A primary function of muscles is to create tension and produce force for movement of the body’s skeletal system. The intrinsic property of muscles and joints to go through a full or optimal range of motion (ROM) is referred to as flexibility. It is developed through the use of various stretching procedures. Presently, uncertainty exists about some proposed benefits of flexibility, including its effect on injury avoidance, muscle soreness prevention, muscular strength training and performance improvement. This review article will attempt to clarify these issues with existing evidence-based science and will present a current research update on this component of fitness.
Andersen (2006) suggests that the foundational determinants of flexibility are a multifactorial cluster of elements (see Figure 1). It is also acknowledged that flexibility is a characteristic specific to each joint or group of joints.
Flexibility has been shown to decrease up to 50% with age in some joint areas. From a base of 1,000 elderly men and women, Bassey et al. (1989) showed that shoulder abduction decreased gradually and consistently with age and was about 25% less in these elderly subjects compared with norms for a younger population. Einkauf et al. (1987) examined the changes in spinal mobility for 109 women aged 20–84 years. The results indicated that spinal mobility decreased with age by 20%, 33% and 50% for anterior flexion, lateral flexion and extension, respectively. Brown and Miller (1998) showed that sit-and-reach ROM decreased approximately 30% for women between 20 and 70-plus years of age. Buckwalter (1997) proposes that a gradual deterioration with age in the cell function within cartilage, ligaments, tendons and muscles is the mechanism for this loss of flexibility. Misner and colleagues (1992) add that collagen, a main constituent of connective tissue, becomes dense (and stiffer) with aging. However, Bassey and associates suggest that this loss of motion can be minimized with regular stretching and ROM exercise.
It has been shown that due to minor differences in joint structures and connective-tissue anatomy, women have slightly greater ROM than men for most joint motions. With a sample of 190 male and female subjects ranging in age from 18 to 88 years, Bell and Hoshizaki (1981) measured 17 joint actions in eight specific joints. The female subjects did have greater overall flexibility than the males. In an assessment of the upper-body joints (shoulder, elbow, wrist, trunk and neck) of a group of 41 subjects (22 young male and female subjects aged 25–35 years and 19 mature male and female subjects aged 65–80 years), Doriot and Wang (2006) also found females to have significantly greater ROM in several joint actions. However, these researchers note that the effect of gender on ROM is much less than that of age.
For the most part, active individuals have greater flexibility in the joints they regularly use than in their inactive counterparts. Voorrips et al. (1993) confirmed with a population of 50 mature women (mean age 71 years) that those subjects who regularly did more walking had greater flexibility in the hip and spine (assessed by sit-and-reach test) than their less active counterparts. Kerrigan et al. (2001) declare that these data suggest a very meaningful application with fall prevention. Their comparison of 16 elderly subjects (8 men and 8 women; average age 77 years) with a history of falling to 23 healthy nonfallers (10 men and 13 women; average age 73 years) showed an association between hip tightness and more falls. The authors specifically recommend hip extension stretching as a necessary intervention for fall prevention. Misner and colleagues (1992), in a long-term study with 12 women aged 50–71 years, showed that regular exercise (15–30 minutes of stretching and 30–60 minutes of walking or water aerobics) 3 times per week for 5 years increased shoulder and hip ROM significantly (3%–22% in various joint actions). Exercise also helped the subjects perform activities of daily living more efficiently. Indeed, ACSM (2006) recommends that preventive and rehabilitative exercise programs should include activities that promote the maintenance of flexibility.
Stretching Methods to Increase Flexibility
There are several known methods (and variations within each method) to increase flexibility. These include passive, dynamic, ballistic and static stretching, contract-relax stretching, proprioceptive neuromuscular facilitation (PNF) techniques and Resistance Stretching®. (The figures demonstrate the methodology of each, yet readers should be aware that there are numerous variations and techniques that can be used within each method.)
Passive stretching is usually performed with a partner who applies a sustained stretch to a relaxed joint. It requires close communication between client and trainer, along with a slow application of the stretch in order to prevent a forceful manipulation of the body segment and possible injury. The client is not actively involved in the stretch.
Dynamic stretching incorporates active-ROM movements that tend to resemble sport- or movement-specific actions. For instance, a volleyball player might do some shoulder flexion and extension actions prior to a game. The rhythmic nature of a controlled dynamic stretch has a functional application owing to its similarity to the primary movement task. Dynamic stretching is often incorporated in the “active” phase of class warm-ups.
Ballistic stretching involves a bouncy approach to reach the target muscle’s motion endpoint. A concern with ballistic stretching is that it is often performed in a jerky, bobbing fashion that may produce undesirable tension or trauma to the stretched muscle and associated connective tissues. It may produce a potent stretch reflex that will oppose the muscle lengthening.
Static, or Hold stretching is probably the most commonly used flexibility technique and is very safe and effective. A muscle or muscle group is gradually stretched to the point of limitation (a mild, even tension) and then typically held in that position for 15–30 seconds.
