Introduction
Throughout childhood, children are encouraged to participate in a wide range of physical activities. A common form of organized physical activity is resistance training (RT), the method of physical conditioning that uses progressively increasing resistive loads and various training techniques to achieve desired muscle strength, power, muscle hypertrophy, local muscular endurance, or a combination thereof (Miller et al., 2010). In the 1970s and 1980s, RT was often not recommended for children due to a perceived injury risk. Subsequent research found that much of the reported injuries were due to inappropriate training techniques, excessive loading, poorly designed equipment, ready access to the equipment, or lack of qualified adult supervision (Faigenbaum et al., 2009). In further support, research found that injury rates in adolescents engaged in RT and weightlifting were lower than common sports such as football, basketball, and soccer (Faigenbaum et al., 2009). On top of RT being a low-risk activity, it has also shown to improve confidence and self-efficacy in untrained adolescent girls (Holloway et al., 1988). While research is not conclusive on this subject, no studies have found RT to negatively impact psychological well-being. The effects of RT in youths may encourage lifelong engagement in physical activity, as lifelong habits are typically established in childhood (Westcott et al., 1995).
So, RT in children has shown to be low-risk, improve confidence, and perhaps engrain habits that will trigger lifelong physical activity. That being said, a common goal of RT programs is to improve physical capabilities. In pubescent and adult individuals, RT has shown to increase muscle mass, strength and power production and that has been attributed to role the endocrine system plays in growth (Haywood & Getchell, 2014). However, in prepubescent boys, muscle cross-sectional area did not increase following a RT program despite increases in strength (Ramsay et al., 1990; Ozmun et al., 1994; Granacher et al., 2010; Faigenbaum et al., 1993). Additional research has shown increases in rate of force development (Waugh et al., 2014) and utilization of the stretch shortening cycle (Radnor et al., 2017) despite no changes in muscle size. So, that begs the question, what mechanisms are responsible for increasing physical performance in youth populations?
Understanding the mechanisms that change or do not change is important to study to gain a better understanding of what physically, mechanically, and neurologically changes in children as they go through RT. A better understanding of these mechanisms can provide greater insight for practitioners to individualize programs for a given child. Perhaps, this understanding can guide programs to achieve better results from RT.
The purpose of this paper is to evaluate the mechanisms that drive improvement after a RT program in children.
This paper will review empirical studies that have evaluated different mechanisms of improvement following RT programs in children. The first section will evaluate tendon stiffness, as increased stiffness has been shown to improve power production. Next, the paper will address the role motor unit activation (MUA) contributes in driving improvement. The following section will review studies that found strength increases without increases in muscle CSA. Finally, the paper will discuss implications for practitioners as well as areas for further research.
Review of the Research - Mechanisms of Improvement
Tendon Stiffness
Tendons are interposed between muscles and bones to form a musculo-tendon unit (MTU) that transmits muscular forces directly to the bone, creating movement or stability around a joint (Radnor et al., 2017). As tendons are exposed to stress, they become tougher, thus increasing the capacity to store and release elastic energy (Shadwick, 1990). Tendons act as biological springs, aiding in force production. Long, pliable tendons can be described as force amplifiers which take advantage of the tendon’s ability to store and reuse elastic energy. On the other hand, short, thick tendons are stiffer and are more effective at transferring muscular forces to bone due to their resistance being stretched, leading to greater rate of force development (RFD) and reduced electro-mechanical delay (EMD; the delay between muscle activation and the onset of force production) (Radnor et al., 2017). So, the dimensions of the tendon could lead to more economical utilization of the stretch-shortening cycle (SSC) or increase speed of movement due to faster force transfer between the muscle and bone.
Comparison of Tendon Stiffness in Adults and Children
A 2013 study (Waugh et al.) compared the mechanical and neural contributions of rapid force production in prepubertal children and adults. 47 children (24 boys, 23 girls) ages 5-12 years (8.3 ± 1.6 years), 10 men (27.0 ± 1.9 years), and 9 women (25.3 ± 3.4 years) participated in the study. For analysis, children were broken up into age groups 5-6 years, 7-8 years, and 9-10 years. (Note: the study reported sample ages of 5-12 years, yet groups only went up to 10 years.) Adults were only separated by gender. All participants were free from known neuromuscular and musculoskeletal disorders and did not partake in competitive sports.
The purpose of the study was to partition the neural and mechanical contributions to the developmental changes in force production capacity in prepubescent children.
The authors measured Achilles tendon stiffness, RFD, electromyographic (EMG) activity of the gastrocnemius medialis (GM) muscle, and electromechanical delay (EMD) of the GM. To measure Achilles tendon stiffness, participants performed five to eight submaximal isometric plantarflexion contractions followed by three to five maximal contractions. All consecutive contractions were separated by a 30 sec rest period. To measure RFD, participants were instructed to rotate the foot away from the body “as hard and as fast as possible.”
