The idea of weight training for children (preadolescent and adolescent) has always raised questions. The concern most often raised is whether or not weight training puts children at an increased risk for injury. A secondary question that is raised is whether children would benefit from weight training, if they are not physiologically mature (e.g., low androgen levels). This chapter will review these issues and offer guidance to the proper (safe) training techniques and programs, followed by a review of the expected training responses and outcomes as characterized in the research literature.
Often the risk of the epiphyseal growth plate injuries is mentioned; however, to date there have only been a few published papers that document such injuries and often these injuries are owing to lack of adequate education (training) and supervision of the children. Often injuries are owing to improper lifting techniques or the participant’s workloads are progressed to quickly. However, injuries may increase for the child athlete, who has inadequate strength for his or her chosen sport and in these cases weight training offers protection from potential injuries.
Children have been shown to increase their strength from participating in weight training programs. Two separate meta-analytic reviews1,2 showed that children increased their strength with resistance training by 13% to 30%. These analyses also determined that there was a greater effect size seen in isotonic training, followed by isometric exercise and finally by isokinetic training. However, the reviews could not determine the optimal training program because the studies were limited by gender (mostly males), age range, differences in intensity, duration, and frequency. It is well documented that the strength (force) of a muscle is directly related to its cross-sectional area. Increase in muscle size is related to the androgen hormone. Because prepubertal children lack adequate androgen hormone, the strength gains at this age are largely believed to be because of neuronal adaptations. Various aspects of muscle strength testing, strength training, growth and maturation, metabolism, and muscle fiber types have been the subject of extensive body of published literature.1–20
When muscles respond in the manner for which they are activated, a specific motion either lengthening or shortening or no movement occurs. A concentric muscle action refers to a shortening of the muscle fibers accompanied by movement of the respective joint. The muscle is shortening because the actions of the muscle fibers are greater than the external resistance and the muscle shortens (Figure 5-1). Since the internal muscular forces are greater than the external resistance, it has also been described as performing “positive” work.
An eccentric muscle action results in a lengthening of the muscle accompanied by joint motion because the resistive forces are greater than the concentric forces of the muscle fibers. An example of an eccentric muscular action is the lowering of weight during a bicep curl (Figure 5-2). The weight is lowered in a slow and controlled manner resulting in the term “negative” work.
An isometric muscle action has no change in length of the muscle, and is not accompanied by joint motion, because the external forces equal the contractile forces of the muscle. Because the total length of the muscle/tendon unit does not change, the work is said to be “zero.”
Both concentric and eccentric muscle actions comprise the type of exercise called isotonic. Isotonic means same tension or same mass. In weight training, isotonic exercises include moving a person’s body weight during an exercise or lifting free weights. While the mass of a weight being lifted during an exercise does not change, the external force will vary depending upon the joint angle and joint angular velocity. The muscle tension during isotonic exercise will vary based upon the weight, joint velocity, muscle length, and type of muscle contraction (eccentric or concentric).
Isometric exercise places tension or resistance on the muscle or muscle group without the joint movement. Isometric exercises are well suited for individuals beginning an exercise regimen or for individuals who have sustained an injury and have limited range of motion or strength21; however, the strength gained is usually limited by the specific joint angle when performing the exercise.
Isokinetic exercises are those in which the movements occur at a constant angular velocity resulting in the same speed of movement. The speed at the joint angle is controlled via a machine called an isokinetic device with a constant resistance (Figure 5-3). Isokinetic devices are frequently used in rehabilitation and testing of muscular strength and power because of the reliability and safety of the devices. A drawback of using isokinetic devices is that the speed controlled by the devices seldom mimics the speed produced in natural movements.
Although, strength usually refers to the weight a person can lift, it is more appropriately defined as the maximal force that a muscle or a muscle group generates at a specific velocity.
Power is defined as the time rate of performing work (Power = work/time) and work is defined as the product of force on an object and the distance the object moves (Work = force × distance over which the force is applied). These formulas are used in designing, implementing, and testing a strength-training program.
Another type of exercise that combines power and strength is plyometrics. Plyometrics involves an eccentric loading of a muscle or a muscle group followed immediately by a concentric muscle action. The eccentric/concentric actions utilize the stretch reflex or stretch-shortening cycle in which the muscle is preloaded with energy during the eccentric phase (much like stretching a rubber bad apart) and the release of that stored energy for subsequent muscular actions (letting go of the rubber band). The stored elastic energy within the muscle is used to produce more force than can be provided by a concentric action alone.22–24 Researchers have shown that plyometric training can contribute to improvements in vertical jump performance, acceleration, leg strength, and muscular power.25–32 Most plyometric exercises include activities such as hops, depth jumps, bounds, skipping, jumps for the lower extremities, medicine ball drills, throwing, and other activities for the upper extremity.
Plyometric training program must follow recommended intensities and volumes and progress over time to avoid injury.33 Training volume are usually categorized according to the number of foot contacts per training session, starting from a low number of foot contacts and progressing upward over several weeks. In addition, it is recommended that plyometric activities be incorporated no more than two to three times per week to prevent undue muscular injury or soreness.
