To the best of our knowledge, this is the first systematic review to evaluate the efficacy and safety of plyometric training in older adults. The results indicate that plyometric exercises might have potential for improving various performance (muscular strength, jump and physical performance), functional (postural stability, daily function), and health-related (bone health, body composition) outcomes in older persons (Table 4).
Despite a recent proliferation of published articles on the effect of plyometric training in various populations [39], we identified only 12 randomized trials (289 subjects) that examined the effect of plyometric exercises in older adults. In addition, most of the trials were relatively small, with the largest one including only 36 subjects. Furthermore, in only six studies [43, 52, 64, 65, 67, 68] could the effect of plyometric exercises be evaluated in isolation and, of those, only two studies [64, 67] compared plyometric exercises with an alternative form of exercise with equalized volume. Therefore, it would be scientifically unjustifiable to draw conclusions related to the effects of plyometric training alone versus plyometric training combined with other exercise methods. Given the potential benefits of plyometric training in older adults and the small number of studies allowing for direct comparisons between exercise modes, more research with larger sample sizes and well-designed active control groups is needed in this population.
With one exception, the studies identified in this review recruited healthy older adults. Thus, in future studies, it would be worthwhile to verify these findings in chronically ill older patients, who may respond differently to plyometric exercise and may require unique safety precautions.
In contrast to the findings of the recent scoping review that reported that less than one-quarter of plyometric jump training studies included females [39], we found that females constituted the majority of subjects (176 females and 113 males), which might be explained by increased interest by researchers in the positive effects of jumping exercises on bone composition in postmenopausal women. Although many other studies have investigated the effects of weight-bearing and impact exercises on bone mineral density, especially in women, those studies were excluded from our review because they did not include a plyometric component.
The methodological quality of the studies in this review was good (mean PEDro score of 6.0), at least in comparison with other reviews of plyometric training in which the PEDro score ranged from 4.5 to 5.25 [28, 45, 46]. To further improve the methodological quality of future studies, researchers should consider the blinding of assessors and aim for effective allocation concealment. Some of the papers notably omitted important training descriptors. For example, only a handful of the papers described exercise intensity [59, 60] or mentioned the type of training surface [53]. Reporting on these intervention details is crucial for leveraging the findings of such studies for future research and practice.
Interpretation of Study Results
Muscular Strength
The strength of various leg muscle groups was reported as an outcome in eight studies [54, 55, 59, 60, 63, 64, 67, 68], primarily using dynamometry. While the majority of these studies [55, 60, 63, 64, 67, 68] found an improvement in muscle strength when comparing a plyometric or combined training with a control group, comparisons between different training modalities yielded ambiguous results. For example, a study that compared resistance training, balance-jumping training, a combination of resistance and balance-jumping training, and a control group found that relative isometric leg press force improved in the resistance and combination (+ 21.7% [+ 3.6 N/kg], effect size (ES) 0.86, p < 0.01) groups, but not in the balance-jumping group [55]. Similarly, in a study comparing plyometric and pneumatic power training two to three times per week, the pneumatic training group showed significantly greater rapid knee extension torque production after only 4 weeks of training (p < 0.01), while the plyometric group showed a significant change only after 12 weeks (p < 0.01) [64]. While a frequency of two to three sessions per week did not increase strength after 4 weeks in that study, another study showed that 4 weeks of jump training five times per week was sufficient to increase hip extension strength (+ 49% [+ 8.5 kg], ES 1.67, p < 0.001) [68]. Together, these results suggest that training duration, frequency, and volume are important variables that need to be considered when designing plyometric interventions for older adults [39, 69].
