Date Published: January 25, 2019
Publisher: Public Library of Science
Author(s): Michael Helm, Ramona Ritzmann, Albert Gollhofer, Kathrin Freyler, Kei Masani.
With an emphasis on ballistic movements, an accurately anticipated neural control is an essential prerequisite to deliver a motor response coincidentally with the event of ground contact. This study investigated how previous knowledge of the ground condition affects proactive and reactive motor control in drop jumps (DJ). Thereby, human anticipatory capacity of muscle activation was investigated regarding neuromuscular activation, joint kinematics and peak forces associated with DJ performance. In 18 subjects, the effect of knowledge of two different surface conditions during DJs was evaluated. Peak force, ground-contact-time (GCT), rate of force development (RFD) and jump height were assessed. Electromyographic (EMG) activities of the m. soleus (SOL) and gastrocnemius medialis (GM) were assessed for 150ms before (PRE) and during ground contact (GC) for the short-, medium-, and long-latency responses. Ankle and knee joint kinematics were recorded in the sagittal plane.In the unknown conditions peak force, RFD and jump height declined, GCT was prolonged, proactive EMG activity (PRE) in SOL and GM was diminished (P<0.05). During GC, a decline in EMG activity in the unknown condition was manifested for SOL and GM for the SLR, MLR and LLR (P<0.05). Ankle and knee joint deflections during GC were increased in the unknown vs. known condition (P<0.05). Peak force, RFD and jump height were positively correlated to GM-EMG in PRE, SLR, MLR and LLR (P<0.05). Results revealed that proactive and reactive modulations in muscle activity prior and during GC are interrelated to the force-time characteristics and height of the jumps. The unknown condition revealed a comparable neuromuscular activity during pre-activation for both conditions, followed by an inhibition in the subsequent phase after touch down. These findings underline that anticipation is a determining factor influencing timing and adjustment of motor responses to accomplish ballistic movements regarding precision and performance.
Anticipation is one of the key factors which determines human motor performance . Scientific evidence concerning the role of anticipation revealed an impact of major significance on movement accuracy, timing and efficiency [1–4]. Thereby, the temporal and spatial predictability of the stimuli is the most potent determinant of anticipation.
No subject dropped out. Fig 1 displays changes in GRFs, EMG activity and joint kinematics in dependency of SGS anticipation in one representative subject. Table 1 contains the grand means of the jump characteristics and joint kinematics. A summary of grand means of the neuromuscular data can be found in Table 2.
This study investigated the effect of anticipation on forces, leg muscle activities and kinematics observed in response to changes in surface stiffness during drop jumps; the set was manipulated by providing either known or unknown SGSs. The main findings were (Fig 6): (a) muscle activation and kinematic pre-set prior to the event of GC depends on knowledge of the environmental condition. Muscular pre-activation and joint angles at initial GC did not vary in the unknown condition for both hard and soft surfaces, but varied distinctively in the known condition. (b) Regarding the reactive phase during GC, muscular activity in SOL and GM is downregulated for the eccentric phase in the unknown condition, and ROM of joints is increased in knee and hip. (c) Positive correlations of the GM EMG (PRE, SLR, MLR and LLR) with peak force and RFD underline its importance in terms of performance markers. (d) No knowledge of the upcoming condition led to a decreased performance, reflected by a decline in peak force, RFD and jump height and longer GCT.
Our knowledge about the effect of anticipation on the underlying motor control strategies that humans adopt to cope with the challenges provoked by unanticipated SGS conditions is limited. The findings of this study highlight that when ground stiffness is uncertain, the CNS is not able to adequately adjust muscular activation to accommodate a fast transition from eccentric to concentric action during drop jumps. Thereby, proactive and reactive modulations in muscle activity prior and during GC are interrelated to the force-time characteristics and height of the jumps. Further, the strategy of the CNS appears to be protective in nature through dampening impact loads via an inhibition in shank muscles as well as increased knee and hip joint deflections during GC in situations when unknown conditions are faced. In view of the interrelation of neuromuscular activity and performance, these findings underline that anticipation is a determining factor influencing timing and adjustment of motor responses to accomplish ballistic movements regarding precision and performance.