GARCÉS, GERARDO L.; MEDINA, DANIEL; MILUTINOVIC, LJUBO; GARAVOTE, PEDRO; GUERADO, ENRIQUE
The incidence of functional disorders of the neck, typically associated with pain and muscular fatigue, is becoming a severe problem, even among young and middle-aged people, in industrial countries (2). Since the cervical muscles must support the weight of the head, head and neck pain often originate as a result of muscular weakness or from fatigue resulting from sustained muscular contracture (3,4,10,15,17,21,24,25). In addition to being an area susceptible to minor pain, the neck is frequently injured in accidents and during athletics, particularly in contact sports (8,10,17,22,24).
Patients with upper cervical pathologic abnormalities show reduced neck strength (2,13). Although strengthening the neck may help to prevent neck pain and injury, effective training programs for the cervical muscles have not been established (10,17). The importance of strengthening neck musculature to reduce the risk of injury and alleviate neck pain has been well documented (2,6,7,10,11,17,18,26). Unfortunately, most of these studies have not been well-controlled investigations, so the effectiveness of training programs remains uncertain. This could be because of the lack of equipment available to safely and accurately quantify cervical flexion and extension strength (9,10).
Strength values of the flexor and extensor of the cervical spine have recently been studied using handheld dynamometers (21). However, it has been shown that these measurements provide poor reliability because strength measurements may vary according to the strength of the investigator (9). There are several studies regarding strength levels of the cervical musculature; however, these studies have used very few subjects (9). There have been two primary reasons for this paucity of data. The first has been the lack of instrumentation available to provide valid and reliable assessment of extensor and mainly flexors of the cervical spine. The second (possibly preventing the development of such instrumentation) has been the previously held feeling among clinicians that strength testing and training of the non–cord-injured cervical spine could be dangerous, further aggravating the injury (7,9,10). However, clinical evidence points to the efficacy of muscular strengthening in neck rehabilitation programs (2,7,10,11,17,18,26). Thus, there is a need for a reliable and accurate test of cervical flexion and extension strength for evaluation of exercise training protocols for the development and rehabilitation of the cervical muscles (10).
Clinical instrumentation is now available that can provide reproducible quantitative assessment of both extensor and flexor strength (9,23). Therefore, the purpose of this study was to describe the maximal and mean isometric strength of the cervical musculature at 0°, 5°, and 10° of flexion and extension of the neck in a healthy population using a Kin-Com® (Chattanooga Group, Inc., Hixson, TN) computerized dynamometer. A reliability study of the Kin-Com® computerized dynamometer measuring system was also undertaken.
In order to determine if isometric cervical musculature strength is age, sex, and body mass dependent, a descriptive cross-sectional study was carried out. After receiving oral and written information, 94 healthy individuals (51 men and 43 women) agreed to participate in the study. For this sample size, considering a precision of ± 5 Nw, the study had a statistical power of 0.89. Descriptive characteristics of the subjects participating in the study are presented in Table 1. Documented consent was obtained from each volunteer and approval to carry out the study was obtained from the institution’s ethics committee. None of the participants had experienced neck pain for at least 1 yr before testing, and none had engaged in training of the neck and/or shoulder muscles for at least 6 months before testing. Volunteers with cardiac or circulatory disorders were excluded. Before the strength assessment was carried out, an orthopedic surgeon evaluated all patients to rule out any spinal disease or deformity.
Before testing, subjects performed 5 to 10 min warming-up of flexion-extension and right-left rotation of a standardized protocol. Testing was performed using a Kin-Com® computerized dynamometer. Volunteers were seated and then restrained via a seat belt and a crossed shoulder harness that restrained the torso to prevent any additive strength effect from trunk musculature during the testing procedure (Fig. 1). The seat belt aided in a similar fashion by securing the pelvis in the seat. After the torso was stabilized and the testing position standardized, the subject’s head was moved into its neutral position. The lever arm was then placed in the nasal-frontal area in order to evaluate the isometric strength of the cervical flexor musculature. Each patient was then asked to push the lever arm pad forward as much as possible for 3 s at 0°, 5°, and 10° of flexion of the neck. In order to test the extensor musculature, the lever arm pad was placed at the occiput and then each patient was encouraged to push backward as much as possible for 3 s at 0°, 5°, and 10° of extension of the neck. One set of three repetitions was performed for each angle, allowing 1 min of rest between sets. Flexion was always tested before extension, starting at 0° for both flexion and extension. Subjects were verbally encouraged to exert maximal effort. A preload of 25 N was used in order to counterbalance the effects of gravity during the testing.