Contract-Relax and PNF stretching techniques were developed by Dr. Herman Kabat in the 1950s as part of his therapeutic work with patients suffering from paralysis and muscular diseases (Sharman, Cresswell & Riek 2006). There are several variations. The contract-relax method involves initially contracting the target muscle, then relaxing and stretching it with an assist from a partner or an applied force (i.e., towel or rope). A variation (contract-relax agonist-contract method) involves performing a contraction of the opposing muscle during the stretching phase to take the target muscle to a new, farther motion endpoint (Sharman, Cresswell & Riek 2006). Traditional PNF techniques involve doing the stretches in diagonal or spiral motions to promote movement through various planes of motion, while contract-relax movement patterns tend to involve single-joint motion through one plane (see the sidebar “Proposed Mechanisms of PNF Stretching: The Controversy”).
Resistance Stretching has gained much attention and interest. It focuses on contracting the target muscles as they are lengthened. In the first phase, the target muscles are placed in the shortened position. Then the client contracts the target muscle(s). While contracted, the muscles are taken through a full ROM (lengthened). So, Resistance Stretching incorporates a strengthening component through the entire ROM. In essence, it is a carefully performedeccentric contraction. The originators of Resistance Training incorporate some very detailed rotational patterns (to challenge the muscle in multiple planes). For further information, go to www.innovativebodysolutions.com.
The most controversial issues with flexibility include injury avoidance, muscle soreness prevention, impact on muscular strength training and performance improvement. Several hundred studies have been conducted on these topics (from randomized and controlled to general observation). To specifically address these questions, recent review articles published in influential, peer-reviewed publications were selected for this current discussion, as these review articles rigorously evaluate and summarize findings across a number of scientific studies and provide the overall “state of knowledge.”
PRE-EXERCISE STRETCHING AND INJURY RISK
Perhaps one of the most exhaustive and comprehensive research reviews on this topic was completed by Thacker et al. (2004). The authors conclude that pre-exercise stretching does not prevent injury among competitive or recreational athletes. Thacker and colleagues support the proposal that this is due to an alteration in joint connective-tissue compliance (ability of the tissue to extend appropriately in response to applied pressure). In some cases, this alteration may lead to greater joint instability. They point out that studies incorporating a pre-exercise combination of resistance exercise, body conditioning and warm-up show promise for better injury prevention. Perhaps this will be a new direction for fitness professionals to pursue.
PREVENTING OR REDUCING MUSCLE SORENESS
Summarizing their findings, Herbert and de Noronha (2009) state that stretching before and after exercise has not been shown to impart any additional protection from muscle soreness. Therefore, stretching does not reduce some of the mechanisms of muscle soreness, including damage to the ultrastructure of muscle, accumulation of calcium ions, cell inflammation, swelling and activation of pain receptors.
IMPACT ON MUSCULAR STRENGTH
When viewing the acute (immediate) effects of stretching before strength training, Rubini, Costa and Gomes (2007) note that static and PNF stretching have shown decreases in maximalstrength ranging from 4.5% to 28%. Yet most of this research used more than one stretching exercise for the same muscle group, with total stretching times of 120–3,600 seconds, which is much more than the recommended four stretches of 30 seconds, totaling 120 seconds, for optimal flexibility increases (ACSM 2006). Rubini, Costa and Gomes add that when the total flexibility session is shorter (30–480 seconds), the research shows little or no compromise from stretching right before maximal force production. Importantly (and practically), exercisers do not train daily to their maximal voluntary contraction, where compromises in strength are observed. Interestingly, Rubini and colleagues highlight that there is no scientific consensus in the research for the underlying mechanism explaining the force production loss in muscle after stretching.
IMPACT ON PERFORMANCE
The studies center attention on the areas of jumping ability, torque (rotary force), running economy and maximal force production. Shrier (2004) reviewed 23 studies, which combined have included static, PNF and ballistic stretching techniques with both genders (from children through adults and untrained individuals through highly competitive athletes). The findings, supported by other reviews (Haff 2006), reveal that regular stretching, when performed at times other than before performance, may elicit positive long-term performance outcomes. However, preperformance stretching may educe (bring out; elicit) insignificant or negative performance outcomes.
This article symbolizes an important triumph for applied research. For many decades, coaches, athletes and others have touted numerous benefits of flexibility. As seen in similar disciplines, the practical beliefs of key pioneers often guide the field. However, as observed with flexibility, many of these beliefs have not proved accurate when challenged through the benchmark of scientific investigation. This does not minimize the importance of flexibility as a component of fitness, yet it better directs professionals who wish to incorporate it into program designs.
The personal trainer applies a slow stretch of the hamstrings. The client is not actively involved in the stretch.
With dynamic stretching the client goes through a controlled active ROM. Ballistic stretching incorporates a rapid bobbing action from the start of the movement to the endpoint of motion.
The client actively brings the target muscle group to the point of limitation.
With contract-relax (Figure 5, top), the stretched target muscle group (hamstrings) is placed into a static contraction, followed by a relaxation. With contract-relax agonist-contract (Figure 6), the opposite muscle group (hip flexors) is contracted against applied resistance. At release, the client stretches to a “new” endpoint of motion. PNF stretching follows a similar technique, except that physical therapists additionally take the movements through various planes of motion.
The client starts in a shortened position (Figure 7, top) against applied resistance (provided by the personal trainer). The trainer then takes the client’s hamstrings through an eccentric action to the motion endpoint (Figure 8). The trainers who developed Resistance Stretching often go through different planes of motion with this type of stretching.