Relationship Between Isokinetic Lower-Limb Joint Strength, Isometric Time Force Characteristics, and Leg-Spring Stiffness in Recreational Runners

Neuromuscular characteristics, such as lower-limb joint strength and the ability to rapidly generate force, may play an important role in leg-spring stiffness regulation. This study aimed to investigate the relationship between isokinetic knee and ankle joint peak torque (PT), the force-time characteristics of isometric mid-thigh pull (IMTP), and leg stiffness (K(leg))/vertical stiffness (K(vert)) in recreationally trained runners. Thirty-one male runners were recruited and underwent three separate tests. In the first session, the body composition, K(leg), and K(vert) at running speeds of 12 and 14 km⋅h(–1) were measured. In the second session, isokinetic knee and ankle joint PT at 60°⋅s(–1) were tested. The force-time characteristics of the IMTP were evaluated in the final session. Pearson’s product-moment correlations, with the Benjamini–Hochberg correction procedure, showed that the knee flexor concentric and eccentric and extensor concentric PT (r = 0.473–0.654, p < 0.05) were moderate to largely correlated with K(leg) and K(vert) at 12 and 14 km⋅h(–1). The knee extensor eccentric PT (r = 0.440, p = 0.050) was moderately correlated with the 14 km⋅h(–1) K(vert). The ankle plantar flexor concentric and dorsiflexor eccentric PT (r = 0.506–0.571, p < 0.05) were largely correlated with K(leg) at 12 km⋅h(–1). The ankle plantar flexor concentric and eccentric and dorsiflexor eccentric PT (r = 0.436–0.561, p < 0.05) were moderate to largely correlated with K(vert) at 12 and 14 km⋅h(–1). For IMTP testing, high correlation was only found between the IMPT peak force (PF) and K(vert) at 14 km⋅h(–1) (r = 0.510, p = 0.014). Thus, superior leg-spring stiffness in recreational runners may be related to increased knee and ankle joint strength, eccentric muscular capacity, and maximal force production.