 years; Mass: 69.3 ± 14.9 kg; Height: 175.5 ± 11.0 cm) were recruited from the Western Australian Institute of Sport Hockey and Netball programs, respectively. At the time of the investigation (summer season), all the athletes were in the general preparation phase of their preseason training cycle. The participants were informed of the requirements and risks associated with their involvement in this study, before written consent acknowledging these details was obtained. Institutional Review Board (IRB) approval for the use of human subjects during this investigation was granted by The University of Western Australia's Human Research Ethics Committee (IRB # RA/4/1/4373).
Before commencing testing, all the athletes completed a 4-week period of sand familiarization. During this time, they attended 2 beach (sand) sessions a week, starting with light jogging and walking, and building up to higher intensity activity, which was ultimately similar to that performed during the ensuing experimental trials. Such an approach ensured that all the athletes had an equal exposure to sand training leading into the experimental trials, thereby reducing any variance arising from unfamiliarity with the training surface.
The athletes were instructed to attend all testing sessions in a well-rested and hydrated manner. Before each session, the athletes were instructed to refrain from consuming alcohol and caffeine, or from participating in intensive exercise for the previous 24 hours. In addition to these restrictions, the participants also recorded and replicated their nutritional habits during the 12 hours before each testing session. This included the consumption of no food or beverage other than water within 2 hours of each trial. Water was allowed ad libitum through each testing session, up to a maximum of 1 standard sized drink bottle (600 ml), but no other food or fluid was permitted. Furthermore, to control for arousal levels during testing, a consistent level of encouragement was provided to the athletes by the same researchers, especially during maximum efforts. All testing sessions were run at the same time of the day to avoid any diurnal influences of circadian rhythm. The SAND session was conducted on soft dry beach sand, on a level area of beach removed from the water's edge. The regional characteristics of the beach can be defined as carbonate rich, fine to medium grain sand (ø ∼ 0.4 mm) (29). The GRASS session was conducted on a well-maintained sporting ground (Kikuyu grass). The athletes were barefoot during the SAND trial, in comparison to GRASS where they wore running shoes. The mean ambient air temperature and relative humidity recorded during the SAND and GRASS trials were 28.5 ± 1.6° C and 47.8 ± 2.3%, respectively; and were 29.6 ± 1.1° C and 50.4 ± 11.4% during the baseline, and 24 hours postexercise performance trials.
Before commencing the tests, the athletes completed a 10-minute warm-up consisting of a 4-minute continuous jog at a moderate intensity, followed by a 3-minute period of self-selected stretching of at least 3 different muscle groups, and 3 minutes of dynamic run-through exercises (i.e., high knees, bounding, etc.). A 3-minute recovery period followed, after which participants entered the gymnasium for the VJ and RSA tests. Before the VJ test, each athlete had their body mass assessed (±50 g; UC-300 Precision Health Scale; A & D Company Ltd., Milpitas, CA, USA). After this, their maximal reach height was recorded using a commercially available yardstick (Swift Performance Equipment, Wacol, Qld, Australia), before they were required to perform a stationary countermovement jump, with the instruction to then displace the highest possible vane of the yardstick. The maximum absolute jump height was recorded after the athlete failed to advance in 2 consecutive jumping attempts, with each jump separated by a minimum of 10 seconds. Absolute jump height was recorded to the nearest 1 cm, and relative jump height determined as the difference between absolute jump height and maximal reach height. Leg power was then calculated in accordance with previously used methods (12). The reliability of this yardstick and VJ testing procedure has previously been investigated in our laboratory, yielding a typical error of measurement (TE) of 1.2 cm.
