# Increased Motor Time in the Lower Extremity Is Related to Fall History in Older Women
## 1 Introduction  
Falling is a serious threat to seniors' safety and health. It is a leading cause of physical injury, disability, or unnatural death in this population [1]. Most fall events are initiated by unexpected postural perturbations and result directly from the failure to perform a quick and effective reaction, i.e., latency of reaction time (RT) [2]. It is also well documented that the latency of RT increases with ageing, especially in those over 70 [3] and women [4, 5]. RT is defined as the length of time elapsed from the presentation of a stimulus to the initiation of a reactive movement, influenced not only by the central processing capacity, but also by peripheral effectors, such as a muscle's architecture and properties [6, 7]. Through synchronous recording of surface electromyography (sEMG) and body reactive action, RT can be fractionated into two temporal components for central and peripheral processing, called the premotor time (PMT) and motor time (MT), respectively [6]. The PMT is estimated as the timing interval from the presentation of a stimulus to the initial sEMG burst, while MT indicates the latency from the initial sEMG burst to the initiation of movement. The fractionated RT technique has been frequently employed to observe the relationship between components of RT and ageing. Previous studies demonstrated that both the PMT and MT [8], or just MT [9, 10], were significantly longer in the older women than their young counterparts. Importantly, recent studies have focused on fractionated RT in older adults with high fall risk in comparison with their counterparts without fall risk, or with low fall risk. [PERSON] al. fractionated RT by defining the onset time of a muscle as the first EMG burst exceeding the baseline by 3 SD for more than 25 ms in a choice step reaction test. They both found that PMT and MT latencies were longer in the fallers than the non-fallers [11]. However, using a similar step reaction task, [PERSON] et al. failed to find any difference in PMT and/or MT between the high- and low-fall-risk groups when the fractionated RT was determined by the first normalized EMG burst to be greater than baseline by 2 SD [12]. During these choice step reaction tests, the motor time was possibly confounded with the movement time due to stepping movements. In a simple RT paradigm, [PERSON] et al. demonstrated that the fallers produced greater composite scores for MT from four muscles in the lower extremity compared to the non-fallers, although no difference in PMT was detected [13]. On the other hand, [PERSON] et al. did not find any differences in PMT or MT between the fallers and non-fallers by using a simple RT test with a seated knee or ankle response, in which EMG onset was defined by the first value greater than 55.0 of peak EMG [10]. It appears that the different research protocols, i.e., sEMG onset and the type of RT, might cause inconsistent results for fractionated RT by fall conditions among older adults. In addition, these previous studies did not make efforts to test how the subject responded to a slip or a trip. First, each of those studies used a visual stimulus in the RT test, while the natural stimulus of a fall is usually unexpected body perturbation. Second, participants in the previous studies reacted to signal stimulation with a pre-planned movement, such as stepping or knee/ankle rotation in the sagittal plane, but in restoring upright stability, an automatic postural response (APR) is typically adopted to counteract a perturbation like stumbling or slipping [14]. Thus, whether the PMT or MT predisposes the reaction time to being longer and makes older adults more susceptible to fall risk is unclear, and a postural perturbation should be used as stimulus to induce fall-related recovery action with the aim of measuring fractionated RT. Muscle weakness in the lower extremity due to ageing has been well documented as contributing to falls [9; 15; 16].