Introduction
According to the National Institute on Aging, by 2050 the population of adults aged 65 years and older is expected to reach 1.5 billion people (34). As humans age, muscular strength decreases 1–2% per year while the loss of muscular power may be even greater (approximately 3–4% per year) (31,33). Previous research has demonstrated the benefits of strength training in older adults; however, muscular power training may be of more importance (19,20,22). The preservation of muscular power in older adults can decrease the risk of disability while increasing functional ability (11). Muscular power has also shown a greater influence than strength on balance in older adults when prescribed with a lower load and higher velocity of exercise movement (29). Incorporating more rapid, force-generating exercises at lower intensities may be an important consideration in the exercise programming for older adults (11).
Resistance training has been demonstrated to be a safe and effective method for strength development in elderly adults and contributing to improvements of physical function and maintaining independence (14). Strength training for older adults is an important part of their exercise prescription; however, power training may have more practical outcomes. Although there are various methods of training for muscular power, a lower amount of an individual's 1 repetition maximum (1RM) moved with a higher velocity is often employed (19–22). Plyometrics (i.e., box jumps, depth jumps, and squat jumps (25)) are a common training mode for improving muscular power. Plyometrics have been shown to increase muscle force, power, and agility in adolescents and recreationally active adults (8,12,23,25). Although an increasing amount of evidence supports the application of plyometrics for the enhancement of muscular power, there are no published data on functional strength and power in older adults undergoing a plyometric training program. It may be difficult to use this mode of training in this population because of the complicated and demanding movements that require great neuromuscular control. Level of fitness must also be considered when prescribing plyometric exercise. The required strength and balance criteria that are commonly recommended for populations training with plyometrics are often difficult for older populations to achieve (16). Because of these quite challenging guidelines and the difficulty for older adults to meet these guidelines, plyometrics are not typically prescribed for exercise in older populations.
The design of equipment to maximize functional movements in people with decreased strength, balance, coordination, and exercise capacity has become increasingly popular. Body mass–supported treadmills may be one solution for individuals with these problems. These treadmills allow for improved mobility, strength, and safety while improving functional capacity related to endurance, strength, and power (30). Because of the stability and support experienced while using the treadmill, the risk of falls are virtually eliminated, and participants can perform more complex and dynamic exercises with the benefit of unweighing a portion of their body mass allowing for many plyometric exercises to be performed safely. Using this equipment may dramatically reduce the recommended minimum strength, speed, and balance requirements needed to safely perform such movements. Because of the stability, support, and reduced intensity that the body mass–supported treadmill provides and the evidence shown on plyometric training's effect on muscular power, this may provide the option for plyometric training in older adults. Therefore, the purpose of this study was to determine if performing plyometrics using a body mass–supported treadmill would lead to an increase in power output and functional movement in older adults when compared with traditional strength training.
Methods
Experimental Approach to the Problem
This study was designed to determine if a plyometric training program could lead to greater functional strength and power compared with traditional strength training in older adults. Participants were randomized to a strength training group (SG), a plyometric training group (PG), or a control group (CG). The SG and PG performed supervised exercise sessions 3 times per week for 8 weeks with at least 48 hours between sessions, whereas the CG performed no exercise. Strength training group performed 3 sets of 10 repetitions at 65–80% 1RM of a seated leg press, seated leg extension, and single leg lunge in a Smith machine. Plyometric training group performed 3 sets of 10 repetitions at 65–80% of their body mass of squat jumps, single leg bounding, and explosive skipping.
Subjects
A group of 23 older adults between 51 and 80 years volunteered for the study (Table 1). Exclusion criteria included any resistance training during the previous 6 months, uncontrolled diabetes or hypertension, previous cardiac event, orthopedic joint replacement surgery, use of any type of mobility aid (walker, cane, etc.), or any physical impairment that would limit their mobility. Health history questionnaire, signed physician's consent, and an informed consent form approved by Boise State University's Institutional Review Board were obtained before participation and after the subjects were informed of the risks and benefits of the procedures. Participants were randomly assigned to groups before completing the pretesting sessions.
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Table 1.: Participant characteristics (
±
SD).*
Procedures
Height and mass were measured using a calibrated stadiometer (Seca, Chino, CA, USA) and digital scale (Tanita Corporation, Arlington Heights, IL, USA). Body fat percentage was measured via air displacement plethysmography (BodPod; Life Measurement Instruments, Concord, CA, USA). On day 1, the following assessments were completed: timed sit-to-stand and stair climb, isokinetic knee flexion and extension, and estimated 1RM. A study investigator demonstrated each test, and participants were asked to complete a practice trial for each. In this randomized, pre-post study design, all dependent measurements were completed before and after the 8-week training intervention.
