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The anterior cruciate ligament (ACL) is a frequent injury sight in football (1) and is followed by surgical reconstruction and a lengthy rehabilitation time. The rehabilitation phase for an athlete to return to unrestricted training is commonly referred to as return to sport (RTS). An athlete’s progression throughout an RTS program following an ACL injury is measured through a host of functional performance assessments. Following an ACL injury, dramatic declines are found in strength and power levels in both bilateral and unilateral (injured and non-injured limbs) assessments (2). Thus, common performance assessments used during a RTS program to measure progress include: 1 repetition maximum (1RM), countermovement jumps (CMJ), single leg jumps, and symmetry between limbs.
There have been many proposed training modalities to progress an athlete throughout a RTS program. An emerging modality is flywheel resistance training (FRT) due to the high forces it can create in both the concentric and eccentric phases of the exercise. FRT can create similar concentric demands as traditional resistance training (TRT) exercises (i.e. barbell back squat), yet FRT can yield higher forces and muscle activation in the eccentric phase (8). Eccentric training has been shown to positively modify several factors such as muscle morphology (fiber type, cross sectional area, and pennation angle) and the remodeling of both peripheral and central neural activity (3,7,10). Other important and more easily quantifiable factors in a RTS program include general strength and power assessments (4). Petré et al. (10), reported in a systematic review that FRT was an effective method for improving strength, power, and speed among healthy athletes. Additionally, a recent meta-analysis suggested that FRT may be superior in improving CMJ performance when compared to traditional resistance training (TRT) among athletes (11). Thus, suggesting FRT could potentially be utilized in a RTS program following an ACL injury.
To date only two investigations have attempted to examine the effects FRT has on RTS factors for athletes with ACL injuries. Henderson et al. (3), examined 11 college athletes during an 8-week intervention which consisted of 2 training sessions per week of flywheel Bulgarian squats that were performed for 1 set to exhaustion. Following the intervention, the authors reported increases in rate of force development and enhancements in maximum voluntary isometric contraction symmetry between legs. The implementation of FRT in a RTS program was further supported by Stojanovic et al. (12), in which the authors compared the effect of 6 weeks of FRT vs TRT programs in 134 professional athletes that were in the late stages of their ACL-rehab. Although both groups significantly improved all testing parameters, the FRT program resulted in greater respective improvements in lower body strength (27% vs 18%), CMJ (13% vs 7%), single-leg jump with injured leg (24% vs 14%), single-leg hop with injured leg (24% vs 8%), and triple-leg hop with injured leg (14% vs 5%) performance, (FRT vs TRT, respectively). Thus, the researchers of both studies strongly suggest that FRT can be effective in an RTS program for athletes with an ACL injury and it may allow for athletes to regain performance assessments at a faster rate in comparison to TRT.
Coach Vinnie Cagliostro and other members of the Stony Brook performance staff decided to put the Exerfly ultimate to the test with 2 football players with ACL injuries. The staff not only utilized a flywheel similar to the two studies above, but also utilized Exerfly’s motorized technology. Non-motorized FRT creates roughly a 1:1 ratio (concentric:eccentric) of resistive load. Meaning the energy that the athlete applies in the concentric (upward) phase is also applied equally in the eccentric (downward) phase. The motorized technology gives you the ability to boost the proceeding eccentric repetition up to 80% greater than the concentric repetition, driving the ratio past 1:1 and creating an accentuated eccentric load.
The primary purpose of this case study was to investigate the effect of motorized flywheel training following an ACL injury on:
This case study examined 2 Stony Brook University football players that had ACL injuries (height: 73.5 ± 4.5 inches, weight: 197.5 ± 32.5, post-op: 7.5 ± 1.5 months). Athletes 1 and 2 were 6 and 9 months post-op at the start of the case study, respectively. Neither subject had previous FRT experience.
The RTS training program occurred in the early off-season. Each week the athlete performed 3 exercises on the flywheel (Squat, Split squat, and RDL). The number of rest days in between each flywheel session was 3 days. The FRT exercises were performed immediately following their prescribed TRT training program and were administered by a member of Stony Brook’s strength or athletic training staff. The athletes also completed their speed and conditioning training throughout this time period. Athlete 1 running sessions were modified and administered by the athletic training staff throughout this time, whereas athlete 2 had no major modification to their running program.
