Force-time characteristics during sustained maximal handgrip effort according to age and clinical condition

Muscle fatigue, a prominent symptom in older patients, can be assessed by sustained maximal handgrip testing. The force decline during sustained maximal contraction is described for young adults, but data for elderly persons are scarce. The aim of this study was to investigate force-time characteristics during a sustained maximal handgrip effort according to age and clinical condition.

Force-time data were continuously recorded during sustained maximal grip effort in 91 elderly patients (aged 83±5years), 100 elderly controls (aged 74±5years) and 100 young controls (aged 23±3years). The force-time curve was divided in 4 parts per 25% strength drop observed. Time (representing fatigue resistance (FR)) was measured during which grip strength (GS) dropped to 75% (FR₇₅), 50% (FR₅₀), 25% (FR₂₅) of its maximum and to exhaustion (FR_exhaustion). Grip work ((GW), the area under the force-time curve) was measured for the 4 parts as well as for the first 20 and 30s of the fatigue protocol test. Strength decay (GW_decay), defined as the difference between the area under the curve (% GW) and a theoretical maximal area under the curve (assuming there's no strength drop), was also studied. In the elderly participants, relationships (controlling for age and sex) of GS, FR and GW with circulating IL-6 and TNF-α were analyzed.

FR_exhaustion was similar for all groups, whereas the duration of each of the 4 parts was significantly different between the 3 groups. FR₇₅ was shortest in old patients (p=0.004), FR_75-50 was almost twice as long in old community-dwelling compared to old patients and young controls (p<0.001). This contrast was inverted for FR_50-25 which was significantly shorter in old community-dwelling compared to the other groups (p=0.013). FR_{25-exhaustion}was significantly longer in young controls compared to the groups of older participants (p=0.017). Old patients showed lower GW for the first 2 parts compared to old community-dwelling and young controls. Also, GW_decay values during the first 20 and 30s were significantly higher in old patients compared to old community-dwelling and young controls (both p<0.001). IL-6 was significantly related to lower GS_max, FR₇₅, FR₅₀, FR₂₅, FR_exhaustion, GW_75, GW₅₀ and GW_75-50.

This is the first study reporting differences in strength decay during a sustained maximal handgrip effort according to age and clinical condition. Old patients showed a particularly rapid decline in GW during the first part of sustained handgrip. GW was also significantly related to circulating IL-6. Future studies should confirm whether a shorter FR test protocol (i.e. until FR₇₅) but using a continuous registration of the strength decay could be more informative in a clinical setting compared to the classical FR test (measuring only FR₅₀).