Biomechanics in Physically Disabled Monoskiers Versus Conventional Downhill Skiers and Snowboarders

Jerrold Petrofsky, PhD, JD

John Meyer, DPT, OCS

Roger Magsino, BS

Shawn Zook, BS

Jerry K. Kao, BS

Michael Laymon, DPT Sc OCS

The Department of Physical Therapy, Loma Linda University, Loma Linda, California

Department of Physical Therapy, Azusa Pacific University, Azusa, California

KEY WORDS: snow ski, paraplegia,
exercise, exertion, oxygen consumption, biomechanics.

Abstract

Eleven male and six female subjects participated in monoskiing, downhill skiing and snowboarding for a study comparing the biomechanics of the three types of sports. All subjects were well-experienced skiers who could handle advanced slopes. Expert skiers who were disabled were compared (when skiing on monoskis) to instructors participating in downhill skiing and snowboarding. Downhill skiing involved a continuous fluid movement, whereas monoskiing involved a high isometric component in upper body muscles and long lever arms to hold the body in position over the ski through the use of outriggers. This would inherently make this form of exercise more inefficient. Furthermore, the long lever arms and high use of upper body muscles should also predispose monoskiers to a variety of orthopedic injuries. Snowboarders used their ankles more than downhill skiers and had the least protection afforded by their equipment against ankle injuries.

Introduction

Snow skiing has been a popular winter sport since the 1930s.1 The recent addition of snowboarding to the sport has brought back the sagging popularity of downhill skiing for college-aged people. Disabled skiers began attempting this sport in the 1950s.2 Persons with a wide variety of disabilities such as cerebral palsy, multiple sclerosis, spinal cord injury, hemiplegia, amputation, blindness, spina bifida, and muscular dystrophy can participate.3

As the popularity of skiing grew among the physically disabled, many types of adaptive equipment were developed. Some examples are three-track skiing, four-track skiing, sit-skiing, and the most widely known, monoskiing, which allow safe and accessible enjoyment of the sport. Patients with complete spinal cord injuries ranging from T2 to L5 are able to use the monoski.3 Today, all three types of skiers can be found on most slopes, even on the most advanced slopes. However, all three types of skiing have been associated with injuries.

Many articles have examined the injury occurrence in downhill skiers. For example, Myles et al.4 examined injuries over a 5-year period in Calgary, Canada. They reported five deaths and 145 injuries to the head and spine. Men were three times as prone to injuries as women. The average age of the injured skiers was 23.8 years. The most common cause was a simple fall on the hill, only seconded by a collision with a tree. Although 41% of all injuries were due to collisions with other skiers overall, these skier-to-skier collisions did not result in a large percentage of head and supine injuries. The authors believed that the main contributor was reckless skiing. The incidence of spinal and head injuries ranged from 5 per million skier days on easy terrain to as high as 122.6 injuries per million skier days on more difficult terrain. The greatest number of head injuries occurred in March, when the snow was icy because of spring melt.

Alcohol only contributed to 4% of head and spinal injuries. These types of severe injuries are common in other ski areas but are a small percentage of total injuries. For example in Sweden, 11% of all ski injures are to the head and spine.5 In Vermont in the United States, the head injury incidence is lower, averaging only 4% over a 10-year period.6 Most downhill ski injuries appear to be to the knee.7,8 Anterior cruciate ligament injuries are common.9 Of 7155 ski patrollers (one third women and two thirds men) and instructors over a 6 year period, these authors found 4.2 injuries per million skier days to the anterior cruciate ligament.