There are various methods for assessing muscular power, including vertical jump on a force platform, pneumatic resistance machines, stair climbing, or isokinetic dynamometry for specified joints. An appropriate power movement for an older adult could be a functional task, such as climbing a flight of stairs or standing from a seated position (22). Although a power clean and sit-to-stand movement have different objectives, they both require relatively rapid force development for success. In this study, power was assessed via timed sit-to-stand, timed stair climb and isokinetic dynamometry.
Timed Sit-to-Stand
Each participant performed a 5-repetition sit-to-stand protocol as fast as possible using previously established procedures (15). With their arms folded across their chest, participants were asked to stand up from 43-cm-tall chair until the legs were fully extended and sit back into the chair with buttocks touching the chair as fast as possible for 5 repetitions (24). Time was measured with a handheld stopwatch to the nearest 0.1 seconds. The faster time taken from 2 trials was recorded.
Timed Stair Climb
The stair climb is an inexpensive and quick test that can be used as an indicator of leg power (3). Participants were asked to ascend nine 17-cm steps as quickly and safely as possible. Average power output was determined by calculating the product of participant body mass and vertical height divided by time. Timing was measured to the nearest 0.1 seconds with a handheld stopwatch with the faster of the 2 times recorded.
Isokinetic Knee Power
Isokinetic knee power was assessed using the Biodex isokinetic dynamometer (Biodex Medical Systems Model B-2000; Biodex Medical Systems, Shirley, NJ, USA) (9,18). Before each trial, participants completed 3 submaximal warm-up repetitions at 60, 120, and 180°·s−1 velocities to become accustomed to testing procedures. Participants then performed 5 maximal effort repetitions at each of velocity. Vocal encouragement was given throughout the trials. Average power was measured at all 3 velocities for both knee flexion and knee extension. Participants were allowed 5 minutes to rest between each testing velocity.
Maximal Strength
Strength measurements during 3 different exercises were assessed for each participant. An estimated 1RM was calculated by performing a 3–5RM in the plate loaded leg press, single-leg lunge using a Smith machine, and machine leg extension. Participants performed each exercise using a light resistance for 10 repetitions to gauge intensity and evaluate range of motion. Once completed, participants rested for 3–5 minutes, and a resistance was estimated by the researcher that would allow participants to reach no more than 5 repetitions. If the participant performed 5 repetitions, 2.5–10 kg were added until the participant could not reach 5 or more repetitions. Participants rested for 3–5 minutes between sets. The participant performed no more than 3 sets of the exercise to prevent fatigue from being the limiting factor. The 1RM was estimated from the lifts and was used during the strength training program to assign appropriate resistance percentages (4,31,32). The equation for the estimation of 1RM incorporates both mass lifted and repetitions met: [(100 × mass lifted in kg) ÷ (102.78 − (2.78 × reps)] (4).
Training Intervention
The SG and PG participants were asked to complete 3 exercise sessions per week for 8 weeks for a total of 24 training sessions. Training logs were kept by the researcher with pertinent information for each exercise, including amount of resistance and repetitions performed. Before each training session, participants completed a general warm-up consisting of either treadmill walking or ergometer cycling for 10 minutes. Strength training sessions were performed following protocols set forth by the American College of Sports Medicine (31). All exercises were performed at 3 sets of 10 repetitions at 65–80% of the estimated 1RM with 60 seconds between sets and up to 2 minutes between exercises. SG completed 3 sets of 10 repetitions on the leg press, bilateral leg extensions, and single leg lunges performed on a Smith machine. Participants were instructed to give maximal effort and to keep proper form and complete the exercises using full range of motion. Once the participant could complete all 3 sets and repetitions for the exercise comfortably, 2.5 kg was added for the next training session.
The PG followed the same protocol as SG but with different exercises performed in the AlterG treadmill (AlterG, Fremont, CA, USA). The AlterG treadmill is a body mass–supported treadmill that allowed the participants to unweigh their body mass by up to 80% during exercise (13). This is accomplished through lower-body positive pressure, which uses air pressure applied within a sealed chamber surrounding the participant's pelvis and legs to support the user's body mass (27). The PG completed 3 sets of 10 repetitions of squat jumps, single leg bounding (10 each leg), and explosive skipping (20 total jumps). These exercises were selected because of the similarity of the activated muscle groups used in the exercises by the SG, to keep specificity of training between the groups as comparable as possible, and to keep volume between the 2 training groups equal by using similar exercises, sets, and repetitions. These exercises are also generally considered to be good beginner exercises when starting a plyometric training program (7). Similar to the prescribed intensity of 65–80% 1RM used by the SG, the PG also began with an exercise intensity of approximately 65% of their body mass and then increased the exercise intensity by increments of 1% of body mass until the highest intensity was achieved in which the participant could complete all 3 sets and repetitions for the exercise. The subjective exercise intensity was determined by using a modified Borg rating of perceived exertion scale (RPE, 0–10; 0 being no exertion and 10 being maximal exertion). Participants were also instructed to give maximal effort in all their jumps. The CG performed no exercise for the duration of the study.
