Ultrasound measurement of the size of the anterior tibial muscle group: the effect of exercise and leg dominance

Gaelic football is a high speed field sport played in Ireland and internationally. In common with other field sports involving kicking, physical contact and running, there is a high incidence of ankle and anterior tibial symptoms in Gaelic football players [1, 2]. These symptoms may be associated with dysfunction of the muscles of the anterior compartment of the leg, located anterolaterally between the tibia and fibula, bounded deeply by the interosseous membrane and superficially by the deep fascia of the leg. The muscles in the anterior tibial muscle group (ATMG) include tibialis anterior, extensor hallucis longus, and extensor digitorum longus. As well as kicking from the hand and the ground, Gaelic football also involves an additional kicking skill, called "soloing". In order to travel with the ball, for every four steps taken the player must either bounce the ball or drop it onto the foot, kicking it back to his or her own hand-this is a 'solo', and would be likely to involve significant ATMG activity in dorsiflexing the foot.

Knowledge of normal muscle characteristics is crucial in planning rehabilitation programmes for patients with muscle injury, as reduction in muscle size has been reported in injured populations [3, 4]. Real-time ultrasound has been shown to provide both a reliable [5] and valid measure of skeletal muscle size when compared to gold standards such as MRI and CT [6]. Specifically in the ATMG, Martinson & Stokes [7] found ultrasound to be a reliable method of muscle size measurement, and that cross-sectional area is correlated with thickness measurements. These authors took the scans in transverse section, as this is the appropriate view for CSA measurements, and measured muscle thickness also in this plane. In longitudinal section, muscle fascicles, and fascial and bony interfaces are viewed along their length. In transverse section, they are viewed in cross-section. In an area such as the lower leg, which is convex in shape, aligning the probe in longitudinal section may allow better congruency to the tissue and thus better image quality. However it may be easier to define the area of thickness measurement in transverse section, as due to the circular shape of the muscle, the area of maximum thickness can be easily determined. No studies could be found where muscle thickness measurement in the two planes has been compared in humans. A study comparing scans in the two planes would provide information as to whether one scanning direction has superior reliability or ease of use, and thus make it the recommended direction to use.

OSullivan et al [8] demonstrated a positive effect of limb dominance on muscle size and strength in the hamstrings of Gaelic footballers, however little is known about the effect of limb dominance and sport-specific skills on the dimensions of the anterior tibial muscle group (ATMG). Kelly and Stokes [9] examined cross-sectional area of the ATMG in healthy females by ultrasound scanning, and noted a 7.2% side-to-side difference, however it was not stated whether this difference favoured the dominant limb, or whether there might have been any activity or sport-specific hypertrophy in the subjects.

The results found in this study highlight some important aspects for consideration in measurement of muscle size in athletes. Inter-rater reliability of scan measurement was high, similar to previous research using ultrasound, which reported high reliability for both linear and CSA measurements between scans and days [5]. Measurements from longitudinal and transverse images were not significantly different, and were equally reliable to measure; therefore either method may be used. Longitudinal measurements may allow multiple areas of thickness to be measured on a single image which may be more appropriate for studies assessing change over time, while also providing greater ease of transducer alignment on the lower leg, therefore this would be our method of choice.

Secondly, this study found that a small, but statistically significant, difference exists in the size of the anterior tibial muscle group in dominant, compared with non-dominant, legs in Gaelic Footballers, representing a 7.3% increase in thickness on the dominant side. This significant side-to-side difference was not seen in age-matched, non-football playing controls and therefore, the difference seen in footballers can be attributed to sports specific performance. This is consistent with previous research on soccer players which found a similar dominance effect in the medial gastrocnemius muscles, where a side-to-side difference of 0.18 cm, equating to an overall difference of 6%, was found [10]. Similarly the study by O'Sullivan et al [8] of muscle size in Gaelic Footballers found a significant dominance effect in hamstrings (representing a 6.5% difference), but no such difference in quadriceps muscles. When comparing with the Kelly and Stokes study [9] of the ATMG, a similar side-to-side difference has been identified, however no association was made in that study with side of dominance or sport-related hypertrophy. There was no dominance effect on muscle size in the control group in this study. This finding is relevant for the clinician in terms of gauging recovery in the dominant limb post-injury. The use of ultrasound scanning in rehabilitative settings is increasing [11] and for clinicians with the relevant training, the measurement of muscle dimensions could form a useful outcome assessment for patients undergoing post-injury rehabilitation.

Footballers also had a significantly larger muscle thickness on their dominant leg, compared to the dominant legs of controls, however no such difference existed in comparison of non-dominant legs between groups. This indicates training-induced hypertrophy of the dominant limb in the footballers. Training, and in particular, resistance training, has been shown to result in altered muscle architecture and muscle producing force [12] and hence, the sports-specific actions of Gaelic football (kicking and soloing) may be likened to a strengthening exercise for the ATMG. Martinson and Stokes [7] concluded that the potential of linear dimensions in assessing changes in muscle size with atrophy and hypertrophy has yet to be established. This study provides some evidence that linear dimensions can detect muscle hypertrophy in the dominant limb of active subjects. However, it is not known whether such a measure can detect longitudinal changes in muscle dimensions.

Muscle atrophy following injury has traditionally been measured using measures of limb girth, with atrophy defined as a greater than one centimetre difference between limbs [13]. However, this has proved inaccurate and often grossly underestimates muscle wasting, as it includes a measure of the subcutaneous tissue and fat. Ultrasound imaging is a quick, easily available, reliable alternative [6]. The measurement protocol in this study could be reliably used to examine muscle size changes in athletes with ankle injury to determine whether atrophy occurs in the ATMG, with an appropriate correction for the size of the dominance effect. A longitudinal study examining subjects over a given period would give more information on the time course of the changes to the muscle as a direct result of training, or of injury. Such a study would also be beneficial in the case of injury, to quantify the extent of weakness and the rapidity with which it occurs and so plan an appropriate rehabilitation. Previous studies have found selective muscle hypertrophy, for example within the quadriceps muscle group [14], and therefore a more detailed assessment of the individual muscles within the anterior tibial muscle group may be warranted in order to gain more information regarding structural changes.

The relationship between muscle size and strength was not assessed in this study. However it would be important to assess the relationship between the size and morphology of this muscle group and muscle performance. The anterior tibial muscle has a fibre type composition ranging from 65% up to 90% type 1 fibres [15], which may change under various conditions. Although muscle force and CSA are strongly and directly related in studies of healthy muscle, muscle volume appears to be better related to muscle torque [16]. Therefore the calculation of muscle volume may allow for a more accurate prediction of the contribution of atrophy to muscle weakness [17].

Limitations to this research include small sample size, convenience sample, and possibility of measurement error. Error in scan measurement was reduced by the use of two examiners in measuring each of the scans, and the provision of close training and supervision by a researcher experienced in rehabilitative ultrasound scanning. A sample of convenience was used, however the sample recruited could be considered representative of the Gaelic football playing population. In addition, only one part of the muscle group was measured, and the results may not reflect those that would be obtained using other scanning sites.

In conclusion, a side-to-side difference, favouring the dominant leg, exists in the size of the anterior tibial muscle group in Gaelic footballers. This difference is not seen in inactive, age-matched controls and so is likely to be attributable to the action of kicking in the sport. The clinician may take this difference into consideration when treating football players with a lower limb or ankle injury, by using ultrasound scanning to monitor change in muscle dimensions. Future research could examine this dominance effect in other sports and muscle groups, as well as the association between muscle size and strength or performance.