Time Course of Improvements in Power Characteristics in Elite Development Netball Players Entering a Full-Time Training Program

Abstract

McKeown, I, Chapman, DW, Taylor, KL, and Ball, NB. Time course of improvements in power characteristics in elite development netball players entering a full-time training program. J Strength Cond Res 30(5): 1308–1315, 2016—We describe the time course of adaptation to structured resistance training on entering a full-time high-performance sport program. Twelve international caliber female netballers (aged 19.9 ± 0.4 years) were monitored for 18 weeks with countermovement (CMJ: performed with body weight and 15 kg) and drop jumps (0.35-m box at body weight) at the start of each training week. Performance did not improve linearly or concurrently with loaded CMJ power improving 11% by Week 5 (effect size [ES] 0.93 ± 0.72) in contrast, substantial positive changes were observed for unloaded CMJ power (12%; ES 0.78 ± 0.39), and CMJ velocity (unloaded: 7.1%; ES 0.66 ± 0.34; loaded: 7.5%; ES 0.90 ± 0.41) by week 7. Over the investigation duration, large improvements were observed in unloaded CMJ power (24%; ES 1.45 ± 1.11) and velocity (12%; ES 1.13 ± 0.76). Loaded CMJ power also showed a large improvement (19%; ES 1.49 ± 0.97) but only moderate changes were observed for loaded CMJ velocity (8.4%; ES 1.01 ± 0.67). Jump height changes in either unloaded or loaded CMJ were unclear over the 18-week period. Drop jump performance improved throughout the investigation period with moderate positive changes in reactive strength index observed (35%; ES 0.97 ± 0.69). The adaptation response to a structured resistance training program does not occur linearly in young female athletes. Caution should be taken if assessing jump height only, as this will provide a biased observation to a training response. Frequently assessing CMJ performance can aid program design coaching decisions to ensure improvements are seen past the initial neuromuscular learning phase in performance training.

Introduction

Talented athletes who are identified early in their sporting career are often fast-tracked through the sporting development pathway for a variety of reasons (²). Athletes who are fast-tracked through the pathway are at risk of overcompeting and are undertrained. This can then manifest itself in poor athletic ability, movement dysfunction, and injury. Academy or junior development level squads are an introduction to high-performance sport athlete pathways. Once an athlete reaches this standard, they are expected to train and compete at the highest possible level to prepare for future senior success (²). The training load at this level is based on the technical and tactical requirements of the sport that are essential in preparation for later competition. However, due to underpreparedness at earlier levels or more accurately, overcompeting in junior sport, it is hypothesised that athletes are not able to cope with this increase in training load. Athletes can also lack the fundamental movement ability and skills necessary to progress technically and physically (^2,4–7). The training load is often not only an increase in training volume, but the intensity and complexity of movement is often greatly increased without sufficient foundation capacity for improvement to occur (¹²). This deficiency can lead to plateaus in technical and tactical development, as athletes are not able to expose themselves to an adequate amount of training to improve sport-specific skills (^3,19). The dysfunction or deficiency in athletic ability must therefore be addressed first before physical training can focus on improving sporting performance, in the investigated example it was seeking improvements in on-court speed, acceleration, and agility.

Academy or junior development level is often the athletes' first exposure to a full-time supervised strength and conditioning program. Given the inexperience of athletes in these situations, the majority of training tends to involve fundamental movement coaching to develop a strong foundation of movement ability and core lifting skills as described by the Australian Strength and Conditioning Association and National Strength and Conditioning Association position articles on youth resistance training (¹⁶). A program will also aim to elicit strength gains from which to base more complex training modalities. During this stage, the improvements in strength training are neural in nature, with a common plateau in improvement rate after 6–12 weeks (¹⁶).

Due to athletes at this development level often exhibiting fundamental movement dysfunction and in most cases a low work capacity tolerance, programs should be careful not to overload too early. Programs must ensure that improvements in physical ability are made on top of a strong fundamental movement capacity (^17,23–25). Strength training gains illustrated in the meta-analysis of resistance training effects in adolescents show a relationship between longer programmed interventions and programs involving more sessions per week, although in female athletes the effect size (ES) of this increase is still unclear particularly after the initial increase in performance due to neural adaptation (⁹). The aim of this investigation was to systematically describe the time course of adaptation to strength and conditioning practices when junior female athletes begin a full-time supervised program.