The authors found a significant main effect of age on tendon stiffness. Stiffness was significantly greater in adults (259.3 ± 41.9 Nmm-1) than in CG9-10 (165.4 ± 39.9 Nmm-1), CG7-8 (146.1 ± 42.1 Nmm-1), and CG5-6 (85.7 ± 29.8 Nmm-1). Achilles tendon in the CG5-6 group was significantly lower than all others.
The authors found a significant main effect of age on EMD. EMD in CG5-6 (96.0 ± 14.0 ms) was significantly longer than in CG7-8 (77.4 ± 17.7 ms), CG9-10 (74.5 ± 13.7 ms), and adults (50.4 ± 12.2 ms). No difference in EMD was observed between CG5-6 and CG9-10, but both groups had a significantly longer EMD compared to adults.
There was a significant main effect of age on RFD. RFD and RFDnorm (RFD30%, RFD50%, and RFD70%) were significantly lower in all children than adults.
There was a significant main effect of age on all measures of REI (rate of EMG increase) except REI to 25 ms. REI was significantly greater in adults compared with CG5-6 and CG7-8 when calculated to 75 ms after EMG onset and all child age groups when calculated to 150 ms. REI at 75 and 150 ms was greater in CG9-10 than in CG5-6 and CG7-8.
The main findings in the study were the following: 1) EMD, RFD, and REI in the plantar flexor muscle group substantially changed between 5 years of age and adulthood; 2) EMD was negatively correlated with Achilles tendon stiffness in children; and 3) Achilles tendon stiffness and muscle activation rate of the GM have a cumulative effect on the prediction of plantarflexion RFD in children. This indicates that rapid force production is dependent on increased rates of muscle recruitment and changes in tendon mechanical properties in children. These findings also point out the age-related improvements in muscular force production and have implications for the performance of complex motor tasks.
Effects of Resistance Training on Tendon Properties and Rapid Force Production in Children
After establishing the differences in Achilles tendon mechanical properties between adults and children, the same group of authors studied the effects of RT on the Achilles tendon in children (Waugh et al., 2014). The main purposes of the study were to examine the effects of plantar flexion RT on the mechanical properties of the Achilles tendon in prepubertal children, and to determine the mechanisms underpinning potential adaptations.
Ten boys and ten girls (age 8.9 ± 0.3 years) volunteered to participate in the study. Participants were evenly divided (5 boys and 5 girls) into control and experimental groups. Children in the experimental group followed a twice-weekly, plantar flexion RT program. Children were trained on a recumbent (45-degree incline) calf-raise machine. The first author was present during all training sessions, and progressively loaded the children based on their perceived effort.
There were no significant differences between the control and experimental groups for any variable measured pre-RT. EMD decreased on average by 9.8 ms (~13%) in the experimental group (pre-RT 76.3 ± 11.0 ms; post-RT 66.5 ± 10.2 ms) and remained unchanged in the control group (pre 74.5 ± 10.8 ms; post 74.2 ± 12.3 ms). However, the time-by-group interaction for EMD did not reach statistical significance (P = 0.096). Changes in EMD were moderately correlated with changes in tendon stiffness in the experimental group (r = -0.59, P = 0.066) and poorly correlated in the control group (r = -0.24, P = 0.278). No time-by-group interaction effects were found for RFD (P = 0.185-0.924) or REI (P = 0.106-0.926).
The main finding was that the mechanical properties of the Achilles tendon were significantly altered by 10 weeks of twice-weekly RT in previously untrained prepubertal children. In the experimental group, tendon stiffness increased by 29% when calculated using absolute tendon force and elongation. These values are similar to those reported previously for adults (15-64%) performing moderate duration RT programs. The results suggest that immature human Achilles tendons respond to chronic loading in a similar way to mature tendons, thereby adding to previous finds by confirming, for the first time, that strength training can increase tendon stiffness.
Importantly, no change in tendon CSA was detected after RT, leading to the conclusion that tendon stiffness increased as a result of a significant (25%) increase in the Young’s modulus. Increases in the Young’s modulus are indicative of changes in the tendon’s underlying microstructure.
Motor Unit Activation
A motor unit is the functional unit of motor control for the innervation of the muscles involved in a movement (Magill & Anderson, 2017). To increase the amount of force exerted by a muscle, a process known as motor unit recruitment occurs in which the number of motor neurons activated increase (Magill & Anderson, 2017). In children, it is hypothesized that increases in strength following a RT program may be attributed to an increase in motor unit activation (MUA) amongst other factors (Ramsay et al., 1990). This is attributed to evidence indicating that prepubertal children do not gain muscle mass after engaging in a RT program (Vrijens, 1978), so other factors must contribute to observed strength gains.
