How the Skyhook Modular Plyo Box changes the game in athletic fitness training
Introduction
Besides designing and supervising workouts to improve athletic performance, strength coaches must also teach and implement “prehab” techniques to reduce the risk of injury. After all, an injured athlete can’t contribute much to their team beyond cheering from the sidelines. To help athletes move faster in all directions and stay in the game, one training tool that offers the most “bang for your buck” is the Skyhook Modular Plyo Box.
To understand why this unique variation of plyometric boxes is a game-changer in a strength coach’s exercise equipment arsenal, consider the extent of lower extremity injuries.
At the professional level, many playoff games and championships have been lost due to Achilles tendon injuries (ATI). In the 2025 NBA playoffs, three players suffered season-ending ATIs, including Tyrese Haliburton in Game 7 of the Finals. The NFL doesn’t fare much better.
A study in the 2024 International Journal of Sports Physical Therapy found that the rate of ATI in the NFL has risen sharply over the past 20+ years. From 1980 to 2001, an average of four ATIs were reported annually. In 2023, there were 22, 12 during the preseason, and 10 during the regular season. There’s more bad news, as full recovery from an Achilles rupture can take a year or more, and it’s estimated that less than a third of those who sustain this injury return to their pre-injury performance levels.
When it comes to ankle sprains and strains, these injuries consistently rank among the most common across all levels of competition. One study estimated that over a million Americans seek medical treatment each year for ankle injuries. Additionally, once someone injures an ankle, they are five times more likely to experience another injury to that ankle. This trend continues despite significant advances in shoe design and orthotics.
Given these numbers, a strength coach should adopt a proactive approach to preventing lower extremity injuries. This strategy begins with understanding the structure and function of tendons.
A strength coach should adopt a proactive approach to preventing lower extremity injuries, says Kim Goss
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Image 1: In 1986, Sports Illustrated discussed the high risk of injuries in professional football, especially to the lower extremities. The trend continues, with a significant annual increase in Achilles injuries.
The Fragile Achilles—Not!
In 1982, podiatrist Dr. Bob Roy McGregor and Stephen E. Devereux published EEVeTec, an acronym for Equipment, Environment, Velocity, Technique, and Conditioning. The basic premise of their book is that there are two main ways to decrease the risk of injury: 1) Reduce the stresses on the body, and 2) Increase the body’s ability to tolerate stress. Let’s focus on the latter approach.
First, understand that tendons are not stiff, cable-like structures connecting muscles to bones. The Achilles tendon, along with ligaments and fascia, functions as a powerful biological spring, stretching and recoiling to release strain energy that amplifies muscle power. This energy release helps athletes run faster and jump higher than they could with muscle strength alone. It also explains why kangaroos, which have exceptionally long tendons, can hop over 25 feet and reach speeds up to 40 mph.
It’s estimated that a tendon can stretch up to approximately 8-10 percent of its length and withstand forces of over nine times an athlete’s body weight before rupturing. However, a sedentary lifestyle, early sports specialization, and the use of NSAIDs and certain steroids may reduce a tendon’s elasticity and stress tolerance.
A sedentary lifestyle, early sports specialization, and the use of NSAIDs and certain steroids may reduce a tendon’s elasticity and stress tolerance, says Kim Goss
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In contrast, weightlifters often reach maximum ankle flexion during the bottom phase of a snatch or clean at high speed, and they have done so with loads exceeding three times their bodyweight. Yet, ankle and Achilles injuries are so rare in that sport that most weightlifting coaches will never see them in their careers. These results validate Dr. McGregor’s idea that an athlete’s conditioning (represented by the “c” in EEVeTec) can significantly help prevent lower extremity injuries.
With conventional barbell and dumbbell exercises, the ankles rarely move through a full range of motion (so, use it or lose it), and specific exercises for them are seldom performed quickly. Using slow, limited-range bodybuilding exercises that isolate muscles may also create “internal resistance” that restricts the range of motion of connective tissues, such that seemingly simple movements can cause injury. Additionally, overusing athletic tape and braces that limit an athlete’s range of motion may reduce tendon elasticity and negatively affect their functions. These factors may explain why approximately 70 percent of lower extremity injuries are non-contact.
Certainly, exercises such as jump roping, bounding, and dot drills develop reflex-like motor patterns that may enhance athletic performance. And like when an arm wrestler’s muscles instinctively relax during a match, causing their arm to slam to the table, these reflexes help prevent muscles and connective tissues from tearing and bones from breaking. Angled boxes take this conditioning a step further by strengthening muscles and connective tissues through their full range of motion—case in point: the soleus.
Image 2: Sprinting and soccer place high levels of stress on the Achilles throughout a large range of motion, factors that contribute to the high risk of injury in these sports. (Left photo by Karim Ghonem, right photo by Joel Morel)
Foundation Training for Athletes
The soleus is the calf muscle beneath the gastrocnemius. It becomes most active when the knee is bent and can be best isolated with seated calf raises, which position the upper and lower legs at a 90-degree angle.