Methods

Experimental Approach to the Problem

An observational descriptive study design to examine the time course of changes in countermovement (CMJ) and drop (DJ) jump characteristics in young adult female athletes as they enter a full-time supervised strength and conditioning program. We monitored each athlete for CMJ and DJ performance at the start of each training week for 18 weeks. Importantly, this period included phases of full-time training as well as periods of travel and competition where strength and conditioning training was limited, thus allowing us insight into the process of adaptation and detraining in such a resistance training–naive yet elite athlete cohort.

Subjects

All athletes (n = 12) were female netball players, aged over 18 years and were recruited from the residential program of the national sports institute (age 19.9 ± 0.4 years; height 1.82 ± 0.54 m; mass 74.1 ± 6.6 kg, and sum of 7 skin folds 76.3 ± 6.0 mm). All athletes received clear verbal and written explanation of the study including the risks and benefits of participation before each athlete provided verbal and written informed consent. Data collection was in accordance with and approved by the institutional ethics committee in accordance with the declaration of Helsinki for research with human subjects. The study conforms to the Code of Ethics of the World Medical Association (approved by the ethics advisory board of Swansea University) and required players to provide informed consent before participation.

Procedures

Training Program

The residential program, from which the athletes were recruited, focused on a multidisciplinary approach to performance gains; the athletes' priorities were structured on their individual physical, technical, and tactical development required for elite competition. An example of the physical conditioning workload for a typical week is shown in Table 1. During the data monitoring period, a typical training week consisted of 7 sport-specific (netball) sessions on-court, 3 strength sessions, and 2 off-feet conditioning sessions. Strength training was periodized into 5-week blocks with individual exercise selection based on training competency; however, the goal of each block was consistent within the squad. The first 5-week block aimed to increase general strength and power and improving their lifting technique proficiency while introducing each athlete to the rigorous formal strength and conditioning training. This program was approximately 40% injury prevention focus, 50% strength and power, and 10% specific trunk components.

Table 1: An example of the squad's training schedule during 18 weeks of athlete monitoring.

The goals of the second 5-week block were to progress each athlete's strength and power and consolidate technical gains in the main lifts. Outside the monitoring sessions, various plyometric and dynamic activities were introduced, such as loaded jump squats, DJs, hurdle jumps, clap push-ups, as well as hang cleans, push press, and progressing in load with squats, bench press, and a variety of other upper-body supplementary exercises in rep ranges of 3–5 for the dynamic movements and 4–8 for the strength exercises; sets ranging from 3 to 5. The program ratios were 20% injury prevention, 65% strength and power, and 15% trunk control.

During the competition period of weeks 11–15, no formal strength sessions were programmed because of travel and competition constraints. However, during this period, individual injury prevention programs were given to each athlete and completed as often as possible in consultation with the travelling physiotherapist.

The final 4-week block of strength training (weeks 15–18) was an identified intensive strength and conditioning block, as this was the finish of the scholarship year and sought to prepare the athletes for inclusion back into their respective semiprofessional league teams. The purpose of this program was to maintain injury prevention work while increasing maximal strength and power development. Strength sessions included a wide variety of plyometric and dynamic activities, various medicine ball throws, loaded and unloaded jumping, and contrast of heavy squats and box jumps with a maximum of 5 reps per exercise, and only 2–3 sets with the focus on speed and quality of movement.

During each week, the program support team of medical staff would meet to consider and provide consent for each athlete to complete the full training planned for that week. In the event of consent not being granted, these athletes performed a modified loading for the week and were excluded from the jump assessment protocol. During the initial 10 weeks of resistance training, loading prescriptions were based on an athlete's tolerance to complete additional repetitions in the prescribed lifts and to encourage lifting technique proficiency. Before and at completion of the final strength and power training block (weeks 15 and 18), formal assessments of strength in the three fundamental prescription lifts (1 repetition maximum [1RM] Hang Clean, 3RM Back Squat, and 3RM Bench press) were conducted, as technique proficiency was deemed appropriate. During the period of investigation, there were no injuries that required more than 3 days of reduced training load or time off from full training but due to the timing of the days of reduced training load, some athletes did miss jump assessment with an average attendance at jump monitoring of 73% for the investigation period (Figure 2, lower panel).