Statistical Analyses
For the estimated maximal strength and isokinetic strength, mean data in absolute values were expressed relative to participant body mass. Descriptive statistics were reported for all variables ( ± SD). A 2 × 3 (time × group) repeated-measures analysis of variance was used to determine if there were differences between the groups. When appropriate, post hoc analysis was completed using a 1-way analysis of variance with the Bonferroni adjustment to determine group differences between pre- and posttesting. Percent change in scores was calculated on the individual data as {[(post − pre) ÷ pre] × 100}, and the mean of the group change was reported. The α level was set at 0.05. Where significance was reached (p ≤ 0.05), Cohen's dz effect size (ES) was also calculated to determine magnitude of effect (0.8 large effect, 0.5 moderate, and 0.2 small (6)). All analyses were completed on SPSS 23.0 (SPSS, Inc., Armonk, NY, USA).
Results
All 23 participants completed the study. Exercise session adherence was excellent with an average of 91% attendance and with no participant missing more than 4 sessions. There were no statistical differences for any participant characteristics or performance measures at baseline.
Functional Strength and Power
After training, there was a significant effect for time (p < 0.001) and group × time interaction (p < 0.05) for the timed sit-to-stand (Figure 1). The PG was significantly faster than the CG (p = 0.013) with a large ES (ES = 2.12), with no other differences between the groups (p > 0.05). The average percent reduction in time was 22.11 ± 8.48%, 12.11 ± 7.59%, and 8.02 ± 3.99% for the PG, SG, and CG, respectively.
Figure 1.: Mean times in seconds for sit-to-stand pre- and posttest. *PG > CG (p = 0.013). PG = plyometric training group; CG = control group.
For the timed stair climb, there was a significant effect for time (p < 0.001) and group × time interaction (p < 0.01) (Figure 2). The PG was significantly faster than the CG (p = 0.002) and had a large ES (ES = 2.34) with no other differences between the groups (p > 0.05). The average percent reduction in time was 14.68 ± 6.28%, 7.13 ± 6.07%, and 1.05 ± 5.51% for the PG, SG, and CG, respectively. Power output in watts was also calculated via the timed stair climb measure. There was a significant effect for time (p < 0.01) and group × time interaction (p < 0.01) (Figure 3). Both the PG (p < 0.001) and SG (p = 0.03) significantly increased their power output compared with CG, whereas PG and SG did not differ. The PG demonstrated a large ES (ES = 2.95), whereas the SG also showed a large ES (ES = 1.93) compared with the CG. The average percent increase in watts was 16.59 ± 9.07%, 8.39 ± 6.49%, and −2.19 ± 5.12% for the PG, SG, and CG, respectively.
Figure 2.: Mean times in seconds for stair climb pre- and posttest. *PG > CG (p = 0.002). PG = plyometric training group; CG = control group.
Figure 3.: Mean power output in watts during stair climb pre- and posttest. *PG > CG (p < 0.001); #SG > CG (p = 0.035). PG = plyometric training group; CG = control group; SG = strength training group.
Estimated Maximal Muscular Isotonic Strength
For the leg press, the PG and SG increased their estimated 1RM, whereas the CG decreased in their 1RM, although not significantly (p > 0.05) (Figure 4A). In the seated leg extension, there was a significant effect for time (p < 0.001) and group × time interaction (p < 0.01) (Figure 4B). Estimated maximal seated leg extension strength for both the PG (p = 0.009) and SG (p = 0.007) were significantly greater than CG. The PG group demonstrated a large ES for the leg extension (ES = 1.83), as did the SG (ES = 2.93). The average percent increase in estimated 1RM was 18.43 ± 11.18%, 20.38 ± 6.03%, and 2.86 ± 6.09% for the PG, SG, and CG, respectively. For the single leg lunge, there was a significant effect for time (p < 0.001) and group × time interaction (p < 0.001) (Figure 4C). Estimated maximal single leg lunge strength in the PG (p = 0.03) and SG (p < 0.001) were significantly greater than CG with a large ES in the PG (ES = 1.16) and for SG (ES = 2.33). The average percent increase in the lunge was 85.74 ± 62.23%, 274.70 ± 244.60%, and 38.38 ± 35.95% for the PG, SG, and CG, respectively.