After a 5-minute rest, the athletes then performed the RSA test. This test required each participant to complete 8 × 20-m maximal sprints, separated by 20 seconds of active recovery in which they made their way back to the same start point (2). Sprint times were recorded using electronic timing gates (±0.01 seconds; Swift Performance Equipment, Lismore, NSW, Australia) placed at the zero- and 20-m marks of a straight-line track. Three performance indices were calculated; the fastest 20-m sprint time, the total sprint time for all 8 sprints, and percentage decrement score across the 8 sprints as per (10). Total sprint time was taken as the primary measure of the athlete's performance, and the reliability of this measure among elite team sport athletes has previously been reported as a TE of 0.42 seconds (2). After another 5 minutes of rest, the athletes then moved outside for the 3-km RTT. This maximal effort test consisted of two 1.5-km laps along a straight footpath (pavement). The reliability of a 3-km time trial performance among elite team sport athletes in similar outdoor conditions has previously been reported as a TE of 24 seconds (11). Blood lactate and RPEs were measured immediately after both the RTT and RSA test. The HR was also monitored throughout both of these tests.
Interval Training Session
Before each of the interval training sessions, the athletes provided a preexercise venous blood (Vb) sample. They then performed a 10-minute warm-up on the training surface, consisting of 4-minute continuous jogging at a moderate intensity, followed by 3 minutes of self-selected stretching of at least 3 different muscle groups, and 3 minutes of dynamic run-through exercises. A 5-minute recovery followed the warm-up, in which time BLa and RPE were recorded. The interval training session consisted of 3 identical interval sets, each separated by 5 minutes of rest. The interval workout, which is displayed here as a work to recovery ratio (W:R), included 3 sets of [2 × 45:90 seconds, 3 × 20:60 seconds, and 2 × 15:45 seconds].
The athletes performed each effort at maximum perceived intensity that could be maintained for the entire effort. An active recovery period (walking or light jogging) followed each work effort. The HR was monitored throughout the entire session, with BLa and RPE taken immediately postexercise. Additional measurements of RPE were taken in between the 3 interval sets. After the session, a 6-minute cooldown was conducted, consisting of a 3-minute jog and 3 minutes of stretching. A postexercise Vb sample was taken within 15 minutes of finishing the cooldown, after which athletes left the testing venue, with the instruction to perform no further exercise or recovery procedures (compression, ice bath, massage, etc.) for a 24-hour period.
At 24 hours postexercise, another Vb sample was taken, before the athletes again performed the performance tests (i.e., VJ, RSA, and RTT). Additionally, subjective perception of delayed onset muscle soreness (DOMS) was measured preexercise via a perceived muscle soreness questionnaire (1).
Ground Surface Stiffness Determination
Surface stiffness for SAND and GRASS trials was determined immediately before each interval training session using a 2.25-kg Clegg impact hammer (SD Instrumentation, Wiltshire, United Kingdom) dropped from a height of 0.457 m. On each occasion, 10 samples (spread over the entire training area) were taken to determine the peak impact deceleration forces exerted by the training surface. Additionally, for the SAND trial, the samples were taken posttraining in areas of heavy use to quantify any compacted sand areas. The peak deceleration force (newtons) of each training surface was calculated in accordance with previously used methods (25–27).
Blood Sampling and Analysis
Earlobe blood samples were taken after the site had been cleaned with a sterilized alcohol swab, and the first blood droplet discarded. Samples were analyzed for plasma lactate concentration (BLa) using a lactate test meter (Lactate Pro; Arkray, KDK, Kyoto, Japan) with a coefficient of variation (CV) of 5.7% (31). Venous blood was collected via venipuncture of a forearm antecubital vein, after the athletes had laid down for 5 minutes to control for postural shifts in plasma volume (25). In accordance with previously used methods, a 21-gauge needle, together with minimal stasis and a gentle movement of the syringe plunger were employed to avoid unnecessary hemolysis in the blood collection process (25,33). Blood was then deposited down the side of two 8.5 ml SST II Gel collection tubes (BD Vacutainer, Franklin Lakes, NJ, USA) with the vacuum seals removed. The samples were left to clot for approximately 60 minutes at room temperature, after which they were centrifuged at 10° C and 3,000 rpm for 10 minutes. Subsequently, the serum supernatant was divided into 1-ml aliquots and stored at −80° C until further analysis. Duplicate samples were then analyzed for markers of muscle damage (creatine kinase [CK] and myoglobin [Mb]), inflammation (high sensitivity C-reactive protein [CRP]) and hemolysis (serum haptoglobin [Hp]). Serum Hp, and Mb concentrations were analyzed at preexercise and postexercise time points only, CRP concentrations at the preexercise and 24 hours postexercise time points, and CK concentrations at preexercise, postexercise, and 24 hours postexercise time points. Blood analysis was conducted at PathWest Laboratory Medicine, Royal Perth Hospital (Perth, WA, Australia). The CV for CK determination at 138 and 450 U·L−1 were 1.8 and 1.7%, respectively; for Mb at 50.0, 87.8, and 213.0 ng·mL−1 were 13.8, 5.9, and 3.9%, respectively; for CRP at 0.62 and 4.55 mg·L−1 were 8.35 and 7.69%, respectively; and for Hp at 0.65 and 2.58 g·L−1 were 8.67 and 9.93%, respectively.