When beginning a strength-training program, the first factor to consider is the physical maturity of the individual. The physical maturity may prevent a child who is too small from properly being fitted to a machine when strength training or not coordinated in completing an exercise with the proper form. Alternate exercises are recommended for the safety of the children to prevent injuries. In addition to physical maturity, mental maturity should also be considered when developing an exercise program. Mental maturity may limit participation when the children cannot adequately follow directions or conduct themselves in an appropriate manner in the weight room to avoid risk of injury. Injuries occur less frequently during resistive training compared to actual athletic participation in children and maturity may be a predominate factor.34
When conducting a strength-training program, adequate supervision and teaching the proper use of weight training equipment is imperative to decrease the likelihood of accidental injuries in the weight room facility.35 Improper form, even at low intensity or resistance levels can lead to the risk of injuries when the intensity and resistance increases. Supervision should include not only verbal feedback about the proper form, movement, and breathing but also visual feedback where the athlete can see his or her movements using mirrors to be aware of poor biomechanics.36 Research has shown that a properly supervised strength-training program can improve strength gains and exercise adherence versus unsupervised strength-training programs.37,38 Children who participate in strength-training programs should also be taught the correct form and sound lifting techniques regardless of an equipment or body weight. Proper form and techniques will help the adolescents develop appropriate muscle strength and muscle balances and limits the potential for injury. This can be better accomplished with a lower athlete to supervisor ratio, especially in the early developmental phases of the strength-training program. It has been recommended that ratios of 1:10 up to 1:25 are adequate, depending upon the maturity and complexity of exercises performed.
Before initiating the strength-training program, it is imperative that the strength-training professional conducts a needs analysis to evaluate the physical requirements of a sport or athletic endeavor and evaluate the physical attributes of the children. Evaluation of the sport or athletic endeavor includes determining movement patterns, the physiologic requirements needed to participate in the sport (power, strength, or hypertrophy of the muscle), and an assessment of common injuries sites or potential for injuries.39,40 Assessment of the children includes previous training background, training experience, age, maturity, and experience.
Determining the selection of exercise is dependent upon the goals and objectifies and the needs analysis. Most exercises can be classified as they relate to the body. Core exercises mean using large muscle groups that involve two or more joint movements (multijoint exercises). These include the muscles of the back, shoulders, chest, thigh, and hip. Smaller muscle groups that usually involve one joint (single-joint exercises) such as the biceps, forearm, calf, neck are called assistance exercises.
Training frequency refers to the number of times strength-training sessions are completed in a given period. The training frequency is dependent upon many factors such as experience, training status, and other physical and sport requirements. Individuals who are inexperienced should begin a training program with relatively fewer sessions per week then increase as the training level of the individual progresses. In most cases, training sessions of one to two times per week is sufficient to begin stressing the body’s systems to adapt to the strength-training programs. As with training sessions, it is recommended that at least 1 day but not more than 3 days of rest should be taken before the next session begins, but it is also dependent upon the child. As the children become more advanced, training sessions can increase up to three times per week.
More training sessions can be accomplished and provide adequate rest, if a split routine is used. A split routine divides the training session into groupings, split between the upper body and lower body exercises. For example, a 4d/wk training session can occur by exercising the lower body on Tuesdays and Fridays and the upper body on Mondays and Thursdays. This type of program allows enough rest and recovery for the muscle group before the next session.41 Another alternative is to perform a “push” and “pull” exercise, where the strength-training program is divided into exercises in which the individual pushes weight (bench press, triceps extension) then pulls the weight (latissimus dorsi pull down or bicep curl). If the training loads are near the maximum capacity, more time for recovery will be required to minimize soreness and provide adequate rest. Some evidence exists that in previously trained individuals, recovery is quicker for training upper body muscles than lower body muscles when training with heavy loads.42
In a strength-training program, the term load refers to the amount of weight used in that specific exercise. As the load becomes heavier, the number of times an individual can lift (repetitions or reps) the load decreases, whereas the load becomes lighters, more reps can be accomplished. Load is often described as a percentage of what a person can lift. Most strength and conditioning specialists describe the load as a percentage of a repetition maximum (RM). The RM can be expressed as the greatest amount of load lifted 1 time (1 RM) or as the amount of load lifted for a specified number of reps, usually expressed as a 5 RM or up to 10 RM when lifting with proper form.
Training load is useful to establish the baseline or estimated amount of load to lift per session. Usually, the training load is based upon the percentage of the 1 RM. It has been suggested that 1 RM be used for testing strength with core exercises and use multiple RM testing for the assistance exercises.43 Determining the RM is dependent upon the goals of the individual and the demands of the sport. Heavy loads are useful for strength or power, moderate loads for hypertrophy, and light loads for muscular endurance. If an individual has strength-training experience, a 1 RM testing method can be used (Table 5-1).