Measurements were expressed in newtons (N). Average, peak torque, and coefficients of variations for each test of each angle were computed. Results were expressed as absolute values and values normalized to index of body mass (weight in kilograms over height in square meters). In order to compare results, individuals were separated by sex and age. According to ages, they were integrated into group A (between 20 and 40 yr old), group B (between 41 and 60 yr old), and group C (older than 60 yr). Descriptive statistics were calculated using the SPSS® statistical package (SPSS, Inc., Chicago, IL). A bivariate correlation analysis was performed on the values obtained for peak and averaged isometric strength from both the men and women to determine the association between age and isometric force. A Student’s t-test (two-tailed) or ANOVA was used to compare results among groups. A Student’s t-test for paired samples was used to compare test-retest differences.
To determine reliability of the study, 22 volunteers (12 men and 10 women) underwent maximal isometric strength testing during two different sessions, the second session being undertaken a week after the first one. The measurements were performed with a 30-s rest period between contractions, and the peak values as well as the average values were used for the analysis.
Statistical analysis was performed using the two-tailed Student’s t-test for paired data on the basis of the individual peak and averaged values obtained from the two sessions performed, and the paired-samples correlation derived. These values were used to express both intratest and intertest variability.
For the intratest study, no significant differences were found between the second and third trials, both for flexion and for extension, proving a correlation ranging from r = 0.951 to r = 0.989. For the intertest study, no significant differences were found among trials both for flexion and for extension. Correlation was found to range from r = 0.731 to r = 0.969. The intratest and intertest coefficient of variation for both peak and averaged isometric strength was less than 15% for all tests in flexion and for tests in 0° and 5° of extension. However, the coefficient of variation reached 26% for measurements of the peak isometric strength at 10° of extension.
A bivariate correlation analysis was performed on the values obtained for peak and averaged strength from both men and women to determine the association between anthropometric data and isometric cervical strength. A strong negative correlation (r = −0.517 to −0.607) was observed for both men and women between age and strength (P < 0.0001). A strong positive correlation (r = 0.464 to 0.535) was found between weight and strength (P < 0.001). The strongest correlation (r = 0.540 to 0.638) was found between height and strength (P < 0.0001) for women and men.
When comparing the strength of men and women, men proved to be around 30 to 40% stronger for averaged flexion and extension at all angles for all ages (Table 2). However, maximal isometric strength differences between sexes decreased with age (the youngest group showing a difference of 37–45%, with the other groups showing a difference of 30–35%, and men showing greater force than women in all groups (Table 3)). No significant differences between men and women or between groups of age were found when comparing flexor/extensor ratio at any angle of flexion or extension. This ratio oscillated between 0.5 and 0.9 for both sexes (Table 4).
When comparing men for age groups, highly significant differences were found for average flexion and extension between the youngest group and the other older groups. However, no statistical differences were found between the two oldest groups. The youngest group was around 25% stronger in flexion and extension than the middle-aged group and around 40% stronger than the oldest group. The middle-aged group was 10 to 30% stronger than the oldest group (Tables 2 and 5). Mean strength was greater at 10° than at 5° and greater at this angulation than at 0° for both flexion and extension. Maximum strength differences between groups were similar to average strength differences. However, peak torque was greater at 0° than at 5° and greater at 5° than at 10° (Tables 3 and 6).
For average strength in flexion and extension, the youngest group of women proved to be 20 to 30% stronger than the middle-aged group (P < 0.001), whereas the middle-aged group was 20 to 30% stronger than the group over 60 (P < 0.001) (Tables 2 and 5). Equally to men, mean strength was greater at 10° than at 5° and greater at this angulation than at 0° for both flexion and extension. For peak strength in flexion, significant differences were only found between the youngest and the oldest groups. The former was less than 10% stronger than the middle group at 0° and 5°, reaching 22% at 10°. This middle group was 20 to 30% stronger than the group over 60 in all positions (Table 3). There was less than 10% difference for peak strength in extension between the youngest and the middle groups (Table 6). The middle group was 20 to 30% stronger than the oldest group (P < 0.001). Also, the female group showed that peak torque was greater at 0° than at 5° and greater at 5° than at 10°. When values were normalized to index of body mass, differences were similar to those found for absolute values (Tables 7 through 10).
The importance of strengthening the neck musculature to reduce the risk of injury, alleviate neck pain, and in rehabilitation has been well documented (17). Therefore, the knowledge of cervical isometric strength is very useful in sports medicine and rehabilitation. It has been proven as a reliable tool to objectively measure the functional improvement after the rehabilitation process in patients with cervical pain. Moreover, increases in cervical isometric strength are accompanied by a reduction in perceived neck pain in patients undergoing cervical rehabilitation (2,7,18,26). The isometric cervical strength increases significantly in healthy populations, as well as after specific training (10,11,17).
Currently, the lack of literature published about cervical isometric dynamometry is an obstacle to obtaining valid conclusions. The few articles that use dynamometry to evaluate cervical isometric strength use different dynamometers, making comparisons of results difficult. This problem is even aggravated by the absence of a standardized evaluation protocol.