Heart rate was monitored at 5-second intervals throughout the testing sessions via the use of short-range radio telemetry (Polar Team Sport System, Polar Electro Oy, Kempele, Finland).
Ratings of Perceived Exertion
Ratings of perceived exertion were collected using the Borg scale (5). The scale incorporated the anchor points, 6 = No exertion at all, through to 20 = Extreme exertion.
Delayed Onset Muscle Soreness
Delayed onset muscle soreness was measured via a muscle soreness questionnaire, where the athletes identified the degree of perceived muscle soreness of an entire muscle area on a scale from 0 (normal; without stiffness or pain) to 10 (very painful) (1). The areas rated for soreness were the quadriceps, hamstrings, gluteals, triceps surae, and tibialis anterior/peroneal muscles.
All results are expressed as mean with 95% confidence intervals (±95% CI), or SEM for clarity of figures. The effect of each training surface condition on resultant training intensities was analyzed by a repeated measure analysis of variance (ANOVA) of the time and trial differences in BLa, HR, and RPE experienced on both SAND and GRASS surfaces. To determine the effect of training surface on recovery at 24 hours postexercise, a series of 1-way ANOVAs were used to analyze any differences in DOMS ratings and performance measures (VJ, RSA, and RTT) between the 2 training surfaces, including a comparison to BASE measures. Finally, a repeated measure ANOVA was used to analyze the time and trial differences in blood variables, to identify any further physiological differences throughout the 24 hours postexercise period. Post hoc (Fisher's least significant difference), paired samples t-tests were used in the event of a main effect to determine specific differences between the surface trials. The alpha level was set at 0.05. Finally, trends in performance were interpreted using Cohen's d effect sizes. However, only effect sizes >0.50 were reported here. The thresholds for the qualitative descriptors of these effects were set at 0.50–0.79 as ‘moderate,’ and ≥0.80 as ‘large’ (7).
Ground Surface Conditions
Surface stiffness measures showed peak impact deceleration forces recorded on SAND (388.5 ± 56.3 N) were significantly lower (p < 0.001) than for GRASS (1,240.5 ± 160.8 N).
Physiological and Perceptual Responses to Training
A significant time effect (p < 0.001) showed that BLa increased from post-warm-up to posttraining in both SAND and GRASS (p < 0.05; Table 1). Furthermore, significant trial (p < 0.001) and time × trial interactions (p = 0.006) revealed that both BLa readings were significantly higher on SAND (p < 0.05), and that the relative change in BLa was also greater on SAND (p = 0.006).
Ratings of Perceived Exertion
A significant time effect (p = 0.006) showed that RPE increased over both SAND and GRASS, with each measure higher than the previous time point (p < 0.05; Table 1). The trial effect (p = 0.051) and time × trial interaction (p = 0.053) for RPE approached significance, with large effect sizes suggesting RPE was higher on SAND after the warm-up (d = 1.15), after the second and third interval sets (d = 1.03 and d = 0.86, respectively) and also for the average session RPE (d = 0.89).