Yuri “The Father of Plyometrics” Verkhoshansky believes the soleus is a key muscle for speed. In a 2011 book he wrote with his daughter, Natalia, titled Special Strength Training Manual for Coaches, Verkhoshansky cited a study highlighting the vital role of the soleus in tennis. The study involved four common change-of-direction movement patterns seen in tennis while the athlete faces the net: Advance to Net, Short Shuttle, Long Shuttle, and Fan.
The study compared results before and after training on a seated calf raise machine and a leg press, a popular lower-body exercise that targets the powerful quadriceps. Despite targeting a relatively small muscle, especially compared to the quadriceps, the seated calf raise group achieved overall better outcomes in these movement tests.
Image 3: The classic athletic fitness training books EEVeTeC and Special Strength Training Manual for Coaches discuss the role of lower extremity conditioning in preventing injuries and achieving maximum performance.
Regarding the calves, standing on an angled box and performing quarter squats is a simple and effective way to stretch both the gastrocnemius (with the legs straight) and soleus (as the legs bend). I often used this exercise before weight training workouts with my sprinters, jumpers, and distance runners at Brown University. Furthermore, Posturologist and strength coach Paul Gagné states that stretching the calves may help prevent shin splints. He also says that athletes with flat (i.e., valgus) feet are often prone to hamstring pulls and tend to have tight calves, so stretching the calves is part of his plan to address this issue. There’s more.
When athletes change directions or move laterally, they briefly support themselves on one leg. At high speeds, the forces on the knees increase significantly. I don’t have specific data on this, but I’ve read that ground reaction forces when a person drops from a height of 12 inches can reach five times body weight. Similar forces occur when a soccer or football player suddenly brakes, evades, or chases another athlete on the field. Unfortunately, bad things happen when an athlete has weak single-leg strength.
Unfortunately, bad things happen when an athlete has weak single-leg strength, says Kim Goss
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Video 1: Single-leg strength is a key component of lateral quickness. Angled plyo boxes are an effective way to focus on this aspect of athletic fitness. Video courtesy of Mark Hoover.
First, it takes athletes with poor single-leg strength longer to stabilize themselves on the ground, which decreases their movement speed. At the Air Force Academy, our football team mainly relied on a running offense (because, as running teams are fond of saying, “Passing is for cowards!”). To create running lanes for our ball carriers, we often had to pull our guards to double-team blocks. Such lateral movement requires exceptional single-leg strength. I found that performing lateral jumps on an angled box significantly improved the 20-yard shuttle times of our linemen.
Furthermore, a lack of single-leg strength can lead to knee alignment issues, potentially increasing the risk of knee injuries. Sports Scientist Dr. Mel Siff told me that when sudden changes in direction produce excessive torque on the knee, damage to the ligaments and inner structures of the knee, particularly the menisci, can result. Since most lower extremity injuries are non-contact, Siff’s comments suggest that single-leg training benefits both performance enhancement and injury prevention for the Achilles, ankle, and knee. However, for the best results, these exercises must also be practiced at the speeds that occur in competition (and to cite another Air Force motto, “Train the way you are going to fight!”).
Training with angled boxes gained popularity in the athletic fitness community through the 1999 book Explosive Power and Jumping Ability for All Sports, written by accomplished track coaches Tadeusz and Henryuk Sozanski. The authors discussed the value of training with concave and convex boxes, including curved versions of these shapes, called Polish plyo boxes.
That was then, but this is now, and coaches can elevate their plyometric training to a higher, more practical level with the Skyhook Modular Plyo Box.
Image 4: Explosive Power and Jumping Ability introduced Polish plyo boxes to the fitness community. At right, Polish boxes by Skyhook.
Enter the Modular Plyo Box
When I used angled boxes with my athletes at the Air Force Academy, I had to jam them against a wall to prevent them from slipping. The Skyhook Modular Plyo Box solved this issue with sturdy V-shaped connectors that securely lock the units together, allowing for various configurations to expand exercise options, especially when multiple boxes are used. For example, the boxes can be arranged in a U-Shape to emphasize explosiveness, and in an L-Shape or a star shape for agility and direction-change drills.
Image 5: The Skyhook Modular Plyo Box features sturdy connectors that provide stability and enable the components to be arranged in numerous configurations.
Following the motto “you only pay for quality once,” The Skyhook Modular Plyo Box is crafted from premium-grade plywood, visually refined, and free of defects such as gaps or voids. This quality enables athletes to perform exercises on them with bare feet. Each box features built-in handles for easy carrying, and the basic unit folds up to a 24” x 24” unit. A standard set includes two 10-degree angled boxes, one flat platform box, and two heavy-duty connectors.
Image 6: The basic Skyhook Modular Plyo Box unit folds up to a 24” x 24” unit and features built-in handles for easy carrying.
The angle of the box is important, just as is the angle of a hill when sprinting. For example, an excessively steep downhill angle shifts the training focus from improving acceleration to braking. Similarly, an angle greater than 10 degrees is optimal for preventing excessive torque at the ankle.
Image 7: Five types of Skyhook Modular Plyo Box configurations: Concave, Convex, Concave with Flat Center Platform, L-Shaped, and Star.