Performance Monitoring Assessments

Athletes performed maximal effort CMJs at body mass (unloaded) and with the addition of a 15-kg external load (CMJ+) for 3 reps after a 5-minute warm-up incorporating dynamic movements and submaximal jumping variations. Under each condition, the athlete was instructed to perform the movement with the intent to jump as explosively and as high as possible with a self-selected depth of countermovement. Jumps were performed and measured using a commercially available Linear Position Transducer (GymAware Version 4; Kinetic Performance Technology, Australia). The position transducer was connected to a lightweight (<0.4 kg) wooden pole (CMJ) or 15-kg Olympic Bar (CMJ+) and positioned across the shoulders. The displacement-time data were directly recorded through Bluetooth and after interfacing with commercial software (GymAware Pro), the reliability of the system and this methodology has been extensively reported (31). As a first-principles measurement device, a LPT can most effectively provide measures of displacement and velocity (rate of change by units of time), thus the preferred performance outcomes in this scenario from the CMJ were the variables of peak movement velocity (m·s⁻¹) during each jump's concentric phase, mean concentric power (W), and maximum jump height (m). In consultation with our coaches, these variables have provided them with the most intuitive, easily understood feedback on their squad of athletes.

After all CMJ assessments, athletes then performed five maximal effort DJs off a 0.35-m box onto a force plate (400 Series Force Plate; Fitness Technology, Adelaide, Australia), which was interfaced with the commercially available software (Version 2010.01; Ballistic Measurement System; Fitness Technology) which facilitated direct measurement and analysis of force-time characteristics at a sampling rate of 200Hz. Athletes were instructed to jump as high as possible off the force plate with minimal contact time. The DJ performance was monitored using the variables of contact time (s), flight time (s), and the calculation of a reactive strength index (RSI) by dividing the jump height by contact time (¹⁸). The change in collection devices between the CMJ and DJ was necessary because of time constraints imposed on the data monitoring procedures by the coaching staff and our desire to monitor the variables that we believed to be most pertinent to change and a monitoring training response in these athletes. The rational for choosing these monitoring variables of interest among the multitude available is presented in Table 2 along with an indication of the variables reliability (coefficient of variation%) within our collection methods.

Table 2: Program monitoring variables and associated adaptation response that each variable is hypothesized to represent including CV%.*

Statistical Analyses

The time course of performance changes is presented as the difference from baseline (±90% confidence limits). All data was log-transformed to reduce nonuniformity of error, and the differences were derived through back-transformation as percent changes (²¹). Standardized differences of the mean were used to assess magnitudes of effects between time points by dividing the differences by the appropriate between-athlete SD. Standardized changes of <0.20, <0.60, <1.2, <2.0, and >2.0 were interpreted as trivial, small, moderate, large, and very large effects, respectively (^8,22). To make inferences about the true (large-sample) value of an effect, the uncertainty in the effects is expressed as ±90% confidence limits. For the time-course data, a substantial true change in performance (compared to baseline) was accepted when the likelihood of a positive effect was greater than 75%, where the smallest standardized change was assumed to be 0.20 (¹¹). Other likelihood values interpreted qualitatively as follows: <1%, almost certainly not; <5%, very unlikely; <25%, unlikely; 25–75%, possible; >75%, likely; >95%, very likely; >99% almost certain (²²). This procedure was replicated to assess the likelihood of further improvements in performance during the different training phases.

Results

During the final strength block (weeks 15–18), the mean (±SD) strength in the 3 fundamental prescription lifts were 1RM Hang Clean (51 ± 5 kg and 52.5 ± 5 kg), 3RM Bench press (43.5 ± 5 kg and 42.5 ± 5 kg), and 3RM Back Squat (70 ± 5 kg and 78.5 ± 5 kg) for pre (week 15) and post (week 18), respectively. Large improvements in CMJ power (24%; ES 1.45 ± 1.11) and velocity (12%; ES 1.13 ± 0.76) under the unloaded jump condition were observed over the 18-week study (Figures 1A,B). The change in jump height was less clear and may have been trivial to large based on the spread of the confidence limits (9.6%; ES 0.58 ± 0.74) (Figure 1C). Similar patterns were observed for CMJ+ with large improvements in power (19%; ES 1.49 ± 0.97), moderate improvements in velocity (8.4%; ES 1.01 ± 0.67), and a small improvement in jump height that was possibly trivial to moderate (11%; ES 0.59 ± 0.55).

Figure 1: The mean (±90% CL) changes for female development athletes (n = 12) in CMJ performance variables, (A) concentric power, (B) concentric velocity, and (C) Jump height, across an 18-week monitoring period. CL = confidence limit; CMJ = countermovement jump.