Strength Training Effects on Prepubescent Boys
Ramsay et al. (1990) conducted a study to evaluate the effects of a 20-week RT program on prepubertal boys. They had three purposes: 1) to examine the effects of a 20-week high intensity progressive RT on maximal voluntary strength, evoked contractile properties, muscle cross-sectional area (CSA), and motor unit activation in prepubescent boys; 2) to determine the time course for strength adaptations; and 3) to identify the mechanism(s) underlying possible training induced strength gains in this population.
Participants were boys (N = 26) aged 9-11 years and were split into experimental (N = 13) and control groups (N = 13). The experimental group trained three times per week for 20 weeks under supervision. Training was divided into two, 10-week phases. Each phase was progressed according to the author’s guidelines.
RT resulted in significant increases in the 1 RM bench press (34.6%) and leg press (22.1%). Significant gains in the bench press occurred during phase 1 (+20%) and phase 2 (+14.6%), whereas significant gains in the leg press occurred only during phase 1 (+16.8%). There were no significant differential effects of training on any of the measured CSAs. However, there were significant main effects in both groups for time, indicative of growth, for lean arm (+2.6%), elbow flexor (+7.7%), lean leg (+5.3%) and thigh extensor (+8.1%) CSAs. Neither training nor growth had any significant effect on the %MUA of the elbow flexors or knee extensors. However, there was a trend toward increased %MUA for both the elbow flexors (13.2%) and knee extensors (17.4%) in the experimental group.
This study showed significant increases in strength in prepubertal boys who undergo a 20-week RT program. Despite specific and heavy loading, CSAs of the elbow flexor, knee extensor, or total lean arm and leg did not significantly change. These results are consistent with other findings. It is assumed that muscular hypertrophy is lacking due to inadequate levels of testosterone in prepubertal boys. MUA has been suggested to increase following RT. However, the results of this study indicate that increases in strength can only partly be attributed to increases in MUA.
Neuromuscular Adaptations Following Prepubescent Strength Training
Ozmun et al. (1994) conducted a similar, yet shorter study to observe the causes of strength gains in prepubertal children. The purpose of their study was to examine the effects of an 8-week weight training program on muscle strength and integrated EMG activity in prepubertal youths.
Participants were boys (N = 8) and girls (N = 8) ages 9-12 years. Participants were split into control (boys = 5, females = 3) and experimental (boys = 3, females = 5) groups. The experimental group participated in RT three times per week for eight weeks. Resistance was progressed in accordance with increases in performance, and participants were supervised throughout.
Subjects who participate in a weight training program exhibited a significant (27.8%) increase in isokinetic strength of the biceps brachii. The training group also demonstrated significant isotonic strength gains (22.6%) compared to the control group (3.8%), who showed no significant isotonic strength gains. There was a significant increase in neural activity pretest to posttest scores in the trained subjects (16.8%). There were no significant changes in upper-arm circumferences or skinfolds in either the trained or control groups.
The authors believe that the strength and EMG increases may reflect an enhancement in motor unit recruitment, improvement in the firing rate of activated motor units, or alteration of EMG firing patterns. Their results support other findings that show increases in strength of prepubertal children without increases in muscular hypertrophy.
Strength Improvements without Muscular Hypertrophy
Effects and Mechanisms of Strength Training in Children
Granacher et al. (2010) conducted a study to further investigate the mechanism of high-intensity strength training in children. The purposes of the study were the following: investigate the effects of a standardized high-intensity strength program on knee extensor/flexor strength, countermovement jumping (CMJ) height, static postural control, soft lean mass, and CSA of the quadriceps muscle of the dominant leg.
Participants (N = 32, age = 8.6 ± 0.5 years) were randomly assigned to experimental (boys N = 8, girls N = 9) and control (boys N = 10, girls N = 5) groups. Children in the experimental group trained two times per week for 10 weeks. A subsample of 13 subjects (n = 6 experimental; n = 7 control) participated in pre and post-training MRI scans of their quadriceps to measure changes in CSA.
The results indicated significant Group x Test interactions for the knee extensors at movement velocities of 60°/sec and 180°/sec. The statistical analysis also indicated significant Group x Test interactions for the knee flexors at movement velocities of 60°/sec and 180°/sec. Group x Test interactions were not significant for soft lean mass of the dominant leg and the CSA of the quadriceps muscle.
