Reliability of doming and toe flexion testing to quantify foot muscle strength

The ability to measure foot muscle strength accurately and reliably is necessary for clinicians and researchers to enable them to monitor strengthening or identify weakness [1]. Direct strength measurements of these muscles is challenging, due in part to the lack of commercially available equipment. In addition, the intrinsic foot muscles (IFM) have many of the same actions as the extrinsic lower limb muscles, therefore it can be difficult to isolate and assess the strength of only the intrinsic foot muscles. It may be more appropriate to test strength during specific movements rather than individual muscles, due to the simultaneous involvement of both intrinsic and extrinsic muscles.

In recent years, studies have shown that IFM strength can have varying effects on lower limb pathologies and balance [2]. In particular, IFM weakness or altered activation has been associated with multiple issues in the foot and lower leg including, pes cavus in patients with Charcot-Marie-Tooth disease [3], heel pain [4, 5], claw toe deformity [6, 7], hammer toe deformity [6, 8], hallux valgus [2, 9], and posteromedial shin pain [10, 11]. People with plantar fasciitis have been found to have weaker IFM [5] and/or lower IFM volume [12] than those with healthy feet. Recent research has also shown that IFM weakness has been associated with an increased risk of falls in older people due to their role in stabilizing the foot and maintaining balance [2, 13, 14].

Previous studies have used a variety of methods in an attempt to measure the strength of intrinsic foot muscles. Most studies have measured toe flexor force using hand-held dynamometry where the dynamometer was fixed stationary as the participants maximally pushed onto the dynamometer with their toes [1, 8, 15]. Other tests have included the Paper Grip Test, plantar pressure, or the Intrinsic Positive Test [2, 10, 16‐19].

The Paper Grip Test and plantar pressure, combined, resulted in reliable measures of the intrinsic plantar flexor muscles (plantar pressure ICC 0.87 for lesser toes and 0.88 for hallux; no reliability calculated for the Paper Grip Test) [19], similar to those found when using hand-held dynamometry (ICC range 0.81–0.94, depending on the rater and the muscle group tested – ankle plantar flexion, lesser toe plantar flexion, or hallux plantar flexion) [1]. Another approach to measuring intrinsic foot muscles is known as the Intrinsic Positive Test. This test involves the researcher evaluating the participant’s ability to perform intrinsic foot muscle contractions where the participant extends the hallux while simultaneously flexing the lesser toes at the MTP joint and extending the interphalangeal joints [10]. This method of testing is less reliable than using the Paper Grip Test or hand-held dynamometry because it is not quantifiable [2]. Each test described here has a major drawback and, as such, the use of the test for research or repeated clinical purposes is less than ideal.

Another limitation to existing testing methods is the lack of data generated regarding strength during movements other than toe flexion. The doming exercise (also known as the short-foot exercise) is used by many clinicians to engage and strengthen the intrinsic foot muscles. To this point, no one has tried to quantify strength associated with performing this movement.

In order to measure strength during functional movements including doming, hallux flexion, and lesser toe flexion, we developed new methods of assessment using dynamometry, acknowledging the contribution of the intrinsic and extrinsic foot muscles in performing these movements. This included developing the only method that we are aware of for quantifying strength during the doming motion, as well as an alternative method of testing toe flexion strength that may be advantageous with regard to simplicity, expense, and/or reliability. The purpose of this study was to evaluate the reliability of these new methods.

ICCs showed good to excellent reliability for all tests (T1, T234, and doming) between days, raters, and sessions (Table 1) [20]. When tests were performed on the same day, the ICCs ranged from 0.71 to 0.93 for inter-rater reliability and 0.94 to 0.99 for inter-session reliability. The ICCs for testing performed on different days ranged from 0.90 to 0.95 for the same rater, and 0.80 to 0.82 for different raters. Results for ICC_(2,k) and ICC_(3,k) were numerically the same, as explained in Portnoy and Watkins (2009).

Average doming strength was 99.96 ± 47.04 N. Average hallux flexion strength was 65.66 ± 24.5 N. Average lateral toe flexion was 50.96 ± 22.54 N. Average force values for each statistical analysis are included in Table 2.

The purpose of this study was to determine the reliability of novel methods of measuring muscle strength during doming and toe-flexion exercises. Inter- and intra-rater reliability were good to excellent for all tests. These results indicate that these methods of testing are repeatable among our rater group, which were moderately trained. The results of the ICC_(3,k) indicate that this repeatability can be expected from any group of raters who perform this testing. Therefore, these tests may be used for research and/or clinical purposes. The results from the inter-rater, inter-day and intra-rater, inter-day comparisons suggest, however, that for the most reliable comparisons, when a patient or research participant is tested multiple times, the same rater should perform the testing.