Here are a few exercises that can be performed with these five configurations:
1. Concave and Convex
These two designs, along with the other three, can be used to stretch both calf muscles, either through isometric holds (knees straight for the gastrocnemius and knees bent for the soleus) or dynamically with quarter squats. For these exercises, position your feet so that your toes are higher than the heels; you may need to hold onto a sturdy object for balance.
Next, standing between the boxes and facing perpendicular, perform small vertical hops to strengthen the foot during inversion and eversion (see Image 8). Plyometric experts, including Dietmar Schmidtbleicher, believe that if the heels touch the ground during depth jumping, the load is excessive. This guideline suggests that with all these plyo box hopping and jumping exercises, you should stay on the ball of your feet throughout the entire set.
Finally, face parallel to the boxes in a split stance (with one foot on each box) and perform small vertical hops to strengthen the calves through plantar flexion (toes down) and dorsiflexion (toes up). From here, you can progress to the more advanced version of both designs by shuffling your feet back and forth.
Image 8: Four basic movements of the foot: plantar flexion, dorsi flexion, eversion, and inversion. (Drawings by Sylvain Lemaire, Hexfit.com)
2. Concave with Flat Center Platform
This configuration involves hopping laterally (from side to side), jumping on one leg from one angled box to another, and avoiding the center platform altogether.
You can also stand on the center platform and jump back and forth with both feet onto one angled box and the platform, then onto the other angled box, and then back to the center. The advanced version involves one leg; initially, you may need to hold onto a sturdy object to become accustomed to the movement.
3. L-Shaped
This design enables you to turn and jump backward. Several variations are possible. For example, start in the center, jump laterally and then back to center, then jump forward and backward. An advanced version would be to perform the drill with one leg at a time.
4. Star
The Star configurations offer the most variations and include all the movements described in the last two sections, as well as the calf stretches.
One challenging variation of this design is to form a cross pattern by hopping on all four angled boxes, returning to the center with each movement. This drill requires you to hop forward, backward, and laterally in both directions. Start with both legs, then progress to one leg at a time. There are many other drills possible with this design—see what you can come up with!
Resistance increases as movement speed rises. Therefore, an athlete begins these drills slowly and gradually accelerates (as long as they keep proper knee alignment). Starzynski and Sozanki illustrated in their book how resistance can be increased by using a barbell across the shoulders; however, a weight vest is a more practical and safer alternative.
Plyometric training is an effective method for enhancing athletic performance and reducing the risk of injury. To stay competitive, build your speed and agility training from the ground up with the Skyhook Modular Plyo Box.
References
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Marrone, W. et al. “Rehabilitation and Return to Sports after Achilles Tendon Repair. International Journal of Sports Physical Therapy. 2024 Sep 1;19(9):1152-1165.
Hsu AR and Anderson RB. “Foot and Ankle Injuries in American Football.” American Journal of Orthopedics. 2016;45(6):358–367.
Siu R, Ling SK, Fung N, Pak N, Yung PS. “Prognosis of elite basketball players after an Achilles tendon rupture.” Asia-Pacific Journal of Sports Medicine, Arthroscopy, Rehabilitation and Technology. 2020 Apr 10;21:5-10.
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McGregor, Rob Roy and Devereux, Stephen E. EEVeTeC. Pp 19-33. Houghton Mifflin Company, Boston, 1982.
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King, Ian. Understanding Plyometrics. 1999, p. 58. King Sports Publishing.
Myer, Gregory D., et al. Did the NFL Lockout Expose the Achilles’ Heel of Competitive Sports? Journal of Orthopaedic and Sports Physical Therapy. Vol. 41, No. 10, October 1, 2011, pp. 702-705.
American Orthopaedic Society for Sports Medicine. “Sports specialization may lead to more lower extremity injuries.” ScienceDaily, July 23, 2017.
Van der Linden PD, Sturkenboom MC, Herings RM, Leufkens HG, Stricker BH. “Fluoroquinolones and risk of Achilles tendon disorders: case-control study.” BMJ. 2002 Jun 1;324(7349):1306-7.
Carroll CC. Analgesic Drugs Alter Connective Tissue Remodeling and Mechanical Properties. Exercise and Sport Sciences Reviews. 2016 Jan;44(1):29-36.
Albright JA, Lou M, Rebello E, Ge J, Testa EJ, Daniels AH, Arcand M. “Testosterone replacement therapy is associated with increased odds of Achilles tendon injury and subsequent surgery: a matched retrospective analysis.” J Foot Ankle Res. 2023 Nov 11;16(1):76.
Charniga, Andrew. Achilles Tendon Ruptures and the NFL. SportivnyPress.com, 2016, February 9, 2017.
Yerkhoshansky, Yuri, and Verkhoshansky, Natalie. Special Strength Training Manual for Coaches. Pp 54-57. Verkhoshansky SSTM©.
Gagné, Paul. Personal Communication. Dec 2007.
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Starzynski, Tadeusz and Sozanski, Henryuk. Explosive Power and Jumping Ability for All Sports, Stadion Publishing Company, July 31, 1999.
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