A significant time effect (p < 0.001) showed that the HR was higher during the second and third interval sets when compared with the first, for both SAND and GRASS (p < 0.05; Table 1). Furthermore, significant trial (p = 0.001) and time × trial interactions (p = 0.006) showed that HR during the warm-up, and through all 3 interval sets was significantly higher on SAND than for GRASS (p < 0.05), as was the average HR for all 3 interval sets (p = 0.009).
No significant differences were observed between trials for relative VJ height or leg power (p > 0.05; Table 2).
Repeated Sprint Ability
No significant differences were observed between trials for fastest 20-m sprint time, but total sprint time (for all 8 repetitions) was significantly greater after GRASS compared with BASE (p = 0.015; Table 2). A moderate effect size (d = 0.71) also showed a greater percentage decrement in sprint performance for SAND compared with BASE (p = 0.082). Furthermore, BLa recorded after RSA was significantly lower after GRASS compared with BASE (p = 0.028). No differences were observed between trials (p > 0.05) for the HR recorded during RSA.
The 3-km Running Time Trial
The RTT was performed significantly faster after SAND compared with GRASS (p = 0.001; Table 2), but neither trial was different to BASE (p > 0.05). Moderate effect sizes suggested that BLa accumulation during RTT was higher at BASE when compared with both SAND (d = 0.58) and GRASS (d = 0.51).
Delayed Onset Muscle Soreness
The 24 hours postexercise DOMS rating for the hamstring muscle group was significantly higher after GRASS (4 ± 1) compared with SAND (3 ± 1) (p = 0.049). Additionally, a moderate effect size (d = 0.50) showed that DOMS rating for the triceps surae muscle group was higher after SAND (5 ± 1) compared with that after GRASS (4 ± 1). However, there were no differences observed in any of the other muscle groups (p > 0.05). Furthermore, no significant differences existed in total DOMS rating between SAND (15 ± 6) and GRASS (16 ± 6) (p > 0.05).
A significant time effect (p < 0.001) showed that Mb increased from pretraining to posttraining in both SAND and GRASS (p < 0.05; Figure 1A). A significant trial effect (p = 0.009), but not a time × trial interaction (p = 0.427) existed between the 2 training surfaces. Pretraining and posttraining readings for Mb were higher on SAND (p < 0.05), but no differences were observed between trials for relative change in Mb (p = 0.427).
A significant time effect (p = 0.003) showed that Hp levels were decreased after exercise on both SAND and GRASS (p < 0.05; Figure 1B). No significant trial (p = 0.435) or time × trial interactions (p = 0.560) existed between the 2 training surfaces, although a moderate effect size (d = 0.60) suggested that the change in Hp from pretraining to posttraining was greater on GRASS.
A significant time effect (p = 0.026) revealed that CRP levels were increased from preexercise to 24 hours postexercise on both SAND and GRASS (p < 0.05; Figure 1C). However, no significant trial (p = 0.525) or time × trial interactions (p = 0.856) were recorded between the 2 training surfaces.
No significant time effects (p = 0.084) existed for changes in CK on either training surface; however, a large effect size suggested that CK increased from preexercise to postexercise (d = 3.21) on GRASS (Figure 1D). Furthermore, no significant trial effects (p = 0.101) or time × trial interactions (p = 0.762) for CK concentrations were recorded between training surfaces, although moderate effect sizes suggested the preexercise (d = 0.68), postexercise (d = 0.57), and 24 hours postexercise (d = 0.51) CK levels were higher on SAND. Finally, no differences were observed between trials for relative change in CK from preexercise to postexercise (SAND: 79 ± 16, GRASS: 84 ± 24 U·L−1), from preexercise to 24 hours postexercise (SAND: 26 ± 95, GRASS: 56 ± 92 U·L−1), or from postexercise to 24 hours postexercise (SAND: −53 ± 101, GRASS: −28 ± 95 U·L−1) (p > 0.05).