The doming strength measurements presented in this study represent a unique assessment that has not been analyzed before. Therefore, we are unable to compare our results to others. However, this is an important measurement to be able to quantify because this movement involves the recruitment of the tibialis anterior and posterior, flexor digitorum brevis, quadratus plantae, and abductor hallucis. These muscles have been shown to play an important role in arch support, the control of pronation, and postural control [13, 21]. Strengthening of the abductor hallucis has also been suggested as a treatment for hallux valgus [22], and pes planus [23]. Therefore, doming is a common exercise used by healthcare professionals in the treatment and rehabilitation of these and other foot pathologies. Having a reliable, non-invasive, and relatively cheap method of measuring the strength and function of muscles that support/lift the arch would allow clinicians and researchers to better monitor the effects of interventions (such as strengthening or orthotic use) and/or pathologies on the function of the foot. However, based on feedback from our subjects, doming is an unfamiliar movement and, for the most reliable results, it should be taught and practiced prior to the testing session for best results.

Average toe flexion strength values from the current study are comparable to, though often lower, than those from previous studies that employed dynamometry and tested toe flexor strength from the hallux and lesser toes separately. Direct comparisons between the current data and previously published data is difficult due to the differences in equipment and the positions used to test. With that said, the hallux flexion strength measurements reported here are very similar to the pre-intervention measurements in subjects with pes planus reported by Jung et al. [23] (62.2 ± 34.6 N and 62.5 ± 29.2 N). Meanwhile, our results are much lower than those reported by Spink et al. [1] (132.9 ± 31.1 N), possibly due to differences in positioning. Our tests were performed with the ankle in a neutral position. This was chosen because it is within the range of motion for common activities, such as standing, balancing, and walking. In this neutral position, both long and short flexor muscles contribute to the strength measured during toe flexion. In contrast, Spink et al. [1] tested subjects lying supine, with the ankle in plantarflexion and with a hand-held dynamometer placed at the interphalangeal joint of the hallux, which was in an extended position. This would alter the length-tension relationship and the force production of the muscles. Subjects were then instructed to flex the toe against the operator’s resistance. This may have allowed for increased contribution of the flexor hallucis longus when compared to our testing position, in which subjects started the test with the toe already flexed. Quek et al. [24] reported hallux flexor strength (79.58 ± 37.83 N), but the data was collected while subjects were standing rather than sitting. Spink et al. [1] were the only previous researchers to report flexion strength of the lesser toes in isolation from the hallux. Once again, our values are quite a bit smaller than those reported in that study (103 ± 27.5 N), likely for the same reasons previously discussed.

Many researchers have performed functional measures of foot muscle strength, though most test just the toe flexors. Some of these methods include qualitative testing, such as the Paper Grip Test, pushing against examiner resistance, or the Intrinsic Positive Test [1, 10, 16, 17, 25]. Our tests showed as good or better reliability than the Paper Grip Test and pushing against examiner resistance. No reliability has been reported for the Intrinsic Positive Test. In order to measure IFM strength and/or strength changes more accurately, various quantitative methods have been used as well. These include various uses of dynamometry, plantar pressure, and magnetic resonance imaging (MRI) [1, 5, 8, 15, 18, 19, 26‐30]. While MRI may be considered a gold standard for measuring muscle size, it is not a direct measurement of muscle strength. In addition, MRI is expensive and difficult to obtain and analyze. Therefore, it is imperative that reliable quantitative methods of measuring functional foot muscle strength are developed. These quantitative methods have shown better reliability than the qualitative ones. The reliability of our measurements are similar to those reported from the aforementioned quantitative methods.

Quantitative strength testing methods vary by equipment used, as well as the number and action of the muscles tested. Studies that have reported foot muscle strength obtained via dynamometry have had subjects use all toes together [5, 15, 24, 29, 30], each toe separately [8], or (as in the current study) the hallux and a group of the lesser toes separately [1, 19]. Most of these tests were performed by having the subject push down on a bar or cuff with their toe(s) [5, 8, 15]. Others have had subjects grab a bar or cuff with their toes and pull into maximal flexion.

While the current data shows that the tests performed during this study are reliable, there are a few limitations that should be noted. Although our subjects reported that the carabiner and bar were comfortable, gripping and pulling with individual toes (particularly the lesser toes) may be an unfamiliar task, resulting in greater concentration on gripping the device than contracting the muscles. During the lateral toe flexion testing, most subjects naturally wanted to invert their foot to assist with the flexion movement. However, since the goal was to isolate flexion strength, the researchers made sure the foot was planted flat on the board during these tests, thereby limiting inversion. As previously stated, the testing positions were chosen because they placed the ankle in a neutral position, allowing for recruitment of the intrinsic and extrinsic muscles as they would be during a portion of common activities such as standing and walking. However, because of the small range of motion involved in these tests, these strength measurements may not be reflective of maximal activation during movements requiring larger ranges of motion and greater force, such as walking and running. Of the three tests performed for this study, the lesser toe flexion test was the most difficult to set up, which may explain the lower ICCs between raters. Getting the subject to grip the T-bar effectively, achieving the baseline threshold by adjusting the turnbuckle, and positioning the foot on the support boards are all potential sources of variability between raters. In addition, the lesser toe flexion tests were the last set of tests performed during each session. It is possible that subjects may have experienced some fatigue of the foot muscles.

In conclusion, our three novel tests showed good reliability between testers and on repeated days of measurement. These are simple tests using relatively low cost equipment which could be used in a variety of situations to compare feet within a subject and/or monitor foot muscle strength changes in a population of interest.