There are several morphological parameters that influence the stability of a cervical spine under external constraint. Those elements have been cited in the literature review of Stemper in 2011 [25].

These parameters define passive stability, whereas active stability is ensured by the paraspinal cervical muscle.

Even with the lack of studies that define paraspinal muscular incompetence as a risk factor of serious accidents, it has been known for several years that this musculature intervenes during impacts to decrease the extent of the motion [26, 27]. This protective mechanism consists of a simultaneous contraction of all the muscles inducing an axial contraction of the whole cervical spine [28, 29].

At the experimental level, some evidence seems to imply that the muscle mass plays a role of “active protection” during a cervical spine trauma. Recently published laboratory measurements suggest that an important muscle force of the cervical spine reflects more efficient active protection in the case of a violent trauma of the head [30]. These results seem like they would be useful for screening of individuals at risk of severe injury but they were not confirmed by a study Mihalik undertook on 37 ice hockey players during one season [31]: Analysis of the acceleration meter in the players’ helmets showed there was no less-violent trauma in subjects with higher muscle force peaks.

The biomechanical role of the paraspinal muscles as a passive protector has been significantly less studied. Nonetheless, its importance seems to be far from negligible and its role increases along with the stretching constraints. The deep muscles are involved first then the superficial larger ones which have more significant leverage [32, 33]. Regarding this subject, analysis by finite element method seems to be promising, allowing simulations of different accidents. The use of a valid experimental model has recently established that the muscular absorption of constraints linked to a traumatic shock were minimal in the case of traumas in hyperextension. This result suggests higher spine vulnerability in this position [33].

Finally, as it ages, the cervical spine becomes rigid and loses its flexibility [34]. Biomechanically, this loss of flexibility exposes the subject to an increased risk of spinal lesions [35, 36]. This will in turn expose athletes at the end of their career to an increased risk of cervical osteoligamentary rupture in the case of an excessively hyperflexed posture. Therefore an athlete at career’s end should be extra vigilant about preparing the cervical spine muscles for action.

When exposed to a violent impact, the response-time is one of the critical parameters. It has been studied in particular for whiplash type accidents. EMG of the sternocleidomastoid muscle reveals that the latent time after sudden acceleration is slightly longer for men than women, about 8 ms and becoming even longer with age [29, 37, 38]. Overall, displacement of the cervical spine will occur between 20 and 40 ms after an impact causing displacement of the head [39, 40]. The muscular delay in activation occurs around 120 to 150 ms with the peak of activation ranging between 200 and 250 ms in subjects aged between 20 and 30 exposed to the most violent traumas [41]. During a direct shock impacting the top of the head, which happens with the spear tackle in American football [4, 42], spinal lesions were noticeable within 2 ms [40].

This muscular contraction is divided into two phases: a reflex contraction and a voluntary contraction [29]. Brault also notes the risk of a muscular lesion if the reflex contraction arises after the head has already started a movement. As for trunk muscles, Moorhouse and Granata estimate that the reflex muscular activity is around 40 % responsible for maintaining stability when facing a postural perturbation [43].

These activation delay notions are of extreme importance in some collision sports where spinal cervical trauma is associated with skull trauma that might lead to a concussion [44]. At that moment, the player on the verge of losing consciousness can no longer “defend” himself through muscular recruitment allowing him to rigidify his spine.

As with all striated muscles of the human body, spinal muscle masses are also subject to fatigue. In fact, maintaining the head in a desired position requires a large recruitment of muscles, particularly if the spine is subject to supramaximal constraints during an intense physical effort. A contraction is called exhausting if it is maintained at a tension level superior to 15 % of the maximal force of a given subject, the endurance being defined by the amount of time during which such a contraction can be maintained [45]. These elements might explain why the majority of accidents occur in the beginning of the season when physical preparation hasn’t yet reached its objectives, as suggested by Mall who has analyzed the collected data from the (American) National Football League over a period of 11 years [46], although other authors find that these lesions become more frequent as the season progresses [47]. As for age related effects, studies indicate that the delay in action of the muscular recruitment becomes progressively longer and the muscular healing following intense physical activity becomes longer as well, although this has not been specifically proven in the case of spinal muscles [48].

Many authors recommend implementation of cervical spine physical preparation programs for high risk activities following a traumatic event [49, 50]. Furthermore, in some disciplines, the condition of the cervical spine constitutes a performance factor that allows the player to get the better of the adversary.

Numerous studies have proved that a reinforcement program is effective for improving the performance of the cervical musculature [51–53]. Various preparation schemes using isometric or static protocols have been described, although neither is clearly superior. Note that even though the benefits of conditioning the cervical musculature are recognized with regards to maximal isometric force, dynamic stability under constraint does not seem to be significantly improved.

After a problem affecting the cervical spine, many authors agree that return to play cannot be expected until the muscular force has returned to normal [49].

On the other hand, paraspinal muscular incompetence would be an interesting risk factor to screen for in a severe accident.

On this basis, it seems useful to evaluate two parameters:

The maximal muscular force can be measured through isometric or dynamic methods. Isometric measurements are mostly used. To mention a few:

Muscular fatigability is defined as the loss of the muscle’s capacity to generate a force. This refers us to the definition of endurance as the capacity to maintain repeated muscular efforts or generate a force in a given period. Assessment of these parameters is rarely used in clinical practice. To mention a few:

So far, there is no normal value for these measures. In fact, it seems that the large variability in evaluation protocols as well as the heterogeneity of the studied population explains to a high extent the difficulty in obtaining reliable and reproducible values [54, 55]. In this context, consider the study by Oliver who has assessed a sample of premier league rugby players in an attempt to define some norms in regard to their positions, keeping in mind that some are more exposed to spine trauma [56]. Unfortunately, these measuring systems and the expertise of a well-trained operator can be hard to obtain. Couvet considered this problem and asked how to determine whether performance on isometric tests could be correlated to simple evaluations. The results, awaiting confirmation on larger series, seem to indicate that simple and inexpensive evaluations could contribute in keeping away, from some high risk positions, those subjects who failed to pass these tests. The French Rugby Federation has since implemented a technical passport partially based on those results [57].

Finally, qualitative analyses of muscular mass seem promising in high level sports fields but they are currently still at their beginning and do not allow definition of precise objectives in terms of physical preparation. The development of dual-energy X-ray absorptiometry (DXA) which is a three component model, allows analysis of bone mineral density in addition to muscle mass and fat mass [58, 59]. Volume analyses are not routinely used, although it seems that single muscle area analysis would help to account for the importance of the whole musculature [60].

Rugby is a contact sport and players are exposed to cervical spinal cord accidents [47, 61–64]. Many studies have identified the risks associated with periods of the game and with positions. (Fig. 12.2).

For several years now, the French Rugby Federation has implemented a preventive policy aiming to limit the number of severe medullary accidents related to the practice of rugby. We can clearly describe the annual prospective epidemiological data collection of these accidents, rules modifications, as well as screening measures of subjects at risk based on the clinical assessment with cervical spine MRI cuts for professionals and players registered on the sports ministry lists [5, 65].

In France, the ability of a player to move towards a high risk position is evaluated on two levels:

The player receives a certificate stating that he is “capable of playing in first line”, and which the referee should verify before each game.

This evaluation has two stages.