Yet another study compared the effect of isokinetic eccentric actions and stretch–shortening cycle (plyometric) contractions on quadriceps strength [67]. Both training programs produced similar improvements in maximal voluntary isometric and eccentric torque and stretch–shortening cycle function. However, the rate of torque development during isometric contraction increased only after plyometric exercise (+ 29% [+ 0.42 Nm ms−1], ES 0.55) [67]. Therefore, according to the studies included in this review, it is likely that plyometric training directly or indirectly increases muscular strength in older adults, but probably not to the same magnitude as resistance training [70, 71]. Additionally, limited evidence [67] suggests that plyometric exercises are superior to eccentric training in improving explosive muscle strength, which is a key deficiency of aging muscle. Of the many possible mechanisms underpinning strength adaptations that occur after plyometric training, the inhibition of Golgi tendon organs combined with repeated activation of muscle spindles may be the most likely explanation [42]. As muscle spindles are stretched during plyometric training, a neuromuscular reflex likely occurs, which may activate higher threshold motor units that would normally not be used [72, 73]. Long-term exposure to such stimuli may decrease neuromuscular inhibition, which would likely result in greater muscle activity and, in turn, greater strength. However, the myriad of mechanical and physiological variables that contribute to strength adaptation are extremely complex and would require greater elaboration, which is outside the scope of this review. Nevertheless, the data extracted from the studies of this review indicate that plyometric training likely increases muscle strength, with no studies indicating that muscle strength decreased as a result of plyometric training.
Bone Health
Of the six studies [52, 54, 55, 59, 60, 65] assessing bone health, usually by dual energy X-ray absorptiometry (DXA), only two showed positive results [52, 54]. A 12-month study of high-impact unilateral exercise (up to 50 multidirectional hops a day, 7 days a week) resulted in significant improvements in femoral neck bone mineral density (+ 0.6% [+ 0.006 g/cm2], ES 0.34, p < 0.05), bone mineral content (+ 0.7% [+ 0.04 g], ES 0.30, p < 0.05), and geometry [52]. Yet another study comparing a 52-week multicomponent intervention with non-exercising controls found a non-significant benefit of exercise on mean total hip bone mineral density (+ 0.4% [+ 0.003 g/cm2], ES 0.04); however, this was significantly greater than in the control group (p < 0.05) [54]. Other studies did not find a significant effect of exercise on bone composition [55, 59, 60] or bone turnover [59, 65]. Of note, in both studies showing positive results [52, 54], the length of the intervention was 52 weeks, while in the remaining studies, the length of the intervention was shorter (11–40 weeks), with the exception of Karinkanta et al. [55], which also lasted 52 weeks but only included a minor plyometric component. Thus, these results are in line with previous findings that sufficient training duration (and volume) are required to achieve significant improvement in bone health [74].
As these data indicate limited benefits of plyometric training for improving bone health, at least in the short-term, it is important to note that there is likely a trade-off between training to enhance neuromuscular performance and training for bone health. For example, studies included in the present review must have included plyometric training characterized by a rapid eccentric muscle action followed by a forceful and rapid concentric action. Consequently, impact forces are largely absorbed during the eccentric phase of landing, and the resultant elastic energy is then coupled with concentric force to execute the following jump, ultimately resulting in very little impact compared to jumps with “hard landings” [75]. To achieve harder landings, subjects are actually instructed to jump and land as heavily as comfortably possible, likely with the legs in a straighter position, without purposefully and eccentrically absorbing force [76]. As a result, it is likely that these impact forces are much greater than those experienced during plyometric training, where the initial impact forces are better absorbed through flexion of the hips and knees. Therefore, although plyometric training likely does not play a large role in increasing bone health, it should not be confused with jump training that includes hard landings and higher impact forces, which are likely to be more effective at increasing bone health [76]. Also, it is important to keep in mind that bone health naturally decreases in older adults, and although the findings of this review indicate that plyometric exercise may not increase bone health per se, any maintenance of bone health should still be considered a positive clinical outcome. Therefore, although there are data presented here and in Table 4 indicating that there may not have been a ‘positive effect’, as seen for other variables, the fact that there were no ‘negative effects’ of plyometric training on bone health is clinically and practically significant.
Body Composition
The body composition category includes assessments of either whole-body masses (four studies [52, 59, 60, 62]) or thickness of quadriceps muscles (two studies [63, 67]). Of the three studies assessing total lean and fat mass by DXA, two studies failed to demonstrate any improvement [52, 59] and one study showed that in the group with a plyometric component, fat mass decreased (– 5.4% [– 1.7 ± 2.0 kg], ES 0.28, p < 0.01) more than in a non-exercising control group (p < 0.001) but similarly to that of another group that completed non-plyometric exercises [60]. Interestingly, the study by Ramírez Villada et al. [62] showed that the percentage of muscle mass increased when calculated by body composition equations (+ 5.2% [+ 1.9], ES 0.48, p < 0.05) but did not observe any effect on absolute skinfold measurements measured by Holtain callipers. Regarding quadriceps thickness, one study showed an increase (+ 2.1% [+ 106 mm2], ES 0.12), which was the same as in the comparative exercise group [67], and the results of another study were unclear [63]. Additionally, it is important to note that the multi-factorial nature of the exercise programs utilized in many of these studies likely meant that the researchers were interested in the effect of the exercise programs as a whole on body composition. As such, if changes in body composition are desired, plyometric training is likely not to be the primary exercise choice for inducing changes in body composition but may be included in a periodized program to result in additional functional adaptations that may not arise from other forms of exercise interventions. Therefore, the results indicate that, similar to any other physical activity, plyometric training is associated with changes in body composition, but its effects are not likely different from those of other exercises of similar volume and intensity.
