Risk of Bias Assessment
Of the 25 studies reviewed, 21 were deemed to have low risk of bias in all five categories (see Table 1). Two studies were considered to have high selection bias, as the participants were not randomly allocated to practice groups [7] or the conditions of testing favored scaled equipment [8]. In the latter case, testing involved five trials with a women’s basketball followed by five trials with a junior basketball for every participant, thereby creating a potential learning effect that favored the junior ball. For two studies, it was unclear whether the risk of detection bias was high or low, because technique was subjectively assessed by “observers,” but it was unclear whether the observers were independent from the research team and/or blinded to treatment allocation [7, 9]. However, it must be noted that Hammond and Smith [7] did not explicitly state whether it was hitting technique or hitting accuracy that was assessed in their skills tests. Intra- or inter-reliability was also not obtained in these two studies. None of the studies reported missing data and only one was considered to be at high risk for selective reporting [10]. Specifically, Pellett et al. [10] discussed the skill learning advantages when practicing with the modified volleyball, despite no supporting evidence from the skills testing results. The Pellett et al. [10] study was also deemed to have another bias in its design, as the skills tests were only performed with the regulation volleyball; thus, children who practiced with the lighter volleyball during the study were likely to be disadvantaged in the skills test. Indeed, this may explain the lack of differences observed in this study. The remainder of this article discusses the findings of the reviewed studies in the context of these limitations.
Table 1 Risk of bias assessment
Overview of Findings
The reviewed studies examined a total of 989 children, with most studies focusing on basketball (n = 343 children) and tennis (n = 313 children). As such, our discussion may appear to focus largely on these sports (see Table 2); however, we suspect that the findings can be generalized to a wide range of skills across multiple sports. We discuss the findings of the review in four sections: psychological factors, skill performance (and acquisition) factors, biomechanical factors, and cognitive processing factors. We acknowledge that several of the reviewed studies provide evidence related to multiple sections (e.g., both psychological factors and skill performance factors) and, therefore, some of our discussion crosses sections.
Table 2 Studies examining the influence of equipment scaling on children’s sport performance
Psychological Factors
Five articles reported psychological benefits for children when using scaled equipment that simplified the task. For instance, 8-year-old children playing tennis with low compression balls on smaller courts reported more engagement during practice sessions compared with children playing with standard tennis balls on a full-size court [29]. The scaled condition created an environment that increased the number of viable opportunities to hit the ball, which consequently heightened engagement in the task. Children in the unscaled or full-size condition had fewer opportunities, which probably caused them to feel that the task was too difficult and to be less engaged. Children of a similar age have elsewhere reported preference for, and presumably greater engagement when, using scaled tennis equipment, including smaller racquets and lower compression balls [24] and lower nets [23]. In a basketball study involving 77 10-year-old children [9], 48 (62 %) preferred using a junior ball (as opposed to a women’s or men’s ball) and only seven (9 %) preferred using an adult men’s ball. Whilst the junior ball did improve shooting performance for all children, it was observed that shooting performance was significantly better when children used the ball of their preference, which was typically a ball smaller than the adult men’s ball.
Greater ‘shot-efficacy’, or the belief of a child that they have “the capacity to achieve the desired or expected effect from shooting” (p. 54) [12], has also been found in children playing basketball with a lighter ball [12] and a lower basket [17]. This was reported to be a consequence of the increased shooting success that children experienced when shooting in the modified conditions. Importantly, a heightened sense of skill mastery is considered to be an indicator of motivation for the task [32, 33]. The relationship is cyclical, as greater motivation tends to lead to greater physical activity levels, which in turn provides children with the opportunity to attain actual motor competence (or skill mastery). Significantly, actual motor competence is thought to be a strong predictor of physical activity in adolescent and adult years [34–37]. As such, it is possible that scaling the equipment and play area for children also contributes to future or ongoing participation in physical activity, which is inextricably linked to a number of health benefits, such as greater physical fitness and a reduced risk of obesity [38, 39].