The loading parameters utilized throughout this study are displayed in Table 1. In weeks 1 – 4 the primary goal was to drive strength-based adaptations as slightly heavier loads were utilized. In addition, intensity (both load and % boost) increased while the volume (reps) decreased, which is commonly seen throughout a strength training program. The secondary goal was to familiarize the athlete to FRT and to increase coordination/stability. In week 5 – 8 the primary goal was to drive power-based adaptations as moderate loads were utilized. The secondary goal was to increase work capacity as the eccentric intensity (% boost) increased, hence why the reps were switched to duration, in which the athletes were instructed to achieve as many repetitions as possible within a 10 second period. For each 4-week block, the % boost was increased by 3% each week to increase the eccentric intensity. The starting point of 0% boost was due to the lack of FRT experience that the athletes had. Athletes with more FRT experience could start off with a higher % boost (i.e., 5 or 10%) or greater % increases each week (i.e., increase of 5% vs 3%).
Before and after the 8-week program the squat 1RM, bilateral CMJ, single-leg jump with injured leg, single-leg jump with uninjured leg and braking force were quantified. Jumps and braking force were quantified via force plates (Hawkin Dynamics, Maine, USA). Limb symmetry index was calculated by [injured side/uninjured side] × 100%. Mean ± SD for the group is displayed in Table 2. Individual changes are displayed in Figures 1 and 2.
Following an 8-week FRT program, all variables dramatically improved at both a group and individual level with the exception of one. This is aligned with a previous FRT study, in which they reported similar, slightly less, improvements in jumping and strength metrics among 134 professional athletes that were in the late stages of their ACL-rehab (12). The current case study found greater improvements in the majority of the variables assessed (i.e., CMJ, single leg jump height, strength) in comparison to the previously mentioned study. Although limited sample size must be accounted for, it is possible that the use of FRT with the motorized technology drove greater adaptations in comparison to the use of a non-motorized flywheel. These findings support the implementation of FRT with motorized technology in an RTS program among football players that have an ACL injury.
The ability to improve strength and power are thought to be two important factors of a RTS program (2). From this case study it seems that both of these factors were dramatically improved following an 8-week FRT program. There was roughly a 40% or 100 lbs improvement in strength. There were also similar improvements in power, with improvements of 14% in bilateral vertical jump, 16% in single leg jump with the non-injured leg and most importantly 58% in single leg jump with the injured leg. It has been suggested that single leg jump height is the most discriminative parameter in comparison to other force plate variables regardless of time point in an athlete's ACL rehabilitation (6). Thus, this improvement and coinciding with the improvements in the other aforementioned assessments suggests that FRT with motorized technology can be implemented in a RTS program to address strength, and unilateral and bilateral power deficits that accompany an ACL injury.
Limb symmetry index is commonly assessed throughout a RTS program. A symmetry of 85 – 90% for a single leg jump test has previously been proposed as a return to play criterion (9). Although less investigated, similar criteria have also been proposed for braking force (5). A symmetry index of >85% was achieved for both single leg jumps and braking force. The largest improvement was found in single leg jump (34%). However, it is important to note that increases on an individual level were vastly different, in which athlete 1 improved his symmetry from 47% to 84%, whereas athlete 2 improved his symmetry from 81% to 89%. This discrepancy in percent change can likely be attributed to differences in RTS time points, in which athlete 2 was in the later stage of his RTS program in comparison to athlete 1. As for braking force symmetry, there was only a 5% improvement. This is likely due to the pre-test symmetry (82%) already being near 85 – 90%. As an athlete gets closer to this criterion, slower/less improvements are observed through training. Despite the marginal gains in symmetry, there was roughly a 45% improvement in braking force with the injured limb and 40% improvement with the uninjured limb. These large improvements highlight FRTs ability to train braking capabilities, which may have a high transfer effect to sport specific tasks such as deceleration, change of direction, and jumping.