The absorptive qualities of a sand training surface can contribute to a significantly greater EC, and reduced loading forces experienced during exercise when compared with a firmer and more conventional training surface such as grass (16,19,23,26–28,37). This study showed that BLa and HR responses were significantly higher during an interval training session performed on sand compared with grass. Specifically, BLa accumulation was 1.6 times greater, and average HR for all 3 interval sets was 1.1 times greater on sand. The magnitude of the difference in these variables is slightly lower than values previously reported comparing sand and grass training surfaces (26,27). Surface characteristics, such as the granulation of the sand, moisture content, or the depth and consistency of the substratum on a sand training surface (26), and the type of grass, moisture content, or areas of overuse and wear on a grass surface, can contribute to the degree of stiffness (peak impact deceleration forces) recorded on a training surface. Here, the surface stiffness readings measured on both sand and grass were higher than those previously reported (26). Furthermore, the difference in these surface stiffness readings between sand and grass were lower than in previous research; the stiffness reading for grass being 3.2 times greater than that for sand, in comparison with the report of Pinnington and Dawson (26), in which the stiffness reading was 4 times greater on grass. Therefore, the surface conditions used here were more similar than in previous investigations and may partly explain the smaller difference in physiological responses (BLa and HR) observed between training surfaces.
Alternatively, the discrepancy in these acute physiological responses may be because of the faster running speeds encountered during the interval training used in the current investigation, compared with the slower steady-state (8–11 km·h−1) running bouts previously examined (19,26,27,37). Previous sand running research has shown a trend for EC values observed between sand and grass training surfaces to become more similar as running speed increases (19,26–28). A greater EC difference at slower speeds is thought to be associated with a significantly increased stance time (T s) when running on sand, leading to a relatively greater amount of active muscle mass required during the support phase of the running gait (28). More specifically, Pinnington et al. (28) reported that running on sand vs. grass resulted in a significantly greater stance time (T s) at 8 km·h−1, but this difference was not significant at 11 km·h−1. Although running speed and T s were not measured in the current investigation, it is reasonable to suggest that running speeds reached during interval training exceeded 11 km·h−1, given the high intensity nature of the intermittent running protocol, and the trained status of the participants. Future research, however, should investigate the kinematics associated with running on sand at higher speeds (>11 km·h−1), to better understand the effective range of running speeds at which sand training can provide additional performance benefits to firm ground training venues.
A higher training intensity (as indicated by the HR and BLa responses) encountered during interval training on sand vs. grass might lead to greater aerobic adaptations being gained over the preseason, such as a larger improvement in aerobic capacity, lactate threshold, exercise/work economy, and oxygen uptake kinetics (30). A high level of aerobic fitness is important for recovery and maintenance of high-intensity efforts experienced over the entire game duration of team sport activity (3). As a result, high-intensity interval training has been shown to improve team-sport specific endurance (Yo-Yo Intermittent Recovery Test) to a greater degree than moderate-intensity intervals (15). Furthermore, the accumulation of H+ during exercise is suggested as an important stimulus for improving muscle buffer capacity (βm) (8,36). An improved βm may assist anaerobic performance in team sport athletes, with a positive and significant relationship reported between βm and RSA (4). Therefore, the significantly higher BLa accumulation seen in sand training in this investigation may underpin an ability to improve βm to a greater degree over the preseason in team sport athletes, ultimately leading to improved anaerobic performance and RSA during the competitive season. However, further research is needed to investigate the long-term effects of interval training on a sand training surface, and the implications of a higher training intensity on training adaptations in team sport athletes.