Analysis of the time course of the changes in unloaded CMJ performance showed a clear substantial improvement by week 7 for CMJ power (12%; ES 0.78 ± 0.39) and velocity (7.1%; ES 0.66 ± 0.34), whereas jump height did not show clear improvements until week 15 (13%; 0.78 ± 0.56). Further small improvements in power (7.2%; ES 0.47 ± 0.59), velocity (3.0%; ES 0.29 ± 0.32), and jump height (5.5%; ES 0.34 ± 0.51) from week 7–11 coincided with the second training block focusing on improving power. Changes in jump performance from weeks 11–15 were unclear but unlikely to be positive (power = −1.4%; ES −0.10 ± 0.67, velocity = −2.5; ES −0.24 ± 0.41, jump height = −2.9%; −0.19 ± 0.40). Improvements from weeks 15–18 were small for power (4.4%; ES 0.29 ± 0.49), velocity (2.5%; ES 0.24 ± 0.43), and jump height (5.6%; ES 0.35 ± 0.37).

Loaded CMJ (CMJ+) changes were similar to the unloaded CMJ condition for velocity with clear substantial improvements evident by week 7 (7.5%; ES 0.90 ± 0.41). Power (ES 0.93 ± 0.72) and jump height (ES 0.59 ± 0.27) improved 11% by week 5 and week 7, respectively, which is earlier than the corresponding time points under the unloaded condition. In contrast to the small improvements in unloaded jump performance during weeks 7–11, changes in CMJ+ performance were likely trivial during the same period. Observed trivial changes were also likely from week 11–15 for velocity (0.5%; ES 0.07 ± 0.18) and jump height (1.3%; 0.08 ± 0.21), whereas there was a probable small improvement in power (5.2%; ES 0.44 ± 0.51). Trivial (unclear) changes were observed between weeks 15 and 18 in power (1.2%; ES 0.10 ± 0.19), velocity (0.3%; ES 0.04 ± 0.35), and jump height (1.4%; ES 0.08 ± 0.46).

Drop jump height improved by 10% (ES 0.52 ± 0.40) from week 1–18 (Figure 2). The change in contact time between week 1 and 18 was unclear, but a moderate improvement in RSI was very likely (35%; ES 0.97 ± 0.69). Drop jump height substantially improved 8.1% by week 5 (ES 0.42 ± 0.44), whereas substantial reductions in contact time were not evident until week 7 (−15%; ES −0.69 ± 0.86). Moderate negative changes from week 16 to 18 were very likely for DJ height (10%; ES 0.58 ± 0.43) and likely for RSI (12%; ES 0.42 ± 0.55).

Discussion

The results from this study provide an insight into the time course of jump performance improvement in elite developmental athletes who are entering a full-time sports training program for the first time. Clear improvements in jump performance were observed over the 18-week training block, whereas most of the improvement occurred within the first 7 weeks, with minimal further improvements throughout the remainder of the training period. There were no improvements during the competition period with unlikely small improvements in unloaded CMJ performance and only trivial changes in CMJ+ performance. The final brief strength development focused period of the training program resulted in small improvements in CMJ, CMJ+, and DJ performance; however, these changes are unclear and possibly trivial.

An important aspect to this work is that the substantial changes over time were identified in less traditionally reported monitoring variables. The majority of previously published training monitoring literature with female athletes has only reported jump height changes with no consideration of power or velocity. We accept that vertical jump height performance provides a marker of muscular power and therefore is useful as an indicator of athletic performance (¹⁰). However, the moderate-to-large initial improvements in jump performance observed in this study contrast with the unclear change over the full time course, suggesting other performance variables should be investigated to better understand the complex interaction of performance variables in response to training. The initial magnitude of change in jump height is similar to magnitude of change seen in training studies of untrained participants to strength or power training (^13–15). However, it is less than that reported in male strength and power-focused athletes, where a significant 9.7 ± 7.9% change in jump height performance has been observed in young rugby league athletes on entering a supervised strength training program for the first time (¹⁵). In regard to studies involving young female athletes, the improvements seen in this study are greater, which may be indicative of the previous studies focusing on injury prevention and neuromuscular training and not on strength or power development. For instance, small significant jump height improvements in junior female basketball players have been observed after 6-week (²⁵) or 7-week (²⁴) exposure to a neuromuscular training intervention. Thus, in order for more substantial improvements to be observed, greater emphasis must be placed on strength and power development and not only injury prevention indices (²⁹).