Snowboarders also experience both neurologic and orthopedic injuries. In one study, Hentschel et al.10 found snowboard injuries to be less severe than downhill ski injuries. Of all head injuries, snowboarders experienced, for example, 21% as concussions in one study and skiers suffered 60% of all head injuries associated with a concussion. On the other hand, snowboarders experienced intracranial hemorrhages in 71% of head injuries and downhill skiers only experienced a hemorrhage in 28% of the cases.10 Many of the injuries in snowboarding involve injuries to the ankle and feet.11 Young et al.11 reviewed snowboard injuries in 9 previous studies and found that fewer than one third of injuries occurred at or below the hip. Over three-fourths of such injuries are at the ankle area and depended on the boot type being used. Most ankle injuries resulted in fractures (50%). Soft boots, as commonly used, were associated with twice the ankle injury rate as hard boots.

Wrist and shoulder injuries accounted for about one third of snowboard injuries, while elbow injuries only represented about 4.4% of injuries. Severe injuries usually involved the head (54%). Curiously, 8% of injuries were in lift lines waiting to get on the lift. Idzikowski et al.12 studied snowboarding injuries over 10 seasons from 1988 to 1998 in Colorado. They found that, of 7430 snowboading injuries, 49% were upper extremity injuries and, of those, 56% were fractures. The most common site of injury was the wrist, accounting for 21.6% of the injuries. They also found that changing equipment technology shifted the injuries from one area of the body to the other over the 10-year period.

Although few studies have examined the injuries in monoskiing in the past 10 years, two studies around 1990 classified the injuries associated with this type of sport. Laskowski and Paul13 looked at 13 ski areas and compared the incidence of injuries with the incidences in other sports. They compared the incidence of emergency room visits over a 4 year period. During this period, 3.4 million non-handicapped skier emergency room visits were seen. Over this same period, 64,000 disabled skier injuries occurred. For able-bodied skiers, this was 3.5 injuries per 1000 skier visits and for the group with disabilities, the injury rate was 3.7 injuries per 1000 visits to the slopes. These injury rates were not significantly different from each other.13 The fracture rate in non-disabled skiers was 15.1% of injuries and 11% in disabled skiers. In both groups, the largest category of injuries was sprains, averaging 42.1% and 44.1% of the non-disabled and disabled groups, respectively. The only category with higher rates of injury was the bruise category; bruises were suffered by 18% of disabled compared with 11% of non-disabled skiers.

Ferrara et al.14 also examined the same problem. They found 1.4 times more upper than lower extremity injuries. Acute injuries occurred to the thigh in 30% of the cases and 25% to the shoulder. Fifteen percent of the injuries occurred to the neck and head.14 Of all injuries, 73% were to the elbow.

Therefore, in all, numerous studies have examined the type and extent of injuries in these three types of skiing. However, it is not just the technique that is different. Clearly, the stress on the body is also different. In recent studies, we found that the stress on the monoskiers' upper body was 3 times that of downhill skiers, while the energy cost and stress of snowboarding was 50% higher than in downhill skiing.15 Furthermore, few studies have examined the techniques used by the skiers and related these to the types of injuries that are seen. The papers on injuries in monoskiers were never correlated to technique . This is particularly important because skiing has changed so much in the past 10 years. Ten years ago, the monoski equipment was hand-made, and no manufacturer made such equipment.3 Today, almost all equipment has been standardized and manufactured so that the technique is now not only different but should be similar for all skiers.3 Downhill skiing has also changed from the old straight ski to the shaped ski.16,17 This change has dramatically changed the way people ski, because the technique for skiing on a shaped edge is different from skiing on a straight edge. Although injury studies are recent, studies of the technique for downhill skiing on the newer skis are rare. Finally, snowboarding is an evolving sport with yearly equipment changes. Little has been done to document the technique used in this sport and correlate the technique to injuries. This is the purpose of the present investigation.

Subjects

Eleven male and six female subjects participated in these experiments. Their level of disability, if any, and their height, weight, and ages are listed in Table 1. The average height of the subjects was 173.3 ± 7.2 cm, the average age was 27.9 ± 8.73 years, and the average weight was 72.3 ± 11.4 kg. There were 11 non-disabled and 6 disabled skiers who participated in these experiments. Non-disabled skiers were largely either ski instructors at the Big Bear Ski Resort or students from Azusa Pacific University. All skiers were considered intermediate to expert on the ski type they used in these studies.