Postural Stability
Postural stability was assessed in five studies [43, 54, 64, 65, 68], either by various balance platforms (both static and dynamic) or by a functional test (Berg balance test). Two studies that evaluated additional jumping exercises in addition to a combined training program demonstrated improvements of various stability scores, such as overall stability where a negative value is a positive finding (– 34.0% [– 1.02 ± 0.29], ES 0.83, p < 0.0001) [43] and Berg Balance score (+ 16.6% [6.6 ± 2.7], ES 1.69, p < 0.001) [68]; these improvements were greater in the group with additional jumping than in the group with the combined training program alone (p < 0.05 and p < 0.001, respectively). However, two other studies that compared an exercise program with non-exercising controls failed to show any between-group differences [54, 65]. This is surprising and contrary to the findings of others [77]. Bolton et al. [54] explained the lack of effect as an insufficient intensity of the intervention and partially unsupervised home-based training with low adherence to exercise. Similarly, insufficient volume, insufficient intensity, or a combination of both can likely explain the lack of effect in the study by Rantalainen et al. [65], as their intervention was not effective at improving any of the study outcomes. Therefore, it appears as though plyometric training positively affects postural stability provided the training program has sufficient volume and intensity. In practice, increasing static stability, dynamic postural stability, or both may translate into better balance during activities of daily living. As a result, an older adult’s fall risk and fear of falling may decrease, which in turn may lead to increased levels of habitual physical activity [78, 79] and reduced disability and morbidity [80].
Jumping and Power-Based Measures
Jump performance was usually assessed on a force plate and was reported in five studies [52, 62,63,64, 66]. In an 11-week study, hopping training improved jump height (p < 0.01) in older men by decreasing the contact time (p < 0.05) and increasing the ground reaction force (p < 0.01) and reactive strength index (p < 0.01) [66]. Furthermore, a study comparing three exercise modalities found improved counter-movement jump height (+ 25%, p < 0.05) in older women whose training included plyometric jumps in comparison with women in traditional resistance training and power training groups (p < 0.05) [63]. Lastly, another study reported significant improvements in explosive strength measured by the subjects’ response to various types of jumps (e.g., height of countermovement jump with arm swing improved by 30% [+ 4.5 cm], ES 1.17, p < 0.05) in training groups that included jump training [62]. Therefore, according to the principle of specificity, it is not surprising that jumping exercises have a large impact on jump performance, especially compared with other types of training that are not task specific. Although jumping is unlikely to be part of daily life for older populations, its strong relationship to other performance measures highlights its use in scientific research. Specifically, jump and power-based performance are positively related to physical function [81] and quality of life [82], and inversely related to chronic diseases such as osteoarthritis, diabetes mellitus, and cardiovascular diseases [83]. Therefore, implementing plyometric training to increase lower-limb power output likely results in positive adaptations that reach far beyond the realm of jumping and other force plate measures.