The authors concluded that peak torque of the knee extensors and flexors were significantly improved at movement velocities of 60°/sec and 180°/sec following 10 weeks of high-intensity strength training. The lack of significant changes in CSA following RT support the findings from other studies who examined these effects (Vrijens, 1978; Ramsay et al., 1990; Ozmun et al., 1994). The authors believe that the increases in strength observed can be attributed to increased MUA.
The Effects of a Twice-a-Week Strength Training Program on Children
Faigenbaum et al. (1993) conducted a study with the purpose to evaluate the effects of a short-term, twice-a-week strength training program on children.
25 boys and girls were split in an experimental (mean age 10.8 years; 11 boys, 4 girls) and a control (mean age 9.9 years; 6 boys, 4 girls) group. Children in the RT group trained twice per week on nonconsecutive days for 8 weeks. Instructors were present to supervise and appropriately adjust training load and repetition.
The experimental group showed significant increases in the 10-RM leg extension (64.5%), leg curl (77.6%), chest press (64.1%), overhead press (87.0%), and biceps curl (78.1%). The control group gained on average 13.0% (range 12.2 to 14.1%) for the five motions tests. The strength gains in the experimental group were significant for all five motions compared to the control group. Strength training resulted in a significant decrease in the sum of seven skinfolds (-2.3%) compared to the control group (+1.7%).
An interesting result of this study showed that the magnitude of strength gain (mean 74.3% for the five motions tested) was greater than the typical response despite reducing training frequency from thrice to twice per week. This may not be indicative of strength gains over the long-term, but is still noteworthy when children are beginning RT. The experimental group reduced their levels of body fat, as measured by skinfolds, which conflicts with most of the literature on children and strength training. However, the training program did not affect limb circumferences, which is consistent with the literature that notes children’s strength gains without increases in limb circumference or muscle CSAs.
Implications for Practice
There are two main takeaways from the information presented above: 1) children improve strength without increases in CSA following a RT program, and 2) while mechanisms are not known in totality, increases in tendon stiffness and increases in MUA may be sources for strength gains.
While improvements in strength are well-researched in children, there is less research on the impact of increased tendon stiffness in children. That being said, tendon stiffness may have substantial effect on movement performance, as stiffer tendons take less time to transfer force (Waugh et al., 2014). Research has also suggested that a stiffer tendon will be more reactive to stimuli (Proske & Morgan, 1987). Thus, a stiff tendon may develop force faster and react faster in situations where force must be applied. When practitioners design RT programs for children, exercises designed to improve tendon stiffness should be implemented.
Another important consideration when working with children is that programs should be well-supervised and progressed appropriately. While strength training is considered a very safe activity, injuries may occur when there are inappropriate training techniques, excessive loading, poorly designed equipment, or lack of supervision (Faigenbaum et al., 2009). A negative experience with exercise could have implications over the lifespan, so again, RT should be done with a professional.
Implications for Further Research
In one study that trained tendon stiffness, the primary means of training was a recumbent calf raise machine (Waugh et al., 2014). While participants did improve their tendon stiffness, it is unknown if that increased tendon stiffness expresses itself in more child-like activities, like jumping, leaping, or hopping. These fundamental movement skills are inherent in a variety of activities, so evaluating if increases in tendon stiffness translate to improved RFD in these skills is still unknown.
Another area that warrants more research is the frequency at which children should engage in RT. Interestingly, one study (Faigenbaum et al., 1993) children participated in RT twice-weekly, yet realized greater strength increases than many studies where children participate three times per week. One possible explanation is that three times per week is too much for children, and they are overtraining. Another factor to consider is that children may not be as engaged in each session when the frequency is higher.
Conclusion
There is abundant research showing that RT in children is effective in improving physical characteristics such as strength and power. However, there is fewer research on what mechanisms actually drive those adaptations. In pubescent and adult populations, RT leads to increases in muscular hypertrophy which is largely attributed to gains from training. However, children do not secrete enough testosterone to develop muscular hypertrophy, so gains in strength and power from RT must come from other sources. One mechanism, tendon stiffness, has been shown to be much lower in children compared to adults (Waugh et al., 2013). However, following RT, it increases, leading to greater force production in a faster period of time (Waugh et al., 2014). Another mechanism that is believed to drive strength and power increases from RT is MUA. Although research is not conclusive, studies have shown trends in increases in MUA following RT.
The research presented in this paper can have several implications for practitioners: 1) RT is safe for children; 2) tendon stiffness can be trained in children; and 3) MUA plays a role in the increases seen from RT. Also, there are areas that require further research regarding the transferability and frequency of RT. To close, in order to promote physical well-being over the lifespan, practitioners are strongly encouraged to create a fun and safe environment for children who choose to participate in RT.
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