All subjects were informed in detail of the methods and procedures in the study. Each of the subjects signed a statement of informed consent approved by the Committee on Human Experimentation before participating in the study.

Methods

Analysis of Muscle Use and Ski Technique

Video and digital still photos were shot of the skiers on the slopes. The techniques of the skiers were then analyzed and compared using stop-action single-frame video analysis. Each frame represented a window of 1/30 of a second. The video capture speed was 1/10,000 of a second, assuring blur-free images. Video was shot with the camera about one third meter above the snow and at various angles to assure good representations of the movements.

Procedures

The experiments were conducted largely at the Bear Mountain ski resort. The subjects all rested for 2 minutes. They then were taken on a lift to a point where they could ski down an intermediate slope to the bottom of the mountain. The maximum elevation was approximately 2800 m above sea level, and the slope was about 4.4 km long and dropped about 670 m. The skiers were matched in speed and time so that the runs would be nearly equal. The investigators went ahead of the skiers and shot video and stills from various angles to be analyzed for technique biomechanics. This, then, was a cross-sectional study of different groups of people.

RESULTS

The average times to ski the slope for the 3 groups of skiers are shown in Table 2. As can be seen in this table, the times were virtually the same averages for the monoskiers, downhill skiers, and snowboarders: 283, 293, and 293 seconds, respectively.

Analysis of Muscles Use and Ski Technique

After sufficient speed was attained, all skiers went into reflexive or learned patterns of movement, trying to control the speed and maintain upright position during right- and left-hand turns. The technique used to manage the turns and maintain balance during downhill skiing apparently required significant muscle control in the lumbopelvic and especially the knee joints (full analysis reported subsequently). Snowboard skiers required muscle control in the lumbopelvic region, hip, and ankle joints. Monoskiing required primary muscle control in the thoracoscapular region. These regions required the greatest amount of eccentric and concentric muscle control to manage the turns in the upright position for all three types of skiing. Furthermore, each type of skiing required different biomechanical functions to manage the turns.

Downhill Skiing

Six non-disabled skiers participated in this series. Analysis of the video showed that downhill skiing used the gliding action of the ski on the snow to shift the legs laterally, but the major control and directional changes came from the muscles in the pelvis, hips, and knees. The right- and left-handed turns were similar in all 5 subjects. Therefore, a right-handed turn will only be described here.

There was minimal neck and head movement during turns in these subjects, but an average of 45˚ of rotation was needed to scan the slope. The eyes were maintained in the horizontal plane during the turns so that there was freedom to tilt the head at least 15˚ to each side. The scapula and neck complex was maintained mostly in neutral with minimal movements. Most of the movement occurred in the thoracic and lumbar regions.

The shoulders and upper extremities were kept adducted except to plant the pole to balance the turn. The shoulders were seen to abduct as much as 60˚ in these six subjects to maintain balance during the turns. The elbows were kept flexed to an average of between 30˚ to 60˚ as the poles guide in the turns (Figure 1).

The degrees of flexion in the hips, knees, and lumbar areas were apparently dependent on the amount of shock absorption the skiers needed to maintain balance. The hips and knees and lumbar spine were also used to lower the center of gravity to gain control.

During a right turn, the left hip was flexed by an average of between 5˚ and 10˚ and abducted by about 5˚ to 10˚ and the right hip was flexed 10˚ to 30˚ and adducted 10˚ to 20˚ (Figure 2). The flexed lumbar spine side bended right and rotated right, compensating for the apparent pelvic tilt that was seen to be about 15˚ to 20˚. The upper thoracic and scapular region rotated left to counterbalance the rotational mechanics of the pelvis and maintain the shoulders facing downhill. Consequently, the greatest amount of repetitive rotation stress would seem to be placed on the knees, which were flexed on an average from 5˚ to 70˚ and rotated from 0˚ to 45˚ on top of a fixed tibia. When the ankles were fixed both in the sagittal and coronal planes, the femoral condyles rotated on the fixed tibia as the transfer of balance went from left to right during the turns in these subjects.

