Handgrip and quadriceps muscle endurance testing in young adults

Grip strength is widely used due to the portability and practicality of grip dynamometry (Bohannon 2002). Grip strength has been shown to be a predictor of general body strength, postoperative complications, mortality and functional decline (Bohannon 2001; Bohannon 2009; Boissey et al. 1999; Massey-Westrop et al. 2004).

Many activities of daily living (ADL) require a sustained effort exerted over a period of time. Therefore, muscle endurance is an important aspect of physical performance and needs to be considered when assessing musculoskeletal function. Recent studies have examined the association between impairments of muscle endurance and underlying pathology relating to morbidity (Hulsmann et al. 2004; Januadis-Ferreira et al. 2006; Coronell et al. 2004). Particular emphasis was placed on the association between respiratory problems, such as chronic obstructive pulmonary disease, and quadriceps endurance (Januadis-Ferreira et al. 2006; Coronell et al. 2004; Van’t Hull et al. 2004; Allaire et al. 2004).

Suitable measurement protocols need to be established to quantify endurance in clinical and epidemiological settings. Differing methodologies and lack of consistency in the use of outcome measures for quantifying muscle endurance, makes comparisons between studies difficult. For example, studies on muscle endurance have used various equipment and protocols, including maximal intermittent contractions (Taylor et al. 2000) and sustained submaximal contractions (Januadis-Ferreira et al. 2006; Van’t Hull et al. 2004; Allaire et al. 2004; Swallow et al. 2007). Intermittent contractions cause slower loss of force than sustained contractions (i.e. less fatiguable), so the results produced by the two types of protocol cannot be compared (Taylor et al. 2000).

Previous research on grip endurance utilised a sustained sub-maximal measurement protocol, representing 50% of the participant’s maximal voluntary contraction (MVC) (Desrosiers et al. 1997; Reuter et al. 2011). A grip dynamometer was utilised but no visual feedback was provided to participants. The above methodology had limitations, as it required high levels of concentration and motivation on the part of the participant, and consistent verbal encouragement from the assessor was needed as some participants could not effectively maintain the exact target (Desrosiers et al. 1997). Maximal intermittent tests involving a 10- repetition maximal contraction was used previously to quantify grip endurance (Nicolay and Walker 2005). Endurance was quantified by comparing the mean value of the first 3 repetitions to the mean value of the last 3 repetitions, and most participants showed a significant decline over the 10 repetitions (Nicolay and Walker 2005).

An investigation using the isometric maximal intermittent endurance test of four muscle groups; finger flexors, thumb abductors, dorsiflexors and plantarflexors, reported significant differences in endurance among the four muscle groups, with lower extremity muscles having a greater endurance capacity than those of the upper extremity (Bemben et al. 1996). There is some evidence on endurance of different muscle groups but given that grip strength is used as a surrogate for quadriceps strength, comparative data on the endurance of the two muscle groups is lacking.

Therefore, the aim of the present study was firstly, to quantify the muscle endurance characteristics of grip and quadriceps muscles and secondly, to examine the association between quadriceps and grip endurance in young adults.

An exploratory study utilising an experimental design was conducted to investigate muscle endurance in healthy young adults. Ethical approval was obtained from the Faculty of Health Sciences Ethics Committee at the University of Southampton. All participants gave written, informed consent.

The present study provides comparative data on the fatiguing characteristics of grip and quadriceps muscles in a sample of healthy young adults. The results highlight that the quadriceps muscle group may have a greater ability to maintain endurance in comparison to hand grip. In addition, males appeared to show greater fatigue than the females for quadriceps but the numbers are too small to draw a definitive conclusion on the effect of gender. Previous research has indicated that men were more fatigable than women during both intermittent and sustained contractions (Hunter et al. 2004). Women had a significantly longer time to task failure (1408 ± 1133 s vs 513 ± 194 s, p < 0.05) compared to men (Hunter et al. 2004).