In addition to the BLa and HR responses, the duration and intensity of the interval training session was enough to cause a significant increase in common blood markers of muscle damage (Mb) and inflammation (CRP) on both training surfaces. However, there was no difference in these blood markers observed between trials, despite the higher training intensity experienced on the sand training surface. It was initially hypothesized that there would be a smaller degree of muscle damage and inflammation after the interval training session performed on sand. This prediction was based on research by Miyama and Nosaka (23) who reported a significantly reduced degree of muscle damage and soreness after plyometric exercise on a sand training surface in comparison with grass. These authors attributed this effect to the shock absorbing qualities of the sand resulting in a reduced stress placed upon the muscle-tendon complex during impact with the surface. To show this, Miyama and Nosaka (23) had their athletes perform a set number of drop jumps (200) on both the sand and grass training surfaces. For a fixed number of contacts with the training surface, the cumulative loading forces, and stress placed on the musculoskeletal system was significantly lower during the sand trial, resulting in lower levels of muscle damage and soreness. However, in the current investigation, the workload during the interval training session was matched for time, and therefore, other factors may have contributed to the levels of muscle damage observed. Pinnington et al. (28) reported that running on a sand training surface at high speeds (11 km·h−1) resulted in a significantly reduced stride length and increase in cadence when compared with running at similar speeds on grass. In addition, previous research has reported a trend for muscle damage and inflammation levels to increase with exercise intensity (6,21). Therefore, an increase in the frequency of foot contacts, and the higher relative training intensity associated with interval training on a sand training surface may have offset any effect of a reduced impact force that may have been experienced.
In contrast to these findings, the results for the secondary blood marker of muscle damage (CK) were inconclusive, with high resting levels reported and no significant change over time observed on either training surface. The lack of response seen for CK levels may be because of the high training status of the athletes used in the current investigation, and the relatively short duration (i.e., 45 minutes) of the interval training session (6). This is supported by previous research that has demonstrated higher resting levels of CK (14), and a lower responsiveness of CK to exercise in trained vs. untrained subjects (35). Therefore, on the basis of previous research and the findings from the current investigation, the use of CK in future research looking to examine the muscle damage response in a similar testing scenario might be questioned. Despite the inconclusive findings for CK, the results of this study have shown that for a standardized workload time, training on a sand surface can result in a significantly greater training intensity experienced by the athlete, without a resultant increase in muscle damage and inflammation.
This pattern was also seen in the hemolytic response after the interval training session on both the sand and grass surfaces. There was a significant decrease in Hp levels from preexercise to postexercise in both the sand and grass trials, indicating the presence of a hemolytic episode (25); however, there was no difference evident here between trials. The degree of change in Hp levels was similar to values reported in previous research indicating a hemolytic episode after running exercise (22,25). It was initially hypothesized that training on a sand surface would result in a significantly lower degree of hemolysis because of the decreased impact forces experienced at heel strike (22). However, despite the significantly lower stiffness readings measured on the sand, there was no difference in the amount of hemolysis incurred. Therefore, it is likely that other factors contributed to the amount of hemolysis encountered in training on this surface type. Hemolysis has been shown to increase with exercise intensity (25), possibly because of the compression of capillary networks by a greater active muscle mass (22), or from an increased level of tissue hypoxia leading to oxidative stress within the RBC (25). These factors, coupled with a higher frequency of ground contacts (because of a potential increase in stride frequency) experienced on the sand training surface, may have counteracted any reduction in hemolysis caused by the lower impact training surface.