Skill Performance (and Acquisition) Factors
It has been well established that scaling equipment generates greater task success and better performance in a range of skills compared with unscaled or ‘adult’ equipment. For instance, in tennis, children playing with lower compression balls are able to strike the ball with greater ease [24, 25, 28]. Low compression balls bounce lower than standard tennis balls,Footnote 2 allowing children to strike the ball in an optimal location relative to their height (i.e., waist height) [26]. Furthermore, children generate greater ball velocity whilst maintaining (or improving) hitting accuracy when using low compression balls,Footnote 3 which indicates that children strike the softer ball with greater power and without the fear of the ball travelling too far [28]. In addition to these findings, it appears that performance is further enhanced when low compression balls are combined with scaled racquets [24]. However, results indicate that ball compression has a greater impact on hitting performance than racquet size, with the lowest compression balls generally producing the best performances.
Scaling the task for children also enhances skill learning opportunities during practice. Farrow and Reid [29] found that a combination of low compression balls and smaller court size increased the volume of practice in 8-year-old beginners, whereas practice with standard balls on a full-size court led to concomitant impairments in learning. The ‘adult’ practice conditions reduced the number of hitting opportunities, which effectively diminished chances for practice repetition and consequently learning. Furthermore, the combination of decreased hitting opportunities and a more difficult practice environment resulted in the children in the adult practice condition executing fewer successful forehands and backhands relative to the scaled conditions.
In a similar vein, other research has demonstrated that children (beginners to tennis) displayed the greatest improvements in a range of skills tests when using scaled racquets (17- and 24-in. racquets) compared with larger racquets (26-in. length racquets) following 16 sessions of practice [31]. Interestingly, the only skill in which performance with a larger racquet was commensurate with a smaller racquet was volleying, which may not be as influenced by the greater moment of inertia of a larger racquet. It is apparent that lighter racquets allow children to wield the racquet with greater ease, thereby facilitating the development of stroke-making ability.Footnote 4
In addition to optimizing the practice environment, scaling equipment also leads to better performance during match-play conditions. For skilled children in tennis, low compression balls (compared with a standard ball) result in faster rallies, more shots played at a comfortable height (between hip and shoulder, as opposed to above the shoulder with the standard ball), and more shots played at the net [26]. In essence, playing with a low compression ball resulted in tennis match play that more closely resembled a professional adult match. Logically, if similar characteristics were observed in practice, it could be reasoned that this would lead to improved long-term outcomes for players learning the sport. A similar study with skilled children showed that lowering the net also had a positive influence on tennis match-play performance [23]. When the net was lowered from 0.91 to 0.67 m, children hit more shots at a comfortable height and in front of the baseline (which typically represents more aggressive play in tennis), and more volleys and winners.
Research in basketball also demonstrates the advantages of scaling equipment for children during match-play conditions. Arias and colleagues examined the effect of ball weight on children’s basketball match-play performance. Five of Arias’ studies [11–13, 15, 16] examined the same cohort of children,Footnote 5 but the results suggested that children exhibited more dribbling and passing [14], increased shot frequency and greater shot success [12, 15], and a higher percentage of attempted lay-ups [11] when playing with a lighter ball (440 g) as opposed to a regulation ball (485 g) or a heavier ball (540 g). Additionally, the lighter ball resulted in more one-on-one situations, presumably because the lighter ball provided children with the opportunity to dribble and take on their opponents [16]. Similar results have also been reported in volleyball, with seventh grade girls displaying a higher percentage of successful sets and serves during match play when using a lighter ball (25 % lighter than a standard volleyball) [10]. In essence, these results are symptomatic of environments that have been constrained, via a lighter ball, to allow children to perform skills with greater success.
To summarise the skill performance (and acquisition) literature, it is apparent that children (a) perform skills better when the equipment and play area are scaled, (b) are presented with increased opportunities to practice skills, and (c) are able to play matches in a style that more closely resembles an adult match. Consequently, skill acquisition should be enhanced when children play sport in a scaled environment. However, no study has examined the influence of scaled equipment over a practice period longer than 8 weeks [31], so we cannot be certain that scaling equipment leads to greater learning in the long-term compared with the use of adult equipment. Future research programs need to place a major emphasis on longitudinal studies to provide a comprehensive analysis of the learning process.