The muscles that controlled the hips and knees are located in the anterior, posterior, medial, and lateral regions of the lower extremities.18 The anterior region contains the rectus femoris, with three single joint muscles around the hip: vastus medialis, lateralis, and intermedialis. Laterally, the iliotibial band has two muscles attached to it, the gluteus maximus and tensor fascia lata, and fibers of the biceps femoris. Posterior, the biceps femoris and semitendinosus and semimembranosus are noted. Medially, the adductor magnus, gracilis, sartorius, semimembranosus, and semitendinousus stabilize the knee.18 Therefore, from analyzing the video, as we have done here, for skiing in which the ankles are fixed and protected by boots and the tibia is stationary, the femoral condyles should rotate on top of the meniscus as the skier makes turns. The hips will translate across the skis from right to left during turns. Furthermore, anterior and posterior cruciate and medial and lateral collateral ligaments should guide the movements of the femoral condyles on the fixed tibia (Figure 3).

Snowboard

Snowboarding involves skiing on either edge of a flat board. The skill of snowboarding is in the ability to control and balance during two distinct types of turns. Analysis of the video on seven snowboarders showed the following: With the right foot behind and the left foot forward, a left-handed turn is made by leaning backwards into the hill with the knees. The hips were seen to be flexed from 15˚ to 45˚; the lumbar spine flexed 10˚ to 50˚; and the ankles dorsiflexed from 5˚ to 20˚ depending on the sharpness of the turn. The right-handed turn was much more complex. On average, the ankles were plantarflexed 0˚ to 20˚ with the right knee flexed about 25˚ and the left knee about 10˚. The right hip was flexed about 10˚, adducted about 10˚, and internally rotated about 30˚, and the left hip was flexed about 5˚, abducted 10˚, and externally rotated about 5˚, allowing for the transfer of weight from the posterior foot to the anterior foot. The lumbar spine was side bent left in all subjects and rotated left secondary to the created leg length discrepancy by flexing the right leg more than the left, creating a pelvic tilt. The upper thoracic and scapular region rotated left to counter balance the rotational mechanics of the pelvis. Consequently, the greatest amount of rotational stress was transferred down into the forefoot because of the rotational stress the forefoot has to pronate and evert about 30˚. The 20˚ of dorsiflexion needed to carve the turn adds even greater stress on the ligaments and joints of the ankle and foot (Figure 4).

The whole body was kept perpendicular to the board during the turns as much as possible. The arms were used as counter balances to initiate the turns and maintain the body perpendicular. The right turn showed the greatest and most complex biomechanical challenge because the thoracic spine was rotated left and side bent left with the lumbar spine in the flexed position, while the pelvis was facing the hill but tilted with reference to the snowboard. The movement of so many joints over the snowboard and the freedom of movement at the ankles placed a particular challenge on the muscle to control upright balance during the right-handed turn (Figure 5).

During the left rotation of the spine, the left external oblique and the right internal oblique muscles apparently performed a concentric contraction while the rectus abdominis and the back extensors seemed to control forward and backward balance. The shoulder flexors, extensors, and abductors must have controlled the arms to facilitate the counterbalance.

The pelvic tilt must be controlled by the quadratus lumborum in the lumbar spine and gluteus medius of the ipsilateral hip and possibly by the contralateral hip adductors. The hip flexors are normally controlled by the iliopsoas and rectus femoris and the hip extensors are normally controlled by the gluteus maximus and hamstrings.18 The assumption would be that these muscles are active to control the movement we noted in these subjects. The knees, when in the flexed position, are normally controlled by the quadriceps, which can create significant stress on the kneecaps during the rapid knee flexion, extension we observed here in the snowboarders.