Figure 4.: Mean estimated 1-repetition maximum relative to body mass for leg press (A), leg extension (B): #SG > CG (p = 0.009), *PG > CG (p = 0.007), and single leg lunge (C): †SG > PG, CG (p < 0.05), *PG > CG (p < 0.05). PG = plyometric training group; CG = control group; SG = strength training group.
Isokinetic Knee Strength
There were significant group × time interactions at all 3 velocities in both extension and flexion (p < 0.001) (Table 2). The PG was significantly greater in all 3 velocities in both flexion and extension than the SG and CG (p < 0.001) except for at 60°·s−1 extension in which SG was greater than PG and CG (p < 0.001). The average percent increase for PG ranged from 24.75 ± 19.94% to 85.74 ± 62.23%.
Table 2.: Average knee power pre and post testing during 8 weeks of resistance training or plyometric training in older adults.*†
Discussion
The results of this study indicated that performing plyometric exercises improved muscular power, isotonic strength, and isokinetic strength in a population of older adults who were not participating in any previous resistance training. Moreover, the PG significantly improved several of these measures compared with a sedentary CG while completing less total work than the SG (19–21,30). During a typical exercise, less total work is reflective of a reduced cumulative amount of weight lifted. For example, an 80-kg participant who is performing 3 × 10 on the leg press using 110 kg of resistance will lift a sum of 3,300 kg for that exercise. In comparison, that same participant performing 3 × 10 squat jumps in the body mass–supported treadmill at 80% of their body mass will lift a total of 1,920 kg. Although this calculation may be an oversimplification of the total work completed during a plyometric exercise, it gives a better understanding of the lower total work being performed during a plyometric exercise vs. a traditional resistance training exercise. The reduced volume of work during plyometric exercises may be advantageous (especially in older adult populations) because of a reduction in the volume of stress placed on joints of the lower body. Furthermore, the power and strength improvements demonstrated in the PG in this study occurred with less absolute work performed, which has been previously demonstrated in other studies (19,20). To the authors' knowledge, this is the first plyometric-specific training program studied in older adults, as well as using a weight-supported treadmill for power training.
The PG was shown to significantly improve their functional strength in both the chair sit-to-stand and stair climb. In the chair sit-to-stand, the PG improved by 21.7%, whereas the SG improved by 12.5%. The PG was significantly greater than the CG (p = 0.013), with no statistical difference between SG and CG. The 21.7% improvement is nearly double the previous findings of resistance training programs that incorporate a lower resistance with a higher velocity of movement in which improvements of 10.4–12% have been reported (19–21). The results of this study found that the PG improved the timed stair climb by 14.8%, whereas the SG improved it by 7.4%. Strength training alone has previously been shown to improve stair climb time. Capodaglio et al. (5) found a 12% improvement in their time to climb stairs compared with control after performing only 2 lower-body exercises 3 times per week for 1 year. Although the SG in the present study improved their stair climb time similar to the results by Capodaglio et al. (5), when comparing strength training with power training, participants performing power training had greater improvements than those performing strength training in functional measures, such as the time to climb stairs compared with controls. For example, training using maximal movement velocity significantly improved stair climb time after 8 weeks of training compared with control, whereas the strength group performing slower velocity movements did not (20). The present study supports the previous findings that resistance training that incorporates lower resistance with high movement velocity can improve functional ability in older adults (19–21). The present study also suggests that improvements in maximal muscular isotonic strength occurred via plyometric training. Interestingly, there were no differences in strength gains between PG and SG in the leg press or leg extension. The large increases seen in the SG for the single leg lunge are likely because of a training effect of the exercise, as it was a new movement for many participants.
Another key finding of this study was the impact of plyometric training on average knee extension and flexion power output during the isokinetic tests. Specificity of training should also be considered when interpreting the results from this study. After 8 weeks of plyometric training, the PG demonstrated a significantly higher power output in knee flexion at 60, 120, and 180°·s−1 and knee extension at 120 and 180°·s−1 compared with the SG and CG. This can likely be attributed to the rapid knee flexion that occurred during the plyometric movement, whereas the SG typically completed each repetition at a slower velocity. Both groups demonstrated specificity of training due the velocity of exercise movement in their respective exercises. This specificity of training was demonstrated in the results at the lower velocity isokinetic measures, specifically the 60°·s−1 extension. During the 60°·s−1 extension test, the SG significantly improved their power by 166%, compared with the PG who improved by 66% (p < 0.001). However, during their training program, the PG typically performed exercises at a higher velocity. Thus, the PG demonstrated greater improvements in the moderate and higher velocity measures because of the much quicker movement patterns that occurred during their plyometric training program. Similar to the present study, Earles et al. (10) found a substantial difference between muscle power improvements and strength improvements. They found that a group of older adults performing resistance training at high velocities demonstrated power improvements of 150%, whereas strength improved by only 22% (10). Similarly, the differences between improvements in power at different velocities were evident in our group of older adults and were likely because of the PG training to develop force much quicker during their exercises.