Biomechanical Factors
The primary argument of the constraints-led approach is that the body is biologically designed to discover and self-organize optimal movement patterns in response to the constraints imposed on the neuro-musculoskeletal system [42]. Thus, if a child plays tennis with a scaled racquet, their body will self-organize its movements in accordance with the constraints imposed by use of that particular racquet (whilst also within the boundaries of other task, environmental and organismic constraints). Indeed, it is evident across a number of studies that scaling equipment leads to the production of more functional movement patterns. For instance, when Buszard et al. [24, 25] asked children to perform a tennis forehand with low compression balls, two technical benefits were identified: the racquet was swung in a desirable low-to-high swing path and the ball was struck in front and to the side of the body.Footnote 6 The benefits were most evident when children used the lowest compression ball of the three types tested, suggesting that a ball that bounced lower and travelled slower through the air provided children with an opportunity to adopt a more desirable technique. Likewise, in basketball, when the basket height was reduced, children adapted their movement patterns and shot with a slightly flatter trajectory [18]. Unfortunately, however, the results reported in this particular study provided no indication as to whether this adaptation was advantageous to shooting performance.
Significantly, a study involving 20 participants in four age groups—(a) 5–6 years, (b) 7–8 years, (c) 9–10 years and (d) 18–33 years—observed that throwing technique regressed when balls were used that were too large in relation to hand size [20]. Specifically, throwing technique showed most regression in the backswing and forearm componentsFootnote 7 when the diameter of the ball exceeded the size of the participant’s hand width. Typically, participants adapted to the larger ball size by shortening their backswing, therein removing the ‘forearm lag’ by adopting a shot-put style of throw, and using two hands to control the ball. In comparison, participants displayed a more desirable throwing technique according to the fundamentals of overarm throwing when they were able to grasp the ball easily.
Similar results were also found when observing children’s catching performance. Seven-year-old children displayed a more mature catching style when attempting to catch a small ball compared with a large ball [21]. Indeed, children were more likely to catch the small ball cleanly in their hands without using their body for assistance. These findings have obvious ramifications for practitioners teaching throwing and catching, as children will require a smaller ball in order to perform these skills in a manner that is desirable for most sport and physical education settings.
There is also evidence that scaling equipment will reduce the risk of injury by constraining children’s technique to more efficient movement patterns. For example, shortening pitch length in cricket not only simplifies the skill for junior fast bowlers, but it also generates more efficient movement kinematics, particularly for younger bowlers [19]. Lower back stress fractures are very common in junior fast bowlers [43], and Elliott et al. [19] concluded that the shortened pitch length would decrease the likelihood of lower back injuries by reducing shoulder counter-rotation. Thus, constraining the task to optimize movement patterns ultimately has potential to reduce the risk of injury.
An interesting question is whether it is possible to quantify the amount of scaling required for each child, to allow desirable movement patterns to emerge. Gagen et al. [30] examined 4- to 10-year-old children who were required to perform a forehand hitting task in which they were instructed to “swing as hard as possible and hit the ball as closely to the centre of the racquet” as they could. Children performed this task using four different racquets that varied in length and mass. Gagen et al. [30] anticipated that the unique physical characteristics of each child (hand size, arm length, height, weight, functional leg length, grip strength, shoulder strength) would predict which racquet produced the most desirable performance, as measured by racquet-head speed and accuracy of contact on the racquet. The results showed that for each child one specific racquet produced better speed and accuracy than the other racquets; however, physical characteristics did not predict this ‘optimal’ racquet statistically. Thus, further research is required to understand the mechanisms underpinning the production of optimal movement patterns when using various equipment sizes.Footnote 8
Finally, a novel approach to understanding the effect of equipment and play area modifications, among other constraints, on the performance of the tennis forehand was offered by Lee et al. [27]. In a point of difference from the other studies critiqued in this review, their approach focused on creating a variable practice environment by constantly manipulating key task constraints, including net height and court size. Children exposed to these practice conditions, in what was termed the non-linear pedagogy group, achieved similar skill improvements but with greater degeneracy in their movement patterns than the linear pedagogy group (in which children used the one size of equipment in an environment that emphasized repetition). The authors interpreted this disparity in degeneracy to mean that the children in the non-linear group discover more movement strategies to achieve the task goal. However, the children who used the one size of equipment and participated in more traditional practice settings (the linear group) rated better than their counterparts on an assessment of forehand technique fundamentals, which in turn, might cause practitioners to contemplate the importance of form versus function. Significantly, in the context of this review, this study chose not to detail the timing or type of scaled equipment that was used, therein clouding direct comment on the efficacy of specifically scaled constraints. Nevertheless, the findings do provide a novel method of modifying the equipment and play area to facilitate the self-organization of movement patterns, which might prove a fertile area of future scaling research.
