You have undoubtedly heard warnings about the use of exercises referred to as “contraindicated.” Exercises such as the deep squat, unsupported forward flexion, the plow, the hurdler’s stretch, the back bend, and both the full and straight-leg sit-up have typically been considered unsafe for the general population, and you have been encouraged to avoid incorporating them into your programs. Yet you may have noticed that many of these same movements are commonly used in yoga and Pilates programs, seemingly without major concerns about safety.
In fact, a review of the research in numerous textbooks, professional articles and scientific journals reveals little direct evidence to justify the total banning of many of these exercises. In this article we will discuss two of them—the deep (full) squat and unsupported forward flexion—reviewing the research and evaluating the risk of injuries.
of Risk Evaluation
Information evaluating exercise risk comes from research in biomechanics and ergonomics, two fields that consider the principles of physics, forces, movement and tissue adaptation.
Tissues that play a role in movement are subjected to forces, or loads, that act on the tissues and contribute to either their adaptation and strengthening, or their failure and injury. The goal of the fitness professional is to promote positive adaptations while minimizing injury risk. The safety of a given load is influenced by several factors, including the type and size of the load, the nature and health of the tissue subjected to the load, how and when the load is applied, and the frequency with which it is applied. (See “Factors Influencing Exercise Injury Risk,” left.)
In addition, a client’s postural habits and daily routines (e.g., prolonged sitting, repetitive lifting) are important considerations when assessing risk, because these factors can alter the tissues’ ability to tolerate loads (McGill 2002).
The Deep Squat
Some of the earliest research examining the safety of squats comes from the United States military, which found a link between squatting and increased knee instability (Anderson 2003). The reasoning was that squats compromised the stability of the knee by stretching the ligaments. Later research has presented mixed findings. While some researchers have found that squats do not increase knee instability, regardless of the degree of knee flexion, other researchers have found that knee instability increases with deep, or full, squats (those that go beyond 90 degrees of knee flexion) (Chandler, Wilson & Stone 1989; Chandler et al. 2000). Many studies have pointed out that factors other than ligament flexibility (e.g., muscle strength, motor control, prior injury) can also influence knee stability.
Concerns About the Deep Squat. Deep squats have been found to increase loads placed on the femoral cartilage (Raske & Norlin 2002), knee joints (Escamilla et al. 2001) and posterior cruciate ligament (Toutoungi et al. 2000), thereby increasing injury risk—especially among individuals with a history of knee injury (Toutoungi et al. 2000). Another concern is that during deep squats, pressing the posterior leg muscles (calf and hamstrings) together causes compression stress, which, in turn, causes the axis of rotation to shift away from the knee and places greater stress on the connective tissues of the knee joint (Kreighbaum & Barthels 1996). Individuals with larger posterior leg musculature are probably at greater risk of injury from this particular stress.
The literature on knee injuries does not answer the question of whether exercises similar to the deep squat (e.g., yoga’s balasana, or child’s pose) increase the risk of injury.
Safety Precautions. Completely eliminating the squat from all exercisers’ programs may be an overreaction. The squat is a functional exercise that is helpful in knee rehabilitation and in promoting
coordinated muscle function at the hip and knee for both sport and everyday
activities (Alter 1996; Toutoungi et al. 2000). But the following precautions should be taken into consideration when incorporating squats into a client’s program:
- The depth of a squat should be based on the individual’s ability and training goals. A client who lacks the muscle strength to stabilize the knee, or whose knee is already compromised from a previous injury, is more likely to be injured by deep squats (Anderson 2003; Escamilla et al. 2001; Toutoungi et
al. 2000) and should perform them cautiously—if at all. (Generally, researchers discourage the performance of deep squats.)
- If a client chooses to perform deep squats, decrease the risk of knee injury by gradually progressing the knee flexion depth to safely match load and ability.
- Limit the duration of full flexion to minimize the effect of end-range-of-motion, compression and shear (friction) forces.
Exercise #2: Unsupported Forward Flexion
The spine is one of the most complex structures in the body, and the risk of placing various loads on it is not fully understood. Many studies have looked at back injury and rehabilitation; yet the safety of unsupported forward flexion (UFF) is still debated.
UFF can be performed with the back flat, hips flexed and spine in neutral; or with the back rounded and the hips and spine flexed (see illustrations). Opinions differ about which position is safer (Alter 1996; Phillips & Clippinger-Robertson 1987).
One of the most comprehensive scientific sources on back health is the research of Stuart McGill in Low Back Disorders: Evidence-Based Prevention and Rehabilitation (Human Kinetics 2002). According to McGill, UFF places unique stresses on the spine, including compressive and shear loads, which are commonly linked to spinal injury. These loads interact with each other and are influenced by spinal stability, repetition of flexion, range of motion, movement speed and time of day (McGill 1998; McGill 2002).