1 | Warm up | Before initiation of RM testing, have the individual perform general warm-up for 5–10 minutes with 1 set of 10 reps using weight that can be easily lifted. Performing too may warm-up sets can fatigue muscles and decrease the accuracy of the testing |
2 | Rest | 1 min |
3 | Lift | Use a load that can be lifted between three and five times by adding weight to the warm-up set. Add 10–20 lbs to, or 5%–10% of, the weight lifted in the warm-up |
4 | Rest | 2 min |
5 | Lift | Use a load estimated to be near maximum load that can be lifted for 2–3 reps. Find load by adding increasing the weight lifted in step 3. Add 10–20 lb to, or 5%–10% of, the weight lifted, for upper body exercises; 30–40 lbs or 10%–20% for the lower body exercises |
6 | Rest | 2–4 min |
7 | Lift | Increase the load (weight lifted) by following weight recommendations in step 5 and have the individual attempt 1 RM |
8 | Rest | 2–4 min |
9 | Lift | If step 7 attempt was successful repeat the step. If step 7 failed, decrease the weight to be lifted as follows: subtract 5–10 lbs from, or 2.5%–5% of weight attempted in step 7 for upper body; 15–20 lb or 5%–10% for lower body. With the new weight attempt 1 RM |
10 | Repeat | Repeat the steps as necessary to find the 1 RM. 1 RM should be determined within five lifting attempts or sets |
If the individual has limited or no experience or he or she is an adolescent, estimating a 1 RM by using a multiple RM testing method is better suited. A 10 RM testing load is often recommended to estimate the 1 RM. The testing procedures are similar to the 1 RM procedures except for the number of reps lifted. Follow the steps for the 1 RM but use 10 reps for each set. Increase the weight for each set by half of the recommended loads for the 1 RM. As with the 1 RM testing protocol, determine the 10 RM within 5 total sets. After the 10 RM is found the individual estimated 1 RM can be calculated based on standard RM table while working with trainer or strength and conditioning specialist.
Although the RM testing has some flaws, it is still one of the best methods to determine loads used for resistance training exercises.44,45 Another method that is not commonly used but can be a good indicator of strength and power is a 1 RM equivalent. The 1 RM equivalent is a formula that takes into account the weight lifted and reps multiplied by an numeric equivalent.46
For example, suppose you have a soccer player who lifted 100 pounds for incline bench press a total of 14 times, his or her 1 RM equivalent will be equal to (100 × 14 × .03) + 100 = 142. Although not as accurate as the 1 RM procedure, it can be used for inexperienced athletes and for any athlete where safety is a concern.
The RM that will be used for determining resistance during a strength-training program, however, will need to be adjusted as the children learn the technique and become more experienced and as the muscles adapt to the stimulus. Many individuals just add weight randomly without determining the best resistance for overloading the muscles to increase gains. One method used that is relatively conservative and perhaps beneficial for children who are strength training is the “2 for 2 rule.”47 This particular method increases the load for the individual when the load becomes too easy. The rule states that the load should increase, if the individual can perform 2 or more reps over the assigned reps for that particular exercise over two consecutive workouts (Table 5-2).
Exercise | Lat pull down |
|---|---|
Goal reps | 10 |
Goal sets | 3 |
Load | Increase if, on the last set, the individual can perform 12 reps for two sessions |
If past training experinece increase load by | 5–10 lb for upper body exercises |
10–15 lb for lower body exercises | |
If limited or no past training increase load by | 2.4–5 lb for upper body exercises |
5–10 lb for lower body exercises |
A set is defined as the number of reps performed before a rest period. For example, if an athlete is to perform three sets of 10 reps, he or she would be lifting the weight 10 times in one set followed by a rest period and repeat the process another two more times for a total of 3 sets. When describing the volume, the sets and reps are written in a format that is easy to decipher. In the above example, the training program would be written as 3 × 10, where 3 represents the sets and 10 represents the reps lifted per set. Begin the training load by performing one set of six to eight exercises with 10 to 15 reps per exercise of all major muscle groups, then progress anywhere from one to three sets with 6 to 15 reps.35
The training volume describes the total amount of load lifted during a strength-training session. The volume is dependent upon the weight lifted, the reps, and sets. The total volume during a strength-training session is dependent upon the goals of the session, training focus, and time of year as reviewed in the section of periodization. Volume is calculated by multiplying the sets times the reps times the weight lifted per rep. For example, suppose an individual is slated to perform 3 × 10 reps with 20 pounds for the biceps curl, the formula would be written as 3 × 10 × 20, with the first number representing the sets, the second number representing the reps, and the third number represents the weight lifted per rep. By multiplying the numbers together, the training volume for that particular exercise would be 600 lbs. If each set has a different weight associated with it, the volume is calculated per each set and then all sets are added together. For example, the scenario above may be assigned as 1 × 10 × 15, 1 × 10, × 20, 1 × 10 × 25, the volume for each set is calculated then all sets added together to find a 600 lb training volume.
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