This study demonstrates that older adults performing plyometric exercises in a weight-supported treadmill can significantly improve knee extensor and flexor power output to a greater extent than traditional resistance training. Although muscular activity was not directly measured in this study, one likely hypothesis for the increase in strength and power is because of the neuromuscular adaptations the participants likely improved. Because the participants had no previous resistance training experience, the initial strength gains were likely because of enhanced neural pathways, which have been shown to be trainable in older adults in the same manner as young people (1,2,28). It is widely accepted that the majority of strength increases during the initial weeks of training are because of neural adaptations, such as increased neural drive and increased muscular activity of the agonist muscles (1,17,26,28). Furthermore, these neural adaptations have been shown to be similar in magnitude in older adults (17). The improvements in strength are important for the older population whether because of enhanced neural adaptations or the combination of both neural and muscle changes. A common dilemma that researchers and clinicians who work with older adults face is to focus the training program on muscular strength or power. Until recently, much of the research has focused on muscular strength. It is evident that training for muscular strength will result in different adaptations than training at high velocities for muscular power. The present study suggests that power training may improve functional outcomes similar to or, in some instances, better than strength training.
Although the overload principle for resistance training is well known, implementing an appropriate overload for the PG has not yet been established in a weight-supported treadmill. In this study, we typically started the participants at a workload of approximately 65–70% body mass and increased the percentage of body mass when the participant reported that performing the exercises became “easy.” A modified Borg RPE (0–10; 0 being no exertion and 10 being maximal exertion) was used after the final set of each exercise. Once the participant could successfully complete 3 sets of 10 repetitions with an RPE of 7–8 (very hard), the bodymass was increased for the next workout by 1%. The rate of progression between both the groups was quite similar. Future research is needed to further clarify the appropriate intensity of plyometric training for older or deconditioned populations using a weight-supported treadmill.
There were a number of strengths to this study. All training sessions were supervised by a study investigator, and participant compliance to the training visits and testing sessions was excellent (91% attendance across all groups). Performance was assessed during each session, and progression was applied as appropriate during the subsequent training session. A sedentary control group was also included in the study design. There were, however, some weaknesses to the study. The sample sizes for each group were relatively low. However, post hoc power analysis indicated that a power of >0.99 was achieved for all test variables. The same study investigators completed the testing and training sessions, which could have inadvertently introduced some bias during the testing sessions. Additionally, this study did not include separate sessions for familiarization testing before the initial testing session. Therefore, a learning effect in the outcome variables measured cannot be ruled out. However, a lack of significant improvements in the control group across testing sessions suggests that a learning effect of the prior testing session was minimal.
In summary, the results of this study indicate that older adults performing 8 weeks of plyometrics in a weight-supported treadmill can improve the time to climb a flight of stairs and rise from a seated position, muscular strength in the leg press, leg extension, single leg lunge, and isokinetic knee flexor and extensor strength. The training protocol was well tolerated with no adverse events reported. These findings are consistent with current power-specific training for older adults and suggest that more velocity training approaches may be used in this population. These significant changes were accomplished through performing less total absolute work than the strength training group. If modified appropriately, older adults may be able to achieve benefits from a plyometric exercise-training program.
Practical Applications
Based on the results of the study, older adults may benefit from performing a modified plyometric exercise program through a body mass–supported treadmill. Many of the participants completed most of their exercises at 80–85% of their body mass, demonstrating the possibility of revisiting the guidelines set forth by the National Strength and Conditioning Association for beginning a plyometric training program. With proper modification, plyometrics may be a beneficial training intervention to increase strength, power, and functional ability in older adults. Exercise professionals working with older adults are always looking for the most efficient methods to get the greatest benefit for their clients. Performing plyometric exercises demonstrates similar if not greater outcomes when compared with strength training alone; thus, they are able to perform less work in an exercise session and still obtain the benefits of a strength training program.
Acknowledgments
The authors would like to thank all of the subjects for their participation in this study. They also thank Cameron Needham, Cassidy Fike, and Chris Mecham for assistance during training sessions and Dr. Tyler Brown for feedback during manuscript preparation. The authors declare no conflict of interest.
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