Net muscle force recorded during voluntary contractions with multiple agonist muscles is the summated result of individual muscle forces. The purpose of this article is to synthesize recent findings from several studies on the modulations of muscle activity and force fluctuations during steady voluntary contractions with multiple agonist muscles in humans. During a sustained low-force contraction with the knee extensor muscles, fluctuations in knee extension force changed concurrently with the involuntary alternate muscle activity between the rectus femoris muscle and the vasti muscles. After prolonged bed rest, the amount of change in the fluctuations in net force was associated with the amount of change in the EMG distribution among individual muscles within the knee extensor group and the ankle extensor group. As an adaptation to bed rest, an increase in the fluctuations in plantarflexion force accompanied an increase in the relative contribution of the medial gastrocnemius muscle to net force. In contrast, an increased contribution of the medial gastrocnemius by an acute increase in muscle length reduced force fluctuations. The discrepancy in the changes in force fluctuations due to an increased contribution of the medial gastrocnemius between two interventions (bed rest and muscle length change) is associated with increased low-frequency power in the EMG of the medial gastrocnemius after bed rest. In summary, the fluctuations in net force were influenced by the alteration in muscle activity among agonist muscles in terms of relative contributions to net force and frequency characteristics.