Measurement and enhancement of power is considered a crucial element of athletic performance rather than simply jump height (^1,26). Sheppard et al. reported small-to-moderate ES changes for jump height (ES 0.23), relative power (ES 0.79), and peak velocity (0.67) (²⁷) in full-time athletes as part of an intensive training program for strength and power development over 12 weeks. In this study, large-to-moderate changes in these variables were observed. Furthermore, senior elite volleyball players improved CMJ jump height by only 1.4% over a 12-month season (²⁶). We contest that these results illustrate the potential for larger changes in performance for development athletes who have a “young” training age in terms of strength and power development as compared with a well-trained group of athletes who have many training years and their potential to improve is over smaller magnitudes in response to training interventions (^13,14,28). We propose that the initial performance gains thus result from an underdeveloped neural adaptation in this athlete group, which is not ideal in regard to the physical age of the athletes as it would be preferred that this pathway for physical improvement is maximized before joining an end-of-development/subelite program.

Previous studies and ours provide evidence that, although competition and travel may hinder the magnitude of improvement, the overall change remains positive and that, encouragingly, the improvements can be maintained in developmental athletes during their first exposure to full-time training. There was potential for the results from this study to show that competition periods could have a negative effect on physical development; however, the net gains of the training period still show substantial improvement. Gaining competition exposure for developing athletes is a key element of any high-performance sport's programme (²⁰), it is important to consider a holistic approach to training where the inclusion of competition may have a positive effect on other elements of on-court performance, even if physical performance was temporarily affected. The net gain in sports performance is an important element for strength and conditioning professionals and sports coaches to consider when planning and administering any programme. The 4-week hiatus in testing between weeks 11 and 15 where the decrements and plateaus in performance originate was due to the competition schedule and not due to a period of zero activity. During this time, the athletes focus was on competition and tactics as opposed to physical conditioning, thus the frequency of performing systematic strength training was reduced.

Although careful consideration was taken in the selection and rationale that underpinned the use of the monitored variables (Table 2), there are 2 important issues that must be considered. Firstly, training adaptations are specific to the movement trained, thus the degree of transfer between training and testing exercises is an important consideration especially if there is little similarity or the test is insensitive to small training gains, resulting in no change being detected. We strongly believe that the combination of monitored variables here in this investigation allows for an understanding of the athlete's adaptation process and rate, such as by considering when changes are seen in an unloaded jumping task as compared with when similar changes are seen under a loaded (additional system stress) condition. Secondly as alluded to, each of these tests and the derived variables measure specific inherently trainable muscle qualities contraction velocity, higher-order motor unit recruitment, power, strength, etc during a prescribed jumping task. It should be understood that interpreting outcomes from tests of muscle function and quality is a function of the method and tool used for data collection, historical and contemporary viewpoints, and the type and quality of training being monitored. It is with these considerations that the plateau in jump performance for instance at weeks 15–18 can be interpreted. This period of training had a heavy focus on strength development (force); however, we had constrained our methods of monitoring to only one of 3 tasks on a device suitable (first-principles measure) to directly quantify force changes. Concurrently, we observed a substantial increase in interathlete variability identified by the large confidence intervals (Figures 1 and 2). Although only 6 athletes were available for testing at week 17 (Figure 2; lower panel), the magnified variability provides a generalized understanding of the cohort's different adaptation rates. Furthermore, supporting the notion that practitioners should be implementing a continual monitoring process and not just at discrete select time points throughout a season as discrete assessments do not provide an individualized picture of the development process.

Figure 2: The mean (±90% CL) changes for female development athletes (n = 12) in DJ performance across an 18-week monitoring period and a histogram of the athlete attendance count for each testing occasion. CL = confidence limit; DJ = drop jump.

This study has described the time course of adaptation to jump performance in a full-time strength and conditioning program. Improvements made over the first period do not continue to increase linearly or at the same magnitude across the jump variables after the initial familiarization period. Time must be taken to monitor the variables accurately and to prescribe adequate training stimulus to improve jump performance after the initial neural adaptation to training in novice performers. Improving the players' long-term athletic development by eliminating as many decrements as possible should be an aim of the strength and conditioning program. Over time, there will however be variations in performance measures, athletes will not continually improve linearly. The role of the strength and conditioning coach should be to understand these interactions and plan accordingly to maintain a positive environment for adaptation, performance, and technical improvement.

Practical Applications

What the outcomes of this investigation highlight for the strength and conditioning professional is that during the first period of training exposure to a full-time training program junior athletes can show improvement across a range of jump performance variables, although importantly an athlete's jump performance will not improve linearly or at the same rate for all jump types. Frequent monitoring of performance variables across different jump types will better inform you as a coach when a plateau has been reached in performance improvement. Thus, the strength and conditioning professional and coach can then vary the program to expose the athlete to a new stimulus to ensure continual improvement in performance. A secondary but important observation is that jump height as a monitoring variable is not the most sensitive to change; therefore, other variables that can be easily measured must be considered in a high frequency monitoring program.