Physical Performance
To evaluate physical performance, the studies used various tests (30-s sit-to-stand test, figure-of-8 running test, timed up-and-go test, 6-m walk, stair climb), with mostly positive effects. For example, in the Park et al. [68] study, the subjects improved in the timed up-and-go test (– 32% [7.3 ± 6.5 s], ES 0.87, p < 0.001) after only 4 weeks of an intervention that combined therapeutic exercises with jumping, and this improvement was significantly greater than in the group with therapeutic exercises alone (p < 0.01). Similarly, Correa et al. [63] showed that older females participating in a 12-week plyometric training program improved in the 30-s sit-to-stand test (+ 17%, p < 0.05), an improvement that was significantly greater than in the group performing traditional strength training (p < 0.05). Yet another study reported a significant improvement in a standardized figure-of-8 running test in groups that performed balance-jumping exercises alone (– 5.8% [– 1.2 s], ES 0.41, p < 0.01) or in combination with resistance training (–8.1% [– 1.7 s], ES 0.62, p < 0.001), but not in a resistance training-only group [55]. The improvement was maintained at a follow-up 12 months after the end of the intervention [56]. However, in this study, the plyometric (i.e., jumping) component formed only a minor part of the balance-jumping training, and thus it is not clear whether the improvement can be attributed to the plyometric component or rather to the balance-specific exercises. Therefore, it seems that plyometric training not only improves physical performance, but in specific tests, it may even be superior to other types of training.
Other Measures and Considerations
Four studies [43, 54, 55, 59] used various questionnaires to assess health-related quality of life and/or daily function. Their results were more or less positive, but did not show any superiority of plyometrics over other types of training. Three studies [63, 64, 66] employed electromyography; of note is the study by Correa et al. [63], who demonstrated that plyometric training improves muscle onset latency (– 30% [– 88 ms], ES 2.01, p < 0.05) and reaction time (– 29%, p < 0.05) of the quadriceps muscle group better than traditional strength training (p < 0.05). Two studies assessed serum levels of testosterone and cortisol hormones but failed to show any effect [59, 67]. One study demonstrated a positive effect of plyometric training on capillary glucose (– 8.9% [– 8.2 mg/dL], ES 1.39, p < 0.001) compared with non-exercising controls [62], indicating that future researchers may wish to further investigate the effects of plyometric training in older patients with diabetes or pre-diabetes. Finally, one study assessed various kinematic, biomechanical, and muscle architectural variables, but its outcomes are unclear [66]. Therefore, the lack of studies and incongruous results of these studies do not allow for conclusive statements to be made regarding the effects of plyometric training on these isolated variables; future research should consider investigating them further.
Safety
Of the 289 subjects who actively participated in exercise programs that included plyometric components, only a maximum of 1.4% incurred an injury that resulted in the subject dropping out of the study. Therefore, data extracted from the studies included in this review indicate that plyometric training is likely safe to perform in older adults, especially under supervised conditions. Though only two studies included a home-based component, we may also assume that practicing plyometric training at home does not incur a significantly increased risk for the older adults, an assumption that is consistent with a previous study of high-speed training under low-supervision conditions in older women [84]. However, as stated in Sect. 3.3, the studies included in this review included mostly healthy subjects, meaning that future research should be conducted on less healthy subjects to determine whether these benefits translate across populations or if additional benefits of plyometric exercise can be identified in specific populations. Nevertheless, the data from the included studies indicate that with proper instruction, and possible supervision, the traditional strength pre-requisites that were established for athletes (i.e., the ability to perform a back squat with 1.5–2.0 times bodyweight) [40, 41] may not apply in older adults, and that basic movement competency followed by periodized progression is likely sufficient for healthy older adults.
Strengths and Limitations
The clear strength of our review is the elaborate search strategy that, rather than relying on searching only for the term “plyometric”, combined various related terms describing potentially plyometric exercises, such as “hopping” or “jumping”. This strategy required all the papers resulting from the searches to be carefully studied to make sure that the exercises truly were plyometric. This approach proved worthwhile, as many of the papers ultimately included in this review did not contain the term “plyometric” and would not have otherwise been found. Interestingly, a recent large scoping review of plyometric jump training up to April 2017 [39] that limited the search to “plyometric” identified 242 eligible papers, but only one of them included subjects over 65 years old [64]. That being the case, future reviews of plyometric training should consider using the search strategy employed in our review instead of relying on just the “plyometric” term.
We also understand that some may view the lack of meta-analysis of the effects of plyometric training on key outcomes as a limitation of the present review. Unfortunately, the limited number of eligible studies and the heterogeneity of the outcomes and assessment methods did not allow for a meaningful meta-analysis at this time, as the number of studies available for each outcome were too low. In addition, the already small number of available studies was further fragmented by the design of the control group. For example, the isolated effect of plyometric training in comparison with the non-exercising group could be evaluated in only three studies [52, 65]. Other studies either compared plyometric training with a different form of exercise or they included plyometric exercises only as a part of multicomponent training.