Compression Loads. When the spine flexes forward, eccentric muscular contractions and vertebral flexion create compressive loads on the disks. These loads exceed those deemed safe by the National Institute of Occupational Safety and Health, since they are greater than 3,300 newtons (N) (McGill 2002). As the spine reaches its end range of motion during full flexion, its tolerance for compressive loads decreases, putting the disks and supportive ligaments at even greater risk of injury (Adams, Dolan & Hutton 1987; McGill 1998).
Shear Loads. Flexion also places shear loads on the spine, and these loads can cause more harm than compressive loads when the spine is fully flexed (McGill 2002). Shear loads greater than 1000 N are considered unsafe (McGill 2002). But performing UFF with the spine in neutral alignment places less load on the posterior spinal ligament (McGill 2002). In addition, in this position, the stabilizing contraction the back extensor muscles must perform to keep the spine in neutral helps resist the shear loads. Hence, minimizing spinal flexion during UFF will decrease the load on the spine and the risk for injury.
Safety Precautions. Although research supports the need for caution, completely banning UFF from exercise programs is unnecessary. In fact, many exercises and daily activities involve spinal flexion. To help reduce the risk of spinal injury during these movements, advise clients to avoid the following:
- spinal flexion during the first hours of waking, when disk hydration, internal pressure and disk injury risk are greatest (Adams, Dolan & Hutton 1987; Dolan, Earley & Adams 1994; McGill 1998)
- spinal flexion at the end range of
motion, where the spine is weakest (McGill 2002)
- prolonged, rapid, repeated or ballistic spinal flexion, all of which decrease the spine’s load tolerance and the spinal ligament’s ability to prevent movement at the end range of motion (Adams, Dolan & Hutton 1987; Dolan, Earley & Adams 1994; McGill 2002)
- spinal flexion when back extensor muscles are fatigued and thereby less able to prevent movement at the end range of motion (Caldwell, McNair & William 2003)
Implications for Practice
The concept of contraindicated exercises was introduced to the fitness industry to promote client safety and decrease the risk of injury. Injury research on deep squats and UFF, however, does not support the complete banning of these exercises. When deciding if any exercise is safe for an individual client, you need to address several factors, including the nature of the tissue to be challenged, the technique used and the load, as well as the client’s injury history and fitness level. Each factor affects risk—and interacts with the other factors that affect risk. In addition, consider your client’s lifestyle. A fitness program should be designed to counteract the daily movement and postural habits that increase the risk of injury. If deep squats and UFF are part of a client’s everyday life or recreation, you can decrease injury risk with a program that improves strength, flexibility and motor control, thereby protecting the knees and back.
Alter, M.J. 1996. Controversial stretches. In Science of Flexibility (2nd ed., pp. 209–30). Champaign, IL: Human Kinetics.
Anderson, O. 2003. Squatting exercise: How safe is squatting? www.pponline.co.uk/encyc/0827.htm; retrieved Feb. 19, 2007.
Caldwell, J.S., McNair, P.J., & Williams, M. 2003. The effects of repetitive motion on lumbar flexion and erector spinae muscle activity in rowers. Clinical Biomechanics, 18 (8), 704–11.
Chandler, J., et al. 2000. Safety of the squat exercise: Current comment from the American College of Sports Medicine. www.acsm.org/Content/Content
Folders/Publications/CurrentComment/2000/SQUAT.pdf; retrieved Mar. 1, 2007.
Chandler, T.J., Wilson, G.D., & Stone, M.H. 1989. The effect of the squat exercise on knee stability. Medicine & Science in Sports & Exercise, 21 (3), 299–303.
Dolan, P., Earley, M., & Adams, M.A. 1994. Bending and compressive stresses acting on the lumbar spine during lifting activities. Journal of Biomechanics, 27 (10), 1237–48.
Escamilla, R.F., et al. 2001. Effects of technique variations on knee biomechanics during the squat and leg press. Medicine & Science in Sports & Exercise, 33 (9), 1552–66.
Kreighbaum, E., & Barthels, K. 1996. Deep squat. In Biomechanics: A Qualitative Approach for Studying Human Movement (4th ed., pp. 203–4). Boston:
Allyn and Bacon.
McGill, S.M. 1998. Low back exercises: Evidence for improving exercise regimens. Physical Therapy, 78 (7), 754–65.
McGill, S.M. 2002. Low Back Disorders: Evidence-
Based Prevention and Rehabilitation. Windsor, ON: Human Kinetics.
Phillips, G., & Clippinger-Robertson, K. 1987. Has
the bend been banned? Aerobics & Fitness (January-February), 20–26.
Raske, A., & Norlin, R. 2002. Injury incidence and prevalence among elite weight and power lifters. The American Journal of Sports Medicine, 30 (2), 248–56.
Toutoungi, D.E., et al. 2000. Cruciate ligament forces in the human knee during rehabilitation exercises. Clinical Biomechanics, 15 (3), 176–87.