Because the snowboarders used flexible boots, their ankles and feet were seen to be used to change the direction of the movement of the snowboard. The muscles around the ankle joint, therefore, must have been very active. Furthermore, the apparent muscle function is determined by the joint angle and position of the snowboarder during the turns. The anterior tibialis, extensor digitorum normally control dorsiflexion. The gastrosoleus, posterior tibialis, flexor hallucis longus, and flexor digitorum control plantar flexion of the foot. Furthermore, peroneus longus and posterior tibialis, anterior tibialis, and more importantly the intrinsic foot muscles control pronation and supination of the foot18 (Figure 6).

Monoskiing

The basic design of a monoski includes a frame chassis equipped with a seat and backrest on top and a standard downhill ski on the bottom. A shock absorber rides between the seat and the ski, which dampens forces transmitted from the ground, providing a smoother excursion over the rough terrain. A release mechanism is included that allows the ski to be lifted into an extended position, enabling the monoski to be picked up by a chair lift. In this manner, the monoskier can ride the lift chair to the top of the mountain without assistance. Once getting off the lift, the monoski drops into a locked position, ready to ski. The monoski is also equipped with lap and leg belts to secure the occupant for the duration of the ride. Monoskiers with injuries higher than T10 can have their monoski equipped with an upper torso seat belt for better stability because they have a deficit in trunk muscle control.

The monoskier also uses outriggers, which can be held in each arm to assist in control, balance, and stopping. Outriggers are a modified version of Canadian crutches. They are short, adjustable poles, consisting of (from top to bottom) cuffs to secure the poles to the arms, handles to grip, and cut-off ski tips at the end. The outrigger is equipped with a cord that can be pulled, exposing a serrated edge on the back of the ski tip that can be used in turning and breaking3 (Figure 7).

Monoskiing can be divided into the initiation phase, steering and hip angulation phase, and the turning phase. The initiation phase consists of three components; "opening the door," which is reaching forward with the outrigger on the same side of the turn; "counter rotation," which brings the body over the ski to transfer the body weight over the ski; and finally, "completion to initiation," which is unweighing the ski before initiating the next turn.

The steering and hip angulation phase consists of two components. The observed 20˚ to 50˚ hip angle into the hill and reaching forward with the outrigger and keeping the body weight over the ski.

The turning phase has three components that allow the monoski to turn; "the initiation component," "the steering component," and "the completion component" (Figure 8). Initiation is defined as looking down the fall line (direction you want to go) with the uphill outrigger extended down the fall line. The ski moves away from the body while hip angulation and body separation are initiated. The "steering component" requires the skier to feel the rhythm and force of the turn. The outrigger continues to be extended down the fall line, but the body bends forward over the ski, causing the thoracic and lumbar spine to rotate in flexion. This also forces the shoulders to face downhill. Finally, the "completion component" also requires the skier to feel the compression as the ski flattens. The uphill and downhill outriggers are at neutral by the side of the trunk. The ski will roll as the next turn is initiated.

Biomechanics of Monoski
Body Movement

Turning is the most difficult skill to master during monoskiing. Therefore, a closer examination of the biomechanics is warranted. The mechanism of the right turn was initiated in these subjects by pushing the right shoulder into flexion of about 120˚ and abduction of about 120˚ in the scapular plane while leaning the inside hip about 45˚ into the hill. Significant shoulder abduction and flexion by as much as 130˚ was seen for short periods of time.

The lumbar and thoracic trunk in these subjects was rotated and side-bent to the left about 30˚ to 40˚ with the flexed lumbar position about 80˚. The lumbar and thoracic coupled motions occurred simultaneously in the monoskiing steering phase. This caused the thoracic and lumbar spine to side bend to the left and rotate to the left. To maintain weight on top, the skier flexed the thoracic spine to the point of touching the chest to the thighs and rotating the head downhill. This motion has to occur every time the monoskier makes a turn either to the left or to the right, and no difference was seen on either turn (Figure 9).