Cognitive Processing Factors
A well-established phenomenon within the motor learning literature is that cognitive processes influence skill acquisition and performance. Acquiring skills with heightened conscious involvement, characterized by the attempt to consciously discover verbal rules about the skill, is referred to as explicit motor learning [46]. Comparatively, acquiring skills via sub-conscious processes, whereby the learner has difficulty verbalizing the step-by-step processes of the skill’s performance, is referred to as implicit motor learning [47, 48]. Research over the past 2 decades has consistently shown that implicit acquisition of motor skills is more advantageous than explicit learning when performance is subsequently required in environments that induce psychological stress [47, 49] or physiological fatigue [50, 51]. Furthermore, dual-task transfer tests have shown that individuals who have acquired a skill implicitly are able to simultaneously perform a cognitively demanding secondary task whilst performing the motor skill [52–54]. In contrast, individuals who acquire a skill explicitly typically have difficulty multi-tasking in these transfer tests.
Several practice methods have been identified that encourage implicit motor learning. Most relevant to this review is the concept of ‘errorless’ or ‘error-reduced’ practice. Research across a range of skills demonstrates that when errors are infrequent during practice, skills are acquired with minimal reliance on cognitive resources (i.e., working memory); thus, implicit learning benefits are evident [53–58]. Given that scaling equipment simplifies skills for children, thereby increasing success experienced, it can be reasoned that scaling will place fewer demands on working memory and, therefore, encourage implicit motor learning.
This hypothesis was recently examined using a dual-task methodology to measure children’s skill performance when attention resources were occupied by a secondary task [25]. Children performed a basic tennis-hitting task in two attention conditions (single-task and dual-task) using two types of equipment (scaled and full size). The scaled equipment included a lower compression ball and a smaller racquet (23-in. length), whereas the full-size equipment included a standard tennis ball and an adult-sized racquet (27-in. length). Results showed that hitting performance and hitting technique were better when scaled equipment was used, demonstrating that scaled equipment did indeed simplify the skill for children. For the less skilled children in the study, hitting performance was not disrupted by a cognitively demanding secondary task when using scaled equipment. However, performance deteriorated significantly when full-size equipment was used, suggesting that equipment that increases skill difficulty places larger demands on working memory resources than equipment that does not (i.e., scaled equipment). While this study only assessed conscious processes during performance on a small number of trials (as opposed to a learning design), the results corroborate the prediction that modification of equipment to simplify a skill reduces conscious processing.
The influence of equipment modification on conscious processing can also be inferred from studies with adults. A golf putter designed to increase skill difficulty resulted in greater preparation time prior to skill execution, which the authors interpreted to represent greater conscious processing [59]. Similarly, equipment that increased skill difficulty demanded greater attention resources during movement preparation and movement execution [60]. Thus, consistent with the findings of Buszard et al. [25], equipment that increases skill difficulty (e.g., full-size equipment for children) places heavy demands on attention resources, thereby leading to a more explicit control of motor performance.
Interestingly, a similar hypothesis regarding equipment modification was expressed over 40 years ago. In a study that examined the acquisition of throwing skill, Egstrom et al. [61] explained, “the adjustments made during the practice periods while learning to throw the light ball accurately resulted in automatic adaptations at a subconscious level. When the subjects then transferred to the heavy ball after a period of practice, the increased weight could have elicited a response … which in turn brought the impulse to consciousness…” (p. 424). Hence, throwing with a lighter ball seemingly encouraged implicit motor learning, whereas the heavier ball more likely activated explicit processes.