Outside of the objective data, there were a host of subjective improvements seen. The staff reported an increase in coordination and stability on flywheel and non-flywheel movements. These improvements could possibly be attributed to the rapid transition from concentric to eccentric, inner set differences in range of motion and torque, and/or the large emphasis on eccentric loading (7) that flywheels offer. The other pronounced subjective quality that improved was the athlete’s confidence. This was attributed to the unique stimulus that FRT offers. Unlike TRT, FRT forces an athlete to absorb a large eccentric load while moving at high velocities. Being able to rapidly absorb eccentric force is similar to many sport movements (i.e., change of direction task). This held true with the athletes in this case study, as they felt that the stimulus was extremely close to the on-field task that they were preparing for during their RTS program.
The incorporation of motorized flywheel resistance training in a return to sport program following an ACL injury resulted in enhancement in a host of performance assessments. To highlight a few, there was an increase of 14% in CMJ height, 39% in squat, 58% in single leg jump height with the injured limb, 46% in braking force with injured limb, and 35% in single leg jump height symmetry index. Although this was a limited sample size, these improvements were larger than what has previously been reported in similar flywheel studies which could possibly be attributed to the use of Exerfly motorized technology. There were also improvements in subjective measures (coordination, stability and confidence). Altogether, the implementation of flywheel resistance training with Exerfly motorized technology following an ACL injury seems to address key objective markers, as well as having additional benefits from a subjective or coaches eye point of view.
References
1. Dodson, CC, Secrist, ES, Bhat, SB, Woods, DP, and Deluca, PF. Anterior Cruciate Ligament Injuries in National Football League Athletes From 2010 to 2013: A Descriptive Epidemiology Study. Orthopaedic Journal of Sports Medicine 4: 232596711663194, 2016.
2. Gokeler, A, Dingenen, B, and Hewett, TE. Rehabilitation and Return to Sport Testing After Anterior Cruciate Ligament Reconstruction: Where Are We in 2022? Arthroscopy, Sports Medicine, and Rehabilitation 4: e77–e82, 2022.
3. Henderson, FJ, Konishi, Y, Shima, N, and Shimokochi, Y. Effects of 8-Week Exhausting Deep Knee Flexion Flywheel Training on Persistent Quadriceps Weakness in Well-Trained Athletes Following Anterior Cruciate Ligament Reconstruction. IJERPH 19: 13209, 2022.
4. Hewett, TE, Di Stasi, SL, and Myer, GD. Current Concepts for Injury Prevention in Athletes After Anterior Cruciate Ligament Reconstruction. Am J Sports Med 41: 216–224, 2013.
5. Jordan, MJ, Morris, N, Lane, M, et al. Monitoring the Return to Sport Transition After ACL Injury: An Alpine Ski Racing Case Study. Front Sports Act Living 2: 12, 2020.
6. Labban, W, Manaseer, T, Golberg, E, et al. Jumping into recovery: A systematic review and meta‐analysis of discriminatory and responsive force plate parameters in individuals following anterior cruciate ligament reconstruction during countermovement and drop jumps. J exp orthop 11: e12018, 2024.
7. Lepley, LK, Lepley, AS, Onate, JA, and Grooms, DR. Eccentric Exercise to Enhance Neuromuscular Control. Sports Health 9: 333–340, 2017.
8. Norrbrand, L, Pozzo, M, and Tesch, PA. Flywheel resistance training calls for greater eccentric muscle activation than weight training. Eur J Appl Physiol 110: 997–1005, 2010.
9. O’Malley, E, Richter, C, King, E, et al. Countermovement Jump and Isokinetic Dynamometry as Measures of Rehabilitation Status After Anterior Cruciate Ligament Reconstruction. Journal of Athletic Training 53: 687–695, 2018.
10. Petré, H, Wernstål, F, and Mattsson, CM. Effects of flywheel training on strength-related variables: a meta-analysis. Sports Med - Open 4: 55, 2018.
11. Shimizu, T, Tsuchiya, Y, Tsuji, K, et al. Flywheel resistance training improves jump performance in athletes and non-athletes: a systematic review and meta-analysis. Int J Sport Health Sci 22: 61–75, 2024.
12. Stojanović, MDM, Mikić, M, Drid, P, et al. Greater Power but Not Strength Gains Using Flywheel Versus Equivolumed Traditional Strength Training in Junior Basketball Players. IJERPH 18: 1181, 2021.
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