The greatest amount of rotation stress was apparently placed on the thoracic-lumbar spine in the flexed, side bent position. The ligaments involved are the supraspinous, interspinous, ligamentum flavum, intertransverse, and posterior longitudinal ligaments, which restrict trunk flexion and side bending.18 The other area for excessive ligamentous stress was probably in the shoulder, especially the anterior and inferior glenohumeral ligaments and the anterior glenohumeral capsule.18

The muscle action is determined by the apparent function during the monoski turns. Primarily, the lumbar and thoracic deep and superficial paraspinal muscles maintain posterior trunk control.18 Superficially, the latissimus dorsi, rhomboids, serratus anterior, and trapezius maintain posterior trunk control, while the internal and external obliques, rectus and transverse abdominus, pectoralis major and minor, and gravity maintain anterior control. The rotatories, multifidus, and semispinalis muscles control the trunk rotation of the thoracic and lumbar spine.18

The shoulders and upper extremities held the outriggers used during the turns. In the shoulder, the pectoralis major and minor, serratus anterior, and scapula stabilizers maintain the scapula on the thoracic wall. The anterior forearm muscles were used to hold the outrigger. The anterior forearm muscles are the flexor carpi ulnaris, flexor carpi radialis longus and brevis, flexor digitorum profundus, flexor digitorum superficialis, flexor policis longus, and hand intrinsic muscles. Although no electromyographic measurements were performed, these muscle must have been active as assessed by the joint angles and apparent torques seen on the video.

Discussion

Snow sports have been a common winter activity for tens of decades. However, only recently have people with disabilities begun to ski with specially adapted equipment.1,3 Even more recent is the advent of snowboarding in the past 20 years. Although the biomechanics of skiing and the physiologic costs of this form of exertion have been well documented,1,17,19-23 little has been documented on the mechanics of handicapped monoskiing. Although injury rates have been studied for handicapped monoskiers,13,14 curiously, few papers have described the biomechanics of this form of skiing to understand these injuries.3 As with monoskiing, numerous papers have been published on injuries in snowboarding.10-12 However, few papers have been published on the mechanics of snowboarding to understand what causes these injuries.24

To complicate matters further, monoskiers, until the past few years, have manufactured their own skis. Today, a variety of manufactures make standardized equipment.3 This should change the injury statistics and biomechanics of this sport. Furthermore, for both downhill skiers and disabled monoskiers, in the past few years, a totally new type of ski, the shaped ski, has been introduced. This ski in downhill skiing was designed to reduce muscle use by making turning easier.16,20,25 The new skis allow the skier to ski with a wider stance and to carve turns at lower speeds.26 If the shape of the ski used now by 95% of all skiers (both downhill and monoskiers) is the new parabolic edge, then the biomechanics of skiing should have changed, making this study even more meaningful. Very little is known about the biomechanics, muscles, and angles of joints while performing a turn with these new technologies. The fact that the metabolic stress is twice as high in snowboarders than in downhill skiers and is three times as high in monoskiers than in downhill skiers, even with the modern equipment, makes this study even more enticing.15

Downhill Skiing

In the present investigation, the knees are the major transitional torsion area during a turn. The hips and L5/S1 spinal segments are also significantly involved when making turns, but to a lesser degree. The L5/S1 spinal segment includes the two facet joints and two intervertebral surfaces that can function independently of each other.18 The transitional torsion that occurs at the knees is on the fixed ankle and tibia. The ranges seen in the knees was approximately 15˚ to 40˚ of rotation coupled with 5˚ to 15˚ of knee flexion. Thus, during a turn, the femur will rotate 15˚ to 40˚ on a longitudinal axis of the tibia, causing the condyles to rotate on top of the meniscus. Because the medial meniscus is shaped like a "C," and the lateral meniscus is shaped like an "O," the natural excursion will be greater on the medial meniscus.27 Furthermore, if excessive knee flexion occurs, coupled with femoral rotation of the knee capsule, the medial and lateral collateral and the anterior and posterior cruciate ligaments become tense.28,29 Abnormal mechanics, coupled with conditions such as fatigued muscles, level of skiing, degree and condition of slope, can predispose skiers to knee injuries.7,8,30,31,32