In addition to these blood measures, the sport-specific athletic performance measures, in the form of the VJ, RSA, and RTT were mostly similar between BASE measures and the 24 hours postexercise performance trials for both SAND and GRASS. However, some noteworthy differences were observed, such as the time taken to complete the RTT being significantly slower following the interval training session performed on grass when compared with sand. Despite a significant result, it should be noted that the difference in RTT performance was only 12 seconds, which falls within the measurement variability previously reported for this test among elite team-sport athletes (TE = 24 seconds; 11). Further to the RTT result, total sprint time in the RSA test was also significantly greater following GRASS compared with BASE, whereas there was no difference observed between SAND and BASE. Overall, these results suggest that next-day performance is unaffected by the higher training intensities experienced on a sand training surface and that SAND may actually incur a lower decrement to performance at 24 hours postexercise when compared with GRASS. The reason for this difference is unclear, because blood indicators of muscle damage and inflammation were similar between trials at this time point. Additionally, overall DOMS ratings were not different between groups. However, the DOMS rating for the hamstring muscle group was significantly higher after GRASS. A possible explanation for this may be that during high-intensity intermittent running on grass, a greater reliance is placed on the hamstrings to decelerate the body's mass after each effort (34), in comparison with sand, where some of these braking forces are absorbed by the surface, reducing the load on the hamstrings. As a result, a greater level of soreness localized to the hamstring muscle group following GRASS may have limited performance to some extent in the RSA and RTT trials. Despite this suggestion, further research is needed in this area to uncover the specific mechanisms by which a sand vs. grass training surface can influence next day performance recovery. Furthermore, it is possible that a higher intensity or longer duration of training session is needed to expose larger differences between the training surfaces at 24 hours postexercise.
The findings from this study suggest that an interval training session performed on sand (in comparison with grass) will allow for a higher relative training intensity, without any additional detriment to next day (24 hours postexercise) athletic performance. In addition, the cumulative effect of a lower impact training surface (sand) may limit the incidence and severity of overuse injuries encountered over a team sport preseason (9,16). Therefore, the use of a sand training surface for such sessions may have valuable implications in a team sport environment, allowing for a higher intensity and frequency of training to be achieved over the preseason.
Despite these suggestions, it is important to address the potential limitations associated with sand training to understand the extent of its application in a team sport environment. Currently, it is clear that running on a sand training surface results in a significant alteration to running kinematics (cadence, joint angles, etc.), muscle activation patterns (28), and physiological responses (19,26,27,37). Therefore, it is possible that habitual sand training will come at the cost of losing training specificity to firm ground performance ability. The findings of this study have shown that a sand training surface is a beneficial alternative to grass during a preseason interval training session where the primary focus is on physiological conditioning. However, its use may be limited when sport-specific skill training is required (i.e., training with a ball, passing, and shooting), because of the irregularity of the surface. Therefore, it may be that sand is a better ground surface suited as an adjunct rather than a primary training surface (i.e., 1–2 times per week), with the majority of team sport training completed on a firm ground surface to maintain specificity.
The results of this study suggest that performing a standardized interval training session on a sand (vs. grass) training surface can result in a greater physiological response experienced by the athlete to a given training session. Furthermore, the lower impact force experienced on sand appears to limit the muscle damage, muscle soreness, and detriments to performance capacity relative to exercise intensity. As a result, the use of sand in a team sport setting may allow for greater training adaptations to be gained over the preseason, while reducing the performance limiting effects, and overuse injuries that may arise from a heavy training load. Despite such suggestions, further research is needed to investigate the long-term effects of sand, and the implications for firm ground performance gains in team sport athletes.
For the coach or sport science practitioner working with team sport athletes, this study shows that sand can be used as an effective substitute to more traditional firm ground training venues for a preseason interval running session. In comparison with grass, the unique surface qualities of sand can offer a higher relative training intensity, without negatively affecting recovery at 24 hours postexercise. Therefore, sand should be integrated with firm ground training venues throughout a preseason conditioning program, because this surface has the potential to allow for superior training adaptations and a more optimal athlete preparation leading into the competitive season. Furthermore, sand should be considered during a period of intensified training or in athletes returning from injury, because the low impact nature of the surface could reduce the loading forces experienced during exercise without sacrificing the quality of the training session.
The authors wish to acknowledge the assistance of David Bell, Michelle Wilkins and the athletes of the WAIS Hockey and Netball programs. They also wish to thank Jim Crandell (Dr. Baden Clegg Pty Ltd.) and Peter Ruscoe (Sports Turf Technology Pty Ltd.) for their assistance and expertise in dealing with the Clegg impact hammer. This research involves no professional relationships with companies or manufacturers who will benefit from the results presented here. Furthermore, the results of this study do not constitute endorsement by the authors or the National Strength and Conditioning Association.
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