The differences in endurance between the two muscle groups suggest that caution should be taken when considering using a grip endurance measurement as a surrogate for quadriceps endurance and vice versa. This is in line with previous literature on the association of maximal isometric strength measurements between quadriceps and handgrip (Samuel and Rowe 2011; Bohannon 2008; Samuel et al. 2012). The findings of present study concur with previous research on different muscles, which reported that the muscles of the lower extremity had greater endurance capacity than the muscles of the upper extremity (Bemben et al. 1996). Our findings were largely unaltered when muscle strength was normalised to body mass with the exception of grip endurance in women where the mean of first and last three repetitions was similar. During the present grip endurance task, a fatigue index of 30% was reported, while a considerably lower value of 18% was noted for the quadriceps. This indicates that the grip may fatigue at a higher rate than the quadriceps and may be attributed to differences in the physiological make-up of the two muscle groups. Maximal contractions were used in the present study to assess their utility for clinical testing and depended primarily on anaerobic metabolism and type II fast twitch muscle fibres (Lexell 1995). The ability to maintain adequate levels of endurance depend on physiological factors including fibre type composition, muscle blood flow and maximum force of the muscle group tested (Bemben 1998; Enoka and Duchateau 2008; Hunter et al. 2004). The fast twitch fibres have greater energy demands but lower capability for oxidative phosphorylation compared with the slow twitch fibres. Hence the proportion of different fibre types will affect the fatigue characteristics of a muscle (Wust et al. 2008). The fibre type composition of quadriceps is 50:50 (type I: type II), and with larger motor units and contractile elements, it has a greater potential for muscle performance (Wynsberghe et al. 1995). This is in comparison to handgrip which utilises a much smaller collection of muscles, with smaller motor units available for contraction and higher proportion of type 1 fibres (Wynsberghe et al. 1995). Therefore, they are more likely to fatigue at a faster pace. Furthermore, maximal contractions were used in the present study and the high level of contraction required should be taken into consideration. For instance, if 20% of maximal voluntary contraction had been used, this would have favoured slow twitch muscles, so hand grip may have shown greater endurance than quadriceps. Endurance protocols therefore need to consider the makeup of muscles and it may not be valid to use the same protocol for muscles that have different contraction characteristics.

In addition, factors possibly influencing the results include gender, motivation, extent of verbal encouragement and feedback provided to the participants (Capodaglio et al. 1997). As described earlier, gender was analysed within the two endurance tests, and males showed a greater decline in isometric strength when compared to females in the maximal intermittent quadriceps test. On the contrary, the grip fatigue index values were not significantly different between males and females. The fatigue indices showed higher variability compared to muscle strength and this might explain the lack of relationship between quadriceps and grip endurance. These observations need to be repeated in larger numbers, to provide conclusive evidence of a difference in endurance between genders, ages and muscle groups.

Due to the fact that a maximal effort was required during the endurance tests, participant motivation was vital. However, maximal values were not confirmed by twitch interpolation. Hence, verbal instructions were standardised to encourage maximal force production (McNair et al. 1996). Effort may have been sub-maximal for grip as the muscle fatigued, as an increase in force was observed between repetitions 10–11 (Figure 2). However, this was not the case for quadriceps torque, which declined throughout the protocol. A few participants reported mild discomfort during the grip endurance test. Similar limitations with equipment comfort were reported in previous studies (Nicolay and Walker 2005).

It would be beneficial to add a grip endurance measurement using protocols that can be replicated in a clinical environment. The maximal intermittent protocol was used rather than a sub-maximal endurance test, as maximal contractions would be simpler to perform in clinical situations. In addition, it is difficult to perform a sub-maximal test without visual feedback of a target and not all systems provide this feedback. Maximal tests might be more relevant to older people who may be working at a higher proportion of their available strength during everyday activities (Samuel et al. 2011). However, as mentioned earlier endurance protocols may need to consider the physiological makeup of a muscle and the same protocol may not be appropriate to test different muscles when contraction characteristics differ.

The present findings provide preliminary data on the fatigue index, indicating the endurance capacity of two important muscle groups, handgrip and quadriceps, in healthy young adults. Due to the increasing proportion of older adults in the population, it would be beneficial to test the endurance protocols in this group. Furthermore, larger studies are needed to provide reference data for groups of healthy males and females across the age spectrum. A measure of muscle endurance along with maximal strength may provide a more accurate picture of muscle function and act as an objective marker for rehabilitation.