The knee becomes the center for many types of injuries. Studies show injuries to the bones,33,34,35 ligaments,9,36 and the meniscus36 during downhill skiing. Two studies revealed severe injuries on the weight-bearing leg, with 65% loading on the weight-bearing leg and associated with 11% tibial fracture and 8% contusions because of a torsional stress.36 The literature is poor in associating minor meniscus injuries, tendonitis, bursitis, chondromalacia, plica, and synovial irritations because of a latency period for these conditions. Based on the advertised benefits of the new shaped skis in ski magazines and on web sites for manufacturers such as Elan, some decrease in knee injuries could be expected with the advent of shaped skis.

However, the mechanics of downhill skiing reported here seem to be the same as that without shaped skis. Although all subjects studied here wore shaped skis, comparing our results with those published previously for straight skis,22,35,37 these data showed no real difference in joint angles or basic ski technique with the new shaped skis. Certainly, Greenwald et al.17 showed no difference in three-dimensional joint biomechanics of the knee when using the side-cut skis of different-cut angles. However, parabolic skis were designed for new skiers.25 The skiers in our studies are all expert, as were those in the study by Greenwald et al.17 These skis, although being used by all skiers, were specifically designed for new skiers.25 Perhaps for the advanced skier, there is no great advantage.

Certainly, the biomechanics seem the same here with and without side-cut skis. In fact, Hintermeister16 points to the fact that the side-cut skies cause some instability at higher speeds that may cost more energy and increase the risk of injury. There seems no clear advantage in the new skis for the expert skier. Furthermore, studies are needed on new skiers to compare the biomechanics and perhaps even oxygen uptake and energy efficiency in ski training with straight versus parabolic skis. Additionally, aggravating the problem in the knees are weak muscles that predispose the skier to injuries.19,32

Two-joint muscles become weaker with decondition and fatigue quicker than one-joint muscles. Two-joint muscles around the knee are the rectus femoris, iliotibial band with two muscle attachments, the gluteus maximus, and tensor fascia lata.18 The posterior muscles, semitendinosus and semimembranosus, and biceps femoris, control the rotation of the tibia.18 Medially, the gracilis and sartorius attach below the knee. These muscles primarily control hip and knee flexion and extension movements. When these two joint muscles fatigue, the proprioceptive feedback and joint protection mechanism are compromised in the knee, predisposing the knee to injuries. The high correlation between the concentric and eccentric strength measurements for muscles that control the knee and performance in elite skiers points to the importance of the knee for this type of skiing.38

Snowboarding

More than 3.4 million people snowboard, and the number of snowboarders has increased by 77%, making snowboarding the fastest growing winter sport in the United States.11 The snowboarders ride an epoxy-fiberglass board with the standard snowboard stance, like surfers, with toes pointed to the side of the board. Snowboarders ride with both feet affixed to the board by non-releasable bindings.11,24

With the right foot behind and left foot in front, making a left turn requires the ankles to dorsiflex up to 20˚, knees and hips flex 10˚ to 30˚, and the lumbar spine goes into flexion about 50˚. Prolonged lumbar flexion should aggravate disc problems in the low back around the ages of 25 to 45 years. Although this was not a problem at the start of the snowboard industry, because most skiers were younger than 20 years old, today people of all ages snowboard, leaving a probability of disc related injuries more real than ever before.12 More complex is a right turn, in which the upper trunk must rotate up to 50˚ with the hips and lumbosacral joint position much like a downhill skier but with increased torsion on the weight-bearing ankles. The ankles are, therefore, the most likely joints to absorb the torsion during snowboarding. The weight-bearing plus the pronation, coupled with the 20˚ plantarflexion need for the right hand turns, place the ankle in the optimal position to sprain or fracture joints. The ankle has no more range to give and is at a hard end point.