Some authors have studied the isometric cervical strength of flexors and extensors (2,9,23,26), and others have only studied strength of flexors (1,20,25). However, the dynamometer most frequently used in the literature is the MedX® Cervical extension machine (MedX Corp., Ocala FL). This dynamometer enables proper isolation of the cervical extensor muscles using belts that restrain any movement from the torso. On the other hand, it only allows the evaluation of cervical extensors using a specific testing protocol (7,8,10,17). Our study validates the use of the Kin-Com® computerized dynamometer as a reliable and reproducible tool with which to study both flexion and extension isometric cervical strength. Both intratest and intertest reproducibility are greater than that shown by other dynamometers (1,9,10,20,21,25).
In our study, besides the dynamometer used, the most important difference with other authors is patient positioning. It is known that isometric cervical strength changes with testing position (5), being significantly stronger in the prone position than in the sitting position. However, most researchers use the sitting position because it is more comfortable for patients (2,7–10,17). Vernon et al. (23) and Ylinen et al. (26) used the standing position in their studies, whereas other authors chose prone (5,21) or supine positioning (1,20,25). In our study, we selected the seated position for two reasons: it is the most used in the literature and the most comfortable for the patients. In this position, it is very easy to evaluate the isometric cervical strength with the head in neutral positioning, the position at which the cervical muscles show the highest and most effective electric activity compared with maximal flexion and extension positioning of the cervical spine (14,19). In this respect, there are also great differences of the studied angles in the literature. We limited our study to 10° of flexion and extension because it is an easily achievable angle even for injured patients.
In addition to positioning and angle, duration of muscle contraction is not standardized. Like other authors (5,17,21,23), and based on the work of McIntyre (16), we decided to take the last 3 s. Even knowing that some researchers perform gravity correction (1,9,10,20,21,25) we did not carry this out because our study took place just between 0° and 10° of flexion and extension. For this arch of movement, our dynamometer registered 0 N at 0°, and 13 N at 10°. Since we programed a preload of 25 N, we believe that gravity did not affect our results.
Muscular strength and body weight have shown a high positive correlation (r = 0.80) in sportsmen (8,21); however, this correlation in a normal population is much lower (9,12,21). Our research shows a high positive correlation between weight and height with isometric cervical strength in men, and height and cervical isometric strength in women. Recently, Jordan and colleagues (9) found a highly positive correlation between height and cervical isometric strength only in men; however, they did not find any correlation between body weight and cervical isometric strength. Although some authors published their results normalized to body weight (20,21), our results expressed both in absolute values and in values normalized to body weight showed similar differences between sex and age groups.
Our results show a statistically significant decrease of cervical isometric strength with age in men and women. Some authors have reported similar results for men, but they found that decrease of strength in women was not significant (9,21). In our study, cervical isometric strength of women proved to be 30 to 40% weaker than men in all age groups studied. These results are slightly higher than those observed by Jordan et al. (9) and slightly lower than those published by Staudte and Dühr (21).
Most researchers have observed the highest isometric cervical strength in flexion and extension at cervical neutral position (2,10,14,17). However, Jordan et al. (9) relate the increase of maximal isometric strength with the increase of the angle of flexion and extension. In our study, we found maximal isometric strength to be higher at neutral position than at 5° and 10°, for both cervical flexion and extension. On the other hand, average cervical isometric strength in flexion and extension increased as the angle of testing increased. These differences between our results and those obtained for other authors may be because of a higher precision of our equipment; the Kin-Com® computerized dynamometer’s software automatically reports the averaged and maximal force during the test.
The cervical flexor and extensor isometric strength ratio in our study was around 0.6 for both sexes in all angles evaluated. This ratio remained constant in all age groups, suggesting that a decrease in strength related to age evenly affected the cervical flexors and extensors. These results are similar to those shown by other authors (9,23).
Although there is not a standardized method for evaluation of cervical isometric strength, our study shows that the Kin-Com® AP dynamometer allows a highly reproducible and reliable evaluation, being very easy to use in comparison with other evaluating devices available. Higher variability observed at 10° of extension may be because of higher difficulties in controlling dorsal flexion of the head at this neck angle in comparison with 0° and 5°.
Several main conclusions are derived from this investigation. Averaged and maximal cervical isometric strength of men are 30 to 40% stronger than women for cervical flexors and extensors. This difference is maintained in all angles explored and in all age groups. There is a significant high negative correlation between cervical isometric strength and age. The ratio between cervical flexors and cervical extensors remains around 0.6 in all age groups for both men and women. The maximal cervical isometric strength, for both flexion and extension, is developed at neutral position of the neck (3,4,6,13,18,22).
Address for correspondence: Gerardo L Garcés, M.D., Ph.D., Instituto Canario de Ortopedia y Traumatología, Presidente Alvear 10, 35006 Las Palmas de GC, Canary Islands, Spain; E-mail: firstname.lastname@example.org.
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