The effect of knee angle on the external validity of isometric measures of lower body neuromuscular function

Independence of reaction time and response force control during isometric leg extension

In this study, we examined the relative control of reaction time and force in responses of the lower limb. Fourteen female participants (age 21.2 +/- 1.0 years, height 1.62 +/- 0.05 m, body mass 54.1 +/- 6.1 kg; mean +/- s) were instructed to exert their maximal isometric one-leg extension force as quickly as possible in response to an auditory stimulus presented after one of 13 foreperiod durations, ranging from 0.5 to 10.0 s. In the 'irregular condition' each foreperiod was presented in random order, while in the 'regular condition' each foreperiod was repeated consecutively. A significant interactive effect of foreperiod duration and regularity on reaction time was observed (P < 0.001 in two-way ANOVA with repeated measures). In the irregular condition the shorter foreperiod induced a longer reaction time, while in the regular condition the shorter foreperiod induced a shorter reaction time. Peak amplitude of isometric force was affected only by the regularity of foreperiod and there was a significant variation of changes in peak force across participants; nine participants were shown to significantly increase peak force for the regular condition (P < 0.001), three to decrease it (P < 0.05) and two showed no difference. These results indicate the independence of reaction time and response force control in the lower limb motor system. Variation of changes in peak force across participants may be due to the different attention to the bipolar nature of the task requirements such as maximal force and maximal speed.

Incidence of injury and physical performance adaptations during military training

OBJECTIVE

Strenuous physical activity, such as military training, is known to demand a high degree of physical performance and to cause overuse injuries. However, the exact relation between injury incidence and physical fitness level and the influence of military training on measures of functional performance, such as intermittent endurance capacity and maximal jump performance, are not fully described.

METHODS

A total of 330 military conscripts were prospectively followed during military basic training. They were divided into 4 self-assessed physical fitness level groups (well trained, trained, less trained, and untrained) and underwent physical performance tests together with registration of injury incidence.

RESULTS

Twelve weeks of military basic training was found to result in an overall injury rate of 28%, with an inverse relation between physical fitness level and incidence of overuse injury (P < 0.0001). Furthermore a fourfold higher injury rate was observed in the previously untrained soldiers compared with the well-trained soldiers. An increase in intermittent endurance capacity (20-m intermittent shuttle run test) was seen in all groups, (13 to 62%, P < 0.05), whereas only the previously untrained group of soldiers improved in aerobic capacity (8 and 16%, P < 0.05; maximal oxygen uptake and Coopers 12-minute running test). Maximal jump performance, both with and without backpack loading (15 kg), decreased (5 to 13%, P < 0.05) in 93% of the soldiers despite weight loss.

CONCLUSIONS

Military basic training has a positive effect on intermittent endurance capacity but a detrimental effect on jump performance. Furthermore, low levels of physical fitness are strongly associated with proneness toward overuse injury development in soldiers going through intense training.

Muscle strength testing: use of normalisation for body size

Assessment of muscle strength tests has been a popular form of testing muscle function in sports and exercises, as well as in other movement-related sciences for several decades. Although the relationship between muscle strength and body size has attracted considerable attention from researchers, this relationship has been often either neglected or incorrectly taken into account when presenting the results from muscle strength tests. Two specific problems have been identified. First, most of the studies have presented strength data either non-normalised for body size, or normalised using inappropriate methods, or even several different normalisations have been applied on the same sets of data. Second, the role of body size in various movement performances has been neglected when functional movement performance was assessed by muscle strength. As a consequence, muscle function, athletic profiles, or functional movement performance assessed by tested muscle strength have been often confounded by the effect of body size. Differences in the normalisation methods applied also do not allow for comparison of the data obtained in different studies. Using the following allometric formula for obtaining index of muscle strength, S, independent of body size (assessed by body mass, m) should be recommended in routine strength testing procedures: The allometric parameter should be either b = 0.67 for muscle force (recorded by a dynamometer), or b = 1 for muscle torque (recorded by an isokinetic apparatus). We also recommend using body-size-independent indices of both muscle strength and movement performance when assessing functional performance from recorded muscle strength or vice versa.