Snowboarders use the edge of the snowboard to carve the turns with body position and ankle movements. Nearly 25% of snowboard injuries occur during the first day of skiing, and 50% occur during the first season.11,12 Falling is the leading cause of injury. Wrist injuries account for 23%; ankle injuries account for 16.7%; knee injuries account for 16.3%; and shoulder injuries account for 8.3%. About 50% of ankle injures are fractures, some of which are known as "snowboarder's ankle," which are difficult to diagnose because they appear to be severe ankle sprains.11

We conclude that the instructor should place emphases in teaching snowboarders to lean forward or backward, instead of plantarflexing or dorsiflexing the ankle, therefore increasing the absorption of torsion by 20˚. The fear of falling and the body's natural instinct lower the center of gravity. Planterflexion and dorsiflexion are used reflexively to control the turns. Further compounding the problem is that most individuals have tight heel cords, which increase pronation. A proper heel cord stretching program should be part of every snowboarder's routine warm up.

Because of the joint positions, the muscles postulated to control ankle pronation are the intrinsic foot muscles, tibialis anterior, peroneus longus, and posterior tibialis.18 These muscles will work concentrically and eccentrically during weight bearing on the ankles, making the feet and ankle susceptible to tendonitis, bursitis, plantar fasciitis, and ligament laxity. All these conditions have a latency and accumulative period before presenting symptoms. The literature did not associate these conditions to snowboarding injuries.

Monoskiing

Although papers have been published in most every country on techniques for snowboarding and downhill skiing, a paucity of literature exists on the techniques of monoskiing. Monoskiing is just one type of ski that people with disabilities can use, but it is used for competition and elite skiers with disabilities.3 New technologies of standardized frames and shaped skis have made monoskiing easier to manage. Biomechanical assessment of the thoracic and lumbar regions and thoracic scapular complex during the hip angulation phase of monoskiing shows that significant torsion takes place in the lumbar and thoracic spine. With the hips flexed 110˚ and knees flexed about 40˚, the spine is flexed about 20˚ to 80˚ during this hip angulation phase. In the hip angulation phase, the hips are shifted into the hill, causing the lumbothoracic spine to side-bend away from the hill in the flexed position. Therefore, coupled motions of side bending and rotation occur to the same direction in the flexed spine.39

Only two studies looked at the injury of disabled skier. No study has been associated injuries with biomechanics of monoskiing. Ferrara et al.14 found the upper extremity affected 1.4 times more often than the lower extremities and that chronic conditions were greater than acute conditions. Of the chronic conditions, 73.3% of the injuries were related to the shoulder, arm, and elbow complex. There are two explanations for this observation. With the lumbar spine flexed to 80˚ and the disabled skier looking down the hill, the thoracic spine is placed into a flexed posture, creating poor muscle control of the scapula. The scapula is placed into a compromised position, protracted and downwardly rotated. At the same time, although the arm reaches forward with the outrigger, the shoulder goes into about 100˚ to 130˚ of shoulder flexion, creating an impingement condition at the subacromial space. The scapular needs to rotate upward to prevent impingement of the supraspinatus tendon and subscapularis bursa. The other mechanism of injury is that the upper thoracic spine needs to rotate to the same direction as the flexed shoulder. This restriction can inhibit proper function of the shoulder complex.

It is our recommendation that teachers, when instructing monoskiers, should place special attention on preventing thoracic spine flexing, and thereby, on promoting thoracic kyphosis. Keeping the chest up while leaning into the anterior quadrants will help minimize shoulder, arm, and elbow injures. Another, less noted problem is that with excessive hip angulation, the spine may side-bend and rotate to the end of range, stressing the ligaments and creating hypermobility of spinal segments. Weak rectus abdominis, internal and external oblique, and paraspinal muscles that control and stabilize the spine would perpetuate these hypermobility problems.

Acknowledgments

The authors wish to acknowledge the United States Adaptive Recreational Center at Big Bear Mountain Resort for their cooperation throughout this project.

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